Whence the beardogs? Reappraisal of the Middle to Late Eocene ‘Miacis’ from Texas, USA, and the origin of Amphicyonidae (Mammalia, Carnivora)

Abstract

The Middle to Late Eocene sediments of Texas have yielded a wealth of fossil material that offers a rare window on a diverse and highly endemic mammalian fauna from that time in the southern part of North America. These faunal data are particularly significant because the narrative of mammalian evolution in the Paleogene of North America has traditionally been dominated by taxa that are known from higher latitudes, primarily in the Rocky Mountain and northern Great Plains regions. Here we report on the affinities of two peculiar carnivoraforms from the Chambers Tuff of Trans-Pecos, Texas, that were first described 30 years ago as Miacis cognitus and M. australis. Re-examination of previously described specimens and their inclusion in a cladistic analysis revealed the two taxa to be diminutive basal amphicyonids; as such, they are assigned to new genera Gustafsonia and Angelarctocyon, respectively. These two taxa fill in some of the morphological gaps between the earliest-known amphicyonid genus, Daphoenus, and other Middle-Eocene carnivoraforms, and lend additional support for a basal caniform position of the beardogs outside the Canoidea. The amphicyonid lineage had evidently given rise to at least five rather distinct forms by the end of the Middle Eocene. Their precise geographical origin remains uncertain, but it is plausible that southern North America served as an important stage for a very early phase of amphicyonid radiation.

Evidence of strong stabilizing effects on the evolution of boreoeutherian (Mammalia) dental proportions

“…Two of the three fossil ursids ( Arctotherium and Ursavus ) fall within the MMC and PMM space of extant bears. Oligocene amphicynodont Campylocynodon and fossil ursid Cyonarctos are early carnivorans (Tomiya & Tseng, ) that fall directly within the MMC and PMM space of extant canids, further supporting the longevity of dental proportions in mammalian evolution and the association between these dental proportions and phylogenetic lineages. Cyonarctos is a fossil carnivoran in the subfamily Hemicyoninae from the Oligocene of Europe, noted for being a very “canid‐like” early ursid (de Bonis, ; Ginsburg & Morales, ).…”

Evidence for the independent acquisition of aquatic specializations in pinnipeds (seals, sea lions, and walruses): insights from the study of the phylogenetic position, locomotor behaviour, and description of the stem pinniped, Puijila darwini

“…A significantly embayed lateral margin of the basioccipital in Puijila is undoubtedly a retention of a primitive feature. This feature was likely present at the base of Arctoidea, as it appears in the earliest members of the Caniformia (Tomiya and Tseng, 2016). A deep basioccipital embayment has been identified in Amphicyonids, ursids and their ancestors, and pinnipedimorphs (Hunt and Barnes, 1994).…”

“…Amphicyonidae have conventionally been placed within the arctoids (Wolsan, 1993;Wyss and Flynn, 1999) though their precise placement within Arctoidea has been volatile. It should be noted Amphicyonidae likely actually falls outside crown Caniformia (Wesley- Hunt and Flynn, 2005;Tomiya and Tseng, 2016), though our analysis is not comprehensive enough to test such a hypothesis. In the present study, Temnocyon is included as an in-group unit in all but one of our analyses, an inclusion that may appear ill-considered in the future, but is necessary for testing conventional hypotheses of caniforme relationships.…”

“…Eighty-two craniomandibular characters, fifty dental characters, and seventy-two postcranial characters were coded when possible for all taxa in the analysis. While some characters are novel, the majority of characters were derived from previous phylogenetic analyses of Carnivora, including those focussing on pinnipeds (Berta and Wyss, 1994;Kohno, 1996Kohno, ,2006Deméré and Berta, 2001, 2002Boessenecker andChurchill 2013,2015;Amson and Muizon, 2014;Churchill and Boessenecker, 2014;Furbish, 2015) musteloids (Bryant et al, 1993;Wolsan, 1993;Ahrens, 2013;Valenciano et al, 2016), arctoids (Wang, 2005;Tedford, 1994;Finarelli, 2008), and carnivorans more broadly (Wyss and Flynn, 1993;Wesley-Hunt and Flynn, 2005;Spaulding and Flynn, 2011;Tomiya and Tseng, 2016) (see Appendix C for full character list). Many of these characters were then modified, either by editing existing character states or by adding character states, to better reflect the diversity of these characters across arctoids.…”

“…Basisphenoid, in ventral view (modified from Berta and Wyss, 1994: character 44) 0=concave 1=convex 49. Basioccipital, inferior petrosal sinus (modified from Tomiya and Tseng, 2016) 0=indistinct embayment on lateral wall of basioccipital 1=shallow embayment; sinus with well-defined medial margin, but not particularly large 2=deep basioccipital embayment; sinus is deep with both medial and lateral edges sharp and well defined This character refers to the portion of the petrosal sinus that is medial to the petrosal. This character is most reliably identified in coronal slices of the basioccipital from CT data, but can also be visible if an unobscured dorsal view of the basioccipital region is possible.…”