Hyainailourine and teratodontine cranial material from the late Eocene of Egypt and the application of parsimony and Bayesian methods to the phylogeny and biogeography of Hyaenodonta (Placentalia, Mammalia)

Abstract

Hyaenodonta is a diverse, extinct group of carnivorous mammals that included weasel- to rhinoceros-sized species. The oldest-known hyaenodont fossils are from the middle Paleocene of North Africa and the antiquity of the group in Afro-Arabia led to the hypothesis that it originated there and dispersed to Asia, Europe, and North America. Here we describe two new hyaenodont species based on the oldest hyaenodont cranial specimens known from Afro-Arabia. The material was collected from the latest Eocene Locality 41 (L-41, ∼34 Ma) in the Fayum Depression, Egypt. Akhnatenavus nefertiticyon sp. nov. has specialized, hypercarnivorous molars and an elongate cranial vault. In A. nefertiticyon the tallest, piercing cusp on M1–M2 is the paracone. Brychotherium ephalmos gen. et sp. nov. has more generalized molars that retain the metacone and complex talonids. In B. ephalmos the tallest, piercing cusp on M1–M2 is the metacone. We incorporate this new material into a series of phylogenetic analyses using a character-taxon matrix that includes novel dental, cranial, and postcranial characters, and samples extensively from the global record of the group. The phylogenetic analysis includes the first application of Bayesian methods to hyaenodont relationships. B. ephalmos is consistently placed within Teratodontinae, an Afro-Arabian clade with several generalist and hypercarnivorous forms, and Akhnatenavus is consistently recovered in Hyainailourinae as part of an Afro-Arabian radiation. The phylogenetic results suggest that hypercarnivory evolved independently three times within Hyaenodonta: in Teratodontinae, in Hyainailourinae, and in Hyaenodontinae. Teratodontines are consistently placed in a close relationship with Hyainailouridae (Hyainailourinae + Apterodontinae) to the exclusion of “proviverrines,” hyaenodontines, and several North American clades, and we propose that the superfamily Hyainailouroidea be used to describe this relationship. Using the topologies recovered from each phylogenetic method, we reconstructed the biogeographic history of Hyaenodonta using parsimony optimization (PO), likelihood optimization (LO), and Bayesian Binary Markov chain Monte Carlo (MCMC) to examine support for the Afro-Arabian origin of Hyaenodonta. Across all analyses, we found that Hyaenodonta most likely originated in Europe, rather than Afro-Arabia. The clade is estimated by tip-dating analysis to have undergone a rapid radiation in the Late Cretaceous and Paleocene; a radiation currently not documented by fossil evidence. During the Paleocene, lineages are reconstructed as dispersing to Asia, Afro-Arabia, and North America. The place of origin of Hyainailouroidea is likely Afro-Arabia according to the Bayesian topologies but it is ambiguous using parsimony. All topologies support the constituent clades–Hyainailourinae, Apterodontinae, and Teratodontinae–as Afro-Arabian and tip-dating estimates that each clade is established in Afro-Arabia by the middle Eocene.

Deciduous dentition and dental eruption of Hyainailouroidea (Hyaenodonta, “Creodonta,” Placentalia, Mammalia)

“…The Borths et al (2016) analysis places Leakitherium within Hyainailourinae, a result congruent with earlier phylogenetic hypotheses (Lewis and Morlo, 2010). One feature uniting hyainailourines is the metacone morphology on M 1 and M 2 , reduced into a buccolingually compressed cusp that is partially or almost entirely fused to the paracone Savage, 1965) is a left maxilla fragment dP 3 and dP 4 shown in 1) occlusal view; 2) buccal view; and 3) lingual view.…”

“…The KNM-RU 15182 (Figure 3) specimen suggests that in Leakitherium, dP 3 was retained longer than dP 4 . This deciduous replacement order is consistent with the deciduous replacement order that Bastl and Nagel (2013) observed in Hyaenodon, a hyaenodont taxon that likely diverged from the clade that includes Hyainailouroidea during the late Cretaceous or early Paleocene (Borths et al, 2016).…”

“…Specimens are to scale and scale bar is 10 mm. (Borths et al, 2016). Interestingly, the dP 4 of Leakitherium ( Figure 3) exhibits a distinct metacone that is lower than and partially fused to the paracone, anticipating the morphology of M 1 in the holotype of Leakitherium.…”

“…Recent phylogenetic studies (Borths et al, 2016;Borths and Seiffert, 2017) incorporate morphological features of the cranial vault , suggesting Apterodontinae as the sister clade of Hyainailourinae. Like the hyainailourines Paroxyaena and Leakitherium, the dP 4 of Apterodon macrognathus ( Figure 1) exhibits a buccolingually compressed paracone that is distally directed.…”

“…Previous work on the dental development of hyaenodonts focused on the genus Hyaenodon, a genus from the Eurasian and North American clade Hyaenodontinae (Mellet, 1977;Bastl et al, 2011;Bastl and Nagel, 2013;Bastl et al, 2014); deciduous dental materials preserved in other hyaenodonts (e.g., Lesmesodon, Cynohyaenodon, Paroxyaena) have received somewhat less attention (but see Morlo and Habersetzer, 1999;Lavrov, 2007). Recent systematic research demonstrates that Hyaenodon is only distantly related to Hyainailouroidea, a separate hyaenodont clade that contains hypercarnivorous taxa and all AfroArabian hyaenodonts (Rana et al, 2015;Borths et al, 2016). But only a few deciduous teeth have been described from Afro-Arabian hyaenodonts (Morales et al, 2010;Solé et al, 2016), making it difficult to compare the life histories of Afro-Arabian hyainailouroids to Eurasian and North American hyaenodont lineages and to use deciduous dental morphology and dental eruption to probe phylogenetic relationships among key taxa in this important group of early Cenozoic predators.…”