Triconodont Mammals from the Cloverly Formation (Lower Cretaceous), Montana and Wyoming

Triconodont Mammals from the Cloverly Formation (Lower Cretaceous), Montana andWyomingAuthor(s): Richard L. Cifelli, John R. Wible and Farish A. Jenkins, Jr.Source: Journal of Vertebrate Paleontology, Vol. 18, No. 1 (Apr. 10, 1998), pp. 237-241Published by: Taylor & Francis, Ltd. on behalf of The Society of Vertebrate PaleontologyStable URL: http://www.jstor.org/stable/4523887 .

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238 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 18, NO. 1, 1998

A

C B

t 4

FIGURE 1. Corviconodon montanensis, gen. et sp. nov. A-C, OMNH 33457 (holotype), stereopairs. A, lingual. B, buccal. C, occlusal. Scale bar = 5 mm.

cally, a posteriorly directed foramen is present at the anteriormost extent of the masseteric fossa (Fig. 1 B). The mesialmost tooth root and alveolus preserved in OMNH 33457 (Fig. IC) is almost circular (not buccolingually compressed) and is rounded distally. This is unlike the ridged condition seen in molar roots and alveoli of more distal teeth on this specimen and in specimens of Astroconodon denisoni, indicating

that it is the distal root of the last premolar-in turn indicating that five molars were present in Corviconodon montanensis. The molars are strongly interlocked: cusp d fits snugly into a groove on the mesial face of cusp b of the succeeding tooth; the mesial root and alveolus of m4 (which is missing) bear a groove and ridge, respectively, indicating that the interlocking mechanism extends deep into the tooth roots, as it does in Alticonodon and Astroconodon. Although wear facets are well developed and the mesial molars are well worn, the specimen represents an immature individual; the last molar, m5, is still in its crypt and is placed above the tooth row, partway up the ascending ramus, on the lingual side of the jaw (Fig. IA). The molars are somewhat larger than those of Astroconodon denisoni and increase slightly in size from first to third (ml est. W = 0.97; m2 L = 3.05, W = 1.02; m3 L = 3.36, W = 1.05), but the last molar is notably the smallest (m5 L = 1.49); crown height also shows a slight increase distally (with the exception of m5), but the difference is much less than in Astroconodon. Sharp lingual cingula are present; it appears unlikely that buccal cingula were present, but this cannot be determined with certainty because of wear. The molar crowns are low relative to the condition seen in Alticonodon and the cusps are separate for a much greater proportion of their height than in that genus. The buccal faces of the cusps appear to be somewhat flatter than in Astroconodon, but this appearance may be due to wear in OMNH 33457. Cusps a, b, c, and d are subequal in height (except on m5, in which they decrease in height distally, and only cusps a, b, and c are present) and are noticeably recumbent; cusp d is relatively taller on mesial molars than it is in Astroconodon, and is more tightly appressed to cusp c than in that genus.

OMNH 33459 (Fig. 2B, C) is an edentulous mandible (in two pieces lacking contact between them) of a triconodontid from the same locality as the holotype and is tentatively referred to Corviconodon montanensis. The anterior part preserves the root of the canine and paired roots of the first three premolars; the posterior part preserves part of the mesial and distal roots of the last premolar, and the mesial root of the first molar (as indicated by the characteristic tongue-and-groove fit of root to alveolus and the size and shape differential between this root and the one that precedes it) and part of the distal root. It thus appears that four premolars were present in the species. An anterior mental foramen lies below the mesial root of p2, and a larger posterior one is present below the mesial root of p3. The symphyseal surface is rugose and extends posteroinferiorly to a point adjacent to the distal root of p2. The root of the canine is buccolingually compressed and is comparatively enor- mous: it occupies nearly the entire dentary where that bone is broken anterior to pl (Fig. 2C).

Gen. et sp. Indet.

Material-OMNH 23498, distal part of left lower molar with cusps c and d.

Horizon and Locality-OMNH locality V62, unit V, Cloverly For- mation, Big Horn County, Wyoming.

Comments-OMNH 23498 (not figured), obtained through screen- washing, represents a triconodontid of much smaller size than Corvi- conodon montanensis. Although little can be said of the specimen because of its incompleteness, it unambiguously documents the presence of another species of triconodontid mammal in the Cloverly Formation. The preserved cusps are much lower than in C. montanensis, more resembling in this respect a mesial molar of Astroconodon denisoni. The lingual cingulum is well developed; cusp d is strong and does not overhang the distal tooth root, a similarity to North American Creta- ceous triconodontids (Astroconodon, Alticonodon, Corviconodon) and a point of variance with Jurassic taxa (Priacodon, Trioracodon, Tricon- odon).

DISCUSSION

In combination with previously reported taxa, the fossils described herein reveal a surprising diversity of triconodonts in the Cloverly For- mation: four species-level taxa are now known, including the large gob- iconodontid Gobiconodon ostromi (see Jenkins and Schaff, 1988) and three triconodontids-the unnamed species from unit V (Jenkins and Crompton, 1979), Corviconodon montanensis, and a small, unidentified taxon. Fragmentary remains of other mammals are also known from the unit (Cifelli, unpubl. data), but this nonetheless represents a sharp contrast with the mammalian fauna of the presumably time-equivalent Trinity Group, which includes abundant Multituberculata and tribos- phenic taxa (see Cifelli, 1997),

as well as Astroconodon. The wear seen



NOTES 239

A

dca A BC

pl p2 p3 p4 ml

canine

can

canine

FIGURE 2. Corviconodon montanensis, gen. et sp. nov. A, OMNH 33457 (holotype) in buccal view, showing wear facets (stippled) on lower molars. Lower case letters refer to lower molar cusps; upper case letters indicate wear facets produced by corresponding upper molar cusps. B, C, OMNH 33459, edentulous left mandible in occlusal (B) and buccal (C) views; tooth roots are stippled. Scale bar = 5 mm.

on teeth of C. montanensis is relatively heavy compared to that of comparably aged individuals of other taxa (e.g., Trioracodonferox, As- troconodon denisoni, unnamed Cloverly taxon); an interesting but (in the absence of a large population sample) presently untestable specu- lation is that this may reflect differences in habitat preference (see Ci- felli et al., 1988).

Jenkins and Crompton (1979) diagnosed the Triconodontidae on the shared presence of three subequal molar cusps (A/a, B/b, and C/c). We concur that this can be recognized as a synapomorphy for the family. (Ichthyoconodon jaworowskorum, from the Early Cretaceous of Moroc- co, also has subequal cusps a, b, and c and, like North American Cre- taceous Triconodontidae, has a d cusp that is fully incorporated into the tooth crown, but other features of this puzzling species make its affin- ities problematic; see Sigogneau-Russell, 1995.) We add to the diagnosis of Triconodontidae the reduction of buccal cingula and of accessory cusps (other than cusp D/d) on the molars and the buccolingual compression of the lower molars. Thus recognized, the Triconodontidae ranges from Late Jurassic through Late Cretaceous and includes the following genera: Triconodon, Trioracodon, Priacodon, Astroconodon, Alticonodon, Corviconodon, and two unidentified taxa (both from the Cloverly Formation) that may not be generically distinct. Rougier et al. (1996) have identified several features of the basicranium that may prove to be synapomorphies of the Triconodontidae when their distribution is better understood. The mandible of Corviconodon bears a foramen at the anteroinferior margin of the masseteric fossa. This fea- ture is also present in Astroconodon (see Turnbull, 1995), Priacodon ferox (YPM 606), Trioracodon bisulcus (YPM 10340) and, possibly, Triconodon mordax (see Simpson, 1928:pl. 4, fig. 2), at least, among triconodontids. The foramen is lacking in presumably distal outgroup taxa such as Kuehneotherium (see Kermack et al., 1968) and Morgan-

ucodon (see Kermack et al., 1973), but is present in Vincelestes (see Rougier, 1993) and a number of early tribosphenidans, including Pro- kennalestes (see Kielan-Jaworowska and Dashzeveg, 1989) and Kie- lantherium (see Dashzeveg and Kielan-Jaworowska, 1984). As should be evident from the foregoing, the phylogenetic significance of the la- bial mandibular foramen is uncertain.

The Late Cretaceous Patagonian Austrotriconodon was initially referred to the Triconodontidae (see Bonaparte, 1986) but later transferred to its own family (Bonaparte, 1992). The teeth identified as lower molars are enigmatic and somewhat resemble premolars; cusps b and c are much lower than cusp a, unlike the condition among molars of triconodontids. The upper molar (Bonaparte, 1992:fig. 3), however, bears strong similarity to ultimate upper molars of Triconodontidae, and appears to have a deep mesial excavation for an interlocking mechanism analogous to or homologous with that seen in taxa of the North Amer- ican Cretaceous (see below).

Assessing relationships within the Triconodontidae is problematic because of inadequate or dissimilar representation among taxa and because variability and character polarity are poorly understood. Corviconodon probably had a lower postcanine dental formula of p4 m5, similar to that of the unnamed taxon from the Cloverly Formation (Jenkins and Crompton, 1979); the dental formula of Alticonodon is not known, but the ultimate molar was as large or larger than its predecessors, with a fully quadrituberculate cusp pattern (Fox, 1969, 1976). Patterson (1956) suggested that the postcanine formula for Astroconodon denisoni is p4 m4; in agreement, Slaughter (1969:103) commented, ".... the number of molars was no less than four. This seems the probable number, so I interpret the mandibular dental formula as ?-1-4-4." Recent reconstruc- tions by Turnbull (1995), however, show that A. denisoni probably had five molars, the last of which was subequal in size to its predecessors



240 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 18, NO. 1, 1998

(e.g., FMNH PM399, 1029, 588). Hence, it is possible that a lower postcanine formula of p4 m5 may have broad distribution among North American Cretaceous triconodontids. The significance of this (and the unique condition in Corviconodon, in which the diminutive m5 could be interpreted alternatively as "incipient" or "reduced") is difficult to judge: Jurassic triconodontids have a lower postcanine formula of p3- 4 m3-4 (Simpson, 1929); the morganucodontids Morganucodon and Dinnetherium have p3-5 m4-5; and the most proximate sister taxa to Triconodontidae + Prototribosphenida (as identified and diagnosed by Rougier et al., 1996), Monotremata and Multituberculata, have highly modified, non-comparable dentitions.

Somewhat less equivocal but still tenuous support for a special re- lationship of North American Cretaceous Triconodontidae, to the exclu- sion of other taxa, is the fact that all share a tall d cusp that forms part of the shearing surface and that does not overhang the distal root on the lower molars; and the probably related condition of the molar series, in which adjoining teeth are tightly interlocked via a tongue and groove mechanism that extends well down the molar roots (see Fox, 1969:fig. 1). Interlocking molar series are also seen in Jurassic Triconodontidae and in more primitive taxa, such as Dinnetherium (see Jenkins et al., 1983); however, in these instances the mechanism is restricted to inter- position of juxtaposing accessory or cingular cusps on adjacent tooth crowns, and serves to align the molars (Crompton, 1974). As Patterson (1951) observed in Astroconodon, the molar series of Cretaceous taxa form integrated, continuous shearing surfaces, and the interlocking mechanism serves to restrict individual molar movement, otherwise per- mitted by periodontal ligaments, during occlusion. In Corviconodon (Fig. 2A), occlusion results in initial wear on the buccal face of cusp c and adjacent distobuccal flank of cusp a, the buccal side of the notch between cusps a and c, and on the buccal side of cusp b. This pattern is highly similar to that of Morganucodon and Jurassic triconodontids, in which these facets are produced by cusps A, B, and C of the upper molars, respectively, and contrasts with the condition in Megazostrodon, amphilestids, and Kuehneotherium (see Crompton, 1974; Jenkins and Crompton, 1979). Wear facets develop on the flanks of the cusps first, and later extend to the shearing crest as the teeth "hone" during occlusion; precise fit in this Cretaceous triconodontid-as in more primitive taxa but in contrast to the condition in therians-depends on tooth wear and not strictly on crown morphology (Crompton, 1974, 1995). Early in wear the shearing surfaces are developed at angles to one another, resulting in a serrated surface, as in Alticonodon (see Fox, 1969, 1976). With more advanced wear (e.g., m2, Figs. IB, 2A), however, the buccal faces of the lower molars achieve a more planar surface, as is seen in Astroconodon (see Patterson, 1951). Occlusion produced by upper molar cusps A and B expands the wear facets between cusps a-c and a- b, respectively; the lower molar facet produced by cusp C, initially restricted to the buccal flank of cusp b (m3, Figs. 1B, 2A), expands mesially to include also the buccal side of cusp d on the preceding tooth (ml-2, Fig. 2A). These wear surfaces thus show the progressive incorporation of cusp d-occlusally insignificant in Jurassic triconodontids- into the shearing battery, the bridging of lower molar shearing crests by a single upper molar wear surface, and the probable functional ex- planation for the deep interlocking mechanism between adjacent molars. An implication for systematics is that the heightening of lower molar d cusp and its incorporation into the functional tooth crown, together with the tongue-and-groove configuration of adjacent molar surfaces, are probably related and should be treated as a single character complex.

Acknowledgments-We are especially grateful to officials of the Bu- reau of Indian Affairs for access to tribal lands, to the Bureau of Land Management for their cooperation with our work on lands under their administration, and to the Tillett, Tilstra, Parmenter, Keebler, Lande, and Taylor families for access to private lands and for various help. We thank E. M. Larson, S. K. Madsen, R. Nydam, T. J. Gaudin, J. Gardner, K. West, J. Banta, K. Smith, and S. Judd for assistance in the field; Madsen is to be especially thanked for his masterful work in preparing and conserving the fossils described herein. Drs. W. D. Turnbull and D. A. Winkler graciously supplied us with casts of Astroconodon denisoni, Drs. J. Gauthier and R. Emry kindly lent us specimens of Jurassic triconodontids, and Dr. G. W. Rougier provided useful advice and comments; we thank them all for these courtesies. This research was sup- ported by NSF grant DEB 9401094 to RLC and by a research initiation grant from the University of Louisville to JRW.

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Received 5 March 1997; accepted 6 July 1997.



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Triconodont Mammals from the Cloverly Formation (Lower Cretaceous), Montana and Wyoming