THE PEARCE­ SELLARDS SeflCS NUMBER 31 EARLY TERTIARY VERTEBRATE FAUNAS BIG BEND AREA TRANS-PECOS TEXAS: SIMIDECTES (MAMMALIA, INSECTIVORA) Eric Paul Gustafson 1979 Texas Memorial Museum/2400 Trinity/Austin, Texas 78705 William G. Reeder, Director The Pearce-Sellards Series is an occasional, miscellaneous series of brief reports of museum and museum-associated field investigationsand other research. All manuscripts are subjected to extramural peerreview before being accepted. The series title commemorates the first two directors of the Texas MemorialMuseum, both now deceased: J. E. Pearce and Dr. E. H. Sellards, professors of anthropology and geology, respectively, of The University ofTexas at Austin. A complete price list ofPearce-Sellards papers, bulletins, and miscel­laneous publications of the museum will be sent upon request. -James Reddell, Editor EARLY TERTIARY VERTEBRATE FAUNAS BIG BEND AREA TRANS-PECOS TEXAS: SIMIDECTES (MAMMALIA, INSECTIVORA) 1 By Eric Paul Gustafson2 ABSTRACT Several specimens from the Buck Hill Group, Agua Fria area, Texas, represent Simi­dectes magnus, a species previously reported only from the Uinta Formation of Utah. Statistics of known specimens of Simidectes are inadequate to demonstrate the existence of more than one species in the Utah and Texas sections; S. medius may be a junior synonym of S. magnus. In the lower part of the'Section at Agua Fria, Simidectes occurs in the Whistler Squat local fauna of late Bridgerian or early Uintan age; in the upper part of the section it is found in association with an early Hyaenodon and the adapid primate Mahgaritastevensi. INTRODUCTION Bones and teeth of numerous mammals of middle to late Eocene age are known from the Buck Hill Group in the Agua Fria area northwest of BigBend National Park, Texas. Among the specimens selected for a study of the carnivorous mammals of this area (Gustafson, 1977) are several which repre­sent the poorly known late Eocene genus Simidectes. The purpose of this paper is to describe these specimens and to assess their taxonomic and bio­stratigraphic significance. Abbreviations used: AP, antero-posterior measurement; e, approximate measurement; N, sample size; T, transverse measurement; R, numerical range; s, standard deviation; V, coefficient of variation; X, arithmetic mean; AMNH, American Museum of Natural History, New York; CM, CarnegieMuseum, Pittsburg; KU, University of Kansas, Lawrence; LACM, Los Ange­les County Museum; PU, Princeton University; TMM, Texas Memorial Museum, Austin. All measurements are in millimeters. Detailed descriptions of localities are on file at the Vertebrate Paleontology Laboratory, Texas Memorial Museum, The University of Texas at Austin 78705. This work was supported in partby the Geology Foundation, Department of Geological Sciences, The Uni­versity of Texas at Austin. from the Vertebrate Paleontology Laboratory, Texas Memorial Museum, The Uni­ versity of Texas at Austin. departmentofGeology,University ofOregon,Eugene,Oregon97403. PREVIOUS WORK Simidectes has been reported previously only from the Sespe Formation of California and the Uinta Formation of Utah. The taxonomy and morpho­logyoftheknown speciesaresummarizedbyCoombs(1971). The faunas of the Agua Fria area are only partly described. Much taxo­nomic work is available from Wood (1972, 1973; Rodentia), Wilson (1974;Leptoreodon), Stevens et al. (1975; mainly stratigraphy but mentions Hyaenodon), Wilson and Szalay (1976; primate, but the paper also mentions an amynodont rhinoceros, artiodactyls, and Hyaenodon), and Wilson (1977;titanotheres). The detailed stratigraphy of the sedimentary rocks and tuffs of the Buck Hill Group in the Agua Fria area has been worked out by J. A. Wilson and James and Margaret Stevens but is not yet published, though preliminary conclusions have been discussed (Stevens et ah, 1975). Moon (1953) mapped the geology of the Agua Fria quadrangle. The fossiliferous sediments lie between the post-Cretaceous unconformity and the Mitchell Mesa Ignim­brite. A new formational name has been suggested but not yet published. CORRELATIONS Whistler Squat local fauna—Wilson (personal communication, 1977) in­cludes all localities in the Agua Fria section up through Purple Bench in the Whistler Squat local fauna. The rodents from this local fauna were first described by Wood (1973) as coming from two localities in the lowest partof the section. The same part of the section also yielded two specimens of ?Proviverra major, which is otherwise known only from the Bridger Forma­tion of Wyoming and probably only the upper Bridger (Gazin, 1976, p. 10).A suggested Bridgerian age agrees with the determination given by Wood (1973) on the basis of the rodents from Whistler Squat, within 50 feet of the base ot the section. However, of the other carnivorous mammals from the lower part ot the Agua Fria section, Hessolestes is otherwise known from only one specimen from the Lapoint Member ofthe Duchesne River Forma­tion, possibly latest Uintan, and Simidectes magnus is known from both the Wagonhound and My ton Members of the Uinta Formation. Both of these suggest an age younger than Bridgerian. It may be significant that the locali­ties from which the Simidectes specimens and two of the three Hessolestes specimens came are higher in the section than those from which ?Proviverra came (Fig. 1). Other carnivorous mammal specimens are too fragmentary for correlation. The presence of Leptoreodon marshi in the lower 50 feet of the Agua Fria section (Wilson, 1974) further supports a post-Bridgerian ageassignment for the Whistler Squat local fauna. The radiometric dates of 46 m.y. and 43 m.y. at Whistler Squat (Wilsonand Szalay, 1976) would fix the age of the lower part of the section as Uintan if the Uintan-Bridgerian boundary is about 47 m.y. (McKenna et ah, 1973). Fig. 1.—Stratigraphic checklist—Agua Fria section, Brewster Co., Texas, Buck Hill Group. Diagrammatic sections show relative thicknesses of units; unit marked with diagonal lines is informally called “Strawberry tuff.” MM Mitchell Mesa Ignimbrite, = K = Cretaceous rocks; names of some localities shown; numbers next to MM and Whistler Squat are radiometric dates in millions of years. All localities below Skyline are parts of the Whistler Squat local fauna. Skyline channels—An adapid primate (Mahgarita stevensi) was described from this locality by Wilson and Szalay (1976). Two carnivorous mammal species, Hyaenodon cf. H. vetus and Simidectes magnus, are from the Sky­line channels. Hyaenodon vetus was first described from the Pearson Ranch local fauna, Sespe Formation, California, considered by Stock (1948) as being of late Eocene age, and by Golz (1976) and Golz and Lillegraven(1977) as being late Eocene-Duchesnean. It occurs there with Simidectes merriami Stock, in the only other known association of Simidectes and Hyaenodon. Simidectes magnus was first described from the Myton Member of the Uinta Formation, but it is also known from the Wagonhound Member (Coombs, 1971). A specimen of Hyaenodon cf. H. vetus which is very similar to the Texas H. cf. vetus is known from the Lapoint Member of the Du­chesne River Formation (Mellett, 1977). The association of the two generaHyaenodon and Simidectes suggests a latest Eocene (latest Uintan or Du­ chesnean) age. SYSTEMATIC PALEONTOLOGY Order Insectivora Bowdich, 1821 Superfamily Pantolestoidea Cope, 1887,incertae sedis Genus Simidectes Stock, 1933 Synonyms —Pleurocyon Peterson, 1919; Pleurocyon (Simidectes)-Stock 1933\Petersonella Kraglievich, 1948. Simidectes magnus (Peterson, 1919) Whistler Squat local fauna and Skyline conglomerate Type—CM 2928, mandible with P3-M2 Uinta Formation (Uinta “C”), Utah. , Referred material—TMM 41580-1 1, mandible with canine, part of P3 P4 M, and part of M 2; 41672-57, M 3; 41745-5, M 3; 41745-33, M 2 ; ,and, 41745-34, M l . Stratigraphic position—“Pruett Formation,” Buck Hill Group, Brewster County, Texas; M M 1 and M 2 from Serendipity locality; M 3 from Purple 3, Bench; and mandible from lower part of Skyline channels. Age—Uintan. late Eocene. Description—The specimens assigned here show the distinctive characters described by Coombs (1971), particularly the deep, narrow jaw with a large,laterally compressed canine, P4 large and submolariform, relatively rounded cusps on the molars, enlarged hypoconids, and lack of cingula, especiallylack of lingual cingula on upper molars. On 41580-1 1, only the canine and M! are complete, but the nature and approximate size of P4 and M2 3 are apparent from remaining fragments and alveoli. _ Fig.2.—Simidectesmagnus,TMM41580-1 1.LowerjawwithpartialP4,Mjandpartial M 2. A, external and B, internal views. Skyline locality, Brewster County, Texas. Fig. 3 .—Simidectes magnus, TMM 41580-1 1. Dorsal view, anterior to right. RELATIONSHIPS ThreespeciesofSimidectes havebeendescribed,twofromtheUintaFor­mation, S. Magnus from Uinta “B” and “C” and S. medius from Uinta “B.” Simidectes merriami was recovered from the Pearson Ranch local fauna,Sespe Formation, and from Laguna Riviera, San Diego County, both in California. Simidectes merriami Stock (1933) is easily differentiated from the Texas and Utah specimens on the basis of larger size and the absence of an entoconid or pre-entoconid on P 4 . Simidectes magnus is described as being slightly larger than S. medius, but neither of these species is well known. The mandible from the Skyline channels is intermediate between the types of S. Magnus and S. medius in depth of mandible and length of Mlsand probably in length of M 3 judging from alveoli. The M 2 seems to have been slightly larger in the Skyline specimen than in S. magnus. The isolated upper molars from Serendipity and Purple Bench are slightly smaller than those referred to S. medius by Coombs (Table 1), and otherwise differ onlyin that the M 2 from Texas has a stronger parastyle and metastyle. If one considers size alone, the differences are not sufficient to demon­strate the presence of more than one species of Simidectes in the Uinta For­mation, nor the distinction of either of the two described species from the Texas specimens. Table 2 lists basic statistics for Mj and M 2 of the Texas and Utah specimens (together) and the Californian S. merriami. The coeffi­cient of variation can be expected to range up to about 7.5 in relatively homogeneous single quarry samples of a single species (Simpson, 1947). The lumped Texan-Utah statistics show less variability in M 2 length than is present in a sample of S. merriami from a single locality. The V for M j for the Texas and Utah specimens, however, is almost twice the figure for the Pearson Ranch sample of Simidectes. Nevertheless, it is not outside the range for V of Mj in living species (Gingerich, 1974), and the Pearson Ranch sample is unusually invariable in this measurement. With such small samples these statistics can only be suggestions, and should be used with great caution. Other differences between S. Magnus and S. medius, for ins-tance,the longertalonid onMj of£ magnus,mayproveto besignificant. I can see no significant differences between the type ofS. medius (AMNH1966) and the Skyline mandible except the smaller size of the former. The most visible differences between the type of S. Magnus and TMM 41580-11 are as follows: hypoconid on P4 relatively large on S. Magnus, entoconid on P4 smaller, talonid (especially the crista obliqua whose high point is the hypoconid) of Mj relatively longer. No M 3 of either£ magnus or S. medius is available for comparison to TMM 41745-5. The M 3 (41671-57) from Purple Bench is extremely similar to that illustrated for S. medius (Coombs,1971.Fig.8),but M3isunknownfor£magnus. Jaw Mj-Mja pth Specimen/C41745-5,-33,-34referred, Mtypetype, -M5-——r—— -— ___ __ __ __— -— _— __ _ __ _ __ ___—M---_— — ___—— M--—— ______— _____ —— -de­ P4merriami,rmagnus, -— -— — — -—____-4.8M-— -— — ­ measurements. *mean specimens (Coombs, 1971) Tablel.—SimidectesPU (Purple(Skyline)1139,121129283006Bench) TMM 41580-11AP T9.2AP T8.5eT6.59.58.88.3T11.56.57.98.4T7.59.1e4.4T10.555e5.8T50.6eT7.75.17.0 13.223.8e10.8 18.4e18.9 medius, 4.8----APP _ _AP_5.2_ _ ___5.54.96.6—11.18.2AP4.94.7 10.1AP 7.3—8.3eAP— — -5.27.69.4Mj23.520.0—10.6 S. 41672-57 5.3 AP 5.56.0a9.3 below 8.225.0* 16.3 andofreferred,9 LACM (Serendipity) AMNH 1966 CM 1465711317 KU 16114 Table 2 —Simidectes statistics. R, numerical range; s, standard deviation; V, coefficient of variation; X, arithmetic mean. NX R s V My AP, all Utah and Texas specimens 3 8.87 8.3-9.5 .6028 6.8 My AP,S. merriami, Pearson Ranch only 8 10.7 10.1-11.1 .3818 3.6 My AP, all Utah and Texas specimens 4 8.4 7.9-9.1 .6165 7.3 My AP, S. merriami,Pearson Ranch only 5 9.96 8.8-11.0 .802 8.1 My AP,S. merriami,all specimens 8 9.91 8.8-11.0 .62 6.3 REFERENCES BOWDICH, T. E. 1821 An analysis of the natural classifications of Mammalia for the use of stu­dents and travellers. Paris, J. Smith, 1 15 pp. COOMBS, M. C. 1971 Status of Simidectes (Insectivora, Pantolestoidea) of the Late Eocene of North America. American Mus. Novitates, 2455. 41 pp. COPE, E. D. 1887 The classification and phylogeny of the Artiodactyla. Proc. American Phil. Soc., 24:377-400. GAZIN, C. L. 1976 Mammalian faunal zones of the Bridger middle Eocene. Smithsonian Contr. Paleobiol., 26. 25 pp. GINGERICH, P. D. 1974 Size variability of the teeth in living mammals and the diagnosis of closelyrelated sympatric fossil species. J. Paleontol., 48(5):895-903. GOLZ, D. J. 1976 Eocene artiodactyla of southern California. Nat. Hist. Mus., Los Angeles,California, Sci. Bull., 26. 85 pp. GOLZ, D. J., AND J. A. LILLEGRAVEN 1977 Summary of known occurrences of terrestrial vertebrates from Eocene strata ofsouthern California. Contrib. Geol., Univ. Wyoming, 15(l):43-65. GUSTAFSON, E. P. 1977 Carnivorous mammals of the late Eocene and early Oligocene of Trans-Pecos, Texas. Univ. Texas, Austin, unpubl. Ph.D. Diss. 209 pp. KRAGLIEVICH, L. J. 1948 Substitution de un nombre generico. An. Soc. Cient. Argentina, E. 3 146: 161-162. MCKENNA, M. C., D. E. RUSSELL, R. M. WEST, C. C. BLACK, W. D. TURNBULL, M. R. DAWSON, AND J. A. LILLEGRAVEN 1973 K/Ar recalibration of Eocene North American land-mammal “ages” and European ages. Abstracts with Programs, Geol. Soc. America Ann. Mtg.,Dallas, Texas. MELLETT, J. S. 1977 Paleobiology of North American Hyaenodon (Mammalia, Creodonta).Contrib. Vert. Evol., 1. 134 pp. MOON, C. G. 1953 Geology of Agua Fria Quadrangle, Brewster County, Texas. Geol. Soc.America Bull., 61; 15 1-196. PETERSON,O. A. 1919 Report upon the material discovered in the upper Eocene of the Uinta Basin by Earl Douglass in the years 1908-1909, and by O. A. Peterson in1912. Ann. Carnegie Mus., 12:40-168. SIMPSON, G. G. 1947 Note on the measurement of variability and on relative variability of teeth of fossil mammals. American J. Sci., 245:522-525. STEVENS, J. 8., M. S. STEVENS, AND J. A. WILSON 1975 Stratigraphy of Pruett and Duff Formations, Agua Fria and Tascotal Mesa Quadrangles, Brewster County, Texas. Abstracts with Programs, South-Central Section, p. 237. STOCK,CHESTER1933 A miacid from the Sespe upper Eocene, California. Proc. Natl. Acad. Sci.,19:481-486. STOCK, CHESTER 1948 Pushing back the history of land mammals in western North America. Bull. Geol. Soc. America, 59:327-332. WILSON, J. A. 1974 Early Tertiary faunas, Vieja Group and Buck Hill Group, Trans-Pecos Texas; Protoceratidae, Camelidae, Hypertragulidae. Texas Mem. Mus. Bull., 23. 34 pp. WILSON, J. A. 1977 Early Tertiary vertebrate faunas, Big Bend area Trans-Pecos Texas: Bronto­theriidae. Texas Mem. Mus., Pearce-Sellards Ser., 25. 17 pp. WILSON, J. A., AND F. S. SZALAY 1976 New adapid primate of European affinities from Texas. Folia Primatolo­gica, 25:294-312. WOOD, A. E. 1972 An Eocene hystricognathous rodent from Texas: its significance in inter­pretationsofcontinentaldrift. Science, 175:1250-1251. WOOD, A. E. 1973 Eocene rodents, Pruett Formation, Southwest Texas: their pertinence to the origin of the South American Caviomorpha. Texas Mem. Mus., Pearce-Sellards Ser., 20. 40 pp.