Abstract

The ‘Swami's Point local fauna,’ recovered from the Delmar Formation in coastal San Diego County, California, U.S.A., represents the first documented Bridgerian vertebrate assemblage in this region. The Delmar Formation is chronologically constrained (ca. 48–47 Ma) on the basis of paleomagnetic polarities, the superposition of marine strata containing nannofossils of the CP 12b subzone, and occurrences of the Bridgerian index taxa Trogosus castoridens and Hyrachyus modestus. The specimen of Trogosus (SDSNH 40819) is assigned here to the genotypic species, T. castoridens, hitherto known only from incomplete dentaries from the middle Bridgerian (Br2) Bridger Formation, Green River Basin, Wyoming. The present specimen is represented by a skull with diagnostically short rostrum, incomplete stylohyoids, posterior parts of both dentaries, and cervical vertebrae from a single individual. This is one of the most anatomically informative specimens of Trogosus and for the first time provides details of the basicranial region in the order Tillodontia. A well-preserved occipitomastoid process is clearly composed of the mastoid process of the petrosal and the paracondylar process of the exoccipital. Cranial comparison reveals greater similarity with T. hyracoides from the Bridger Formation than with T. grangeri and T. hillsii from the Huerfano Basin, Colorado. The results of a phylogenetic analysis suggest that the short-faced condition evolved at least twice in species of Trogosus and leads to recognition of two sympatric trogosine species pairs: T. hyracoides + T. castoridens from the Green River Basin and T. grangeri + T. hillsii from from the Huerfano Basin.