New Mexico Geological Society Annual Spring Meeting — Abstracts

In 1881, E.D. Cope fIrst recognized and described the highly variable "condylarth" genus Periptychus, based on specimens from the Nacimiento Fonnation, San Juan Basin, New Mexico. This archaic ungulate fIrst appeared during the basal Paleocene (Puercan 1, ~65 Ma), and persisted, in relative abundance, through Tiffanian 5 (~56.2 Ma). Measurements of all available Periptychus specimens give a mean variance generally twice as great as the variance typical of extant mammalian species. The 8+ million year generic span, excessive range of variation, and unusually rapid evolutionary rate estimated for Paleogene mammals, compelled researchers to postulate several Periptychus species. Proposed phylogenies involved up to two genera and 8 species. In recent comparisons, these species distinctions are controvertible. The advocated contrasting characters continuously recur and vary within the proposed divergent populations, and often manifest themselves within individual specimens.

The early collected Periptychus specimens were gathered without geological maps or accurate stratigraphic data. Without temporal correlation, species distinctions were reduced to arbitrary divisions based on a theorized intennediacy in size, proportion, robusticity, etc., rather than any true disparity, adaptive shift or morphological innovation. Our new analysis of recently collected
NMMNH Periptychus specimens, procured with reliable geological and stratigraphic data, has allowed temporal organization. The once excessively broad range of specimens clearly differentiates into two lineages: an early Puercan population consistent with Periptychus coarctatus, and a subsequent, more variable, Torrejonian-Tiffanian group we unify as P. carinidens. These two species, in turn, segregate into smaller temporal populations that now exhibit nonnative mean variance. Despite its lengthy duration, oscillations in mean size and wide geographic range, we interpret P. carinidens as a single evolving lineage due to continuous morphological overlap and lack of evidence of local endemism. Our reconstruction thus involves a parsimonious single genus, Periptychus, and two temporally successive species, Periptychus coarctatus and P. carinidens.