Marsupial Mammal from the Upper Cretaceous North Horn Formation, Central Utah

Paleontological Society

Marsupial Mammal from the Upper Cretaceous North Horn Formation, Central UtahAuthor(s): Richard L. Cifelli and Christian De MuizonSource: Journal of Paleontology, Vol. 72, No. 3 (May, 1998), pp. 532-537Published by: Paleontological SocietyStable URL: http://www.jstor.org/stable/1306651 .

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JOURNAL OF PALEONTOLOGY, V. 72, NO. 3, 1998 JOURNAL OF PALEONTOLOGY, V. 72, NO. 3, 1998

32?39'46"N, 97?19'W, Fort Worth 7.5' Quadrangle. "In 1984 the highway I-35W and Loop 1-20 interchange was converted into a vast cloverleaf to expose the Weno and Pawpaw south of... 714." "The locality is on the southeast side of the access road leading from I- 35W south to 1-20 east. It is a northwest facing cut exposing the top of the Weno, 6.76 meters of Pawpaw and the lower Main Street. In June 1985 this slope was bladed and the Pawpaw more or less ho- mogenized ...." (J. H. Tenery, personal commun.)

Reid Locality 9 (Dirks).-Pawpaw Fm. Less than 1 m Weno, 3.3 m Paw- paw, 0.5 m Main Street Limestone. At this locality, the base of the Pawpaw is tan shale overlain by about 9 cm of flaggy limestone layers; asteroids were collected from the "starfish bed," a claystone interval at the top of the lime-rich interval about 2.7 m above the base of the Pawpaw. Approx. 32?38'N, 97023'52'W, Benbrook 7.5' Quadrangle. Ap- proximately 0.5 km NE of the intersection of Old Granby Road and Columbus Trail, Fort Worth. As of mid-1993, exposure extended several hundred meters along a small, east-west creek bottom.

Reid Locality 10 (Sycamore School Road).-Weno Fm., about 10 m Pawpaw Fm., and about 3 m Main Street Fm. Approx. 32?39'N, 97?19'38"W, Fort Worth 7.5' Quadrangle. Exposure on hill slopes immediately east of Sycamore Creek, 1.8 km SW of the center of the intersection of I-35W and 1-20; west of Highland Terrace S Street, and north of Hallmark W Street, Fort Worth.

Reid Loc. 11.-Main Street Fm. Approx. 32?46'38"N, 97?13'06'W, Hurst 7.5' Quadrangle. About 2 km NW of the intersection of 1-30 and 1-820, SW of the intersection of Bridgewood Dr. and Randol Mill

32?39'46"N, 97?19'W, Fort Worth 7.5' Quadrangle. "In 1984 the highway I-35W and Loop 1-20 interchange was converted into a vast cloverleaf to expose the Weno and Pawpaw south of... 714." "The locality is on the southeast side of the access road leading from I- 35W south to 1-20 east. It is a northwest facing cut exposing the top of the Weno, 6.76 meters of Pawpaw and the lower Main Street. In June 1985 this slope was bladed and the Pawpaw more or less ho- mogenized ...." (J. H. Tenery, personal commun.)

Reid Locality 9 (Dirks).-Pawpaw Fm. Less than 1 m Weno, 3.3 m Paw- paw, 0.5 m Main Street Limestone. At this locality, the base of the Pawpaw is tan shale overlain by about 9 cm of flaggy limestone layers; asteroids were collected from the "starfish bed," a claystone interval at the top of the lime-rich interval about 2.7 m above the base of the Pawpaw. Approx. 32?38'N, 97023'52'W, Benbrook 7.5' Quadrangle. Ap- proximately 0.5 km NE of the intersection of Old Granby Road and Columbus Trail, Fort Worth. As of mid-1993, exposure extended several hundred meters along a small, east-west creek bottom.

Reid Locality 10 (Sycamore School Road).-Weno Fm., about 10 m Pawpaw Fm., and about 3 m Main Street Fm. Approx. 32?39'N, 97?19'38"W, Fort Worth 7.5' Quadrangle. Exposure on hill slopes immediately east of Sycamore Creek, 1.8 km SW of the center of the intersection of I-35W and 1-20; west of Highland Terrace S Street, and north of Hallmark W Street, Fort Worth.

Reid Loc. 11.-Main Street Fm. Approx. 32?46'38"N, 97?13'06'W, Hurst 7.5' Quadrangle. About 2 km NW of the intersection of 1-30 and 1-820, SW of the intersection of Bridgewood Dr. and Randol Mill

Rd. Specimens collected from float. The site was later graded to be developed for housing.

Reid Loc. 12.-Grayson Fm., near Woodbine Fm. contact. Approx. 32?46'06"N, 97?13'W, Hurst 7.5' Quadrangle. About 1 km NW of the intersection of 1-30 and 1-820, east of Bridgewood Dr. between Bridgewood Dr. and 1-820, north of Bridge St. Site since developed as a shopping center.

Ben White Roadcut Locality, University of Texas.-Section 13, Hinote (1978); intersection of Route 360 and South Lamar Boulevard, approximately 0.3 mi (0.5 km) northwest of intersection with U.S. 290; Austin, Travis County, Texas.

Lake Waco Dam Spillway Locality.-Del Rio (=Grayson) Shale. Waco Dam Quarry, Waco Lake Research Area, northwest of Waco, Mc- Lennan County, Texas. Crateraster sp. A, just above the pit floor just east of the north entrance corner; probably community C, sensu Hin- ote (1978). Betelgeusia reidi, several? fragmentary specimens, from the Albian portion of the interval, section 6 or 7 of Mancini (1977, p. 348-349). A pyritized fauna probably belongs to the "Middle Claystone" of Mancini; fragments of Betelgeusia reidi collected from below interval with pyritized fossils, but perhaps not within the Mid- dle Claystone sensu Mancini. (fossils and notes provided M. Listokin and M. Foote). Akaidia sumralli, relatively complete specimen, para- type Texas Memorial Museum 1786TX1, about 10 m above the base of the quarry along the east wall, approximately 100 feet south of the northeast corner of the quarry; fragmentary specimens from lower in the section at unidentified sites within the quarry.

Rd. Specimens collected from float. The site was later graded to be developed for housing.

Reid Loc. 12.-Grayson Fm., near Woodbine Fm. contact. Approx. 32?46'06"N, 97?13'W, Hurst 7.5' Quadrangle. About 1 km NW of the intersection of 1-30 and 1-820, east of Bridgewood Dr. between Bridgewood Dr. and 1-820, north of Bridge St. Site since developed as a shopping center.

Ben White Roadcut Locality, University of Texas.-Section 13, Hinote (1978); intersection of Route 360 and South Lamar Boulevard, approximately 0.3 mi (0.5 km) northwest of intersection with U.S. 290; Austin, Travis County, Texas.

Lake Waco Dam Spillway Locality.-Del Rio (=Grayson) Shale. Waco Dam Quarry, Waco Lake Research Area, northwest of Waco, Mc- Lennan County, Texas. Crateraster sp. A, just above the pit floor just east of the north entrance corner; probably community C, sensu Hin- ote (1978). Betelgeusia reidi, several? fragmentary specimens, from the Albian portion of the interval, section 6 or 7 of Mancini (1977, p. 348-349). A pyritized fauna probably belongs to the "Middle Claystone" of Mancini; fragments of Betelgeusia reidi collected from below interval with pyritized fossils, but perhaps not within the Mid- dle Claystone sensu Mancini. (fossils and notes provided M. Listokin and M. Foote). Akaidia sumralli, relatively complete specimen, para- type Texas Memorial Museum 1786TX1, about 10 m above the base of the quarry along the east wall, approximately 100 feet south of the northeast corner of the quarry; fragmentary specimens from lower in the section at unidentified sites within the quarry.

J. Paleont., 72(3), 1998, pp. 532-537 Copyright ? 1998, The Paleontological Society 0022-3360/98/0072-0532$03.00

MARSUPIAL MAMMAL FROM THE UPPER CRETACEOUS NORTH HORN FORMATION, CENTRAL UTAH

RICHARD L. CIFELLI AND CHRISTIAN DE MUIZON Oklahoma Museum of Natural History and Department of Zoology, University of Oklahoma, Norman 73019, <[email protected]>, and

Laboratoire de Paleontologie-URA CNRS 12, Mus6um National d'Histoire Naturelle, 8, rue Buffon, F-75005 Paris, France

ABSTRACT-Little is known of the non-dinosaurian fauna from the Cretaceous (Maastrichtian) part of the North Horn Formation, despite its biogeographic importance. Herein we describe a new marsupial mammal from the unit, founded on an exceptionally complete specimen of a juvenile individual, and present new information on the incisor region of early marsupials, based on comparison with complete specimens from the early Paleocene of Bolivia. Alphadon eatoni, new species, is the smallest Lancian species of the genus, and departs from a presumed marsupial morphotype in having the second lower incisor enlarged. The species is, however, primitive in lacking a "staggered" pattern to the incisor series and in having a labial mandibular foramen, and in these respects it differs from Paleocene and later marsupials. Poor representation of other taxa precludes meaningful comparison to most other North American Cretaceous marsupials, although Eodelphis, thought to be distantly related, also has an enlarged i2. Although Alphadon is characterized by many primitive features, the relative development of the incisors is not what would be predicted in a morphological antecedent to later Marsupialia.

J. Paleont., 72(3), 1998, pp. 532-537 Copyright ? 1998, The Paleontological Society 0022-3360/98/0072-0532$03.00

MARSUPIAL MAMMAL FROM THE UPPER CRETACEOUS NORTH HORN FORMATION, CENTRAL UTAH

RICHARD L. CIFELLI AND CHRISTIAN DE MUIZON Oklahoma Museum of Natural History and Department of Zoology, University of Oklahoma, Norman 73019, <[email protected]>, and

Laboratoire de Paleontologie-URA CNRS 12, Mus6um National d'Histoire Naturelle, 8, rue Buffon, F-75005 Paris, France

ABSTRACT-Little is known of the non-dinosaurian fauna from the Cretaceous (Maastrichtian) part of the North Horn Formation, despite its biogeographic importance. Herein we describe a new marsupial mammal from the unit, founded on an exceptionally complete specimen of a juvenile individual, and present new information on the incisor region of early marsupials, based on comparison with complete specimens from the early Paleocene of Bolivia. Alphadon eatoni, new species, is the smallest Lancian species of the genus, and departs from a presumed marsupial morphotype in having the second lower incisor enlarged. The species is, however, primitive in lacking a "staggered" pattern to the incisor series and in having a labial mandibular foramen, and in these respects it differs from Paleocene and later marsupials. Poor representation of other taxa precludes meaningful comparison to most other North American Cretaceous marsupials, although Eodelphis, thought to be distantly related, also has an enlarged i2. Although Alphadon is characterized by many primitive features, the relative development of the incisors is not what would be predicted in a morphological antecedent to later Marsupialia.

INTRODUCTION

THE NORTH Horn Formation, exposed on the eastern margin of the Wasatch Plateau (though probably with a greater

distribution; see Hintze, 1988), is one of only a few latest Cre- taceous terrigenous units in the state of Utah and is the only one that has produced substantial remains of fossil vertebrates of Lancian age (see Lillegraven and McKenna, 1986, for discussion and definition). The North Horn Formation has attracted interest because it also includes vertebrate assemblages of early Paleo- cene age (e.g., Archibald et al., 1987), though whether or not the actual Cretaceous-Tertiary boundary is represented within the unit (e.g., Spieker, 1960) is debatable (Cifelli et al., in press). In a broader, geographic context, the North Horn Formation and its contained assemblage of Cretaceous vertebrates are signifi- cant in that they lie between the more northerly, contempora- neous Lancian faunas of Wyoming, Montana, and Canada; and

INTRODUCTION

THE NORTH Horn Formation, exposed on the eastern margin of the Wasatch Plateau (though probably with a greater

distribution; see Hintze, 1988), is one of only a few latest Cre- taceous terrigenous units in the state of Utah and is the only one that has produced substantial remains of fossil vertebrates of Lancian age (see Lillegraven and McKenna, 1986, for discussion and definition). The North Horn Formation has attracted interest because it also includes vertebrate assemblages of early Paleo- cene age (e.g., Archibald et al., 1987), though whether or not the actual Cretaceous-Tertiary boundary is represented within the unit (e.g., Spieker, 1960) is debatable (Cifelli et al., in press). In a broader, geographic context, the North Horn Formation and its contained assemblage of Cretaceous vertebrates are signifi- cant in that they lie between the more northerly, contempora- neous Lancian faunas of Wyoming, Montana, and Canada; and

the southern faunas of the San Juan Basin, New Mexico, and the Big Bend region, Texas (see e.g., Clemens et al., 1979; Weis- hampel, 1990). The North Horn fauna includes one of the few occurrences of the titanosaurid sauropod Alamosaurus (see Gil- more, 1946; Lucas and Hunt, 1989) and appears to represent a distinct, semiarid biogeographic province that characterized the American southwest in the late Maastrichtian (Sloan, 1969; Lawton, 1976; Lehman, 1981, 1987).

With the exception of the teiid lizard Polyglyphanodon (see Gilmore, 1942), the Lancian fauna of the North Horn Formation consists almost exclusively of dinosaurs (Gilmore, 1946; see summary by Cifelli et al., in press). Herein we describe a marsupial mammal from the Cretaceous part of the unit in Emery County, Utah (Fig. 1). As well as adding to the known vertebrate diversity approaching the Cretaceous-Tertiary boundary in central Utah, this small mammal is of special interest because it is

the southern faunas of the San Juan Basin, New Mexico, and the Big Bend region, Texas (see e.g., Clemens et al., 1979; Weis- hampel, 1990). The North Horn fauna includes one of the few occurrences of the titanosaurid sauropod Alamosaurus (see Gil- more, 1946; Lucas and Hunt, 1989) and appears to represent a distinct, semiarid biogeographic province that characterized the American southwest in the late Maastrichtian (Sloan, 1969; Lawton, 1976; Lehman, 1981, 1987).

With the exception of the teiid lizard Polyglyphanodon (see Gilmore, 1942), the Lancian fauna of the North Horn Formation consists almost exclusively of dinosaurs (Gilmore, 1946; see summary by Cifelli et al., in press). Herein we describe a marsupial mammal from the Cretaceous part of the unit in Emery County, Utah (Fig. 1). As well as adding to the known vertebrate diversity approaching the Cretaceous-Tertiary boundary in central Utah, this small mammal is of special interest because it is

532 532



CIFELLI AND MUIZON-CRETACEOUS MARSUPIAL MAMMAL

FIGURE 1-Locator map showing the approximate position (black star) of OMNH site V798, Cretaceous part of the North Horn Formation, Emery County, Utah.

based on a unique specimen of a juvenile individual (Cifelli et al., 1996) that includes morphology not otherwise represented in comparable taxa known from the Cretaceous of North Amer- ica.

CONVENTIONS

In addition to other Cretaceous marsupials from North Amer- ica, we made comparisons with Recent taxa (predominantly Di- delphidae, which retain a primitive incisor pattern; and Dasyu- ridae) and three taxa from the early Paleocene of Bolivia, Pu- cadelphys, Mayulestes, and Andinodelphys, which are represented by exceptionally complete specimens (Muizon, 1994; Marshall and Muizon, 1995; Muizon, unpubl. data). Designa- tions for tooth position follow established convention (e.g., Clemens, 1979), though we note that ontogenetic evidence presented by Luckett (1993) indicates that pl-2 of adult marsupials represent unreplaced dpl-2 and that the marsupial ml may be an unreplaced deciduous tooth as well. Similarly, Hershkovitz (1982, 1995) has presented arguments suggesting that the original number of lower incisors present in marsupials was five, and that the first has been lost in known taxa; we follow convention in numbering the incisors numerically (i.e., il-4). Mea- surements, in mm, are after Lillegraven (1969) and Lillegraven and Bieber (1986), and are abbreviated as follows: AP, antero- posterior (=mesiodistal) length; W, greatest width (buccolin- gual); Wtal, width of talonid; Wtri, width of trigonid. Institu- tional abbreviations: OMNH, Oklahoma Museum of Natural History; USNM, United States National Museum.

SYSTEMATIC PALEONTOLOGY

Order MARSUPIALIA Illiger, 1811 Family Uncertain

Genus ALPHADON Simpson, 1927 ALPHADON EATONI new species

Type specimen.-OMNH 27380, left dentary with 2 incisors, canine, pl-2, dp3, ml, and unerupted m4.

Diagnosis.-Distinct from all Lancian species of the genus on

the basis of its smaller size. Differs from the slightly larger Ju- dithian species, A. attaragos Lillegraven and McKenna, 1986, in having a more anteriorly placed metaconid on ml, and from the similar sized Judithian A. perexiguus Cifelli, 1994, in having lower molars with more flexure (entoconid, metaconid, and par- aconid not in as straight a line), relatively taller trigonid with more acute angle formed by major cusps, and more conical, less bladelike entoconid.

Comments and description.-Cretaceous marsupials are generally characterized mainly on the basis of upper molar structure (e.g., Clemens, 1966, 1979; Lillegraven, 1969), and the definition of A. eatoni is not wholly satisfactory in this respect. None- theless, lower molars have proven useful in characterizing species of Alphadon (cf. Lillegraven and McKenna, 1986), and the features cited in the diagnosis above clearly show A. eatoni to be distinct from all similar species.

OMNH 27380 appears to be the most complete of the few specimens representing juvenile marsupials known from the Cre- taceous of North America: other than the single maxillary frag- ment (bearing dP3-M1) of Pediomys hatcheri illustrated by Clemens (1966: fig. 58), only isolated deciduous teeth have been reported (e.g., Clemens, 1966; Lillegraven, 1969; Fox, 1981; Cifelli, 1994). Internal anatomy of OMNH 27380, based on high resolution X-ray computed tomography, has been presented by Cifelli et al. (1996). These authors found that, in known respects, tooth eruption and replacement is similar to the relatively ste- reotyped pattern seen in Recent taxa. Based on comparison with the didelphid Monodelphis and the dasyurid Sminthopsis, these authors estimated OMNH 27380 to represent an individual 60- 70 days in age.

The hemimandible (Fig. 2) is complete except for some breakage in the incisor region and the loss of the tip of the coronoid process, one or more incisors, and m2-3. In most respects, the jaw is highly similar to that of Paleocene Pucadelphys, or to a small didelphoid such as Marmosa, except that the mental region forms a more distinct angle with respect to the ventral margin of the horizontal ramus (in living taxa the jaw tapers more gent- ly), and the ramus is quite shallow posterior to the position of m2. The shallowness of the mandible is almost surely due to the young age of the individual it represents, as it is comparable to what is seen in Recent juvenile didelphids and in a mandible of a juvenile Pucadelphys. Two well defined, subequal mental fo- ramina are present, one beneath the anterior root of p2 and the other beneath the anterior root of ml. The same position is observed in Pucadelphys (adult and juvenile) and in Mayulestes (adult and juvenile). In Andinodelphys, the anterior mental foramen is variably located below either the anterior or posterior root of pl (CM, unpubl. data). The masseteric fossa is moder- ately well defined; anteroventrally within this fossa is a very small foramen (Fig. 3) that faces posteriorly. Ventrally, the masseteric fossa is defined by a sharp crest which is weaker than is generally seen in Recent didelphids and which extends posteriorly to the mandibular condyle. The condyle itself, which is rarely preserved in Cretaceous marsupials, lies just above the level of the tooth row, as it does in Recent taxa and in Paleocene didelphoids; it is lower than in the borhyaenoid Mayulestes. The condyle differs from that of Pucadelphys, Andinodelphys, Ma- yulestes, Didelphodon, and living didelphids in being much more rounded, less developed mediolaterally, and less cylindri- cal in general appearance. Lingually, the mandibular foramen is well developed and opens posteriorly near the angle of the jaw, at the anterior extremity of the angular process, as in didelphoids in general. Part of the angle has been displaced by breakage, but it is clear that it was strongly inflected, as in most marsupials and several other groups of primitive mammals (see discussions

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JOURNAL OF PALEONTOLOGY, V. 72, NO. 3, 1998

FIGURE 2-Alphadon eatoni, new species. OMNH 27380 (holotype), left mandible in labial (A) and lingual (B) aspects; scale bar = 2 mm.

by Clemens, 1979; Marshall and Muizon, 1995; Sanchez-Villa- gra and Smith, 1997).

Two incisors are present; owing to breakage in the distal part of the series, the original number of incisors cannot be deter- mined with certainty (Figs. 2, 3). The mesial alvelous (which is clearly the most mesial alveolus originally present and which thus housed numerical il) is well preserved; it is mesodistally compressed, and housed a root (and, by implication, a tooth) that was much smaller than that of the succeeding incisor, i2. Despite the fact that one or more incisors are missing, it is clear that i2 (AP = 0.55; W = 0.43) was much the largest of the series. It bears a lanceolate, lingually recurved crown with mesial and distal crests that curve lingually at the base, forming a faint lingual cingulum. There is no bony buttress labial to i2; instead, the alveolar margin is somewhat recessed adjacent to the root. Similarly, the large root to i2 lies near the labial margin of the dentary and shows no sign of curving lingually or being lingually displaced ("staggered"; Hershkovitz, 1982, 1995) with respect to adjacent teeth. At least one alveolus, for numerical i3, is present posterior to i2. This alveolus is covered by the root and crown of a tooth that is morphologically similar to but much smaller than i2. The tooth had come out of its socket and become displaced labially prior to fossilization; the crown (AP = 0.29; W = 0.22) is separated from the root by a seam of intervening matrix. Because this tooth appears to lie very near its original position, we interpret it as having the alveolus immediately distal to i2, and hence identify it as i3. Judged by the size of the root for this tooth in relation to the space it covers, it appears unlikely that it overlies more than one alveolus. Posterior to the root of i3, a strong bony septum, much better developed than in Recent didelphoids, separates the alveolus (or alveoli) that it obscures

from the alveolus for the canine. In Recent didelphids, the septum between i4 and the canine is often thin or incomplete. A slight space separates the canine from the mesial margin of its alveolus in OMNH 27380; hence it is possible that i4 was placed against the mesial margin of the canine, and a separate alveolus for it cannot be observed. However, this seems unlikely: the space is far narrower than the root of i3 (which itself is small relative to the size of the jaw), and a similar space is present posterior to the canine (which is incompletely erupted, explain- ing the small gap between alveolus and tooth). In sum, the first three incisors of the series are represented by teeth or their alveoli. The balance of evidence suggests that only three were originally present and that a short diastema separated i3 from the canine, but the formula cannot be established with confi- dence.

The canine (Fig. 2) is trenchant and bears a sharp distal keel; it is similar to that of Pucadelphys and Andinodelphys but differs from that of Recent Didelphidae in having a weaker mesial keel and in lacking a pronounced posterior curvature at its tip. The premolars are similar to those of other Cretaceous and primitive living marsupials, so that complete description is unnecessary: they bear a salient protoconid which is clearly more developed than in Pucadelphys and Andinodelphys. Lingually, a weak pos- terointernal cingulum, lacking in the Paleocene taxa as well as Recent didelphids, is present. The first premolar (AP = 0.73; W = 0.40) differs from that of Recent didelphids in being relatively taller, anteroposteriorly shorter, and less anteriorly recumbent. In these respects, p is similar to that of Late Cretaceous Alphadon wilsoni (cf. Lillegraven, 1969, fig. 18), although it appears to be taller than in that species. Its implantation in the dentary is more vertical than in Pucadelphys and Andinodelphys, where the tooth

534




FIGURE 3-Alphadon eatoni, sp. nov. OMNH 27380 (holotype); A, oc- clusal view of preserved dentition (i2-3, c, pl-2, dp3, ml); B, post- erodorsal view of masseteric fossa, showing position of labial mandibular foramen; alveoli for m3, upper left, indicated for orientation. Ab- breviations: lmf, labial mandibular foramen; m3, alveoli for m3. Scale bar = Imm for top photo; 0.5 mm for bottom photo.

is clearly anteriorly recumbent. Similarly, the second premolar (AP = 1.09; W = 0.63) is also comparatively tall and erect, as it is in A. wilsoni and A. ?marshi (Clemens, 1966, fig. 61), relative to the condition seen in A. ?lulli (Clemens, 1966, fig. 59).

The deciduous tooth (AP = 1.15; Wtri = 0.57; Wtal = 0.56) is fairly typical for marsupial dp3: it is molariform but consid- erably smaller than ml, differing from the molars in having lower, more poorly defined cusps, an anteroposteriorly elongate trigonid that appears to be rather narrow compared to the talonid, and an anteroposteriorly short talonid. The first lower molar (AP = 1.56; Wtri = 0.89; Wtal = 0.87) is of typical Alphadon-like construction (Clemens, 1966, 1979; Lillegraven, 1969; Cifelli, 1990), except for differences from similar species as noted in the diagnosis, and description is unwarranted.

Hypodigm.-The type only; other, fragmentary specimens may belong to this species (Cifelli et al., in press) but are not formally included.

Type locality.-OMNH locality V798, Cretaceous (Maastrich- tian) part, North Horn Formation (Fig. 1). This site is about 150 m N of the USNM "lizard locality," and lies at the same hori- zon. The "lizard locality," from whence the type and many referred specimens of Polyglyphanodon sternbergi were collected, is described by Gilmore (1942; see his plate 24A, B), who gives a legal description, photographs, and paleontological context. Photographs contained in the USNM archives, as well as evidence of previous collecting at OMNH V798, indicate that this site was conceived by Gilmore, Gazin, and other USNM collec- tors as being part of the "lizard locality."

Etymology.-For Jeffrey G. Eaton, in recognition of his contributions to knowledge of Cretaceous mammals from the State of Utah.

DISCUSSION

The vertebrate fauna of the North Horn Formation is sum- marized by Cifelli et al. (in press). Of the approximately 40 taxa

known from the Cretaceous part of the unit, eight are mammals. Only one mammal species other than Alphadon eatoni is known by a reasonably complete specimen, Pediomys hatcheri, a marsupial also collected from the vicinity of the USNM "lizard locality" (Clemens, 1961). Alphadon eatoni adds to known diversity of the fauna but, because it is presently endemic to the lower part of the North Horn Formation, it is of no biostrati- graphic utility. However, the species is of considerable interest because it appears to share the most nnambiguous apomorphy of Marsupialia, the tooth eruption and replacement pattern (dis- cussed by Cifelli et al., 1996; see also Muizon, 1994, for discussion of morphological criteria defining the group). Several details of the external anatomy in A. eatoni also deserve com- ment. One of these is the presence of a small foramen at the base of the masseteric fossa. A foramen in this position, termed the labial mandibula foramen, has been identified in a variety of primitive mammals, and its distribution in a phylogenetic context is enigmatic (see review by Cifelli et al., in press). The position (at the base of the masseteric fossa) and orientation (facing posteriorly) of the small foramen in Alphadon eatoni are similar to what is encountered in other primitive mammals, although it remains unclear whether or not the structures are ho- mologous. Loss of the labial mandibular foramen has been used as a defining feature of Metatheria (Marshall and Kielan-Jawo- rowska, 1992). However, a foramen in this position has been identified in the primitive marsupial or marsupial-like mammal, Kokopellia, and among South American Marsupialia (Cifelli and Muizon, in press), as well as Alphadon eatoni. This distribution suggests that loss of the labial mandibular foramen is not diag- nostic of either the Metatheria or Marsupialia. Because of its small size, this structure in A. eatoni could alternatively be in- terpreted as a neomorph (e.g., a nutrient foramen), an hypothesis which merely serves to underscore the dubious validity of this character as it is presently understood (see also Cifelli et al., in press).

Other noteworthy anatomical details of Alphadon eatoni con- cern the lower incisor series: the incisors constitute the most poorly known part of the dentition in Cretaceous marsupials. In primitive living marsupials (Didelphidae) and in the three early Paleocene taxa (Mayulestes, Pucadelphys, and Andinodelphys; Marshall and Muizon, 1995; Muizon, in press, unpubl. data) for which the incisor region is well known, there are four lower incisors. They are subequally developed in Pucadelphys and An- dinodelphys (in the latter i2 is slightly larger than il and i3), but in Mayulestes the fourth incisor is reduced. Four incisors are also present in the Mongolian placental Asioryctes (see Kielan- Jaworowska, 1981), and it is generally agreed that marsupials primitively had at least this many (Clemens and Lillegraven, 1986). Hershkovitz (1982, 1995) has suggested that five lower incisors were originally present, and that reduction of the anterior part of the jaw resulted in loss of il and crowding of the remaining teeth, with numerical i2 having a lingually placed root and a labial buttress. Hershkovitz (1995) showed that this pattern is seen in living Australian and South American marsupials and suggested that it is present in two Cretaceous taxa, Eodelphis browni and "Adinodus pattersoni" (which we consider to be a nomen nuduim). Judged by the alveoli, Kokopellia, from the medial Cretaceous of Utah, had four subequally developed incisors in a nonstaggered pattern (Cifelli and Muizon, in press). The staggered incisor pattern is unequivocally present in the three taxa (Mayulestes, Pucadelphys, Andinodelphys) from the early Paleocene of Bolivia, and the condition is present in most other South American and Australian taxa where it can be observed (this is precluded in many cases by poor preservation or extreme modification of the rostrum). Hershkovitz (1982, 1995) observed that a staggered pattern is not present in the South American

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microbiotheres and suggested that this group is primitive with respect to the incisor pattern. However, the distribution of the character relative to the taxonomic and phylogenetic hypotheses of Aplin and Archer (1987), Marshall et al. (1990), and Szalay (1993, 1995) strongly suggests that the microbiothere condition is the result of a reversal.

In Alphadon eatoni, the incisors were arranged in a nonstaggered pattern, and i2 was clearly enlarged with respect to the other incisors; at least three (which we regard as the probable number) incisors were present. Among other marsupials from the Cretaceous of North America, partial incisor regions are known for several taxa, but these are not very informative: Clemens (1966, fig. 58) illustrated a mandible of Alphadon ?lulli in which three incisor alveoli are seen and noted (p. 93) that at least two incisors were present in a mandible tentatively referred to the pediomyid Pediomys krejcii. Clemens and Lillegraven (1986, p. 71) reported that, "at least some species of Alphadon, the only Cretaceous marsupials known from material that pro- vides a full count of lower incisors, had at least four." We are unaware of any published specimens documenting the presence of more than three lower incisors in Alphadon. The incisors are fairly well known in the Judithian stagodontid, Eodelphis browni, in which three are present, the second of which is enlarged (Matthew, 1916), as in A. eatoni. Stagodontidae are hy- pothesized to represent a rather early-diverging but highly specialized clade of carnivorous marsupials known only from the Late Cretaceous of North America (Fox, 1981; Fox and Naylor, 1986). Enlargement of i2 is a derived similarity of A. eatoni to E. browni. The significance of this resemblance cannot be eval- uated at present, because the incisor region is so poorly known among early marsupials that character distribution is largely a matter of speculation. However, the incisor series of A. eatoni is of some interest with respect to the major radiations of marsupials on other continents-South America and Australia. Like Kokopellia and Eodelphis, A. eatoni is primitive with respect to Didelphimorphia (including Borhyaenoidea) and Dasyuromor- phia in lacking the "staggered" i2 (Hershkovitz, 1982, 1995), in turn providing corroboration for monophyly of these taxa (Muizon and Cifelli, unpubl. data). But A. eatoni also appears to be divergently specialized with respect to these taxa, specif- ically in the enlargement of i2 and, possibly, in the reduction to three incisors. Alphadon (a relatively long ranging genus; Eaton, 1993) and dentally similar taxa are often regarded as being close to the ancestry of later marsupials (e.g., Lillegraven, 1969; Mar- shall et al., 1990). The somewhat unusual configuration of the lower incisors in this genus suggests greater divergence from a primitive pattern than has generally been appreciated on the basis of its generally conservative molar structure.

ACKNOWLEDGMENTS

It is a pleasure to acknowledge D. Harber for the cooperation of the U.S. Forest Service and J. Judd for logistic help in the field. We thank K. Smith, W. J. May, and especially R. L. Nydam (who found the specimen described herein) for help in the field, and S. K. Madsen for preparing OMNH 27380 with his usual consummate skill. Figure 1 was drafted by C. McCallister; SEM photos (Figs. 2, 3) were taken by C. Chancogne, URA 12 CNRS. This collaborative research was made possible by a "professeur associe" appointment at the Laboratoire de Paleontologie, Mu- seum National d'Histoire Naturelle, Paris, and we are grateful to Dr. P. Taquet for facilitating our work. Field expeditions of CM for collection of Bolivian marsupials were funded by the National Geographic Society, Institut Frangais d'Etudes An- dines, the Centre National de la Recherche Scientifique, and the Museum National d'Histoire Naturelle. Field work by RLC in the North Horn Formation was supported by grants from the

National Geographic Society (4761-92, 5021-93) and the Na- tional Science Foundation (DEB 9401094).

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Marsupial Mammal from the Upper Cretaceous North Horn Formation, Central Utah