Bartels a Transitional Paleocene-Eocene Reptile Fauna From the Bighorn Basin, Wyoming

Herpetologists' League

A Transitional Paleocene-Eocene Reptile Fauna from the Bighorn Basin, WyomingAuthor(s): William S. BartelsReviewed work(s):Source: Herpetologica, Vol. 39, No. 4 (Dec., 1983), pp. 359-374Published by: Herpetologists' LeagueStable URL: http://www.jstor.org/stable/3892531 .

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December 19831 HERPETOLOGICA 359

Arima, AMNH 87398-401, 87402 (skeleton); Mt. Aripo, BMNH 1940.3.10.96-97; Mt. Tucuche, AMNH 55760-63, BMNH 1934.12.2.14-18, 1935.1.5.6-11, 1965.635-36, 1947.2.22.4 1, MCZ 17894-97, 19895-98; Simla, Arima Valley, AMNH 62888, 79921-24, KU 192399-400; Spring Hill Es- tate, Arima Valley, USNM 166625 (1 + 2 tadpoles); St. Pat's Estate, Arima Valley, BMNH 1956.1.4.63; Tamana Caves, N slope Mt. Tamana, AMNH 87396, 87397 (tadpoles). VENEZUELA: Sucre: Mauraco, N of Pilar, KU 181982-84.

Flectonotus pygmaeus. VENEZUELA: Aragua: Es- taci6n Biol6gica Rancho Grande, AMNH 70434-47 (14 + 1 lot young, 3 lots tadpoles), KU 133346-87, 133463-64, 139518-19 (tadpoles), 166738-54, 166756-59, 167758-59 (skeletons), 167819-23 (tadpoles), 184957-59, 185719-31, 185777-78 (tadpoles), USNM 196335. Barinas: La Soledad, UPR-M 5882-89, 5891-904. Carabobo: Puerto Ca- bello, SMF 2679. Merida: La Azulita, UPR-M 4293- 94. Tdchira: Mata Mula, UPR-M 6081, 6087-88, 6090.

Fritziana fissilis. BRASIL: Espirito Santo: Santa Teresa, 700 m, KU 187470-74. Guanabara: Sumare

(Rio de Janeiro), AMNH 70260; Tijuca, KU 92237- 42. Rio dejaneiro: Angra dos Reis, MNRJ 1998 (tadpoles); Parati, MNRJ 3119 (3); Teres6polis, AL (3), BMNH 1949.1.1.61-62, El 1615. Sdo Paulo: Bo- caina, WCAB 29253-442 (25 examined); Fazenda da Posse, Sao Jose do Barreiro, WCAB 31362-75, WCAB (tadpoles); Paranapiacaba, KU 71826; Saio Jose do Barreiro, Serra da Bocaina, USNM 164026- 30; Serra da Bocaina, AL (25).

Fritziana goeldii. BRASIL: Guanabara: Realengo, MNRJ 2068; Represa dos Ciganos, MNRJ 1875 (3); Rio de Janeiro, AL (5); Tijuca, AL (3), AMNH 70257- 58, 72324-25, El 1607, KU 84720, 84721 (skeleton), 92231-32 (skeletons), 92233-36; MNRJ 1846, 1862; WCAB 18716 (tadpoles), 42496. Rio de Janeiro: Co- Ionia Alpina (Teres6polis), BMNH 1947.2.12.69-70; Teres6polis, MNRJ 267 (2), 269. Sdo Paulo: Campos do Jordato, WCAB 34669-98.

Fritziana ohausi. BRASIL: Rio de Janeiro: Tere- s6polis, AL (7 + 1 lot tadpoles), KU 92225, 92226 (skeleton), WCAB 1077, 7879, 7888, 7891, 7894, 7896, 8872, 9383-84, 19523-25, 36355-56. Sdo Pau- lo: Paranapiacaba, AMNH 69946-47, BMNH 1964.158, KU 71782, 92227-30.

Herpetologica, 39(4), 1983, 359-374 ? 1983 by The Herpetologists' League, Inc.

A TRANSITIONAL PALEOCENE-EOCENE REPTILE FAUNA FROM THE BIGHORN BASIN, WYOMING

WILLIAM S. BARTELS

ABSTRACT: The Clarkforkian Land-Mammal Age is the transitional Paleocene-Eocene interval in the history of North American vertebrates. The Clarkforkian reptile fauna may be distinguished from earlier Paleocene faunas by the occurrence of the homed alligator Ceratosuchus and the anguid lizard Melanosaurus. A more dramatic faunal change occurs at the Clarkforkian- Wasatchian boundary when champsosaurs and Ceratosuchus disappear, emydid turtles and glyptosaurine lizards undergo significant radiations, and several families of turtles presumably mi- grate into the region. The Bighorn Basin Clarkforkian reptile assemblage is dominated by large aquatic taxa such as champsosaurs, alligators (Allognathosuchus in particular), the crocodylid Leidyosuchus, and trionychid and emydid turtles. The terrestrial component of this fauna is far less abundant and diverse, containing no snakes or distinctly terrestrial turtles and only two abundant lizards, Melanosaurus and the small anguid Machaerosaurus.

Key words: Reptilia; Testudines; Choristodera; Lacertilia; Crocodilia; Fauna; Paleocene- Eocene; Wyoming

THE Clarkforkian Land-Mammal Age has recently been redefined, based on a thorough review of its mammalian taxa (Rose, 1981). Clarkforkian deposits are central to understanding the transition from archaic Paleocene mammalian faunas to Eocene assemblages of modern as-

pect. The Clarkforkian is preceded in time by the Tiffanian (late Paleocene) Land- Mammal Age, and followed by the Wa- satchian (early Eocene) Land-Mammal Age. The Clarkforkian represents an interval of two million years, from about 55 Ma to about 53 Ma (Gingerich, 1980).



360 HERPETOLOGICA [Vol. 39, No. 4

Prior to a preliminary version of this study (Bartels, 1980), only a single Clark- forkian reptile (Simoedosaurus) had been described from the Bighorn Basin (Sig- ogneau-Russell and Baird, 1978). The only other published record of a Clark- forkian reptile was that of Ceratosuchus (Schmidt, 1938) from the Plateau Valley Beds of Colorado. Fortunately, late Pa- leocene Tiffanian (Estes, 1975; Gaffney, 1972; Gilmore, 1928, 1938, 1942) and early Eocene Wasatchian (Gilmore, 1942; Hay, 1908; Hutchison, 1980; Mook, 1921, 1924; Sullivan, 1979) herpetofaunas are more completely known and help to provide a framework for elucidating the transitional Clarkforkian reptile fauna.

In the Bighorn Basin, Clarkforkian fos- sils are recovered from two formations of Laramide-age basin fill, the Fort Union Formation and the overlying Willwood Formation. These elastic units are ex- posed in the northern Bighorn Basin (Clark's Fork Basin-Polecat Bench area), a structural and topographic depression in northwestern Wyoming. The Fort Union (Polecat Bench) Formation is bounded below by Lance-equivalent Up- per Cretaceous beds and above by the Willwood Formation. The Fort Union Formation reaches a maximum thickness of nearly 3000 m in a foredeep along the Beartooth Mountains to the west, but it is much thinner throughout most of the basin. It is comprised of drab fluvial grav- els, sands, silts, muds, limestones and lignites, with sands and silts predominating. The upper 100 m of the unit contain the lower Clarkforkian deposits of the Plesia- dapis gingerichi Zone (Rose, 1981). This interval is somewhat unfossiliferous and is the most poorly understood portion of the Clarkforkian. The remainder of the Clarkforkian is contained in the lower 800 m of the Willwood Formation. Distin- guished by the first appearance of red banded beds representing well-drained soils (see Bown, 1979), the Willwood For- mation is comprised of variegated fluvial sands, silts, muds, and limestones. Ov- erbank silts and muds predominate and lignites are locally absent. The Willwood

contains the middle Clarkforkian Plesia- dapis cookei Zone and the late Clarkfork- ian Phenacodus-Ectocion Zone (Ginger- ich, 1976).

SYSTEMATIC PALEONTOLOGY At least 21 species representing 10

families and four orders of reptiles are now recognized from Clarkforkian Big- horn Basin deposits. Many of these taxa need systematic revision beyond the scope of this investigation. Some of the following identifications should, therefore, be considered tentative.

Following Gingerich (1976) and Rose (1981), stratigraphic occurrences of each taxon are segregated into the Plesiadapis gingerichi (early), Plesiadapis cookei (middle), and Phenacodus-Ectocion (late) Clarkforkian Zones. The specimens listed below are in the University of Mich- igan Museum of Paleontology, except where noted. Fossil localities listed for the referred material are University of Michigan Sand Coulee (SC) and Foster Gulch (FG) sites in the northern Bighorn Basin.

Class Reptilia Order Testudines Linnaeus, 1758

Suborder Casichelydia Gaffney, 1975 Infraorder Cryptodira (Cope, 1868)

Superfamily Baenoidea Williams, 1950 Family Baenidae Cope, 1862

Baenid (indeterminate genus and species) Referred specimens.-UM 74606,

77682, carapace fragments that cannot be identified below family.

Occurrence.-Phenacodus-Ectocion Zone (SC-202, SC-234).

Discussion.-Baenids are among the rarest Clarkforkian turtles. In addition to these two specimens, there are a few carapace fragments from various horizons that might be referable to this family. Baenids are well-known from adjacent older (Torrejonian and Tiffanian) and younger (Wasatchian) horizons (Gaffney, 1972; Archibald and Hutchison, 1979), so their scarcity during the Clarkforkian is problematical.



December 1983] HERPETOLOGICA 361

Superfamily Trionychoidea Gray, 1870 Family Trionychidae Bell, 1828

Plastomenus Cope, 1873 Plastomenus sp.

Referred specimens.-UM 65240, 69680, 74596, 74610, portions of carapace and plastron, none of which are complete.

Occurrence.-Plesiadapis cookei Zone (SC-62, SC-136). Phenacodus-Ectocion Zone (SC-29, SC-163).

Description.-Each contains one or more of the following diagnostic characteristics: hyo-, hypo-, and xiphiplastra massive and solidly united along midline; hyoplastron notched anteromedial- ly; eighth costal not significantly reduced.

Discussion.-Plastomenus is the most completely understood trionychid genus; the solid plastron is a clear, easily recognized feature and is unique among tri- onychiids. Specific relationships within the genus are poorly known, however. Hay (1908:467) recognized species ac- cording to the density and style of carapace ornamentation. The systematic val- ue of these characters is questionable, and most specimens do not fall clearly into any of his categories. Specific assignment should be delayed until a more satisfac- tory classification can be developed for the genus as well as for fossil trionychids in general (see Gaffney, 1979).

Details of the carapace (lack of orna- mental ridges, expended seventh costal, reduced eighth costal) and plastron (massive hypoplastron, strongly curved hyoplastron) indicate that the Clarkforkian Plastomenus is distinct from described Wasatchian forms (Hay, 1908). There is enough variation within Clarkforkian specimens to suggest that two species of Plastomenus may have lived in the Big- horn Basin at that time.

Trionychid (indeterminate genus and species)

Referred specimens.-UM 74597, 74600, 74608, 74609, shell fragments.

Occurrence.-Plesiadapis gingerichi

Zone (SC-156, SC-171). Phenacodus-Ec- tocion Zone (SC-71, SC-159).

Discussion.-Scraps of trionychid shell are abundant throughout the Bighorn Ba- sin Clarkforkian. Included here are those specimens that would normally be assigned to Trionyx because of their greatly reduced eighth costal and plastral elements. This genus is so poorly understood that such an assignment would only further confuse the taxonomic status of this form.

Superfamily Testudinoidea Baur, 1893 Family Emydidae Gray, 1825

Emydid A (undescribed new genus and species)

Referred specimens.-UM 65492, 65493,65534,67562,67564,67565,71484, 74593, 74598, 74601, 74605, 74606, fragmentary to nearly complete shells.

Occurrence.-Plesiadapis gingerichi Zone (SC-156, SC-171), Plesiadapis cookei Zone (SC-120, SC-134, SC-135, SC-136, SC-188), Phenacodus-Ectocion Zone (SC- 48, SC-50, SC-53, SC-234).

Discussion.-Included here are specimens belonging to an as yet unnamed genus and species of macrocephalic emydid. This new genus is characterized by a prominent anterior emargination in the carapace and deep gutter-like epiplastra (both for the accommodation of the large skull). This genus is the most common Clarkforkian turtle and is very similar to Wasatchian forms (Hutchison, personal communication, 1981). It differs in detail from a Tiffanian taxon referred to the Eu- ropean macrocephalic emydid Ptycho- gaster by Estes (1975:376). This Tiffani- an form is congeneric with the Clarkforkian genus and should be considered a new, convergently evolved taxon (Hutchison, personal communication, 1980).

The Clarkforkian macrocephalic genus has previously been referred to as the "Echmatemys-like emydid" by Bartels (1980:77) and the "undescribed large- headed emydid" by Hutchison (1980: 117). A full description of this unusual




A

B

~~c?-~

,> C~~~~~

FIG. 1.-Champsosaurus gigas UM 74562, anterior portion of right dentary. (A) Superior view. (B) Internal view showing the relatively smooth sym- physeal surface. (C) Lateral view. Scale = 2 cm.

turtle is currently being completed by J. H. Hutchison and R. C. Wood.

Emydid B (unnamed new genus and species)

Referred specimens.-UM 65547, 74595, 74602, 77666-77678, carapace and plastron fragments.

Occurrence.-Plesiadapis gingerichi Zone (SC-83, SC-250), Plesiadapis cookei Zone (SC-62, SC-84, SC-127, SC-136, SC- 197), Phenacodus-Ectocion Zone (SC-50, SC-72, SC-158, SC-159, SC-162).

Discussion.-Included here are specimens referable to an as yet unnamed new genus and species of emydid box turtle. Hutchison (1980:117) described this form

as "unrelated to extant or other known box turtles .... This genus is the earliest known example of a fully developed box turtle morphology. There is a single transverse plastral hinge separating fully kinetic anterior and posterior lobes. The genus is unique among emydid turtles in having well developed elongate costi- form processes on the nuchal bone."

This species is common throughout the Bighorn Basin Clarkforkian.

Family Chelydridae Gray, 1870 Protochelydra cf. P. zangerli

Erickson, 1973 Referred specimen.-UM 71483. Occurrence.-Phenacodus-Ectocion

Zone (SC-234). Discussion.-A fairly complete but

poorly preserved shell referable to the Chelydridae, and probably P. zangerli. A single incomplete shell of this form is also known from the latest Tiffanian of the Bighom Basin (UM 77658, SC-178).

Family Chelydridae? Gray, 1870 Clemmys cf. C. backmani Russell, 1934

Referred specimens.-UM 69689, 74592.

Occurrence.-Plesiadapis cookei Zone (SC- 127), Phenacodus-Ectocion Zone (SC-202).

Discussion.-Two partial shells representing a second, much larger taxon probably referable to the Chelydridae. This form is closely related to or conspecific with the problematical "Clemmys" backmani, which may be a chelydrid (Hutch- ison, personal communication, 1982).

Order Eosuchia Suborder Choristodera Cope, 1884

Family Champsosauridae Cope, 1876 Champsosaurus Cope, 1876

Champsosaurus gigas Erickson, 1972 Referred specimens.-Jaw fragments

(UM 74562, Fig. 1), and a fragmentary skeleton (UM 71808) referable to C. gigas.

Occurrence.-Plesiadapis gin gerichi Zone (SC-250), Plesiadapis cookei Zone (SC-176).



December 19831 HERPETOLOGICA 363

Discussion.-Most elements of typical Champsosaurus construction. Snout long and very slender, mandibular symphysis very long and shallow. Clavicle and quadrate exceptionally massive. The most complete specimen (UM 71808) possesses the diagnostic distally flattened ribs and long femora of C. gigas (Erickson, 1972).

Simoedosaurus Gervais, 1877 Simoedosaurus sp.

Referred specimens.-Princeton Uni- versity Nos. 22199, 22334, posterior portions of mandibles and some fragmentary postcrania.

Occurrence.-Plesiadapis gingerichi Zone (Burns Mine, Bear Creek, Mon- tana).

Discussion.-Included here are two previously described (Sigogneau-Russell and Baird, 1978) specimens of a short- snouted champsosaur characterized by palatal elements covered with broad patches of teeth and anterior marginal teeth developed into large canines. The mandible is robust and the splenial is ex- cluded from the short, rugose symphysis. First recognized and extensively studied in Europe (Sigogneau-Russell and Rus- sell, 1978), Simoedosaurus has only recently been reported from western North America. Sigogneau-Russell and Baird (1978) assigned two additional specimens to Clarkforkian horizons, but these are probably latest Tiffanian in age (Rose, personal communication, 1979). Compar- ative work is still in progress, but the unnamed American species appears to be very similar to the European material (D. Baird, personal communication, 1979).

Numerous champsosaur remains have been recovered from each of the three Clarkforkian zones. Most of these are too incomplete to assign to either of the above taxa. Champsosaur postcranial material is very abundant in Tiffanian and early Clarkforkian horizons in the Fort Union Formation but is quite rare in Willwood Formation deposits. This apparent drop in abundance is probably related to the fact that champsosaurs, being highly

aquatic and fairly large (body length to 4 m or more), were piscivorous dwellers of large streams and rivers, reminiscent of the modem gharial (Gavialis gangeticus) of India and Burma. The shift from Fort Union to Willwood sedimentation is characterized by a reduction in the num- ber and size of streams, thereby discour- aging habitation by and/or preservation of the champsosaurs.

Order Squamata Suborder Lacertilia

Family Anguidae Cope, 1864 Subfamily Glyptosaurinae McDowell & Bogert, 1954

cf. Odaxosaurus piger (Gilmore, 1928) Referred specimen.-UM 77641, pos-

terior half of right frontal. Occurrence.-Plesiadapis cookei Zone

(FG-6). Discussion.-This specimen repre-

sents a small, primitive glyptosaurine very similar to Odaxosaurus (see Gauthier, 1982; Meszoely, 1970). The width of the armored portion of the frontal along the posterior margin is 3.7 mm, and the pat- tern of its osteodermal ornamentation is generally vermiculate.

Gauthier (1982) erected Proxestops jepseni to include Peltosaurusjepseni and some specimens referred to Odaxosaurus (Pancelosaurus) piger by Meszoely (1970). Proxestops is intermediate between Odaxosaurus (small, vermiculate armor) and Xestops (relatively large, pus- tulate armor). Although this Clarkforkian specimen is most similar to Odaxosau- rus, it is slightly larger than described Odaxosaurus material, and shows incip- ient development of pustules along the vermiculate ridges of the osteodermal roofing. This assignment to Odaxosaurus should, therefore, be considered tentative.

Xestops Cope, 1873 Xestops vagans (Marsh, 1872)

Referred specimens.-Partial right mandible (UM 67188) and left frontal (UM 74616).

Occurrence.-Plesiadapis cookei Zone




A A

FIG. 2.-(A) Xestops vagans UM 67188, posterior portion of right dentary. (B) Machaerosaurus sp. UM 73570, partial right dentary. Scale = 5 mm.

(SC-136), Phenacodus-Ectocion Zone (SC-81).

Discussion.-Xestops characteristics include: frontals strongly sutured but not fused; frontoparietal scutes separated at the midline by interparietal scutes; and a lateral depression demarking the junc- tion of the prefrontal and frontal. The teeth of Xestops vagans are relatively robust and are almost uniform in size except for the very reduced posteriormost teeth (Fig. 2), the crowns are blunt with distinct carinae and striations radiating perpendicular to the weakly developed cutting edge. The intramandibular septum has a free ventral border posteriorly.

Xestops is a small, primitive glyptosaur characterized by unfused frontals with nearly parallel sides (Meszoely et al.,

1978). It may be distinguished from Me- lanosaurus (to which it is very similar and closely related) by its less bulbous and more isodont dentition, more gracile mandible, more concave internal dental border, smaller size, and somewhat less distinct osteodermal ornamentation.

Originally described from the middle Eocene Bridger Formation (Marsh, 1872), Xestops vagans is now known throughout at least half of the Eocene, making it one of the longest ranging fossil lizard species.

Melanosaurus Gilmore, 1928 Melanosaurus maximus Gilmore, 1928 Referred specimens.-Partial skulls

(UM 65550, 66709, 68044, 69258, 71638), frontals (UM 73787, 74618), parietals




A 4

'__

FIG. 3.-Melanosaurus maximus. (A) Partial left maxilla UM 66709. (B) Posterior portion of right dentary UM 73408. Scale = 1 cm.

(74615, 74617), and mandibles (UM 65494,65545,67246, 73408,74614, 76860, 77564, 77692), often associated with a variety of postcrania.

Occurrence.-Plesiadapis cookei Zone (SC-116, SC-117, SC-127, SC-143, SC-197, SC-238, FG-6), Phenacodus-Ectocion Zone (SC-23, SC-24, SC-50, SC-70, SC- 159, SC-164).

Discussion.-Included here are only those specimens clearly referable to Me- lanosaurus maximus. Melanosaurus is a large, heavily armored glyptosaurine characterized by divided external nares, fused parietals with an open parietal fo- ramen, and massive, completely fused frontals that taper anteriorly. Denticles

occur in large patches on the pterygoids, but there are few on the vomers (Gil- more, 1928). The teeth have large, bulbous crowns with well-developed carinae and radiating striations. The shafts are very inflated and possess broad, wrinkled bases containing large basal foraminae (Fig. 3). The posterior teeth are very reduced in size, and the anterior teeth are slightly less robust, but never acute. Post- cranially, Melanosaurus possesses large, thick osteoderms in a variety of shapes and sizes, stout limb elements, and very massive vertebrae.

Melanosaurus is by far the most common Clarkforkian lizard, especially in the more well-drained deposits of the Will-




B

A

FIG. 4.-(A) "Cf. Gerrhonotus sp." UM 74614, middle portion of right dentary. (B) Exostinus rugosus UM 73565, fragment of right maxilla. Scale = 3 mm.

wood Formation. It has not been recovered from the early Clarkforkian, however; thus Melanosaurus, as present- ly known, must be considered an exclu- sively Eocene taxon.

Subfamily Anguidae McDowell & Bogert, 1954

Machaerosaurus Gilmore, 1928 Machaerosaurus torrejonensis

Gilmore, 1928 Referred specimens.-UM 73570,

73608, 73681, 74611, 77006, 77007, fragmentary mandibles and maxillae.

Occurrence.-Plesiadapis cookei Zone (SC-143, SC-188).

Description.-These specimens possess fine vermiculate ornamentation on

the maxillae, very gracile mandibles, and isodont dentition with dagger-like crowns bearing sharp cutting edges that occa- sionally develop a "shouldered" appearance (Fig. 2). When fully developed, the anterior shoulder forms a very small sec- ondary cusp. The teeth are slightly re- curved posteriorly and possess slender shafts.

Discussion.-Machaerosaurus is a small anguid almost certainly referable to the Anguinae (Meszoely et al., 1978). Ma- chaerosaurus torrejonensis was described from the middle Paleocene Tor- rejon Formation of New Mexico (Gilmore, 1928) and the Torrejonian Swain Quarry of southeastern Wyoming (Sullivan, 1982). The Clarkforkian specimens described here are very similar to the Paleocene material and belong to the same species.

Machaerosaurus is an extremely rare element in the Clarkforkian of the Big- horn Basin. Specimens clearly referable to this genus have only been recovered from washing operations at two very fos- siliferous "microsites," where very small vertebrates are concentrated. The rela- tive abundance of Machaerosaurus at SC- 188 (a locality that has provided much of our understanding of very small Clark- forkian lizards and mammals) suggests that it may have been fairly common during the Clarkforkian, but has suffered from preservational and collecting biases. The discovery and washing of additional microsites in the Clarkforkian and Wasatch- ian should add greatly to our knowledge of this and other small lizards.

Subfamily Gerrhonotinae McDowell & Bogert, 1954

"cf. Gerrhonotus sp." sensu Estes, 1964 Referred specimen.-UM 74612. Occurrence.-Plesiadapis cookei Zone

(SC-116). Description.-A single fragmentary

dentary of a very small lizard characterized by laterally beveled crowns bearing extremely fine striations (Fig. 4).

Discussion.-The specimen referred to this "taxon" possesses the chisel-shaped



Decemitber 1983] HERPETOLOGICA 367

crowns of the cf. Gerrhonotus material previously described from the Late Cre- taceous Lance Formation and the middle Paleocene Tongue River Formation (Estes, 1964, 1976). Many additional specimens of this form are needed before m-ore can be said of its taxonomic position and stratigraphic range.

Family Xenosauridae Cope, 1864 Restes Gauthier, 1982

Restes rugosus (Gilmore, 1942) Referred specimen.-UM 73565, frag-

mentary maxilla. Occurrence.-Plesiadapis cookei Zone

(SC-188). Descriptiotn.-This specimen possess-

es characteristic rugose dermal sculptur- ing on the maxilla (Fig. 4). The teeth have nearly circular shafts and bilobate crowns.

Discussion.-Restes rugosus was de- scril)ed as Exostinus by Gilmore (1942). Gauthier (1982) noted that the skull roof osteoderms are more flattened and closely packed than in other xenosaurids (Exos- tinus and Xenosaurus) and erected Restes for this late Paleocene and early Eocene species. It is also characterized by very high-crowned teeth (Estes, 1975), no intramandibular septum, and extreme anterior excursion of the coronoid laterally along the dentary.

Restes rugosus, like many other Clark- forkian vertebrates, is known only from the middle Clarkforkian SC-188 micro- site. It is therefore difficult to speculate on the abundance and stratigraphic dis- tribution of this species, which is quite easily distinguished from other small Early Tertiary lizards.

Superfamily Varanoidea Lydekker, 1888 Varanoid

(indeterminate family, genus, and species) Referred speciient.-UM 74619, iso-

lated parietal. Occurrentce.-Plesiadapis gin gerichi

Zone (SC-215). Discussion.-This parietal (Fig. 5) re-

seimlles that of the varanid Saniwa (R. Estes, personal communication, 1981), but

FIG. 5.-Varanoid parietal UM 74619. Scale in mm.

it may be referable to the poorly known varanid Paleosaniwa or parasaniwid Pro- varanosaurus. This specimen is the only certain record of a Clarkforkian varanoid.

Subclass Archosauria Order Crocodilia Gmelin, 1799 Family Alligatoridae Gray, 1844 Allognathosuchus Mook, 1921

Allognathosuchus wartheni Case, 1925 Referred specimens. -Partial skull and

mandibles (UM 65778) and fragmentary mandibles (UM 65649, 71487, 74541).

Occurrence.-Plesiadapis cookei Zone (SC-62), Phenacodus-Ectocion Zone (SC- 57, SC-235, SC-289).

Description.-All specimens include robust mandibles lacking the strong surangular rise posterior to the tooth row and highly differentiated dentition in the middle portion of the dentary characteristic of other described Eocene Allogna- thosuchus species.




M.~ ~ ~ ~ ~~.

I~~ ~

FIG. 6. Allognathosuchus wartheni UM 65778. (A) Superior and (B) lateral views of partial right and

left mandibles. Leidyosuchusforinidabilis UM 72573. (C) Superior and internal views of anterior fragment of right dentary. (D) Lateral view of posterior fragment of right mandible. Scale = 2 cm.

Discussioni.-Included here are only those specimens clearly referable to A. wartheni (Case, 1925). Many of the un- diagnostic specimens listed below probably belong to this species.

Allognathosuchus is a primitive alligator of small to medium size characterized by a broad, highly vaulted snout with moderately large external nares divided by a poorly ossified nasal septum formed




by the premaxillae and nasals. The su- praoccipital is included in the strongly ornamented cranial table. The choanae are placed posterior to the middle of the robust pterygoids. The mandible contains prominent vertical undulations along the very short tooth row, a small external fenestra in the deep postdentary region, a wide and open adductor fossa, a leaf- like splenial covering of the internal surface of the dentary, and a large symphysis that includes the splenial. The dentition is heterodont, containing enlarged anterior canines, medial spatulate teeth, and bulbous posterior teeth that are often heavily worn (Fig. 6). All the teeth are set close together and bear fine striations and strong carinae.

Allognathosuchus (indeterminate species)

Referred specimens.-UM 64706, 65061, 65102,65551, 65659,66291,66292, 66538, 66546,66583,68423,69681,69914, 71459, 72551-72555, 72557-72559, 72561-72566, 72568-72572, 74542,74546, 74550, 74623, 74624, fragmentary skulls, mandibles, and postcrania not certainly assignable to Allognathosuchus wartheni.

Occurrence.-Several localities in each Clarkforkian Zone.

Discussion.-Allognathosuchus remains are common throughout the Big- horn Basin Clarkforkian. Most of these specimens probably belong to a species of the ill-defined Allognathosuchus het- erodonlpolyodon morphological group loosely characterized by their small size, gracile skull and mandible, poorly developed anterior vertical undulation on the dentary, high surangular rise, and deep postdentary region. The inadequate descriptions and illustrations of A. heterodon and A. polyodon (Cope, 1872; Mook, 1921, 1961) prohibit specific assignment of small, generalized early Eocene Allo- gnathosuchus specimens. A thorough review of these forms combined with the description of additional early Eocene material will add greatly to our under-

standing of this ubiquitous Early Tertiary crocodilian.

Ceratosuchus Schmidt, 1938 Ceratosuchus burdoshi Schmidt, 1938 Referred specimens.-Isolated squa-

mosals (UM 72560), partial skull and mandibles (UM 68238, 71489), fragmentary skulls and skeletons of several indi- viduals (UM 71490), and type skull (Field Museum of Natural History P15576).

Occurrence.-Plesiadapis gingerichi Zone(?) Plateau Valley Beds, Piceance Basin, Colorado, Phenacodus-Ectocion Zone (SC-163, SC-235).

Discussion.-The Bighorn Basin material agrees closely with the type skull (FMNH P15576) described by Schmidt (1938) from the Plateau Valley Beds of western Colorado. These horizons are probably early Clarkforkian in age (Rose, 1981).

Ceratosuchus burdoshi is a broad- snouted alligator closely related to the more common genus Allognathosuchus. Ceratosuchus may be distinguished from the latter (which it most closely resem- bles) by its larger and more rounded external mandibular fenestra, more spatulate and procumbent anterior dentition, deeper anterior splenial covering over the primordial canal, less highly vaulted snout, reduced dermal ornamentation of the skull, and the development of large, horn-like squamosals (Fig. 7). Ceratosu- chus is much rarer than Allognathosu- chus and is thus far known only from Clarkforkian horizons.

Diplocynodon Pommel, 1847 Diplocynodon sp.

Referred specimen.-UM 69867, anterior portion of a left dentary.

Occurrence .-Phenacodus -Ectoci on Zone (SC-289).

Discussion.-This specimen belongs to a form very similar to the narrow-snouted Diplocynodon stuckeri from the middle Eocene Bridger Formation (Mook, 1960). Diplocynodon is a large primitive cai- man. It is the most common Tertiary croc-



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odilian in Europe (Berg, 1966) but is quite rare in North American deposits of similar age. The North American specimens differ from their European counterparts in the possession of more pronounced vertical undulations along the tooth row, somewhat more robust mandible, larger symphysis, and a stronger confluence of the third and fourth dentary teeth (Fig. 8).

The referred Clarkforkian specimen differs in detail from D. stuckeri and probably represents a new species.

Family Crocodylidae Cuvier, 1807 Subfamily Leidyosuchinae Nopsca, 1928

Leidyosuchus Lambe, 1907 Leidyosuchusformidabilis Erickson, 1976

Referred specimens.-Fragmentary skulls and postcrania (UM 72573, 74552) and isolated teeth (UM 74553, 74554, 77001, 77003).

Occurrence.-Plesiadapis gingerichi Zone (SC-171, SC-249), Plesiadapis cookei Zone (SC-108, SC-117, SC-120, SC-134, SC-136, SC-200), Phenacodus-Ectocion Zone (SC-il, SC-49, SC-81, SC-163, SC- 164, SC-183, SC-202, SC-234).

Discussion.-These specimens are referable to the large crocodylid Leidyo- suchus formidabilis (Fig. 6). Leidyosu- chus is a common element of Late Cretaceous and Early Tertiary faunas of western North America. It is a primitive crocodylid characterized by frontal par- ticipation in the anteromedial border of the supratemporal fenestrae, a nasal-frontal contact, and a gently sloping occiput. The posterior portion of the skull is very flat and broad, and there is a pronounced posterior excursion of the squamosals onto the dorsal surface of the quadrates. The mandible is very deep posteriorly and possesses a large external fenestra and extremely long, shallow symphysis. Leid- yosuchusformidabilis is a larger, narrow- snouted species, previously reported from the Paleocene (Tiffanian) of North Da- kota (Erickson, 1976).

Leidyosuchus teeth occur through the Tiffanian, Clarkforkian and Wasatchian of

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FIG. 8.-Diplocynodon sp. UM 69867. (A) Supe- rior view showing cross sections of broken first and third teeth, (B) lateral view, and (C) internal view showing the rugose area of splenial attachment be- hind the large, curved symphysis. Scale = 1 cm.

the Bighom Basin. These teeth (and more complete specimens) are common in Fort Union horizons but become quite rare with the onset of Wiliwood sedimentation. Habitational and preservational biases (similar to those postulated for the champsosaurs) may have diminished the fossil record of Leidyosuchus during the early Eocene. This change from large stream and swamp environments (Fort Union) to smaller stream and broad flood- plain conditions (Willwood) seems to have greatly benefited the small alligators, however, and they become very abundant in Willwood sediments (per- haps at the expense of the larger aquatic predators).

FAUNAL COMPARISONS

Clarkforkian deposits record the transition from relatively archaic Paleocene herpetofaunas to Eocene assemblages of




a more modern aspect. North American Paleocene reptile communities are, in general, little changed from Late Creta- ceous assemblages, and comparisons with Lancian reptile faunas are also pertinent (Estes, 1970; Estes et al., 1969).

Estes (1964) described extensive Uni- versity of California collections from the type area of the Lance Formation in eastern Wyoming. Except for the lizards (and dinosaurs), the Lancian fauna is quite similar to that found in the Clarkforkian. The aquatic component of the Creta- ceous fauna contains species of Clarkforkian genera ("Baena," Champ- sosaurus, and Leidyosuchus), trionychids similar to later forms, and the primitive alligator Brachychampsa (Gilmore, 1911). The major aquatic element lacking from the Lance beds are the emydids (J. H. Hutchison, personal communication, 1980). The Late Cretaceous lizards are very different from those of the Clarkfork- ian. Of the fourteen Lancian lizards, only Odaxosaurus and the problematical "cf. Gerrhonotus" are known from Clarkfor- kian horizons.

A few undescribed specimens of large reptiles are known from the basal Paleo- cene Mantua Quarry (Polecat Bench, Big- horn Basin, Wyoming). This small assemblage contains a baenid, a trionychid, indeterminate species of Champsosau- rus and Leidyosuchus, and Diplocyno- don. Allognathosuchus mooki has not been recovered from the quarry, but it is known from the very early Paleocene of New Mexico (Simpson, 1930). Except for the absence of emydids, the early Paleo- cene reptile fauna differs only slightly from that of the Clarkforkian. No lizards are present in the Mantua sample.

Middle Paleocene reptile assemblages are slightly more advanced than those of the early Paleocene. Estes (1976) described the lower vertebrates from the late Torrejonian Tongue River Formation of Montana. This fauna is a mixture of Late Cretaceous holdovers and new Paleo- cene taxa. Clarkforkian taxa present in the

Bighorn Basin by this time include Champsosaurus gigas, Plastomenus, Ma- chaerosaurus, Odaxosaurus and Allo- gnathosuchus. The middle Paleocene fauna is still somewhat primitive in that it lacks a wide assortment of anguid lizards and alligators. No emydids are known from the middle Paleocene of the Big- horn Basin, although a macrocephalic form is present in New Mexico at this time (J. H. Hutchison, personal communication, 1982).

The late Paleocene (late Tiffanian) rep- tilian fauna of the Bighorn Basin is much more advanced than earlier Tertiary assemblages. Clarkforkian taxa present by this time include Restes rugosus, Leid- yosuchus formidabilis, Allognathosu- chus heterodon, and a variety of turtles, including the macrocephalic emydid and two chelydrids (J. H. Hutchison, personal communication, 1981). The only Clark- forkian elements not present by the late Tiffanian are Machaerosaurus, Cerato- suchus, Allognathosuchus wartheni and the emydid box turtle.

In contrast to the gradual transforma- tion between older Paleocene and Clark- forkian faunas, there is a fairly dramatic faunal break at the Clarkforkian-Wasatch- ian boundary. Extinctions at this time include Ceratosuchus and the champsosaurs. Wasatchian faunas are also characterized by a diversification of emydids and glyptosaurinids, the first appearance of kinosternid, dermatemydid and testu- dinid turtles (Hutchison, 1980), and the occurrence of modern rhineurid amphisbaenians and the problematical crocodilian Orthogenysuchus (Mook, 1924). The Clarkforkian taxa that continue into the Wasatchian with little change include Allognathosuchus, Leidyosuchus, Mel- anosaurus and some of the emydid and trionychid turtles.

In summary, the Clarkforkian reptile fauna represents an assemblage that evolved slowly throughout the Paleocene from the typical Late Cretaceous non-di- nosaurian herpetofauna. The Wasatchian




early Eocene reptile fauna is a diversified Clarkforkian assemblage mixed with a variety of immigrant taxa.

PALEOECOLOGY Reptiles are environmentally sensitive

and provide some general information on the climate and geography of the region in which they live (or lived). Since some Clarkforkian reptiles are closely related to their Recent counterparts, some of these ecological inferences can be ap- plied to the latest Paleocene-earliest Eocene environments of the Bighorn Ba- sin.

The Clarkforkian trionychids are extremely similar to living taxa and provide reliable comparisons. Living Trionyx species inhabit larger water bodies (lakes, large ponds and streams, rivers), indicat- ing the presence of significant amounts of permanent water during the Clarkfork- ian. Trionyx is quite temperature tolerant (Recent species range from Florida to Michigan), so little can be said regarding paleotemperatures.

Few paleoecological inferences can be drawn from the champsosaurs. They are regarded as piscivorous and highly aquatic, and their large size probably restricted them to major streams and ponds.

Clarkforkian lizards are all distantly re- moved morphologically and phylogenet- ically from their Recent relatives and therefore provide little insight into Clark- forkian environments. The predomi- nance of presumably terrestrial glypto- saurs may indicate a considerable amount of dry ground between the streams and ponds, particularly during Willwood de- position.

Snakes, amphisbaenians and amphibi- ans are either extremely rare or absent from most Bighorn Basin Tertiary horizons. This scarcity must be due in large measure to taphonomic and collecting biases. Each of these groups possess small delicate skeletons that are not easily preserved, weathered to the surface, or seen by the collector. It is unlikely that the

absence of these forms indicates any unusual paleoecologic condition during the Clarkforkian.

The great abundance and diversity of Clarkforkian crocodilians suggest an equal abundance and variety of aquatic habi- tats. Recent caimans and crocodylids do not hibernate, and it is unlikely that their primitive ancestors (Diplocynodon and Leidyosuchus) were capable of this com- plex activity. If this is true, it indicates that Clarkforkian temperatures were equable and there was no hard winter freeze. Allognathosuchus is closely related to the Recent Alligator and may have shared its descendants' intolerance to very warm tropical climates (Alligator missis- sippiensis and A. sinensis are found only in subtropical and warn temperate re- gions). Combined, the crocodilians may bracket Clarkforkian temperatures in the subtropical range.

Acknowledgments.-I thank P. D. Gingerich for reviewing this manuscript and allowing me to study the collections at the University of Michigan. Spe- cial thanks go to J. H. Hutchison for providing un- published information on the evolution of Early Tertiary turtles, and D. Baird for the loan of specimens at Princeton University. I also thank W. Langston, Jr. and R. M. Sullivan for their helpful insights on Tertiary reptiles. This research was sup- ported in part by National Science Foundation grants DEB 77-13465 and DEB 80-10846 to P. D. Ginge- rich.

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Accepted: 6 June 1983 Associate Editor: Stephen Tilley

University of Michigan, Museum of Pa- leontology, Ann Arbor, MI 48109, USA



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