Smithsonian miscellaneous collections

Geologic study of the occurrence of Paleocene mammal remains in the Bison Basin of Wyoming. There are four main fossil localities - two in the exposures below the southern rim of the basin, one in the southwestern part and one in the western end. Of these 53 are in the University of Wyoming collections, with occurrences split between the saddle vicinity and the Titanoides locality.

NO. 6 PALEOCENE FAUNAS OF BISON BASIN — GAZIN

ENVIRONMENT AND RELATIONSHIPS BETWEEN THE BISON BASIN FAUNAS

8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I

Most small condylars are found only in the saddle or ridge fauna, and there appear to be no differences worth commenting on. On the other hand, a single example of the saddle tentatively pointing to Litolestcs lacunatus hardly differs from the typical material from the Titanoides locality. Typical materials of each were found as single specimens at the Saddle and Titanoides sites, respectively.

AGE AND CORRELATION OF THE FAUNAS

The fauna of the Titanoides site is quite limited, but there is evidence of a slightly more typical Tiffanian stage. The Thryptacodon here can apparently be compared closely with that of the Tiffany beds in Colorado. Haplaletesserior could possibly suggest a relatively early horizon, but this species is so much larger than that of the Lebo, or even than the form described from the saddle site in the Bison Basin, that its significance seems to have been lost.

12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I Except for their intermediate positions, no particular additional

SYSTEMATIC DESCRIPTION OF VERTEBRATE REMAINS

REPTILIA

ANGUIDAE

MAMMALIA

MULTITUBERCULATA PTILODONTIDAE

In size and exterior, as well as in the spacing of the teeth, of which there were at least 13, the tooth is quite similar to P4 in Ptilodusmontanus. Anconodon russelli can be represented by a fragment of the right mandibular ramus showing the root part of the incisor and P4. Measurements of the teeth are included below with those of the following species believed to be distinguishable.

SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I

The combination of characters seems most closely approximated in the San Jose and Knight material of Diacodon alticuspis. The shortening of the talonid on P4 and the lower molars of Xenacodon suggest Diacodon, but P4 appears to be clearly less progressive in that the paraconid is small and the talonid is not basined. Pa, anteroposterior diameter : greatest transverse diameter 1.8:1.0 P*, anteroposterior diameter : transverse diameter of trigonid 2.4:1.4 Ml, anteroposterior diameter : transverse diameter of trigonid 2.2:1.6.

PANTOLESTIDAE

Discussion.- The structure of P4 and the general shape of the molars suggest a correspondence of this form with Aphrononis and thus with the pantolestids, although the weakness of the hypoconulid appears. P4 of Bisonalveiis has a slightly better developed paraconid at the anterolingual tooth margin and a more pronounced and posterolingually located entoconid, so that this part of the talonid ridge is. The lower molars show elevated trigonids, somewhat smaller than in Aphronorus, but the paraconid is almost as high as the metaconid.

20 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I

None of the material of this species was found at the more westerly and possibly higher horizons in the basin. In the more typical ridge site materials, this is noticeable in the relatively inflated appearance of the cusps. M3 (buccally incomplete), but other specimens with M3 out of the saddle show this tooth to be normally proportioned.

22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I MEASUREMENTS IN MILLIMETERS OF LOWER TEETH IN SPECIMENS OF

Discussion.—Plesiadapis jepseni is one of the better represented forms in the Bison Basin collection, exceeded only in number of specimens by Phenacodus. However, two came from the site in the western part of the basin and two from near the Titanoides primaevus site in the south-western part of the basin.

24 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, I3I

ARCTOCYONIDAE

OXYCLAENINAE

26 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I

An isolated upper molar, probably M^ (U.S.N.M. No. 21003), would also appear in a species of Chriacus about the size of C. The external styles of this tooth are not clearly developed, but the lingual cingulum bears a hypocone of prominent and an equally prominent. protostyle less developed on antero-lingual edge of tooth. 21019, presumably M-, is much smaller, approximately 25 percent smaller in overall size than the M- discussed in the previous section.

In size it appears to be more comparable to Chriacus truncatus, approximately that of Thryptacodon belli. The rectangular appearance of this tooth and the prominence of the antero-internal cusp or protostyle would seem to exclude it from consideration as a form of Thryptacodon. The teeth are three times larger than the type, as perhaps indicated by the size given by Simpson, but the rudimentary state of the mctaconid at P4 and the prominently isolated hypoconulid at M3 may suggest a closer relationship with it.

28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I

However, it should be noted that an approach to this condition is made in certain specimens of T. detnari, so possibly not of diagnostic significance. antiquus, in addition to the development of the metaconid on P4, includes a little less difference in width between M2. and the trigonid of M3, with M3 relatively much shorter. teeth of Thryptacodon demari showacingulum, usually discontinuous, external to the hypoconium, but they do not appear to be developed external to the protocone to the extent seen in T . Nor is it nearly as extended posterior to the hypoconid on M3. 20992, has only M^, There is also an isolated M^ in the University of Wyoming collection which may represent this species. The cingulum is evenly continuous around the anterior and lingual portions of the tooth and the hypocone is a simple rising conical cusp. from the cingulum posterointernal to the the protocone.

There is no protostyle and there are no particularly distinct accessory cuspules along the hypocones as observed in the Eocene materials. lacks the characteristic parastyle, and in no. 20992) there is a very rudimentary protostyle anterolingual to the protocone where the cingulum is somewhat more sharply bent around the edge of the tooth than in M^.

30 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I

A complex similar to that pictured in the Bison Basin is seen in fewer materials, although distinctly better, in the Fort Union area of Montana's Crazy Mountain field. Using Simpson's histogram (1937b, Fig. 35) for the length of M2 in the Torrejon material in the American Museum, I added individuals in each size range to the numbers in Fig. 2 based on information obtained from the Torrejon collections in the National Museum, and included a similar histogram for the teeth of the Bison basin . In the latter, the columns are extended by dashed lines in cases where, in the absence of Mg, the size of the adjacent molar is indicative of one or the other group.

I was unable to find any characters of importance in these specimens by which the shape of the Bison pelvis could be determined as distinct from the earlier C. Mi, anteroposterior diameter: greatest transverse diameter 77:6.1 Ma, anteroposterior diameter: greatest transverse diameter 8.0 :6.7 Ma, anteroposterior diameter: greatest transverse diameter 8.7:5.6.

32 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I

MESONYCHIDAE

MIACIDAE

CONDYLARTHRA

HYOPSODONTIDAE

A very large species of Promioclaenus can be represented by a fragmentary right mandibular ramus (U.S.N.M. No. 21020), preserving only the posterior part of M2 and part of the trigonid as well as the roots of M3. There is a distinct possibility that this is Litaletes and not Promioclaenus; however, the cusps included in the preserved portion of M2, although somewhat worn, suggest fewer crown teeth as in Promioclaenus , significantly less inflated than in Mioclaemis . Horizon and locality.-Tiflfanian Bison Basin, saddle locality on the southern edge of the Bison Basin, sec.

Skaphikos, from Greek/;o/>/joj=anything hollowed out as a basin, in reference to the Bison basin.

38 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I

Discussion.- The type of Haplaletes pelicatusis an excellent lower jaw with the cheek-tooth series in an almost unworn condition. However, the premolars are a little more inflated and as a result the metaconid of P4 is not as prominent. Comparison of Haplletes pelicatus with Haplletes diniinutimis Dorr (1952) is not feasible, as the latter is represented by a partial upper dentition; nevertheless, the very small size of the Dell Creek form leaves no doubt as to their distinctiveness.

40 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I

The paraconid is located in approximately the same position, but on the posterior molars it is more clearly isolated from the adjacent molars. The extent to which this cusp is connected to the ridge toward the protoconid, however, varies in the Torrejon form. Specific characters.— About one-third larger than Liiolestcs notissimusin the size of the carpels and well outside the range given by Simpson (1937a).

Discussion. The form described here as Litolestes lacunatus is the smallest of the condylarths recognized in the Bison Basin collections, yet is clearly larger than the previously described form.

42 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, I3I

PHENACODONTIDAE

Paraconid of lower molars variable but generally low and weak, Trigonid not so prominently based as in Gidleyina superior. The type comes from the locality at the western end of the basin, as do about eight other specimens, although most of these are isolated upper and lower teeth. The material of the ridge site appears to be entirely consistent with that of the western side, and among the ten specimens from the ridge is the upper dentition (U,S.N,M. No. 20795) shown on Plate 9 and about three jaws with two or choose more.

About five specimens from the saddle site, including parts of the upper and lower dentition, could represent a slightly smaller variant, although probably not particularly distinct. Logically, a doubt may arise as to the reasonableness of recognizing Gidleyin as separate from Ecfocion. As noted by Simpson, the upper premolars in Gidleyina are less progressive, and the upper molars show a better development of ridges from the protocone to the protoconule and metaconule.

The noupper premolars appear to be included in the Bison Basin collections, but the molars exhibit the protoconcrests noted above, and comparison with G. P4 in the Montana and Wyoming Gidleyina material would appear distinct when compared with the Eocene material of Ecfocion, primarily in that trigonid . is elongated relative to the length of the talonid, while in E. In relation to the form of Gidleyina known from the Melville Unit of Montana's Fort Union, I strongly suspect that Gidleyina sil-herlingi is a synonym of G.

44 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I

There is uncertainty as to whether the species represented by this material should be referred to Phenacodus or Tetraclaenodon. Although there are differences between them in the degree of development of a number of characters, they are of the nature of average differences, lacking the consistency generally expected at the generic level. Granger attempted a definition based on the development of the mesostyle but certain upper molars of Tetraclaenodon puercensis.

Nevertheless, this is variable in populations of the better-known species of Phenacodus as well as in Tetraclaenodon puercensis and, like the increasing importance of the tritoconus of the upper premolars, is a difference of degree that is not easily defined.

PANTODONTA

CORYPHODONTIDAE

NO. 6 PALEOCENE FAUNAS OF BISON BASIN — GAZIN 49

50 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I31

A revision of the North American Creodonta with notes on some genera which have been referred to this group.

EXPLANATION OF PLATES

Plesiadapis jepseni, new species: I, left ramus of the mandible (U.S.N.M. No. 20586), lateral and occlusal views; 2, left maxilla (U.S.N.M. No. 20781), occlusal view; 3, Left ramus of the mandible (U.S.N.M. No. 20760), type specimen, lateral and occlusal views .

MULTITUBERCULATES AND INSECTIVORES FROM THE BISON BASIN PALEOCENE

TRICENTES AND CHRIACUS FROM THE BISON BASIN PALEOCENE

CUAENODON FROM THE BISON BASIN PALEOCENE

LITOMYLUS AND PROTOSELENE? FROM THE BISON BASIN PAL.EOCENE (SEE EXPLANATION AT END OF TEXT.)