The petrosal and basicranial morphology of Protoceras celer

Abstract

Protoceratids are an extinct family of endemic North American artiodactyls. The phylogenetic position of protoceratids in relation to camelids and ruminants has been contentious for over a century. The petrosal morphology of basal (Leptotragulus) and derived (Syndyoceras) protoceratids has suggested that protoceratids are closely related to ruminants, whereas a prior description of a disarticulated intermediate protoceratid petrosal (Protoceras celer) indicated that protoceratids were closely related to camelids. This contradictory evidence implied that there were several character reversals within the protoceratid lineage and brought into question the utility of basicranial characters in artiodactyl phylogenetics. Here, we provide descriptions of an additional P. celer petrosal. The descriptions are based on data produced by computed tomography scans, which allowed us to image the petrosal in situ in the skull. Our results indicate that the petrosal morphology of P. celer is similar to that of other protoceratids, implying that, contrary to previous evidence, petrosal morphology is conserved within the Protoceratidae.

A reassessment of protoceratid vertebrarterial canal morphology

“…Webb and Taylor (1980) reported that protoceratids—particularly Protoceras —have the same morphology as camelids, citing Flower (1885) and Scott (1895) as authorities. The atypical vertebrarterial canal morphology of camelids and protoceratids is the only synapomorphy uniting the two families (e.g., Joeckel and Stavas, 1996; Prothero, 1998a; Norris, 2000; Robson et al, 2021) and has been used as a character in numerous large-scale artiodactyl taxon-character matrices (e.g., Gentry and Hooker, 1988; Geisler and Uhen, 2003, 2005; Theodor and Foss, 2005; Geisler et al, 2007; Thewissen et al, 2007; Geisler and Theodor, 2009). Oddly, members of the Oromerycidae, the putative sister group to the Camelidae, retain the typical artiodactyl condition (Gentry & Hooker, 1988; Prothero, 1986, 1998b).…”

“…The evolutionary position of Protoceratidae within Artiodactyla is ambiguous. Protoceratids may be closely related to camelids (Patton & Taylor, 1971, 1973Webb & Taylor, 1980) or ruminants (Gentry & Hooker, 1988;Joeckel & Stavas, 1996), but there are few derived features linking the family to either clade (Norris, 2000;Prothero & Ludtke, 2007;Robson et al, 2021Robson et al, , 2022. One of the most compelling arguments for a close evolutionary relationship between protoceratids and camelids is their unusual and supposedly shared vertebrarterial canal morphology (Webb & Taylor, 1980).…”