New insights on the most primitive desmostylian from a partial skeleton ofBehemotops(Desmostylia, Mammalia) from Vancouver Island, British Columbia

Abstract

A partial articulated skeleton of a desmostylian was found in siltstone of the Sooke Formation in the streambed at the mouth of the Sombrio River in Juan De Fuca Provincial Park, Vancouver Island, British Columbia, Canada. Another exposure of the Sooke Formation southeast of the locality has been dated to Chron C6Cr age, 24.1-24.8 Ma. This specimen includes the left side of the skull, two molars, a premolar, canines, partial scapula, nearly complete humerus, and numerous vertebrae and ribs. Molar characteristics are the same as material of Behemotops proteus from the Pysht Formation of Washington State, which is near the type locality of Behemotops proteus of the upper Oligocene Pysht Formation, Washington State. Previous specimens of B. proteus were limited to lower jaws and portions of the upper and lower postcanine dentitions. The slightly smaller Behemotops katsuiei from Japan is known from more elements, yet its cranial material is limited to the posterior portion of the cranium and a small portion of the zygomatic arch. This new material allows us to see that Behemotops cf. B. proteus had cranial features much like those seen in Cornwallius sookensis of North America. These include a postorbital process of the jugal, with the zygomatic process of the squamosal not dorsally expanded, a concave hard palate, enlarged canine tusks that point ventrally, and a narrow, curved incisor arcade on a narrow rostrum. This is different from specimens previously referred to as Behemotops emlongi and then synonymized with B. proteus, which have a broad symphysis with large tusks. These specimens formerly known as B. emlongi are now referred to a new genus.http://zoobank.org/urn:lsid:zoobank.

How can we reliably identify a taxon based on humeral morphology? Comparative morphology of desmostylian humeri

“…RBCM.EH2007.008.0001, a nearly complete left humerus from the late Oligocene of Vancouver Island, British Columbia, Canada, reported by Beatty & Cockburn (2015) . RBCM.EH2007.008.0001 shows epiphyseal fusions in the humerus and is considered as an adult.…”

“…Many diagnostic features of desmostylian genera and/or species have been proposed based on the morphology of the skull, including the mandible and molar teeth (e.g., Reinhart, 1959 ; Domning, Ray & McKenna, 1986 ; Inuzuka, 1988 ; Inuzuka, 2000a ; Inuzuka, 2000b ; Beatty, 2009 ; Chiba et al, 2016 ; Beatty & Cockburn, 2015 ; Santos, Parham & Beatty, 2016 ). Inuzuka (2000a) , Inuzuka (2000b) and Inuzuka (2013) , for example, proposed many diagnostic features in the cranial and postcranial morphology for the genera Desmostylus and Paleoparadoxia .…”

“…Furthermore, Barnes (2013) divided the genus Paleoparadoxia into three genera, Archaeoparadoxia , Paleoparadoxia, and Neoparadoxia . His taxonomic scheme has been accepted in many studies on desmostylians (e.g., Beatty & Cockburn, 2015 ; Matsui & Kawabe, 2015 ; Chiba et al, 2016 ). Accordingly, the taxonomy of Japanese desmostylian from the Miocene needs to reflect this scheme, necessitating the establishment of diagnostic features for these three new genera.…”

A long-forgotten ‘dinosaur’ bone from a museum cabinet, uncovered to be a Japan's iconic extinct mammal,Paleoparadoxia(Desmostylia, Mammalia)

“…For example, the femora described as those of Cornwallius [ 18 ] were primarily identified based on little evidence that its morphology is similar to those of other desmostylians but is smaller and different from Behemotops proteus [ 6 ], the only other desmostylian known from the synchronous age and general location at the time. Since then, evidence of at least one other smaller desmostylian, Seuku , has been found in the same age and region [ 45 ] suggesting that there is greater diversity than previously expected and we need more caution in making assumptions about identities of unassociated specimens. From the above, we should be more cautious in making assumptions about identities of fossil specimens unassociated with primary locality information.…”

The phylogeny of desmostylians revisited: proposal of new clades based on robust phylogenetic hypotheses

“…In the currently-accepted taxonomic scheme, Desmostylia includes two families, 10 to 12 genera, and 13–14 species (Shikama, 1966; Kohno, 2000; Inuzuka, 2005; Domning & Barnes, 2007; Barnes, 2013; Beatty & Cockburn, 2015; Chiba et al, 2016). The two families are Desmostylidae Osborn, 1905, and Paleoparadoxiidae Reinhart, 1959.…”