SIDEBONE IN A FOSSIL HORSE James R. Rooney I and Eric Scott 2 SUMMARY Ossification of the lateral cartilage of the distal hind phalanx of an individual of Equus "occidentalis" from the Rancho La Brea asphalt deposits is described. This is the first known report of "sidebone" among ancient horses. INTRODUCTION The purpose of this report is the description of a unique specimen of ossification of the cartilage of the third pha- lanx - sidebone - in an ancient horse, Equus "occidentalis," from the Rancho La Brea asphalt deposits in California. While sidebone is not uncommon in recent horses, no reports have been found of its occurrence in ancient horses. The Rancho La Brea asphalt deposits comprise the richest known concentration of terrestrial late Pleistocene fossils anywhere in the world. These deposits are located on the Santa Monica Plain in the Los Angeles Basin, approximately four kilometers southeast of the base of the Santa Monica Mountains. At Rancho La Brea alluvial sediments deposited during the last part of the Pleistocene Epoch (from <38,000 years before present to +/-12,000 years before present) overlie oil-bearing marine strata dating to the Miocene and early Pliocene. 1 For at least the past forty thousand years and continuing today, liquid petroleum derived from these marine rock units moves upwards, propelled at least in part by methane gas, through cracks and fissures in the Pleistocene alluvial sediments resulting from local tectonic activity. 1 This petroleum Authors' addresses: 1Gluck Equine Research Center, Department of Veterinary Science, University of Kentucky. Present address: 204 Sportsman Neck Road, Queenstown, Md 21658.2Section of Paleontology, San Bernardino County Museum, 2024 Orange Tree Lane, Redlands, Ca, 92374. collects in shallow depressions at the surface where the lighter, more volatile petroleum elements evaporate to leave sticky pools of asphalt. Thousands of animals living during the Pleistocene were trapped and preserved in these asphalt seeps. 1,2 Two species of extinct horses have been reported from Rancho La Brea. a,4 The present specimen is attributed to the larger of these species, commonly referred to as the taxon Equus occidentalis Leidy 1865. (The validity of this taxon has been challenged on technical grounds; for conve- nience, the species name will be placed in quotations herein as Equus "occidentalis." This large horse is re- ported to have been related to the burchelline zebra lin- eage, 5,6 It died out approximately 11,000 years ago in North America, leaving no descendants. More than 220 individuals of this horse are present in the collections from Rancho La Brea. 7 Casuistic: The specimen, LACMHC 122797, is a distal phalanx of Equus "occidentalis." It appears that it is the distal phalanx of the left hind leg based upon the following observations: 1. The articular surface comprises two parts divided by the sagittal ridge. By fixing one leg of calipers on this ridge and adjusting the other leg to the abaxial border of each articular part, it is apparent that one part is wider than the other. Similarly, fixing one leg of the calipers on the extensor process and adjusting the other leg to measure the length indicates that the wider part is also slightly longer than the other. In Equus the area of the medial part of all the phalangeal joint surfaces is larger than the lateral. The measurements establish that this is the left phalanx. 2. The angle of the dorsum (front) of the phalanx is about 58-59 ~ with the distal or sole border of the phalanx. In modern Equus the angle of the hind phalanx is larger than that of the phalanx of the foreleg which varies around Volume 18, Number 8, 1998 519