Descriptive and Comparative Osteology of the Oldest Fossil Squirrel, Protosciurus (Rodentia: Sciuridae) ;RT J. EMRY I f 4H and RICHARD W. THORINGTON, JR. <%, ^ ONIAN CONTRIBUTIONS TO PALEOBIOLOGY • NUMBER 47 SERIES PUBLICATIONS OF THE SMITHSONIAN INSTITUTION Emphasis upon publication as a means of "diffusing knowledge" was expressed by the first Secretary of the Smithsonian. In his formal plan for the Institution, Joseph Henry outlined a program that included the following statement: "It is proposed to publish a series of reports, giving an account of the new discoveries in science, and of the changes made from year to year in all branches of knowledge." This theme of basic research has been adhered to through the years by thousands of titles issued in series publications under the Smithsonian imprint, commencing with Smithsonian Contributions to Knowledge in 1848 and continuing with the following active series: Smithsonian Contributions to Anthropo/ogy Smithsonian Contributions to Astrophysics Smithsonian Contributions to Botany Smithsonian Contributions to the Earth Sciences Smithsonian Contributions to the Marine Sciences Smithsonian Contributions to Paleobiology Smithsonian Contributions to Zoology Smithsonian Studies in Air and Space Smithsonian Studies in History and Technology In these series, the Institution publishes small papers and full-scale monographs that report the research and collections of its various museums and bureaux or of professional colleagues in the world of science and scholarship. The publications are distributed by mailing lists to libraries, universities, and similar institutions throughout the world. Papers or monographs submitted for series publication are received by the Smithsonian Institution Press, subject to its own review for format and style, only through departments of the various Smithsonian museums or bureaux, where the manuscripts are given substantive review. Press requirements for manuscript and art preparation are outlined on the inside back cover. S. Dillon Ripley Secretary Smithsonian Institution SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY • NUMBER 47 Descriptive and Comparative Osteology of the Oldest Fossil Squirrel, Protosciurus (Rodentia: Sciuridae) Robert J. Emry and Richard W. Thorington, Jr. SMITHSONIAN INSTITUTION PRESS City of Washington 1982 ABSTRACT Emry, Robert J., and Richard W. Thorington, Jr. Descriptive and Compar­ ative Osteology of the Oldest Fossil Squirrel, Protosciurus (Rodentia: Sciuridae). Smithsonian Contributions to Paleobiology, number 47, 35 pages, 16 figures, 3 tables, 1982.—The early history of the Sciuridae is not well known, squirrels being generally poorly represented in the Tertiary fossil record. A nearly complete skeleton, recently discovered in early Oligocene deposits of Wyoming, repre­ sents what may be the oldest fossil squirrel known. For the first time, this early squirrel can be compared fully with its extant relatives. The specimen, assigned to Protosciurus jejfersoni, retains the primitive protrogomorphous zygomasseteric structure, as in other known Protosciurus, but the masseteric fossa of the mandible is farther forward than in most nonsciurid protrogo- morphs. The auditory region of the skull has derived squirrel characters, but it is in the postcranial skeleton where similarities to extant squirrels are most apparent. Except for minor differences in joint construction, the skeleton is strikingly similar to that oi Sciurus niger, the living fox squirrel. It differs from extant ground squirrels in the more gracile proportions of its long bones and asymmetry of foot construction. This early member of the squirrel family was clearly an arboreal squirrel, with morphology, and presumably habits, very similar to those of extant Sciurinae. OFFICIAL PUBLICATION DATE is handstamped in a limited number of initial copies and is recorded in the Institution's annual report, Smithsonian Year. SERIES COVER DESIGN: The trilobite Phacops rana Green. Library of Congress Cataloging in Publication Data Emry, Robert J. Descriptive and comparative osteology of the oldest fossil squirrel (Protosciurus: Sciuridae: Rodentia) (Smithsonian contributions to paleobiology ; no. 47) Bibliography: p. 1. Protosciurus. I. Thorington, Richard W. II. Title. III. Series QE701.S56 no. 47 [QE882.R6] 560s [569'.323] 81-14440 AACR2 Contents Page Introduction 1 Acknowledgments 1 Material 1 Taxonomic Identity of USNM 243981 3 Anatomy 4 Skull, Mandibles, and Dentition 4 Skull 4 Auditory Region 4 Mandible 6 Dentition 7 Vertebral Column 10 Pectoral Limb 11 Scapula 11 Humerus 13 Ulna 15 Radius 16 Carpus 18 Scaphoid 18 Triquetrum (cuneiform) 18 Pisiform 18 Hamate (unciform) 19 Manus 19 Metacarpals 19 Phalanges 20 Interpretation of the Hand 20 Pelvic Limb 21 Inominate bone 21 Femur 22 Tibia 24 Fibula 24 Tarsus 24 Calcaneus 24 Astragalus 26 Navicular 27 First Cuneiform (entocuneiform) 28 Second Cuneiform (mesocuneiform) 28 Third Cuneiform (ectocuneiform) 28 Cuboid 28 IV SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY Pes 28 Metatarsals 28 Phalanges 29 Interpretation of the Foot 29 Summary of Characteristics of the Limbs of Protosciurus 31 Summary 32 Literature Cited 35 Descriptive and Comparative Osteology of the Oldest Fossil Squirrel, Protosciurus (Rodentia: Sciuridae) Robert J. Emry and Richard W. Thorington, Jr. Introduction fossil material, we thank Donald Savage and Barbara Waters of the Museum of Paleontology, The stimulus for this report was the discovery University of California, Berkeley, and Richard in 1975 of a nearly complete skeleton of what was Tedford and Malcolm McKenna of the American apparently a fossil squirrel in the Oligocene Museum of Natural History. For improvements White River Formation of Wyoming. Fossil re­ in the manuscript as a result of their careful mains of squirrels are relatively uncommon in the critical reviews we thank the following: Dr. Craig North American Tertiary record. Postcranial re­ C. Black, Director, Carnegie Museum of Natural mains are even less common, and were completely History, Pittsburgh; Dr. Lawrence R. Heaney, unknown for the Oligocene members of the fam­ Museum of Zoology, the University of Michigan, ily. Thus, the specimen, with excellently pre­ Ann Arbor; and Dr. John H. Wahlert, North served postcranial material, is particularly impor­ Museum, Franklin and Marshall College, Lan­ tant; it gives us the first opportunity to determine caster, Pa. We thank John Wahlert also for shar­ how squirrel-like the early members of the family ing his knowledge of incisor enamel microstruc­ were. It also provides the evidence needed to ture. place early Oligocene taxa confidently in the family Sciuridae, where they were placed only tentatively on the basis of dental and cranial Material morphology. ACKNOWLEDGMENTS.—Our first expression of The specimen that is the focus of this report is appreciation is to Jennifer Emry, who found the USNM 243981 (USNM = the former United critical fossil, without which this study could not States National Museum collections in the Na­ have begun. The specimen was skillfully prepared tional Museum of Natural History, Smithsonian in the laboratory by Leroy Glenn and Frederick Institution), provisionally referred to Protosciurus Grady. The pencil drawings are by Lloyd Logan. jeffersoni. It consists of the palate with complete For access to collections in their care, or loans of maxillary dentition, both auditory bullae with petrosals attached, and numerous other skull frag­ Robert J. Emry, Department of Paleobiology, and Richard W. Thor­ ington, Jr., Department of Vertebrate Zoology, National Museum of ments, both mandibular rami with complete den­ Natural History, Smithsonian Institution, Washington, D.C. 20560. tition, and most of the postcranial skeleton, partly I SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY articulated or closely associated (Figure 1). The represent, on the basis of much other faunal skeleton represents a young individual, with the evidence, the early part of the Chadronian. It is complete mature dentition in place but practi­ probably slightly older than the well-known Pipe­ cally unworn, and with the epiphyses still un­ stone Springs localities of Montana and probably fused; the gray squirrel, Sciurus carolinensis, reaches younger than early Chadronian localities such as this stage of ontogenetic development at about the Yoder of eastern Wyoming and the Porvenir eight months of age (240 days, USNM 528003). of west Texas. Several other specimens of Proto­ The specimen was found in the White River sciurus jeffersom (maxillae, jaws, or teeth) have Formation in the head of Little Lone Tree Gulch, been found in the same stratigraphic sequence, in the Flagstaff Rim area of Natrona County, but this specimen occurred lower in the section Wyoming, at about 13.5 meters (45 feet) below than any other. If we are correct in its being ash B on the generalized zonation section (Emry, slightly older than the Pipestone Springs occur­ 1973:29). The White River Formation here is rences oi P. jeffersom, then it is probably the oldest Chadronian in age, and the part of the section known fossil squirrel specimen. where USNM 243981 occurred is believed to The cranial and dental parts of the fossil were FICURE 1.—Posterior part of skeleton of Protosciurus, USNM 243981, still in the original matrix as it occurred. (Approximately X .75.) NUMBER 47 compared with other Protosciurus specimens, and Taxonomic Identity of USNM 243981 other fossil rodents, as cited in the text. The postcranial skeleton was compared principally