The first hyaenodont from the late Oligocene Nsungwe Formation of Tanzania: Paleoecological insights into the Paleogene-Neogene carnivore transition

Abstract

Throughout the Paleogene, most terrestrial carnivore niches in Afro-Arabia were occupied by Hyaenodonta, an extinct lineage of placental mammals. By the end of the Miocene, terrestrial carnivore niches had shifted to members of Carnivora, a clade with Eurasian origins. The transition from a hyaenodont-carnivore fauna to a carnivoran-carnivore fauna coincides with other ecological changes in Afro-Arabia as tectonic conditions in the African Rift System altered climatic conditions and facilitated faunal exchange with Eurasia. Fossil bearing deposits in the Nsungwe Formation in southwestern Tanzania are precisely dated to ~25.2 Ma (late Oligocene), preserving a late Paleogene Afro-Arabian fauna on the brink of environmental transition, including the earliest fossil evidence of the split between Old World monkeys and apes. Here we describe a new hyaenodont from the Nsungwe Formation, Pakakali rukwaensis gen. et sp. nov., a bobcat-sized taxon known from a portion of the maxilla that preserves a deciduous third premolar and alveoli of dP4 and M1. The crown of dP3 bears an elongate parastyle and metastyle and a small, blade-like metacone. Based on alveolar morphology, the two more distal teeth successively increased in size and had relatively large protocones. Using a hyaenodont character-taxon matrix that includes deciduous dental characters, Bayesian phylogenetic methods resolve Pakakali within the clade Hyainailouroidea. A Bayesian biogeographic analysis of phylogenetic results resolve the Pakakali clade as Afro-Arabian in origin, demonstrating that this small carnivorous mammal was part of an endemic Afro-Arabian lineage that persisted into the Miocene. Notably, Pakakali is in the size range of carnivoran forms that arrived and began to diversify in the region by the early Miocene. The description of Pakakali is important for exploring hyaenodont ontogeny and potential influences of Afro-Arabian tectonic events upon mammalian evolution, providing a deep time perspective on the stability of terrestrial carnivore niches through time.

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

“…As in recent analyses (Borths et al, 2016;Borths and Seiffert, 2017;Borths and Stevens, 2017), our phylogenetic results suggest that the base of the hyaenodont radiation is joined by a sister clade composed of Altacreodus and Tinerhodon. These taxa have strong support as sister lineages (PP = 0.82) and the clade has strong support (PP = 0.84) as the sister clade of Hyaenodonta.…”

“…A. macrognathus also shares with Leakitherium and Paroxyaena a mesiodistally short metastyle on dP 4 , crenulated enamel, and a lobe-like protocone. In contrast, the dP 4 of the teratodontines Metasinopa and Masrasector exhibit extensive metastyles, smooth and thin enamel, and buccolingually narrow protocones, morphology shared with Pakakali, a hyaenodont from the late Oligocene of the Rukwa Rift Basin (Borths and Stevens, 2017). These observations, in conjunction with the phylogenetic analysis conducted here to test the utility of deciduous dental characters, support the close relationship between Apterodontines and Hyainailourines in the clade Hyainailouridae.…”

“…OTUs that have been scored for deciduous characters are indicated in bold, italicized font. Together with the Borths and Stevens (2017) analysis, this study documents the utility of deciduous dental characters in assessing phylogenetic relationships across hyaenodonts, significantly expanding upon ontogentic information previously available for just Hyaenodon (Bastl and Nagel, 2013). Recent work on hyaenodont systematics has improved sampling for Afro-Arabian OTUs (Borths et al, 2016), placing into context these descriptions of deciduous dentition from a broader sample of Afro-Arabian hyaenodonts.…”

“…Deciduous dental morphology is heritable in mammals (Hughes et al, 2000;Townsend et al, 2012) and is a rich source of phenotypic character information. The character-taxon matrix is an expansion of hyanodonts sampled by Borths et al (2016), and Borths and Stevens (2017), building upon previous work by Polly (1996), Zack (2011), andSolé et al (2014), adding additional characters of the deciduous dentition (14 deciduous, 148 total), and expanding the OTUs (84 total). Some of the characters incorporated into this analysis are modified from Bastl et al (2014).…”

“…This extended dental developmental interval would have allowed these hyainailouroids to wear the deciduous carnassial for up to three extra years, a significantly longer interval compared with carnivorans. When P 3 and P 4 finally erupted, they erupted into a fully mature dentary and were spared from years of accumulated wear, wear that is preserved on the dP 4 of Metasinopa (DPC 4544) and dP 3 of Pakakali (Borths and Stevens, 2017).…”