ANAGENETIC EVOLUTION OF TETRACLAENODON, A PALEOCENE "CONDYLARTH" FROM THE SAN JUAN BASIN, NEW MEXICO

The earliest known phenacodontid, the Paleocene "condylarth" genus Tetraclaenodon, appeared in North America at the beginning of the Torrejonian (~63.8 Ma). Tetraclaenodon was prolific and broadly distributed throughout the Western Interior, especially in the San Juan Basin of New Mexico, where stratigraphically and temporally closely spaced samples present evidence of phyletic gradualism in an early mammal lineage. Due to high variability and lack of stratigraphic control of earlier specimens, over 11 taxa have been proposed, but most workers reduced the varied taxa to a single species, Tetraclaenodon puercensis. Nevertheless, some researchers continue to periodically posit additional species chiefly on the basis of larger size rather than on any morphological innovation. The largest fossil sample of T. puercensis is from the San Juan Basin where recent New Mexico Museum of Natural History (NMMNH) stratigraphic studies have allowed precise temporal correlation of fossil localities. Reexamination of the over 600 NMMNH specimens, from six temporally successive faunal zones, permit evaluation of T. puercensis’ evolutionary pattern. The two earliest Torrejonian populations are significantly smaller in overall size than the four successive larger-sized populations, which present a gradual and unidirectional increase in size while displaying more subtle progressive trends. Thewissen’s (1990) professedly larger T. septentrionalis, is invalid because the specimens fall within the range of variability of the upper T. puercensis assemblages. Therefore, despite high variability, our new analysis confirms T. puercensis as a single evolving lineage due to continuous morphological overlap throughout its minimum 2.9 my generic duration and collaborates recent studies indicating anagenesis was common and the dominant mode in early mammalian evolution.