Cenomanian and Turonian (Early Late Cretaceous) Multituberculate Mammals from Southwestern Utah

Cenomanian and Turonian (Early Late Cretaceous) Multituberculate Mammals fromSouthwestern UtahAuthor(s): Jeffrey G. EatonSource: Journal of Vertebrate Paleontology, Vol. 15, No. 4 (Dec. 27, 1995), pp. 761-784Published by: Taylor & Francis, Ltd. on behalf of The Society of Vertebrate PaleontologyStable URL: http://www.jstor.org/stable/4523668 .

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Journal of Vertebrate Paleontology 15(4):761-784, December 1995 @ 1995 by the Society of Vertebrate Paleontology

CENOMANIAN AND TURONIAN (EARLY LATE CRETACEOUS) MULTITUBERCULATE MAMMALS FROM SOUTHWESTERN UTAH

JEFFREY G. EATON Weber State University, Department of Geosciences, Ogden, Utah 84408-2507

ABSTRACT- Multituberculate faunas from the Dakota Formation (upper Cenomanian) and the Smoky Hollow Member of the Straight Cliffs Formation (middle or upper Turonian) are described. The presence of Cimolodon is established in the Cenomanian; a new genus, possibly cimolodontid, is present as well. A new genus (Dakotamys) is described from the Dakota Formation, and another (Bryceomys) from the Smoky Hollow Member. Neither of the new genera are assigned to suborder or family, and both are morphologically closer to Paracimexomys than to other described taxa. Paracimexomys and, possibly, cimolomyids are present in both faunas. The difference in composition of mammalian faunas from the Cenomanian to Turonian may reflect ecologic changes causing geographic shifts in mammalian faunas rather than an episode of catastrophic extinction.

INTRODUCTION

Early Late Cretaceous multituberculate faunas have been described only from southwestern Utah. Eaton (1987) described multituberculates from the Kaipa- rowits Plateau of southwestern Utah, which at that time included few specimens of Cenomanian through Santonian age. Eaton (1988) and Eaton and Cifelli (1988) reviewed the faunas recovered from the Kai- parowits Plateau which were, at that time, dominantly faunas of Campanian age. Since then, many important Cenomanian localities were discovered and extensively worked by Eaton, and new Turonian localities were discovered and worked cooperatively by Eaton and Richard Cifelli (Oklahoma Museum of Natural His- tory). These localities yielded abundant vertebrate re- mains including hundreds of teeth of mammals. The therian mammals of the Cenomanian were described by Cifelli and Eaton (1987), and Eaton (1993a). A preliminary account of the Turonian therians was given by Cifelli (1990). The present paper represents the first systematic description of the Cenomanian and Turo- nian multituberculate fossils recovered from southwestern Utah.

LOCALITIES AND CORRELATION

One Cenomanian locality (Museum of Northern Ar- izona locality 939) is on the west side of the Paunsau- gunt Plateau, near the town of Alton; all remaining localities are in Bryce Valley, east of Bryce Canyon National Park (Fig. 1). All localities were plotted on U.S. Geological Survey 7.5 minute topographic quad- rangles. Detailed locality data for Museum of Northern Arizona (MNA) localities may be obtained from the Department of Geology, Museum of Northern Ari- zona, Route 4 Box 720, Flagstaff, Arizona 86001; Oklahoma Museum of Natural History (OMNH) localities from the University of Oklahoma, Norman, Oklahoma 73019; and University of Colorado Mu-

seum (UCM) localities from Campus Box 315, Uni- versity of Colorado, Boulder, Colorado 80309. MNA locality 1003 and OMNH locality V4 are the same locality.

The Cenomanian localities occur in the middle member of the Dakota Formation and the Turonian localities occur in the Smoky Hollow Member of the Straight Cliffs Formation (Fig. 2). The Cenomanian and Turonian vertebrate localities are separated strat- igraphically by the upper member of the Dakota For- mation, the Tropic Shale, and the Tibbet Canyon Member of the Straight Cliffs Formation (Fig. 2), which represent the marine transgression and regression of the Greenhorn epeiric seaway (Eaton et al., 1987). Ea- ton (1991) suggested that the middle member of the Dakota Formation was probably late Cenomanian in age; argon/argon dates from this member, which range from 92.9 + 0.2 Ma at the base to 90.5 + 0.1 Ma near the top (Bohor et al., 1991), support this. However, Kowallis et al. (1989) provided argon/argon dates for the latest Cenomanian in the same geographic area and stratigraphic horizon of 94.7 + 0.4 Ma and 94.5 + 0.2 Ma, considerably older than those of Bohor et al. (1991 ). These two sets of argon/argon dates are incompatible, making it difficult to assign an isotopic date to the fauna.

The Smoky Hollow Member of the Straight Cliffs Formation overlies the middle Turonian Tibbet Can- yon Member (Eaton, 1991) and may be either late middle or early late Turonian in age. The Turonian is only a few million years long (e.g., Harland et al., 1989) and, as the fauna from the Dakota Formation is probably of late Cenomanian age, the two faunas are separated by about two to three million years.

METHODS

Fossil localities were extensively quarried and recovered matrix (about 10,000 kg) was screen-washed

761



762 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 15, NO. 4, 1995

89Bryce Valley

Tropic

Bryce Canyon National Park

Alton UTAH

Scale

._ . .. _ Miles 0 Kilometers L.- . "A Kilometers

FIGURE 1. Study area location map.

as described in Eaton and Cifelli (1988) and Eaton (1993a). Specimens were measured by use of a camera lucida which superposed the specimen image onto graph paper with line to line distances calibrated to 0.1 mm. All measurements are given in mm; the hundredth of a mm value was estimated when the dimension lay between calibrated lines on the graph paper. Termi- nology and orientation of measurements for each tooth position is shown in Figure 3.

All specimens are isolated teeth. Association of isolated multituberculate teeth is problematic. Appropri- ate width and length ratios, fit of interstitial facets, frequency of occurrence, and shape and depth of cusps were used to taxonomically group isolated specimens of different tooth positions. Such associations must be considered tentative until jaws are found.

A large variety of anterior upper premolars was pres-

ent in the samples. Without maxillaries, associating these with the appropriate taxon would be a speculative task and was not attempted as part of this paper.

All figured specimens (except lower incisors) were cast in photo-gray epoxy and the casts were photo- graphed at a magnification of 1.2. All photographs were enlarged to twelve times natural specimen size so com- parisons can be made directly from figure to figure. Stereopairs are provided for all occlusal views ("osp" in figure captions). In many cases, several specimens of the same tooth position and taxon are illustrated to show variation within the sample.

ABBREVIATIONS All specimen numbers preceded with a "V" are MNA

specimens. All others are OMNH specimens unless another acronym is used.



EATON- CENOMANIAN AND TURONIAN MULTITUBERCULATES 763

MNA Museum of Northern Arizona, Flagstaff OMNH Oklahoma Museum of Natural History, Nor-

man UOSM former acronym of OMNH UCM University of Colorado Museum, Boulder AP anteroposterior dimension LB lingual-buccal dimension H:L height to AP length ratio of fourth lower pre-

molars * actual (not estimated) dimension of an in-

complete tooth SD standard deviation CV coefficient of variation 3:3:4 cusp formula (or ridge-serration-ridge for-

mula for p4) from external to internal cusp row.

R in M2 cusp formulas, external row (ridge) osp with no distinct cusps

indicates occlusal stereo pair in figure captions

SYSTEMATIC PALEONTOLOGY

Cenomanian Multituberculates

Class MAMMALIA Order MULTITUBERCULATA Suborder PTILODONTOIDEA Family CIMOLODONTIDAE CIMOLODON Marsh, 1889

CIMOLODON sp. cf. C. SIMILIS Fox, 1971 Table 1, Fig. 4A-J

Specimens-MNA Loc. 1067: p4-V5863, V5872, V5881, V5885, V6030; P4-V5358, V5898.

Description, p4-External ridges range in number from 9-10, internal ridges from 8-10, with 11-13 serrations. V5863 (Fig. 4A-B) has a serration count identical to that of Cimolodon similis (12; Fox, 1971), is within the range of C. nitidus (12-14; Clemens, 1964), and is less than known for C. electus (15; Fox, 1971). The blade has a high profile relative to its length (H:L = 0.62) diagnostic (Clemens, 1964:56) of Cimolodon. The apex of the arch is at serrations 4-5. The anteroexternal lobe is very deep. The first serration is well up the anterior face (e.g., V5881, Fig. 4C-D) and a relatively weak posteroexternal cusp is present. V5885 (Fig. 4E-F) is unusual in having a very strong, pillar- like posteroexternal cusp. V6030 (Fig. 4G-H) has 13 serrations, with 9 external and 10 internal ridges. It is the largest blade (AP = 4.92) included in this species and has the lowest H:L ratio (0.55). The highest point of the blade is reached anterior to the midpoint, at the fourth or fifth serration. A posteroexternal cusp, with its top surface oriented anteroventrally, is present.

P4-V5358 (Fig. 4I-J) is large (AP = 3.51", LB = 1.90") and has a cusp formula of 2:5. The tooth is broad with a greatly expanded anteroexternal platform. The cusps of the medial row have a low angle of descent and are strongly striated. A groove, present on the posteroventral face, climbs posteriorly to the labial

0c 4

u) formation member m

STRAIGH T Smoky Hollow 40

CLIFFS Tibbet Canyon 50

0 m-

*=*) TROPIC 200

-I SHALE

upper DAKOTA middle x 50

lower

2 -

FIGURE 2. Age and stratigraphic relationship of Ceno- manian-Turonian mammal-bearing sequence within the study area. All fossils described here were recovered from the middle member of the Dakota Formation and the Smoky Hollow Member of the Straight Cliffs Formation. Approximate stratigraphic horizons of vertebrate localities indicated by "X." Cross hatching indicates unconformity with underlying Jurassic rocks.

cusp of the posterior basin. The posterior basin is short relative to the length of the tooth. V5898 is badly worn and incomplete but has the strongly striated cusps present on V5358 and appears to be about the same size.

Discussion - Some characteristics of the p4s, such as the presence of the apogee of the crest at the fourth serration and the shortness of the first and second external ridges, are shared by Mesodma primaeva. These specimens, however, are considered to represent Cim- olodon rather than Mesodma because the anterior face of p4 is initially vertical in Mesodma, whereas on the MNA specimen and p4 of Cimolodon, the anterior face leans strongly posteroventrally. The p4s are highly arched (high H:L ratios) and have the appropriate




m2-Occlusal view p4 - Labial view M 2- Occlusal view internal acusp

r external ---- LB-- row cusp row

- - .. -lst serration 2

S 3.•.

" 2

2 .1 9th external Aridge A jP ) d

SI ridge (RI

externa- internal postero- ---

cusp row cusp row external LB cusp " .... " " " central

ml- Occlusal view rvalley LB ---

-- antero- M1-Occlusal view external

S'-lobe LB

p4- Lingual view

intr internal AP internal

1 : cusp row

external X\central cusp row valley medial p4-Occlusal view cusp row

central '•external - valley cusp row

P4 - Occlusal view posterior -basini

AP P4 - Lingual view

Santero- 5 ...external H3 platform

LB ----+ medial cusp row

FIGURE 3. Terminology and the tooth orientations used for measurements in this report. Anterior is down on page for occlusal views. The height (H) measurement of fourth lower premolars was taken perpendicular from a line formed where the enamel meets each root on the lingual side of the tooth to the apex of the tooth.

number of serrations with the first occurring high on the anterior face typical of p4s of Cimolodon. The p4s have fewer serrations than C. electus (15; Fox, 1971), straddle the range of C. similis (12; Fox, 1971), and are within or just below the range of C. nitidus (12- 14; Clemens, 1964). The profiles of the p4s are most similar to those of C. electus and C. similis (see Fox, 197 1:figs. 3d, e; 5c, d). The largest p4 is just above the

known size range of C. similis and about 80% the size of C. electus (based on Fox, 1971:table 2). The size range of these p4s overlaps that of teeth assigned to the "Genus and Species Undetermined" below, but differs in having a higher H:L ratio and the first serration occurring higher on the anterior face.

The P4s are similar to those of C. electus (see Fox, 1971:fig. 4a, b) and C. similis in overall morphology,



EATON- CENOMANIAN AND TURONIAN MULTITUBERCULA TES 765

AA

EG

a."r5i.~

FIGURE 4. Cenomanian multituberculates; Cimolodon sp. cf. C. similis. A, Lp4, V5863, lingual; B, same, labial; C, Lp4, V5881, lingual; D, same, labial; E, V5885, Rp4, lingual; F, same, labial; G, Lp4, V6030, lingual; H, same, labial; I, RP4, V5358, osp (occlusal stereo-pair); J, same, labial. Scale bar = 1 mm.

including the proportionally short posterior basin, with cusps high on the posterior wall, and the broadly expanded anterolabial platform. As is the case with the p4s, these specimens are within the expected size range of C. similis (4.1; Fox, 1971 :table 2) and are estimated to be about 80% of the size of C. electus (based on Fox, 1971:table 2). Both the upper and lower fourth premolars most closely approximate C. similis of known cimolodontids.

It is odd that only fourth premolars of this taxon are present in the sample. It is apparent that there is a

taphonomic bias at these localities, probably reflecting hydraulic conditions during deposition. Nonetheless, these specimens are adequate to establish the presence of Cimolodon in the fauna.

Genus and Species Undetermined Table 1, Fig. 5A-M

Specimens-MNA Loc. 939: P4-V5625; M2- V6101. MNA Loc. 1067: p4-V5347, V5379, V5870, V5888, V5897, V6026, V6031, V6043, V6045, V6057,




TABLE 1. Measurements in mm of Cimolodon and ?cimolodontids from the Dakota Formation. The "*" indicates actual, not estimated, measurement of an incomplete specimen.

N Range Mean SD CV Cimolodon sp. cf. C. similis

p4 AP 5 3.78-4.92 4.13 0.46 11.1 H 5 2.35-2.70 2.47 0.16 6.5 H:L 5 0.55-0.65* 0.60 0.04 6.3

P4 AP 1 3.51" LB 1 1.90*

?cimolodontid, gen. et sp. indet. p4 AP 10 3.22-3.88 3.51 0.16 4.6

H 7 1.90-1.95 1.93 0.02 1.0 H:L 6 0.50-0.56 0.54 0.02 4.5

m2 AP 2 1.62-1.65* 1.64 P4 AP 4 3.00-3.33 3.14 0.16 5.1

LB 4 1.30-1.45 1.37 0.06 4.6 H:L 4 0.41-0.46 0.44 0.02 5.0

M2 AP 12 1.72-2.05 1.87 0.10 5.3 LB 11 1.40-1.75 1.57 0.10 6.4

V6311; m2-V5365, V5878; P4-V5360, V5873, V6147, V6314; M2-V5351, V5667, V5868, V5895, V6002, V6054, V6335, V6441, V6489, V6774, V6777, V7326.

Description, p4-The number of external ridges ranges from 7(?)-9, internal ridges from 7-10, and serrations from 10-12. V6031 (Fig. 5A-B) is typical of the p4s in having 9 external and 8 internal ridges, with 10 serrations. The anterior face of the tooth is tilted posterodorsally. Ridges on both sides of the tooth are similarly spaced and are oriented anteroventrally. A less typical specimen, tentatively included here, is V5870 (Fig. 5C, D), which is the smallest of the specimens and has fewer internal ridges (7). The crest is more arched than that ofV6031 and the anteroexternal lobe is positioned relatively more anteriorly than on V6031.

Second Lower Molar - Two m2s, both heavily worn, are referred to this taxon. V5878 (Fig. 5E) has crenulated enamel and pits in its central valley. The cusps of the external row do not appear to be divided on the labial wall of the tooth. There is no posterolingual notch behind the internal cusp row.

P4-V5360 (Fig. 5F, G) has a cusp formula of 2:6, with an additional two tiny cuspules on the external row, such that the formula approaches 4:6. The anteroexternal platform is broadly expanded and the tooth narrows posteriorly. The apices of the cusps of the medial row are striated and a line connecting the tips would be slightly convex. The cusp apices descend at a relatively low angle. A broad wear facet is present on the posteroventral surface of the tooth. Relative to its length, the tooth has a long posterior basin (37% of tooth length). The tooth is concave lingually in occlusal view. V5873 (Fig. 5H, I) has more weakly developed

external cuspules than seen on V5360 and the posteroventral surface of the tooth has crenulated enamel (removed by wear on V5360). V6314 (Fig. 5J, K) has a cusp formula of 2:6 with the first cusp of the external row much smaller than the second. The cusps of the medial row descend at a slightly lower angle than those of V5873 or V5360. Within this sample, there seems to be some variation in the number and size of cusps on the external row and some variation in the rate of descent for cusps in the medial row.

M2-V7326 (Fig. 5L) has a moderately high cusp formula of 3:3:5. The enamel is complexly striated and pitted. The internal cusp row is almost continuous, with little cusp division either labially or lingually. V6774 (Fig. 5M) appears to have a higher cusp formula of R:4:6(?).

Discussion--The presence of abundant fourth premolars and second molars without first molars of appropriate size is troubling. Either these fourth premolars and second molars belong to a taxon with atyp- ically sized first molars or there was some taphonomic bias favoring preservation of fourth premolars and second molars at these localities.

The p4s resemble those of cimolodontids in that the anterior face of the tooth is angled posterodorsally, as in Cimolodon (see Fox, 1971 :fig. 5c; Clemens, 1964: fig. 23a) and unlike the condition in Mesodma, Cim- olomys, and Cimexomys (in which the anterior face is initially more nearly vertical). The first serration is low on the anterior face of the tooth, unlike p4s of Cim- olodon, but the arch of the tooth and the number of serrations approach the condition found in Cimolodon. Mesodma, Cimolomys, and Cimexomys have flatter- crested p4s; the p4s of Cimolomys and Cimexomys bear fewer ridges than those of Cimolodon.

The m2s have deep crenulation and pitting of the enamel, and the cusps of the external row are not deeply separated on the labial wall of the tooth as in some specimens of Cimolodon (see Lillegraven, 1969:fig. 11.6a-c; Fox, 1971 :fig. 5b). The length of these m2s is 71% of the mean for m2s of C. similis (calculated from Fox, 1971:table 2), while the p4s are 83% of the length of those assigned to C. similis. Either tooth proportions are different for this species or the m2s and p4s do not belong to the same species.

The P4s are concave lingually (in occlusal view), as in Cimolodon, and the cusps of the medial row descend such that the arch they produce is similar to that of Cimolodon (cf. Clemens, 1973:fig. 30). Some specimens of Cimolodon have a relatively long posterior basin (38% of total length, Clemens, 1973:fig. 30) though it should be noted that specimens assigned to C. nitidus by Archibald (1982:fig. 20) have short posterior basins. The relative size of the posterior basin may not be taxonomically useful because the posterior basin of P4s assigned to the same species of Cimolomys may be either short (Archibald, 1982:fig. 23) or long (Clemens, 1964:fig. 34). The broad anteroexternal platform is typical of Cimolodon, as is the complex enamel




C Al Akc

-ID

*

F

G

.H

as K

M

FIGURE 5. Cenomanian multituberculates; Cimolodontidae, genus and species undetermined. A, Lp4, V6031, lingual; B, same, labial; C, Rp4, V5870, lingual; D, same, labial; E, Rm2, V5878, osp (occlusal stereo-pair); F, RP4, V5360, osp; G, same, labial; H, RP4, V5873, osp; I, same, labial; J, RP4, V6314, osp; K, same, labial; L, LM2, V7326, osp; M, RM2, V6774, osp. Scale bar = 1 mm.

on the posterodorsal face above the posterior basin (Clemens, 1964:fig. 30). The cusps of the medial row are more deeply separated and the tooth does not broaden posteriorly as in Cimexomys hausoi (see Ar- chibald, 1982:fig. 38). The angle of descent is similar to that of the medial cusp rows of P4s assigned to C. antiquus (see Fox, 197 l:fig. 3) but P4s of that species are smaller, lack striated cusp tips, and have fewer cusps.

The M2s have complexly wrinkled enamel in the area of the external row and the internal cusp row is ridge-like, with poorly separated cusps, as in C. electus (see Fox, 1971:fig. 5a). As is the case with the m2s,

these second molars are about 70% the length of those of C. similis.

These fourth premolars and second molars have many characteristics of Cimolodon but are not clearly referable to that genus. The p4s have less arch and are 83% of the length ofC. similis. The P4s are also smaller than those belonging to known species of Cimolodon in being 77% the size of C. similis and 7 1% of the size of C. nitidus. The second molars are about 70% the length of those of C. similis and do not necessarily represent the same taxon as the fourth premolars. It is possible that a new genus and perhaps more than one species of cimolodontid is represented in this sample.




TABLE 2. Measurements in mm of Paracimexomys from the Dakota Formation.

N Range Mean SD CV

Paracimexomys sp. cf. P. robisoni ml AP 5 1.72-1.81 1.76 0.04 2.3

LB 5 1.02-1.17 1.09 0.06 5.5 m2 AP 3 1.52-1.53 1.53

LB 3 1.33-1.43 1.39 P4 AP 3 2.19-2.32 2.24

LB 3 0.90-0.99 0.94 Ml AP 1 1.82

LB 2 1.08-1.13 1.11 M2 AP 1 1.68

LB 1 1.52

Paracimexomys sp. ml AP 1 1.32

LB 1 0.98 m2 AP 2 1.00-1.08 1.04

LB 2 1.00-1.02 1.01 P4 AP 2 1.82-1.83 1.83

LB 2 0.82-0.85 0.84 Ml AP 3 1.51-1.72 1.58

LB 3 0.92-0.99 0.96 M2 AP 4 1.12-1.28 1.17 0.08 6.8

LB 4 1.02-1.12 1.09 0.05 5.5

Suborder and Family, incertae sedis PARACIMEXOMYS Archibald, 1982

PARACIMEXOMYS sp. cf. P. ROBIsoNI Eaton and Nelson, 1991

Table 2, Fig. 6A-F

Specimens-MNA Loc. 939: ml-V5629. MNA Loc. 1067: ml--V6000, V6368, V6380, V6481, V6490; m2--V6313, V6365, V6426; P4-V5357, V6243, V6310; M1-V6238, V6442; M2-V5667, V6006, V6233.

Description, ml--All cusps of the external cusp row of ml (V6481, Fig. 6A) are well-separated and the row broadens posteriorly. The first cusp is low and conical; cusps 2-4 are subequal in height and weakly pyramidal in shape. The central valley is not straight but sinuous. The first two cusps of the internal row are tall and conical but not completely separated. The third cusp is completely separated from the second. The tooth is indented medially on both sides such that it has a weakly developed waist in occlusal view.

Second Lower Molar--On V6365 (Fig. 6B) the external row increases in height posteriorly. The cusps of the external row are not deeply divided labially. The first two cusps are well-separated on the lingual wall and the remaining cusps are less distinctly separated. The enamel on the lingual face of cusps 3 and 4 is crenulated. The cusps of the internal row are not deeply divided lingually and are separated by a deep groove labially. The second cusp is lower than the first. A distinct notch is present behind the second cusp on the posterolingual wall of the tooth.

P4-V6243 (Fig. 6C, D) has a cusp formula of 1:5. The anterior width of the tooth is broader than the posterior because of the labially expanded anterolabial platform. The medial cusp row ascends steeply and the tips of its cusps are striated. The posterior basin is formed by a deep pit with well-developed cusps on either side. On another specimen (V5357), the posterior basin is more open posteriorly.

Ml-This worn Ml (V6442, Fig. 6E) has a cusp formula of 3:4:1. The external cusp row appears to have only 3 cusps, but there may have been a small anteriormost cusp removed as a result of wear. A fourth cusp is present on another specimen (V6236). The cusps of the external row narrow posteriorly, in contrast to the cusps of the medial row, which broaden posteriorly. The internal cusp row consists of a single cusp present at the notch separating cusps 3 and 4 of the medial row.

M2-V6233 (Fig. 6F) has a cusp formula of R:3:4. The external platform is rimmed by a cuspate ridge. The first "cusp" (a swelling of the anterior wall of the tooth) of the medial cusp row merges with the large second cusp. The third cusp is worn. The cusps of the internal row are closely joined and not deeply divided lingually but are divided on the labial wall down to the central valley.

Discussion - The M 1 s of this species of Paracimex- omys, from the Dakota Formation, are almost identical with those of the Early Cretaceous taxon Paracimex- omys robisoni Eaton and Nelson, 1991, from the Cedar Mountain Formation. The P4s assigned here are somewhat narrower than those of P. robisoni, however, and the posterior basin appears to be too narrow for the single complete M1 in this sample. In addition, the Ml:P4 ratio (0.81) is lower than that of P. robisoni (0.9). As such, the inclusion of these P4s with the M1 is considered tentative until a larger sample is available. Assignment of isolated second molars is always tentative but the m2s assigned here are similar to those of P. robisoni and have an m2:ml ratio (87%) appropriate for Paracimexomys (Table 2). No M2s were assigned to P. robisoni by Eaton and Nelson (1991) and the M2s assigned here fit well with both the Mls and m2s and have the high M2:M1 ratio (92%) typical of Paracimexomys (see Krause et al., 1992:table 2).

PARACIMEXOMYS sp. Table 2, Fig. 6G-N

Specimens-- MNA Loc. 939: P4- V6 100. MNA Loc.

1067: ml - V5355; m2s- V6446, V6457; P4 - V6378; Mls--V5857, V6315, V7327; M2s-V6324, V6385, V6783, V6787.

Description, ml -V5355 (Fig. 6G) is a very small ml missing the anteroexternal cusp of the tooth. The cusp formula was probably 4:3 prior to breakage. Cusps of the external row are bulbous, subpyramidal in form, and lower than the cusps of the internal row. Cusps 2 and 3 are well-separated. The cusps of the internal row are tall and conical. The first two cusps are linked by a crest while cusps 2 and 3 are broadly separated. The



EA TON- CENOMANIAN AND TURONIAN MULTITUBERCULA TES 769

A GA Alon- Nom*

1 4 1 A w l -4Ap

FIGURE 6. Cenomanian multituberculates. A-F, Paracimexomys sp. cf. P. robisoni. A, Rml, V648 1, osp (occlusal stereo- pair); B, Rm2, V6365, osp; C, LP4, V6243, osp; D, same, labial; E, LM1, V6442, osp; F, RM2, V6233, osp. G-N, Paraci- mexomys sp. G, Rml (incomplete), V5355, osp; H, Rm2, V6457, osp; I, RP4, V6378, osp; J, same, labial; K, RM1, V6315, osp; L, LM1, V7327, osp; M, RM2, V6787, osp; N, LM2, V6783, osp. O-P, cf. Paracimexomys sp. undetermined. 0, Lm2, V5381, osp; P, RM2, V5627, osp. Scale bar = 1 mm.

central valley is deep and wide, probably as a result of wear.

Second Lower Molar--V6457 (Fig. 6H) has a cusp formula of 3:2. The cusps of the internal row are deeply divided labially, but are incompletely separated lingually. The first cusp is the tallest (the others are subequal) and its tip is oriented posteriorly. The second cusp is buccolingually compressed. The cusps of the external row form a continuous wall and are weakly divided by grooves on the lingual surface.

P4-V6378 (Fig. 61, J) has a cusp formula of 2:5. The anteroexternal platform bears a low, crest-like cusp anteriorly and a second distinct cusp labial to cusps 2 and 3 of the medial row. The cusps of the medial row descend sharply until the apex at the fourth cusp. The tips are well-striated. The posterior basin is broad with a deep central groove bounded by two sharp cusps.

MI- The Mls (e.g., V6315, Fig. 6K) have a cusp formula of 4:4:1. The first cusp of the external row is small and connected to a much larger second cusp. The cusps are conical but are worn on their lingual sides. The cusps of the external row are taller than the first three cusps of the medial row, which are conical, and increase only slightly in size posteriorly. The external and medial cusp rows diverge anteriorly. The internal cusp row consists of a single cusp on a platform that connects to the main body of the tooth lingual to the midpoint of the third cusp of the medial row. V7327 (Fig. 6L) is similar to V6315 but is 13% longer and the second cusp of the medial row is longer than the third cusp.

M2-V6787 (Fig. 6M) is a small M2 with a cusp formula of 1:2:3. The external platform is narrow and bears one distinct cusp. On the medial row the anterior




TABLE 3. Measurements in mm of Dakotamys malcolmi from the Dakota Formation.

N Range Mean SD CV

p4 AP 4 2.63-2.73 2.68 0.04 1.5 H 4 1.42-1.55 1.49 0.06 4.0 H:L 4 0.53-0.58 0.55 0.02 4.3

ml AP 14 1.82-2.02 1.93 0.07 3.6 LB 14 1.03-1.23 1.11 0.06 5.4

m2 AP 5 1.58-1.79 1.66 0.09 5.4 LB 5 1.39-1.45 1.39 0.05 3.6

P4 AP 3 2.38-2.55 2.44 LB 3 1.12-1.34 1.21

MI AP 9 1.82-2.18 2.00 0.10 5.0 LB 8 1.06-1.30 1.15 0.09 7.8

M2 AP 4 1.55-1.65 1.60 0.06 3.8 LB 4 1.28-1.39 1.35 0.05 3.7

ridge of the tooth is merged with the large first cusp. The second cusp of the row is narrower than but as tall as the first. The cusps of the internal row are not well-divided, but form a continuous ridge, with the first two cusps close together and the third shifted posteriorly. V6783 (Fig. 6N) differs from V6787 in that the first cusp of the medial row is weakly ribbed, both cusps of the medial row have apices oriented anteriorly, and the cusps of the internal row are better separated.

Discussion--The medial and external cusp rows of the M1 s referred to this species diverge anteriorly and the molars have a low cusp formula, as specified in the diagnosis of Paracimexomys (see Archibald, 1982). These Ml s differ from those assigned to Paracimex- omys sp. cf. P. robisoni in being smaller, having a proportionally narrower external cusp row, having con- sistently conical cusps in the external row and blocky cusps in the medial row, and in not having the cusps of the medial row increasing in length posteriorly.

The ml lacks the sinuous medial valley typical of Paracimexomys but this may be the result of wear on the single available specimen. The extreme simplicity of the tooth is reminiscent of Loxaulax wealdensis (see Clemens and Lees, 1971:pl. 1-A).

Cusps of the external row of m2 form a continuous ridge. The cusps of the external row are well-divided on Mesodma (see Lillegraven, 1969:fig. 7-4a) and Cim- olomys (see Archibald, 1982:fig. 23e); the first two cusps are well-divided in Cimexomys but are progressively less so posteriorly (Archibald, 1982:fig. 37e); and in Cimolodon, the cusps are deeply separated on the lingual wall of the row.

M2 has the continuous, undivided internal row that is typical of Paracimexomys; cusps of all other genera are better divided on the internal row. The width of the M2s slightly exceeds the width of the Mls (see Cimexomys maxilla in Montellano, 1992:fig. 6); however, it is unclear if the two specimens figured here actually represent the same species, as the recurved

cusps and ribbing present on V6783 (Fig. 6N) would seem more characteristic of Cimolodon than Paraci- mexomys. Several M2s from the Lower Cretaceous Cedar Mountain Formation also have strong ribbing and pitting; these were placed incertae sedis by Eaton and Nelson (1991). This may indicate either that cimolodontids were present in the Early Cretaceous or that this kind of pitting and ribbing is primitive and shared by both Paracimexomys (in which case it is variable in the genus) and cimolodontids. The M2:M 1 length ratio is 74%, which is low for Paracimexomys (see Krause et al., 1992:table 2).

It is possible that more than one species or genus is represented by the specimens included in Paracimex- omys sp. They have been assigned to this single taxon because of their low cusp formulae, size, and relatively simple morphology, but the small number of specimens does not allow for consistent separation based on other morphologic criteria.

cf. PARACIMEXOMYS sp. Undetermined Fig. 60-P

Specimens-MNA Loc. 939: M2-V5627. MNA Loc. 1067: m2-V5381.

Discussion- These two second molars are in the same size range (V5381 AP = 1.18, LB = 1.17; V5627 AP = 1.22, LB = 1.14) as those described above as Par- acimexomys sp., but are different enough morphologically to be described separately. V5381 (Fig. 60), an m2 with a formula of 4:2, differs from the m2 assigned to Paracimexomys sp. (V6457, Fig. 6H) in that it broadens posteriorly rather than being parallel sided and the first two cusps of the external row are divided by a narrow, deep valley rather than forming a continuous ridge.

V5627 (Fig. 6P), an M2 with a formula of R:2:4, differs from M2s of Paracimexomys sp. in having more cusps on the internal row (4 rather than 3) and no distinct external cusps.

DAKOTAMYS, gen. nov.

Type and Only Species -Dakotamys malcolmi. Distribution - Cenomanian, Dakota Formation of

southwestern Utah.

Diagnosis--First molars with low cusp formulae (ml--4:3, M1--4:4:1-2) short internal row on M1, cusps of external and medial rows on M1 offset (do not align transversely) and high second to first molar length ratios (ca. 0.9), as in Paracimexomys. Molars cusps ribbed. Differs from Paracimexomys in that Ml1 has parallel medial and external cusp rows (not di- verging anteriorly), and cusps of external row are mark- edly smaller, lower, and more poorly defined posteriorly; straight central valley (rather than sinuous) on ml; P4 with labially expanded anteroexternal platform; p4 with higher arch.

Etymology-Named after the Dakota Formation and mys, Latin for mouse.



EA TON- CENOMANIAN AND TURONIAN MULTITUBERCULA TES 771

B C

FIGURE 7. Cenomanian multituberculates. A-M, Dakotamys malcolmi, gen. et sp. nov. A, Rp4, V6056, lingual; B, same, labial; C, Rp4, V5874, lingual; D, same, labial; E, Rml, V6368, osp (occlusal stereo-pair); F, Lml, V6384, osp; G, Rm2, V6488, osp; H, LP4, V5359, osp; I, same, labial; J, RM1, V5875, holotype, osp; K, RM1, V6232, osp; L, RM , V6225, osp; M, RM2, V5392, osp. N, ?Dakotamys sp., RM1, V6781, osp. 0, genus and species undetermined A, RM2, V6027, osp; P, genus and species undetermined B, LM2, V5865, osp. Scale bar = 1 mm.

DAKOTAMYS MALCOLMI, sp. nov. Table 3, Fig. 7A-M

Type Specimen --MNA Loc. 1067: M 1 --V5875. Hypodigm-MNA Loc. 939: P4s-V6009, V6010;

M1-V6011. MNA Loc. 1064: m2-V6102; M2- V5392. MNA Loc. 1067: p4s-V5856, V5859, V5874, V5884, V6050, V6056; mls-V5346, V5368, V5380, V5867, V5886, V5891, V6138, V6332, V6368, V6384, V6391, V6484, V6778, V6785; m2s-V6488, V6773, V6776, V6786; P4s-V5359, V6307; Mls-V5350, V5875, V5883, V6225, V6232, V6237, V6309, V6327,

V6334, V6456, V6789, V7328; M2s-V6240, V6370, V6381. UCM Loc. 85358: ml-UCM 54330.

Distribution--As for the genus. Diagnosis -As for the genus. Etymology--For Dr. Malcolm C. McKenna, in rec-

ognition of his support of research, his never ending faith in the value of field work, and for teaching me the relationship of Scotch whisky to finding fossils.

Description, p4 -These p4s have a range of 6-7 external and 6 internal ridges with 9-10 serrations. V6056 (Fig. 7A, B) has a formula of 6:10:6. The anterior face




of the tooth is tilted somewhat posterodorsally and the first 3 serrations (particularly 2 and 3) are widely separated. The first serration occurs more than half way up the anterior curve of the blade. The apex of the blade occurs about midway to slightly posterior of the middle of the tooth. There is only a hint of the posteroexternal cusp which was possibly removed by wear on this specimen, as that cusp is well-preserved on V5874 (Fig. 7C, D). V5874 also differs from V6056 in having the greatest separation between serrations 1 and 2, not 2 and 3.

First Lower Molar--The mls have a cusp formula of 4:3 and have a waist in occlusal view. On V6368 (Fig. 7E) the cusps of the internal row are conical and taller than those of the external row. The first cusp of the internal row is conical and cusps 2 and 3 are pyramidal on their labial sides and conical lingually. On the external cusp row the anteriormost cusp is small, cusp 2 is conical, and cusp 3 is connected labially to cusps 2 and 4 by a low ridge. Cusp 4 is indistinctly divided and is swollen posterolabially. Cusps of both rows are ribbed at their bases along the central valley. V6384 (Fig. 7F) is similar to V6368 but is more robust, is more deeply indented on its labial side, the fourth cusp of the external row is more distinctly divided into two cusps, and the posterolabial part of the tooth is more broadly expanded and bears crenulated enamel. The broad posterolabial "cusp" is counted as a single cusp here because the "fifth" cusp is not distinctly formed and the shape of this part of the tooth is highly variable within the sample.

Second Lower Molar-V6488 (Fig. 7G) has a cusp formula of 4:2. The first 2 cusps of the external row are well-divided lingually and cusps 2-4 form a continuous ridge. The two cusps of the internal row are completely separated and a distinct notch is present at the posterior terminus of cusp 2.

P4-V5359 (Fig. 7H, I) has a cusp formula of 3:6. All cusps are well-striated. The anteroexternal platform is broad and bears three cusps. A line drawn at the tips of the cusps of the medial row would would form an almost straight line which is at a low angle to the base of the tooth. Posterior to the last cusp of the medial row crenulated enamel is present. The posterior basin is bounded by a cusp on each side.

M1 -V5875 (Fig. 7J), the holotype, has a cusp formula of 5:4:2. The first cusp of the external row is tiny and is linked to the second cusp by a sharp crest. A pocket is present labial to this sharp crest between cusps 1 and 2. The remaining cusps of the row are conical and are connected by crests. The cusps of the external row become progressively lower posteriorly. The area behind the fourth cusp is low and pitted. The first cusp of the medial row is subpyramidal in shape and the remaining three cusps are pyramidal, bear ribs on their sides, and broaden posteriorly. The internal cusp row has two subequal cusps and the row is connected to the posterolabial corner of the second cusp of the medial row. V6232 (Fig. 7K) demonstrates variation in having the first cusp of the medial row buccolingually compressed and the posterior cusp of the

internal row larger than the anterior cusp. V6225 (Fig. 7L) has distinct labial pockets separating the cusps of the external row and the anteriormost cusp of the internal row is larger than the posterior cusp.

M2-V5392 (Fig. 7M) has a cusp formula of 2:3:4?. A distinct ridge bearing two cusps encircles the external platform on which the enamel is crenulated. The first cusp of the medial row is formed by a slight swelling of the anterior wall of the tooth. The second cusp is the largest of the tooth and is ribbed on all sides, with its apex oriented anteriorly, as is the apex of the posteriormost cusp. The first three cusps of the internal row are not well-separated and are strongly ribbed lingually. There is a cuspule, not included in the cusp formula, between cusps 2 and 3.

Discussion--The Mls are very similar to those of Paracimexomys, particularly in their low cusp formula and small size. However, Dakotamys also has a few characteristics of Cimolodon, such as strongly ribbed cusps, and the tendency of cusps of the external row to become lower, smaller, and less distinct posteriorly, as in Cimolodon electus (see Fox, 1971:fig. 4d). The cusps of the medial row are somewhat pyramidal, ap- proaching the condition seen in Cimolodon. The con- nection of the cusps of the external row are as noted by Fox (1971:928) for C. electus, in which ".... the most anterior two or three cusps of the external cusps are joined throughout most of their height, only the cusp apices are free." This is not true of the Maastrich- tian taxon C. nitidus. These molars are in the size range of M 1 s of P. robisoni, an early Cretaceous species (Ea- ton and Nelson, 1991), but differ in having more fully developed internal cusp rows, parallel medial and external cusp rows, and a marked height reduction of the external cusp row posteriorly. The M 1s of Dakotamys have an identical cusp formula to Mls of P. priscus but are smaller.

The M2s were assigned to Dakotamys malcolmi because the cusps of the medial row lean anteriorly at the same angle as those on M1, the central cusp of the teeth are strongly ribbed, and the anterior interstitial facets fit well against the posterior facet of Ml. These M2s compare more closely to those of Cimolodon electus (see Fox, 1971:fig. 5a) in their overall shape and complexity of enamel than any M2s assigned to Par- acimexomys. The m2s that were included in this taxon are worn and were placed here largely on the basis of their width and fit against the posterior interstitial facet of the mls.

The complex enamel on the posteroventral surface of P4 is like that found in Cimolodon electus and C. nitidus (see Clemens, 1964:fig. 30a; Clemens, 1973:fig. 30a) and is not characteristic of figured P4s of Mesod- ma, Cimexomys, or Paracimexomys. The labially expanded anteroexternal platform is found both on Cim- olodon and Mesodma (M. primaeva, see Lillegraven and McKenna, 1986:fig. 6D) but not on figured specimens of Cimexomys or Paracimexomys.

The p4s in this sample are more arched than those of Mesodma or Cimolomys. They are proportionally shorter relative to height than p4s of Cimolodon similis




but are similar in shape to the much larger p4s of C. electus (see Fox, 1971 :fig. 3e), although they have fewer ridges and serrations. Compared to figured p4s of Cim- exomys (e.g., Archibald, 1982:fig. 73a), the p4s of Dak- otamys are more arched, with deeper anteroexternal lobes, lower posteriormost serrations on the posterior margin of the tooth, and anterior faces are not as vertical. The labial outline of p4 is more symmetrical than that of Paracimexomys (P. priscus, Lillegraven, 1969: fig. 10-3). The first serration occurs relatively high on the anterior edge of the crest in some specimens, but never as high as in specimens of Cimolodon.

The teeth of Dakotamys share some characters with those of Cimolodon, but it is unclear if these are retained primitive characters, if they are independently derived, or if they are synapamorphies with the Cim- olodontidae. At this stage of knowledge, particularly lacking jaws, it seems most prudent not to place Dak- otamys in any family.

?DAKOTAMYS sp. Fig. 7N

Specimens-MNA Loc. 1067: Mls-V5861, V6781. Description, Ml - V6781 (Fig. 7N) has a cusp for-

mula of 4:4:1 and a strongly developed waist in occlusal view between cusps 2 and 3 of the external row and at the midpoint of cusp 2 of the medial row. The medial and external cusp rows are parallel. The first cusp of the external row is small, low, and connected to the second cusp by a thin wall. Cusps 2 and 3 are conical and well-separated. Cusps 3 and 4 are completely separated; posterior to cusp 4 is a distinct, cuspate(?) wall that attaches to the last cusp of the medial row. There is some indication of ribbing on the lingual side of the cusps of the external row. The cusps of the medial row are weakly pyramidal, well-separated, and increase in height posteriorly. The internal row consists of a single cusp positioned at the gap between the last two cusps of the medial row. V5861 is the anterior part of an almost identical M1.

Discussion--V6781 is similar in size (AP = 2.13, LB = 1.24) to Mls assigned to Dakotamys malcolmi, but differs in the presence of a better developed fourth cusp, the strong separation of cusps 2 and 3 of the external row, the strong waist, and the rudimentary development of the internal cusp row. It is not clear if this represents variation within Dakotamys or indicates the presence of another taxon.

Genus and Species Undetermined A Fig. 70

Specimens-MNA Loc. 1067: M2-V6027, V6439. Description and Discussion - These two M2s are rel-

atively large (V6027 AP = 1.95, LB = 1.90; V6439 AP = 1.98, LB = 1.75) and have cusp formulae of R-2: 3:4. The cusps of the internal row on V6027 (Fig. 70) decrease slightly in size anteriorly. The central cusp of the medial row is the largest of the tooth. There are two distinct cusps on the external platform, which is broadly expanded.

These M2s are similar to, but smaller than, those of Cimolomys clarki (see Sahni, 1972:fig. 11F) in cusp formula, crescentic nature of the ultimate cusp of the medial row, the well-separated cusps on the internal row, and the broad external platform. The broadly expanded external platform is unlike that of M2 of Paracimexomys, Cimexomys, and Mesodma. The few figured M2s of Cimolodon (see Clemens, 1964:fig. 31; Lillegraven, 1969:fig. 11-5; Fox, 1971:fig. 5) have high cusp formulae (1-5:3-4:4-6) and the cusps of the internal row are not as well-separated as in these specimens. The M2s are also similar to those of Bryceomys, gen. nov., described below (compare to Fig. 9M), but the internal row cusps of this taxon are more completely divided.

These two teeth share characters with Cimolomys and Bryceomys, but the sample is inadequate to iden- tify them as either. The occurrence of compact, round- ed teeth such as multituberculate second molars and anterior upper premolars may be favored in localities that represent fluvially transported specimens. At MNA locality 1067 second molars and upper premolars are more numerous and morphologically diverse than teeth of any other tooth position.

Genus and Species Undetermined B Fig. 7P

Specimen-MNA Loc. 1067: M2-V5865. Description and Discussion--V5865 (Fig. 7P) is a

relatively large M2 (AP = 2.10, LB = 2.10) with a cusp formula of 3:2:3. The tooth is squared, with bulbous, inflated cusps. The external row is formed by three low cuspules and the row is not expanded labially far from the medial row. The anteriormost cuspule of the medial row is little more than a swelling on the anterior ridge and is not counted in the cusp formula. Cusps 1 and 2 are conical and large, with the second cusp being the tallest. The first cusp of the internal row is well- separated from the second cusp, and cusps 2 and 3 are connected.

The cusps are conical and is this respect are unlike those of Cimexomys or Mesodma. The tooth does not have the pitted enamel or high cusp formula of M2 of Cimolodon. The cusps of the external row are not as well-separated as those of Cimolomys, yet the specimen is morphologically closer to M2 of Cimolomys (e.g., C. clarki, Sahni, 1972:fig. 1 l1-F) than M2 of any other described taxon.

Turonian Multituberculates

Suborder ?TAENIOLABIDOIDEA

Family, Genus, and Species Undetermined Fig. 8A-B

Specimens -Loc. 995: P4s-V6358, V7407, V7472, V7502.

Description, P4 -These P4s have a formula of 2:5, are the largest P4s in the sample recovered from the Smoky Hollow Member, and range in size from AP = 2.82-3.49, LB = 1.45-1.66. V7502 (Fig. 8A, B) is typical of P4s in the sample in having a strong waist in




B A

I L

*I'

FIGURE 8. Turonian multituberculates. A-B, ?Taeniolabidoidea, family, genus, and species undetermined. A, RP4, V7502, osp (occlusal stereo-pair); B, same, labial. C-D, Paracimexomys sp. cf. P. robisoni. C, Lml, V7473, osp; D, RM2, OMNH 22474, osp; E-T, Bryceomys fumosus, gen. et sp. nov. E, Rp4, V5639, lingual; F, same, labial; G, Rp4, V7476, lingual; H, same, labial; I, Lp4, V7468, labial; J, Rml, V7376, osp; K, Lml, V6116, osp; L, Rml, V7377, osp; M, Lml, V6298, osp; N, Lml, V6769, osp; 0, Lm2, V7448, osp; P, Lm2, OMNH 25448, osp; Q, LP4, V7386, osp; R, same, labial; S, LP4, V7359, osp; T, same, labial. Scale bar = 1 mm.

occlusal view and a well-expanded anteroexternal platform that makes the anterior part of the tooth wider than the posterior. The platform bears one small, low anterior cusp and a larger posterior cusp. Both cusps are striated. The apices of the cusps of the medial row are striated, well-separated, and climb at a low angle. A deep groove is present from the tip of the ultimate cusp of the medial row to the labial cusp of the posterior basin. The basin is deep, open posteriorly, has strongly developed cusps on both sides of the basin, and is high on the posterior face of the tooth.

Discussion--These P4s have several cimolomyid

characteristics (compared to Cimolomys gracilis; Clemens, 1964:fig. 36), such as the low cusp formula, large breadth relative to length, well-developed posterior basin which is high on the tooth, strong waist in occlusal view, and low angle of climb of the cusps of the medial row. As was the case with the cimolomyid- like second molars recovered from the Dakota For- mation, it is unsettling to have teeth of only one position that are cimolomyid-like. As P4s tend to have more diagnostic characters than second molars, these provide some evidence that cimolomyids, although rare, may be present early in the Late Cretaceous.




Suborder and Family Unknown PARACIMEXOMYS Archibald, 1982

PARACIMEXOMYS sp. cf. P. RoBIsoNI Eaton and Nelson, 1991

Fig. 8C-D

Specimens - OMNH Loc. V4: M2 - 22474. OMNH Loc. V60: M2-24593. Loc. 995: ml-V7473; Ml-- V7356.

Description, ml -V7473 (Fig. 8C) has a formula of 4:3 (AP = 2.12, LB = 1.28). The first cusp of the external row is low, cusps 2 and 3 are taller and subequal in size, and cusp 3 is connected lingually to the worn fourth cusp. The first two cusps of the internal row are subequal in size and are closely joined; cusps 2 and 3 are more deeply divided. The cusps of the internal row are much taller than those of the external row. The tooth is deep crowned and there is a slight indentation midway on the labial side of the tooth.

M2-OMNH 22474 (Fig. 8D, AP = 1.62, LB = 1.43) has a cusp formula of R:3:4. The posterolabial margin is formed by a cuspate ridge. The first cusp of the medial row is connected to the large second cusp by a ridge. The four cusps of the internal row are undivided except at cusp apices.

Discussion - A poorly preserved Ml is included here largely on the basis of its size (AP = 2.19*, LB = 1.49*) and occlusal fit with the ml (V7473). The ml is almost identical to the mls assigned to Paracimexomys sp. cf. P. robisoni from the Dakota Formation (Fig. 6A) but is slightly larger and the first two cusps of the internal row are better connected. The M2s have the continuous internal row characteristic of Paracimex- omys and are similar to the M2s from the Dakota Formation (assigned to Paracimexomys sp. cf. P. robisoni) but are not as squared.

BRYCEOMYS, gen. nov.

Type Species -Bryceomys fumosus, sp. nov. Included Species-The type and Bryceomys hadro-

sus, sp. nov. Occurrence - Smoky Hollow Member of the Straight

Cliffs Formation, Turonian age, southwestern Utah. Diagnosis-Mi with cusp formula 3-5:4:2-4; inter-

nal cusp row broader than in Dakotamys or Paraci- mexomys, connects to anterior of third or posterior of second cusp of medial row, unlike Paracimexomys. Cusps of medial and external rows of M1 offset (not aligned transversely) as in Paracimexomys and Dak- otamys but not Cimexomys. Medial and internal cusp rows of M1 are parallel or diverge slightly anteriorly; cusps of external row are taller posteriorly, unlike Dak- otamys. Cusp formula of ml 4-5:3, cusps of external row conical to weakly crescentic, well-divided, separated by deep, wide U-shaped valleys (in side view). Fourth lower premolar with high arch; on m2, first cusp of internal row is connected by a continuous ridge to second cusp of external row. M2:MI length ratio

76%, m2:ml length ratio r78%, less than for Par-

acimexomys or Dakotamys. Cusps of internal row of M2 well-separated, unlike Paracimexomys or Dako- tamys.

Etymology--Named after Bryce Valley, from which

all known specimens have been recovered, and "mys" Latin for mouse.

Discussion -Bryceomys is most similar to Paraci- mexomys. In both the cusp formulae are low and the cusps of the medial and external rows alternate on M1. They differ in that Bryceomys exhibits greater development of the internal cusp row on M 1, lacks the con- nections of the anterior cusps of the external row of M l, Ml lacks a "waist," has a more highly arched p4 and a more greatly expanded anteroexternal lobe on p4, and has a lower second to first molar length ratio. Bryceomys differs from Dakotamys in having a more greatly expanded internal cusp row on M1, in lacking connected cusps on the anterior part of the external row of M1, in having a more weakly developed pit labial to external row cusps 1 and 2 on M1, in having a sinuous rather than straight central valley on ml (as in Paracimexomys), and in having lower second to first molar length ratios.

No specimens of Bryceomys younger than Turonian are known, with the possible exception of Cimexomys gregoryi, from Campanian strata on the Paunsaugunt Plateau of southwestern Utah (Eaton, 1993b). Cim- exomys gregoryi was tentatively placed in the genus Cimexomys and was defined on the basis of a single Ml1. More study will be required to determine if trans- ferral of the Paunsaugunt taxon to Bryceomys is war- ranted.

BRYCEOMYS FUMOSUS, sp. nov. Table 4, Fig. 8E-9H

Type Specimen-Loc. 995: M1 -V6122. Hypodigm-OMNH Loc. V4: mls-22483, 25444;

m2s-22447, 22450, 22477, 22481, 23020, 25445, 25447, 25448, 25449, 25450; P4s-22462, 22476, 23008; Mls-22456, 22463, 22482, 22485, 23021, 25465, 25469; M2s-23013, 25470, 25471, 25472, 25478, 25480. OMNH Loc. V60: p4s-23083, 24511, 24513, 24515, 24517, 24518; mls-23068, 23082, 23084, 23085, 23087, 24520, 24521, 24522, 24524, 24525, 24526, 24527, 24528, 24531, 24532, 24533, 24534, 24562; m2s-23090, 23091, 23093, 23094, 23095, 23096, 24534, 24537, 24538, 24539, 24540, 24541, 24542, 24548, 24550, 24553; P4s-24572, 24573, 24574; Mls-23058, 23070, 23071, 23081, 24575, 24576, 24578; M2s-23076, 23079, 23080, 24582, 24584, 24588, 24589, 24590, 24591, 24592. MNA Loc. 995: p4s-V5639, V5643, V6109, V7468, V7469, V7475, V7476; mls-V6108, V6110, V6116, V6120, V6298, V6346, V6478, V6761, V6764, V6768, V6769, V6772, V6775, V7357, V7376, V7377, V7378, V7379, V7365, V7400, V7412, V7426, V7430, V7464, V7466; m2s-V6117, V6762, V6765, V7360, V7367, V7406, V7422, V7432, V7438, V7439, V7440, V7446, V7448, V7449, V7454, V7487, V7491, V7492, V7493,




TABLE 4. Measurements in mm of Bryceomys from the Smoky Hollow Member of the Straight Cliffs Formation.

N Range Mean SD CV

Bryceomys fumosus p4 AP 11 2.50-3.31 2.78 0.24 8.6

H 7 1.49-1.68 1.57 0.09 6.0 H:L 7 0.54-0.63 0.58 0.03 5.3

ml AP 44 1.32-1.88 1.55 0.11 7.2 LB 44 0.81-1.18 0.94 0.08 8.7

m2 AP 44 0.99-1.42 1.23 0.10 8.2 LB 46 0.88-1.31 1.12 0.10 8.7

P4 AP 14 1.89-2.48 2.08 0.21 10.0 LB 13 0.82-1.11 0.98 0.10 10.7

MI AP 28 1.52-2.10 1.73 0.17 9.7 LB 28 1.05-1.39 1.18 0.09 7.5

M2 AP 36 1.12-1.53 1.33 0.11 7.9 LB 35 1.08-1.44 1.26 0.10 8.0

Bryceomys hadrosus ml AP 2 2.64-2.71 2.66

LB 2 1.60-1.61 1.61 m2 AP 2 1.98-2.22 2.10

LB 2 1.74*-1.78 1.76 M1 AP 1 2.68

LB 2 1.63-1.87 1.75 M2 AP 2 2.18-2.23 2.21

LB 2 1.87-1.92 1.90

V7494; P4s-V6114, V6349, V6496, V7359, V7366, V7386, V7417, V7418, V7425, V7434, V7435, V7447, V7452, V7457, V7467; Mls-V5641, 5642, V5645, V6106, V6112, V6135, V6360, V7373, V7374, V7375, V7389, V7392, V7401, V7403, V7410, V7421, V7427, V7428, V7437, V7443, V7444, V7445, V7458, V7488; M2s-V5649, V6448, V6107, V6494, V6757, V6767, V7352, V7353, V7358, V7393, V7395, V7398, V7399, V7404, V7414, V7415. V7424, V7433, V7436, V7451, V7456, V7481, V7482. MNA Loc. 1003: p4-V5330; mls-V5333, V7380, V7381, V7382, V7383, V7384, V7463, V7470; m2s-V5336, V7485, V7486, V7489; P4-V7459; M1--V7479; M2s-V5335, V7477, V7478.

Occurrence - Smoky Hollow Member of the Straight Cliffs Formation, Turonian age, southwestern Utah.

Diagnosis--Smallest species of the genus. Etymology - Fumosus, Greek for smoky, after the

Smoky Hollow Member of the Straight Cliffs Forma- tion, from which all specimens were recovered.

Description, p4-The basis for including p4s with B. fumosus is the occurrence of left and right p4s (V7468, V7469, probably from the same individual) with a P4 (V7467) of this species in a tiny sandstone clast. In this sample of p4s, external ridges range in number from 7-8; 7 internal ridges and 9-11 serrations are present. V5639 (Fig. 8E, F) is typical ofp4s assigned here. The crest of the tooth is well arched (H:L = 0.56) and a large external lobe is present. The anterior face of the tooth is curved and the upper part leans strongly posterodorsally. The posteriormost serrations form

posterodorsally oriented cusps. V7476 (Fig. 8G, H) has the most serrations (11), and is the least worn p4 included in the species. The crest of the tooth is highly arched and the serrations on the labial side almost reach the anterior margin of the tooth. The posteroexternal cusp is angled at 300, as it is on most of the specimens (though not on OMNH 23083), possibly reflecting wear. V7468 (Fig. 81) has a very strong, pillar-like posterolabial cusp which bears three cuspules. The posteriormost serrations are cusp-like and oriented posterodorsally.

First Lower Molar -V7376 (Fig. 8J) has a cusp formula of 4:3, which is invariant in the sample. The molar is waisted in occlusal view because of indenta- tions between cusps 2 and 3 of the internal row and at cusp 3 of the external row. The central valley is sinuous. The cusps of the external row are slightly lower than those of the internal row. The first cusp of the external row is small and conical, cusps 2 and 3 are subequal in size, and cusp 3 is connected posterolabially to a wall-like fourth cusp that is weakly subdivided though counted as a single cusp. The first cusp of the internal row is conical and slightly smaller and lower than the second cusp, which is weakly crescentic. The third cusp is the largest and tallest of the tooth. The fourth cusp of the external row of V6116 (Fig. 8K) shows no tendency toward the subdivision seen on V7376. V7377 (Fig. 8L) is strongly indented on the external side and has pitting in the central valley. Among the smallest specimens (AP = 1.31) is V6298 (Fig. 8M), which has a more weakly developed fourth cusp on the external row, but otherwise is morphologically similar to the other mls. V6769 (Fig. 8N) is an example of one of the larger ml s. On the larger specimens, the U-shaped (in side view) valleys which separate the first three cusps of the external row are very deep and the cusps are therefore well-separated.

Second Lower Molar--Most of the m2s assigned to this species have a cusp formula of 3:2. V7448 (Fig. 80) is typical of the m2s. The first cusp of the external row is small and the upper third of the cusp is separated from the second cusp; a groove separates the cusps lingually. Cusps 2 and 3 are only slightly divided at their apices. A ridge crosses the central valley and connects the first cusp of the internal row to the second cusp of the external row. The posterior wall of the tooth is cuspate. The first cusp of the internal row is tall, concave posteriorly, and well-separated from the buccolingually compressed second cusp. OMNH 25448 (Fig. 8P) is one of the larger m2s assigned to this taxon; it has more bulbous cusps and is deeper crowned than V7448. The two cusps of the internal row are subequal in size and height; they are slightly separated at their apices and separated labially by a deep groove.

P4-The cusp formulae of these P4s are 1-2:5. V7386 (Fig. 8Q, R) is an example of a specimen with a formula of 2:5. The external platform is broad enough to make the anterior width of the tooth broader than the posterior width. The platform bears two well-developed cusps. The medial row is comprised of five striated




IIA 'B

*9D

*ISE IF

*H

AMAN

aaAs

FIGURE 9. Turonian multituberculates. A-H, Bryceomysfumosus, gen. et sp. nov. A, RM 1, V6122, holotype, osp (occlusal stereo-pair); B, RM1, OMNH 24576, osp; C, RM1, V6112, osp; D, RM1, V7427, osp; E, RM1, V7488, osp; F, LM1, V7444, osp; G, LM2, V7451, osp; H, LM2, V7415, osp. I, Bryceomys sp. cf. B. fumosus, LM1, V7460, osp. J-M, Bryceomys hadrosus, sp. nov. J, Rml, V7371, osp; K, Rm2, V7474, osp; L, LM1, OMNH 25467, holotype, osp; M, RM2, V7483, osp; N-P, Bryceomys spp. N, Lm2, V7484, osp; 0, LMI (incomplete), V7413, osp; P, RM2, V6343, osp. Scale bar = 1 mm.

cusps that descend at a moderately high angle. The posteroventral face of the tooth has crenulated enamel and is tilted lingually. The posterior basin is bounded on both sides by cusps, the labial cusp being the higher of the two. V7359 (Fig. 8S, T) has a single cusp on the anteroexternal platform and a complex posterior basin with large cusps.

M1--V6122 (Fig. 9A), the holotype, has a cusp formula of 4:4:3. The first cusp of the external row is small, medially placed, and is connected by a crest to

the first cusp of the medial row closing off the central valley anteriorly. Cusps 2 and 3 are conical and subequal in size and height. Cusp 4 is attached to a wall which closes the central valley posteriorly. A deep, broad valley separates the medial and external cusp rows. The cusps of the medial row are weakly pyramidal and narrow slightly anteriorly. The first two cusps are subequal in height and the remaining two cusps increase in height posteriorly. The internal row bears three distinct cusps on a low and lingually expanded




platform which is well-supported by the posterior root. The valley between the internal cusp row and the main body of the tooth is deep. The internal row connects anteriorly behind cusp 2 of the medial row at about half the total length of the tooth. The number of internal row cusps can vary from two (OMNH 24576, Fig. 9B) to four (V6112, Fig. 9C). V7427 (Fig. 9D) has a distinct cusp developed on the wall posterior of the fourth cusp of the external row. V7488 (Fig. 9E) has a small cuspule low on the middle of the labial wall of the tooth. V7444 (Fig. 9F) is unusual in having the internal row divided into two segments. The posterior segment has two large cusps posteriorly and one small cusp anteriorly. This segment is separated from the anterior segment at the middle of cusp 2 of the medial row. The anterior segment bears several cuspules adjacent to cusps 1 and 2 of the medial row. An additional cusp is also present at the anterolabial corner of the tooth.

M2-V7451 (Fig. 9G) is typical of the many M2s assigned to this taxon, with a cusp formula of R:2:3. The external row consists of a weakly cuspate ridge surrounding the anterolabial part of the major cusp of the medial row. The anterior wall of the tooth does not swell medially and is not counted as a cusp. The anterior wall is connected to the first and very large cusp of the medial row. The base of the cusp is ribbed and its apex is curved anteriorly. The cusps of the internal row are well-separated, both lingually and buccally.

V7415 (Fig. 9H) has a cusp formula of 3:3:3. The first cusp of the medial row is formed by a slight swelling of the anterior wall of the tooth, is conical is shape, and is irregularly ribbed. The apex of the third cusp is oriented anteriorly. The central valley of the tooth is deep. The cusps of the internal row are well-divided and decrease in size posteriorly.

Discussion--The high arch of the p4s assigned to B. fumosus and continuity of their labial ridges externally are similar to the figured p4s of Cimolodon (e.g., Fox, 1971 :fig. 3), but differ from them in not being as highly arched and in having the first serration occurring low on the anterior face of the tooth. In overall form, the p4s are more similar to those figured for Cimexomys (e.g., Archibald, 1982:fig. 37) than of other figured p4s, particularly in the shape and orientation of the anterior face, the occurrence of the first serration low on the anterior face, and the orientation and cusp-like nature of the posteriormost serrations. The p4s differ from those of Cimexomys in having more ridges that extend farther anteriorly, and in having slightly more arch. There is a wide range in AP dimensions (2.50-3.31, SD = 0.24) and and H:L ratios (0.54-0.63) within this sample, and p4s from more than one taxon may be included here.

The mls are difficult to distinguish from those of Paracimexomys. Few mis of Paracimexomys have been figured (Fox, 1971:fig. 2; Archibald, 1982:fig. 39c, Eaton and Nelson, 1991: figs. 2b, c) but in all figured specimens of Paracimexomys the central valley is open

posteriorly. In the specimens assigned to Bryceomys, the medial valley is closed by the posterior wall. Should this characteristic prove to be undiagnostic, then some specimens of Paracimexomys may well be included within the sample of B. fumosus.

The m2s are morphologically distinctive because of the continuous ridge which crosses the tooth basin. They are assigned here because of the fit against the posterior wall of appropriately sized mls (see Fig. 9N, P).

The P4s assigned to B.fumosus have relatively broad posterior basins that correspond to the anterior interstitial facets of the Ml s assigned to the same species. The anterior end of these P4s is broader and the cusps apices of the medial row are less well-separated than in P4 of Paracimexomys.

Although the M1 s have a better developed internal cusp row than do Ml s of Paracimexomys, they are only superficially similar to Ml s of Cimexomys. The M s assigned to Bryceomys fumosus have lower cusp formulas for the medial and external cusp rows and the internal cusp row is longer proportional to total length than for any species of Cimexomys. M1 of C. antiquus (see Fox, 1971 :fig. 2e) is proportionally much longer relative to width than it is in this taxon; fur- thermore, Bryceomys lacks the subcrescentic to crescentic cusps of Cimexomys (see Fox, 1971:925; Mon- tellano, 1992:fig. 5b). In Cimexomys, the cusps of the medial and external rows are opposite each other (Fox, 1971:fig. 2e; Archibald, 1982:fig. 38d; Montellano, 1992:fig. 5b) such that the valleys that separate the cusps of each row form lines transverse to the long axis of the tooth. The condition in Bryceomys, as in Par- acimexomys and Dakotamys, is that the cusps of each row are offset such that there are no continuous valleys transverse to the axis of the tooth. In all figured Mis of Cimexomys, the central valley cuts the posterior wall such that it is open posteriorly, but in Paracimexomys, Dakotamys, and Bryceomys, the central valley is closed by the posterior wall. These Ml s are most similar to those of Paracimexomys magister (see Fox, 1971:fig. 2c) of previously figured multituberculate specimens but differ in being smaller, in having a longer internal cusp row, less anterior divergence of the medial and external cusp rows, and more squared posterior ends.

The Ml s also demonstrate surprising variability in the length, number of cusps, and development of the internal cusp row. V7444 (Fig. 9F) indicates an internal cusp row the length of the molar could be developed from both ends of the molar. Odd cusps in other positions are also common on the Mis of this taxon.

The M2s of B. fumosus are similar to those of Par- acimexomys in being dominated by the large first cusp of the medial row, but differ from those of Paraci- mexomys and Dakotamys in having relatively well- separated cusps in the internal row.

There is a wide range of tooth sizes included within this species. Two species may be present but no consistent morphologic characters could be found to dis-




tinguish them and there is no distinct break in the size distribution.

BRYCEOMYS sp. cf. B. FUMOSUS Fig. 91

Specimen-Loc. 995: M1--V7460. Discussion, M1-V7460 (Fig. 91) has a formula of

4:4:2 (AP = 1.48, LB = 0.91) and an internal cusp row that terminates at the middle of the third cusp of the medial row. The specimen is very similar to the type of B. fumosus except it is more delicately constructed. It is significantly narrower than the Mls included in B. fumosus (range 1.05-1.39) and has a shorter, less lingually expanded, internal cusp row. Although the specimen is more Paracimexomys-like than other M1 s of Bryceomys, there is no tendency for the medial and external cusp rows to diverge anteriorly. The weak development of the internal cusp row on this small Ml1 suggests it may not belong with B. fumosus.

BRYCEOMYS HADROSUS, sp. nov. Table 4, Fig. 9J-M

Type Specimen--OMNH Loc. V4: M1--OMNH 25467.

Hypodigm - OMNH Loc. V4: ml- 25443, M1 - 22443, 25468; P4-25462. MNA Loc. 995: m2s- V7379, V7474; mls--V6302, V7371; M1--V6344; M2s-V5651, V7483. MNA loc. 1003: M2-V5332.

Diagnosis--Larger than Bryceomys fumosus. Three cusps on external row of Ml, internal row connects anteriorly to third cusp of medial row; ml with wide, deep, U-shaped valleys (in side view) separating cusps of both rows.

Etymology - Hadrosus, Greek for stout. Description, ml -V7371 (Fig. 9J; described and fig-

ured in Eaton, 1987, as UOSM 20000) has a cusp formula of 5:3. Cusps of both rows are conical with the second cusp of the internal row and the second and third cusps of the external row having weakly concave posterior sides. Cusps are broadly and deeply separated by U-shaped valleys (in side view). The first cusp of the external row is low and small, cusps 2 and 3 are much larger than the first cusp and are subequal in size, the fourth cusp is much smaller than the third, and the fifth cusp is small and positioned lingually on the posterior wall of the tooth. The first two cusps of the internal row are completely divided but are not as widely separated as cusps 2 and 3. The central valley is slightly sinuous, less so than in B. fumosus or Par- acimexomys.

Second Lower Molar--V7474 (Fig. 9K) has a cusp formula of 3:3. The external row is damaged and bears three cusps that were probably separated, prior to wear, only at their apices. The central valley is deep and crossed by a ridge connecting the first cusp of the internal row to the second cusp of the external row. The first cusp of the internal row is large and concave posteriorly, the second cusp is buccolingually compressed, and a third small cusp is present just lingual to the

notch in the posterior wall that corresponds to the terminus of the central valley.

P4 - OMNH 25462 is the posterior half of a P4 that is morphologically similar to those assigned to B. fumosus but is much larger (Table 4).

M1 -OMNH 25467 (Fig. 9L), the holotype, has a cusp formula of 3:4:3. On the anterolabial ridge of the external row there is a small swelling, not included in the cusp formula, in front of the first cusp. The cusps of the external row are conical and decrease in size and height posteriorly. The medial and external cusp rows diverge anteriorly. The first cusp of the medial row is anteroposteriorly short; cusps 2 and 3 are subequal in size, and the fourth cusp is tall and formed along the posterior margin of the tooth. The internal cusp row is broadly expanded lingually, with two subequal posterior cusps and a small, low, anterior cusp. The row attaches to the anterior part of the third cusp of the medial row. A broad, closed valley separates the internal row from the main body of the tooth.

M2- V7483 (Fig. 9M) has a cusp formula 4:3:3. The external row is a cuspate ridge that surrounds the second cusp of the medial row anterolabially. The first cusp of the medial row is formed by the slight swelling of the anterior wall of the tooth. The second cusp is broadly based with its apex oriented anteriorly. The third cusp is as tall as the second but has a smaller base. The first and second cusps of the internal row are separated at about half their height; cusps 2 and 3 are more deeply divided, almost to the floor of the central valley. A deep pit, possibly a diagnostic char- acter for the genus, is present on the posterior wall between the posteriormost cusps of the medial and internal cusp rows.

Discussion--The mean lengths of the molars of B. hadrosus range from 34-42% larger than those of B. fumosus. The m2 of B. hadrosus has the ridge crossing the central valley connecting the first cusp of the internal row to the second cusp of the external row diagnostic of Bryceomys.

The cusps of the medial and external rows on the Mls alternate in position as in Paracimexomys and Dakotamys and the cusp rows diverge anteriorly as in Paracimexomys but unlike Dakotamys. The ml s (e.g., V7371, Fig. 9J) included in this species have lengths and cusp depths almost identical to the Ml s. The blocky cusps of the medial row of the Mls fit well into the broad valleys separating the cusps on the mls. The M s differ from those of B. fumosus in being larger, in having fewer cusps on the external row, and in having the internal row connected to the third, rather than the second, cusp of the medial row. Other than their larger size, the M2 of B. hadrosus is morphologically similar to the M2 of B. fumosus.

BRYCEOMYS spp. Fig. 9N-P

Specimens - OMNH Loc. V4: M 1 - 23010; M2 - 25477. MNA Loc. 995: ml-V7411; m2s-V6770,




*L

llr~i..*

FIGURE 10. Turonian multituberculates. Family, genera, and species undetermined. A, Lp4, V6347, lingual; B, same, labial; C, RP4, OMNH 24570, osp; D, same, labial; E, RM2, V6760, osp (occlusal stereo-pair); F, RM2, OMNH 23075, osp; G, Ri, OMNH 25438, lingual; H, Li, V6113, lingual. Scale bar = 1 mm.

V7419, V7484; Mls-V6306, V7413; M2s-V6343, V6449, V7369.

Description, ml -V7411 is a poorly preserved ml with a cusp formula of 4:3 (AP = 2.56, LB = 1.35). The tooth appears to taper strongly anteriorly but this may be in part due to damage. The tooth has a strong waist in occlusal view.

Second Lower Molar--Three m2s have cusp formulas of 3:2, an AP range of 1.69-1.82, and an LB range of 1.45-1.48. V7484 (Fig. 9N) has the first cusp of the external row well-divided from the second. Cusps 2 and 3 are only divided at their apices. A ridge orig- inating on the first cusp of the internal row crosses the central valley and attaches to the second cusp of the external row. The two cusps of the internal row are about the same size and the first cusp is concave posteriorly.

Mi-Two large Mls (V6306, AP = 2.52*, LB =

1.49*; OMNH 23010, AP = 2.78*, LB = 1.72*) are poorly preserved but may be M 1 s of B. hadrosus. The posterior half of another M1, V7413 (Fig. 90), is small (LB = 1.43). The last two cusps of the external row are well-separated and are concave posteriorly. Cusps 2 and 3 of the medial row have their apices oriented anteriorly, are concave anteriorly, and have ribbed walls. The valley between the medial and internal cusp rows is pitted. The internal cusp row is broad and is formed by a ridge with an indistinct number of cusps. The row attaches anteriorly at the midpoint of the second cusp (assuming a total of four cusps) of the medial row.

M2-Four M2s have a cusp formula of R:3:3, an AP range of 1.63-1.73, and an LB range of 1.53-1.62. On V6343 (Fig. 9P) the external row is formed by a cuspate wall. The first cusp of the medial row is merged with the base of the large second cusp. The second cusp

is coarsely ribbed on its posterior and labial sides. The third cusp is slightly taller than the second. The three cusps of the internal row are well-divided lingually and buccally.

Discussion-- The ml approaches the length of ml s assigned to B. hadrosus, but is 0.3 mm narrower and appears to taper strongly anteriorly, unlike mls of B. hadrosus. The m2s are assignable to Bryceomys because of the characteristic ridge that crosses the central valley, but they are intermediate in size between those of B. fumosus and B. hadrosus. The partial M1 (V7413) is about 18% narrower than those assigned to B. hadrosus and is estimated to have been much longer than the M1 s assigned to B. fumosus. The M2s have all the characteristics of Bryceomys and are intermediate in size between B. fumosus and B. hadrosus. Of previously figured M2s, these are most similar to teeth figured as Stygimys in Archibald (1982:fig. 21g) except for the presence of ribs on the sides of the cusps.

Most of the specimens included in Bryceomys spp. are intermediate in size between B. fumosus and B. hadrosus, and have ranges that are disjunct from either species. With the current sample size it is inappropriate to assign these to either species and the material is inadequate to justify a new species.

Genera and Species Undetermined Fig. 10A-H

Specimens - OMNH Loc. V4: lower incisor-- 25438; M2-25476. OMNH Loc. V60: P4-24570; M2s- 23075, 23078. MNA Loc. 995: lower incisor-V6113; p4-V6347; M2s-V6760, V7409.

Description and Discussion --Several specimens cannot be identified even at the family level but may shed some light on the potential diversity of the fauna.




V6347 (Fig. 10A, B) is a large p4 (AP = 4.19, H = 2.40, H:L = 0.57) with a high crowned blade and a formula of 7 external and internal ridges and 9 serrations. The anteroexternal lobe is deep and the first serration occurs about halfway up the anterior face of the tooth. The shape of the crest of the tooth is difficult to estimate, as the posterior part of the crest is heavily worn. A small posteroexternal cusp is present. The large size and few anterior serrations are Cimolodon- like characters (e.g., Archibald, 1982:fig. 18), but both the vertical posterior wall and the degree of arch of this specimen are more like that of Cimexomys (see Archibald, 1982:figs. 35, 37; Montellano, 1992:fig. 8). The largest multituberculate described from the Smoky Hollow Member is Bryceomys hadrosus, for which no p4s were described; however, if this p4 were considered to represent that species, the m1 :p4 length ratio would 0.63, less than for any North American Cretaceous multituberculate. This suggests the presence of yet another taxon in the fauna.

OHNM 24570 (Fig. 10C, D) is a P4 with a cusp formula of 2:6 (AP = 1.98, LB = 1.05). The anteroexternal platform is weakly expanded labially and bears two cusps. The cusp apices of the medial row ascend with an arch to the fifth cusp; the sixth cusp is lower. A sharp ridge descends posteriorly to the labial cusp of the posterior basin. The weakly expanded anteroexternal platform is similar to some P4s of Cimolo- mys (C. gracilis, Lillegraven, 1969:fig. 13-I; C. clarki, Sahni, 1972:fig. 11H); another similarity is that the penultimate cusp is the highest of the medial cusp row (C. gracilis, Archibald, 1982:figs. 23a-c). The specimen is unlike Cimolomys in having a medial cusp row that increases greatly in height posteriorly and in lacking a waist in occlusal view. The rate of descent of the medial cusp row is like that of P4s of Mesodma, particularly M. senecta (see Fox, 1971:fig. 1) and some P4s of Me- sodma have the sharp ridge posterior of the ultimate cusp of the medial row (Sahni, 1972:fig. 10c; Lille- graven and McKenna, 1986:fig. 6D); however, the relatively weak anteroexternal platform is unlike P4s of Mesodma and more like the condition found in P4s of Cimexomys (see Archibald, 1982:fig. 35b). Of figured specimens, only the P4 of Cimolomys gracilis (see Ar- chibald, 1982:figs. 23a-c) has the penultimate cusp of the medial row being the tallest. This group of characters precludes unequivocal assignment of this specimen.

V7409 is a worn M2 too large (AP = 2.59, LB = 2.17) to fit with any of the species described above but inadequately preserved to figure. V6760 (Fig. 10E) is an M2 with a cusp formula of 3:3:4? (AP = 1.59, LB = 1.38), complexly pitted enamel, and a continuous internal cusp row, a combination unlike other M2s in the sample. Three M2s (OMNH 23075, Fig. 10F; OMNH 23078, OMNH 25476) have an AP range of 1.42-1.45 and a LB range of 1.18-1.28. They are more anteroposteriorly elongate than M2s ofBryceomys and, unlike Bryceomys, they have a distinct first medial row cusp developed on the anterior wall and poorly divided

cusps of the internal row. Second molars are more likely to survive fluvial transport than less compact teeth and, not surprisingly, second molars demonstrate more taxonomic diversity than do teeth of other positions (except possibly anterior upper premolars). The few M2s discussed here suggest the presence of several additional multituberculate taxa in the fauna.

Lower incisors are relatively rare in the sample. Most of the lower incisors (e.g., OMNH 25438, Fig. 10G) are completely covered in enamel, taper anteriorly, are somewhat flattened dorsally, and have a medial cin- gulum. As these are the most common incisor type in the sample, it is possible that they represent Bryceo- mys. A slight variation occurs on V6113 (Fig. 10H), on which the enamel appears to thin on the dorsal side but is not sharply restricted to the ventrolateral surface of the tooth, unlike the condition seen in taeniolabi- doids (Kielan-Jaworowska, 1974).

RELATIONSHIPS OF TAXA

Paracimexomys, Bryceomys, and Dakotamys represent a diversification of middle Cretaceous post-pla- giaulacoid multituberculates. Placement of these taxa in appropriate suborders and families is beyond the scope of this paper as it would be necessary to review all Cretaceous multituberculate systematic schemes, especially the recent comprehensive treatment by Sim- mons (1993).

Paracimexomys, Bryceomys, and Dakotamys are unique among North American Cretaceous multituberculates in sharing alternation of cusps on Ml such that the external and medial rows are offset. This is assumed to represent retention of a primitive condition, as it is also present in the Early Cretaceous bol- odontoid (see Simmons, 1993) Eobataar (see Kielan- Jaworowska et al., 1987:fig. 3B). The second to first molar length ratios are lower than those of Cimexomys and Mesodma, but not lower than the wide range among species of Cimolodon (0.6-0.9). Paracimexomys and Bryceomys appear to be taxonomically close in that they share a sinuous central valley on the ml, while Dakotamys has the straight central valley typical of ptilodontoid multituberculates. Also unlike either Par- acimexomys or Bryceomys and more similar to the condition found in ptilodontoids, Dakotamys has weak posterior closure of the central valley of M1. The m2: m l length ratio of Dakotamys (0.86), however, is closer to that of Paracimexomys (0.9) than it is to that of Bryceomys (0.77), as is its M1 cusp formula (4:4:1-2, same as Paracimexomys; Bryceomys has an anteriorly lengthened internal cusp row).

The Cimolodontidae was originally named by Marsh (1889) but is not well-defined, nor have its relationship to the suborder Ptilodontoidea been clearly established. The family is retained here for simplicity and contains only one Cretaceous genus, Cimolodon, which is distinct in the high arch of p4, with its first serration high on the anterior part of the crest. The relationship of Cimolodon to other ptilodonioids is obscure. The




strongly ribbed, robust, pyramidal molar cusps and high crowned p4s are derived relative to the condition found in North American plagiaulacoids, but some of these characteristics are present, though less developed, on some specimens of Early Cretaceous Para- cimexomys (see Eaton and Nelson, 1991); they are also well-developed on specimens of Ptilodus (see Krause, 1977:pl. I, fig. 3). The m2:ml length ratios for species of Cimolodon average 0.7, but the most primitive named species of Cimolodon, C. electus from the Milk River fauna, has an m2:ml ratio of 0.9 (from Fox, 1971:table 2), as in Paracimexomys. Because of the morphologic distinctiveness of Cimolodon, compared to plagiaulacoids, it will be assumed here that Cimo- lodon is more derived in most characters than Para- cimexomys (see Simmons, 1993).

That Dakotamys has special ancestral relationship to Cimolodon is considered unlikely. Dakotamys could be considered morphologically transitional between Paracimexomys and Cimolodon. Both Dakotamys and Cimolodon have a pocket labial to the anteriormost cusps of the external row on M1, Dakotamys shows some tendency toward the ribbed pyramidal cusps and pitted valleys found in some species of Cimolodon, and both taxa have straight central valleys on Ml s. There is, however, at least some indication that cimolodontids were present in the Early Cretaceous (Eaton and Nelson, 1991), and they certainly were present in the Cenomanian, at the same time as the first appearance of Dakotamys. Cimolodon differs from Dakotamys in having higher cusp formulae, non-alternating cusps of the medial and external rows on M1, more strongly developed internal cusp row on M1, in lacking posterior closure of the central valley on M1, and the development of the internal cusp row on M2 into a continuous ridge.

THE CENOMANIAN-TURONIAN EXTINCTION

The Cenomanian-Turonian boundary is considered to be one marking extinction at a global scale (Kauff- man, 1984; Raup and Sepkoski, 1982, 1986; Elder, 1987). The proposed explanations for this event range from impact of an extraterrestrial object or objects (Raup and Sepkoski, 1982, 1986) to changes in climate related to major eustatic sea-level rise (Elder, 1987). Lacking in all previous discussions of the extinction are non-marine biotas.

Certainly, the Cenomanian-Turonian boundary is one of enormous change in terrestrial faunas. The turtle Glyptops is common in the Late Jurassic and makes its last appearance in the Dakota Formation, as do the fish Lepidotes and Ceratodus (Kirkland, 1987). The differences in faunal lists for the Dakota Formation and Smoky Hollow Member of the Straight Cliffs For- mation might, at fist glance, seem to indicate a major extinction event for mammals.

Cenomanian multituberculates include species of Cimolodon, Paracimexomys, Dakotamys, and proba-

bly those of at least one other family, possibly the Cimolomyidae. These taxa, with the possible exception of Paracimexomys sp. cf. P. robisoni, are not present in the Smoky Hollow fauna of Turonian age. Two of the genera (Cimolodon and Paracimexomys) are known from later in the Cretaceous, and it is unknown if Dakotamys had post Cenomanian descendents. All therian genera described from the Dakota Formation (Eaton, 1993a) persist into the later Cretaceous. Dak- otadens is present in the Turonian (Cifelli, pers. comm., 1993), and Alphadon and Protalphadon are both known to occur in the Campanian but are, as yet, unknown from the Turonian (Cifelli, 1990). The older Ceno- manian mammalian fauna from Bryce Valley is actually more like later Cretaceous faunas than is the Turonian fauna.

The marked difference between the Cenomanian and Turonian mammalian faunas in the same geographic location, coupled with the persistence of Cenomanian taxa after the Turonian, suggests that mammalian faunas responded to ecologic changes by shifting geographic ranges rather than suffering a marked episode of extinction. The ecologic controls are unknown. Cen- omanian mammals evolved on broad open floodplains until the close of the Cenomanian, when the rapid and widespread Greenhorn transgression restricted western North America to a narrow, north-south elongate, subcontinent (Elder and Kirkland, 1993). Middle-late Tu- ronian mammals witnessed the regression of the Greenhorn Sea and the re-expansion of the western North America subcontinent. The changes in climate, seasonality, and even the salinity of the rivers (suggested by the large component of brackish water taxa found in late Cenomanian localities, relative to the paucity in comparable Turonian localities), that ac- companied eustatic changes were probably enormous. Environmentally-induced changes in terrestrial faunas are compatible with those suggested by Elder (1987) for marine faunas and do not support a catastrophic episode of extinction induced by bolide impact.

ACKNOWLEDGMENTS

This research was supported by grants from the Na- tional Geographic Society (3965-88 to Eaton) and the National Science Foundation (EAR-9004560 to Eaton; BSR 8507598, 8796225, 8906992 to Dr. Richard Ci- felli). The cooperation of Dr. Richard Cifelli, the Bu- reau of Land Management, and the State of Utah is greatly appreciated. Assistance in the field and labo- ratory was provided by Pam Borne, Elly Evers, Abe Gillett, Jared Morrow, and Cori Yordi. The illustra- tions in Figure 3 were drawn by Jodi Griffith. Chester Tarka is thanked for his suggestions on improving my microphotographic technique, and I apologize to him for being such a poor student. Reviews of the manu- script by Rich Cifelli, Dave Archibald, Dave Krause and Jason Lillegraven are appreciated.




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Received 20 April 1994; accepted 17 August 1994.



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Cenomanian and Turonian (Early Late Cretaceous) Multituberculate Mammals from Southwestern Utah