Yale Peabody Museum of Natural History New Haven, CT 06511

Postilla umber ise

28 March 1983

A New Species of /secto/ophus (Mammalia, Tapiroidea) from the Middle Eocene of Wyoming

Robert Milton Schoch

(Received 12 October 1982) Abstract

A partial right dentary bearing M, from the middle Eocene (Bridger) of Wyoming repre- sents a new species of /secto/ophus here designated /. radinskyi. |. radinsky/ differs from all other /secto/ophus in its smaller size; it is intermediate in size between Homogalax protapirinus and its supposed descendant /secto/ophus /atidens.

Key Words

/sectolophus, Tapiroidea, Bridgerian, Eocene, fossil mammal.

Introduction

Among the previously uncatalogued middle Eocene fossil mammal remains from the Green River (Bridger) Basin, Wyoming, housed in the Yale Peabody Museum, New Haven (YPM), is a partial dentary bearing M, of a tapiroid which rep- resents a new species of /secto/ophus.

© Copyright 1983 by the Yale Peabody Museum of Natural History. All rights reserved. No part of this Publication, except brief quotations for scholarly Purposes, may be reproduced without the written permission of the Director, Yale Peabody Museum of Natural History.

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Systematic Paleontology

Class Mammalia Linnaeus, 1758 Order Perissodactyla Owen, 1848 Suborder Ceratomorpha Wood, 1937 Superfamily Tapiroidea Burnett, 1830 (Gill, 1872)

Family Isectolophidae Peterson, 1919 Genus /secto/ophus Scott and Osborn, 1887

/sectolophus radinskyi Schoch,

new species

(Fig. 1, Table 1)

Holotype

YPM 40262, partial right dentary with posterior root of P3, roots of P,-M, and complete Ms.

Horizon and Locality of the Type

Middle Eocene (Bridgerian) aged strata of the Bridger Formation, Green River (Bridger) Basin, Wyoming. Collected by L. LaMothe and J. W. Chew at Lone Springs, Wyoming, July 1874.

Hypodigm

Known only from the holotype.

Etymology

Named for Leonard B. Radinsky in honor of his many contributions toward a better understanding of tapiroid evolution.

New Species of Postilla 188

/sectolophus

3 Eig ¢ = & Eg

rl 1

UCC VLG) VL Ls PP PPE OLS PLCS a Was Pe EO UP 20 e640 80 2 60 7080 0 Sa 50 60 70 ao. 8

Fig. 1

/sectolophus radinskyi Schoch, new species, YPM 40262 (holotype), right den- tary with M.: A, occlusal view, stereo pair; B, labial view; C, lingual view. Scales are in mm.

3 New Species of /sectolophus

Postilla 188

Diagnosis

Smallest known species of /secto/ophus: length of M,_3 approximately 35.2 mm; dentary slightly shallower than that of /sectolophus /atidens.

Description

YPM 40262 consists of a partial right den- tary which bears a fragment of the posterior root of P., the roots of P,-M, and the com- plete M., Pz-Mg are all double-rooted and increase in size posteriorly. The preserved portion of the dentary is comparable in morphology to that of /secto/ophus latidens, although somewhat shallower and more gracile (depth of ramus under M,_5 = 21.0 mm).

M, of /secto/ophus radinsky/is virtually identical in morphology to the M, of other species of /secto/ophus, although much smaller in size. The transverse protolophid and hypolophid are parallel to one another, and very slightly notched in their centers (such that the protoconid and metaconid, hypoconid and entoconid remain relatively distinct) and trend slightly anterolabially. The protolophid is very slightly higher than

the hypolophid. The metaconid is very slightly higher than the protoconid; there is no metastylid. The paralophid is represent- ed by a low anterolabial and anterior ridge. The metalophid is low and extends from the apex of the hypoconid to the postero- lingual base of the protoconid. The trigonid

__and talonid (excluding the hypoconulid

lobe) are subequal in length and width. The hypoconulid forms a large, broad, posterior heel bounded labially and posteriorly by a ridge which is highest labially (about half the height of that of the hypoconid) and de- creases in height posterolingually. This ridge is minutely cuspidate and bears a shallow, posterior notch positioned slightly lingual to the anteroposterior midline of the tooth. Mz bears a low, minutely cuspidate cingulid on its anterior and labial sides.

Discussion

Radinsky (1963) rediagnosed, revised and described /secto/ophus, recognizing two species, the smaller and earlier / /atidens (middle Eocene, mean length M,_, = 40.4 mm) and the somewhat larger and later / annectens (late Eocene, mean length M,_, = 46.8 mm). |n preserved parts YPM 40262

Table 1

Measurements (in mm) of M,s of /secto/ophus /atidens and /sectolophus radinskyi, new

species.

/sectolophus latidens

YPM 12563 YPM 15297 YPM 15298 YPM 16337

/sectolophus radinskyi YPM 40262

Length Width IAs 0, is) 7 Shi 18.0 + _ 175 8.5+ (er? 78

New Species of /sectolophus

Postilla 188

is identical to / /atidens, except for its smaller size (Table 1). YPM 40262 is within the size range of the early Eocene isecto- lophid Homogalax protapirinus. However, it differs from Homogal/ax in lacking a metastylid on M.. Metastylids are always present on the M,_, of Homogalax (Radinsky 1963). Among other known Eocene North American tapiroids, YPM 40262 is within the size range of the early Eocene helaletid Heptodon ca/ciculus and the middle Eocene He/a/etes nanus. However, YPM 40262 differs from these species, and all Helaletidae, in bearing a relatively unreduced metalophid and a long and broad (large) hypoconulid on M.. In helaletids the M,_, metalophids are greatly reduced and the M. hypoconulid is short and narrow or lost (Radinsky 1963). Thus, based on the morphology of Mz, YPM 40262 is referable only to the Isectolophidae, and of the two genera composing this family it is closest mor- phologically to other species of /sectolophus. Based on the lengths of M,_3 in the known species of /secto/ophus (Radinsky 1963), / /atidens is approximate- ly 14 percent smaller than / annectens and YPM 40262 is approximately 13 percent smaller than /. /atidens. On this basis | coin the name /secto/ophus radinskyl, new species, for YPM 40262, the smallest known species of /secto/ophus.

Radinsky (1963, p. 74) postulated that Homogalax protapirinus gave rise to /sectolophus latidens. The discovery of /sectolophus radinsky/ supports this hypothesis. /secto/ophus radinsky/ is in- termediate between these two forms; it is within the size range of Homoga/ax protapirinus, but in nonmetric morphology is identical to /secto/ophus /atidens.

Acknowledgments

| thank Professor John H. Ostrom (YPM) for allowing me to study specimens in his care. | thank Mary Ann Turner (YPM) for help in locating specimens and locality information. The photographs were taken by William K. Sacco (YPM). Part of the costs of publication were supported by the O. C. Marsh Publication Fund, Yale Peabody Museum.

Literature Cited

Radinsky, L. B. 1963. Origin and early evo- lution of North American Tapiroidea. Bull. Peabody Mus. Nat. Hist. (Yale Univ.) 17:1-115.

The Author

Robert Milton Schoch. Department of Geology and Geophysics and Yale Peabody Museum of Natural History, Yale University, 170 Whitney Avenue, P.O. Box 6666, New Haven, CT 06511.