New Specimens of the Rare Taeniodont Wortmania (Mammalia: Eutheria) from the San Juan Basin of New Mexico and Comments on the Phylogeny and Functional Morphology of “Archaic” Mammals

Abstract

BackgroundTaeniodonta is a clade of Late Cretaceous – Paleogene mammals remarkable for their relatively extreme cranial, dental, and postcranial adaptations and notable for being among the first mammals to achieve relatively large size following the Cretaceous-Paleogene mass extinction. Previous workers have hypothesized that taeniodonts can be divided into two clades: Conoryctidae, a group of small-bodied taeniodonts with supposedly “generalized” postcranial skeletons, and Stylinodontidae, a group of large-bodied, robust animals with massive forelimbs and claws adapted for scratch-digging. However, many taeniodont taxa are poorly known and few are represented by postcranial material, leaving many details about their anatomy, biology, and evolution ambiguous.Methodology/Principal FindingsIn this paper, we describe three new specimens of the rare taxon Wortmaniaotariidens from the early Paleocene (Puercan) of New Mexico. Among these specimens is one that includes remarkably complete cranial and dental material, including associated upper and lower teeth, and another that consists of partial forelimbs. These specimens allow for an updated anatomical description of this unusual taxon, supply new data for phylogenetic analyses, and enable a more constrained discussion of taeniodont biology and functional morphology.Conclusions/SignificanceThe new specimen of Wortmania that includes associated upper and lower teeth indicates that previous interpretations of the upper dentition of this taxon were not accurate and the taxon Robertschochiasullivani is a junior synonym of W . otariidens . New specimens that include partial forelimbs indicate that Wortmania is very similar to later, large-bodied taeniodonts, with marked and distinctive adaptations for scratch-digging. Comparisons with other taeniodont taxa that include postcranial material suggest that all taeniodonts may have had scratch-digging adaptations. A phylogenetic analysis shows that Schowalteria and Onychodectes are basal taeniodonts, Stylinodontidae (including Wortmania) is monophyletic, and a monophyletic Conoryctidae (but not including Onychodectes) is only recovered when certain characters are ordered.

A Digital Endocranial Cast of the Early Paleocene (Puercan) ‘Archaic’ Mammal Onychodectes tisonensis (Eutheria: Taeniodonta)

“…Schoch ( 1986 ) suggested that Onychodectes may have been partially arboreal, based on observations of postcranial features that are likely retained from a smaller, more arboreal ancestor. Our results cast doubt on this assessment, and mesh nicely with Williamson and Brusatte’s ( 2013 ) findings that catalogued postcranial adaptations for scratch-digging in most taeniodonts, including Onychodectes . The brain anatomy of Onychodectes does not rule out the possibility of burrowing, which is certainly a plausible strategy for the animal (the burrowing American badger Taxidea taxus is of a similar size and has a broadly similar skeleton), but there is little affirmative evidence from its brain anatomy to assert that Onychodectes was a habitual burrower.…”

“…Onychodectes and other taeniodonts exhibit skeletal features found in modern scratch diggers, such as an elongated olecranon process of the ulna and robust forelimb claws (Schoch 1986 ; Williamson and Brusatte 2013 ). However, it is difficult to determine whether this anatomy represents adaptations for foraging (for instance, habitually digging for buried foods) or other lifestyle habits such as burrowing.…”

“…Taeniodonts were among the first mammals to evolve dental adaptations for feeding on tough vegetation (e.g., crown hypsodonty) and postcranial specializations for scratch digging and/or burrowing. At least nine valid taeniodont genera are currently recognized (Schoch 1986 ; Rook and Hunter 2014 ; Williamson and Brusatte 2013 ). They became larger and more specialized over time (e.g., Patterson 1949 ), culminating in the last-surviving middle Eocene Stylinodon , which was about 50–100 kg in body mass and possessed rootless teeth, large gliriform canines, and powerful forelimbs with enlarged claws (Schoch 1986 ; Lucas et al 1998 ; Turnbull 2004 ).…”

Quantitative assessment of tarsal morphology illuminates locomotor behaviour in Palaeocene mammals following the end-Cretaceous mass extinction

“…Anatomical and functional insights on Palaeocene mammals have previously either been limited to qualitative inferences [12][13][14][15], or focused on a specific extinct taxon [16][17][18], or a small and/or closely related sample of extinct taxa [19,20]. These findings challenge the concept that Palaeocene mammals were non-specialized generalists and indicate a wider range of morphological diversity than previously alluded.…”