A Carboniferous synapsid with caniniform teeth and a reappraisal of mandibular size-shape heterodonty in the origin of mammals

Abstract

Heterodonty is a hallmark of early mammal evolution that originated among the non-mammalian therapsids by the Middle Permian. Nonetheless, the early evolution of heterodonty in basal synapsids is poorly understood, especially in the mandibular dentition. Here, we describe a new synapsid, Shashajaia bermani gen. et sp. nov., based on a well-preserved dentary and jaw fragments from the Carboniferous–Permian Halgaito Formation of southern Utah. Shashajaia shares with some sphenacodontids enlarged (canine-like) anterior dentary teeth, a dorsoventrally deep symphysis and low-crowned, subthecodont postcanines having festooned plicidentine. A phylogenetic analysis of 20 taxa and 154 characters places Shashajaia near the evolutionary divergence of Sphenacodontidae and Therapsida (Sphenacodontoidea). To investigate the ecomorphological context of Palaeozoic sphenacodontoid dentitions, we performed a principal component analysis based on two-dimensional geometric morphometrics of the mandibular dentition in 65 synapsids. Results emphasize the increasing terrestrialization of predator–prey interactions as a driver of synapsid heterodonty; enhanced raptorial biting (puncture/gripping) aided prey capture, but this behaviour was probably an evolutionary antecedent to more complex processing (shearing/tearing) of larger herbivore prey by the late Early to Middle Permian. The record of Shashajaia supports the notion that the predatory feeding ecology of sphenacodontoids emerged in palaeotropical western Pangea by late Carboniferous times.

Permian hypercarnivore suggests dental complexity among early amniotes

“…Permo-Carboniferous amniote fossils record the deep phylogenetic divergence between synapsids and reptiles, the former represented by pelycosaur-grade synapsids 13 , the latter represented by eureptiles and parareptiles. Among early synapsids, varanopids occupy a unique position in having the longest fossil record ( Supplementary Information ), extending from the late Carboniferous into the late middle Permian (more than 40 million years), a near-global distribution 14 – 16 , and the earliest evidence of parental care 17 .…”

Predatory synapsid ecomorphology signals growing dynamism of late Palaeozoic terrestrial ecosystems

“…2 ) illustrate FG optimisation for biting efficiency and power over biting speed 28 . Growing tooth size and shape variation through basal synapsid evolution 42 supports a shift towards more complex jaw use and feeding behaviour associated with tetrapod-on-tetrapod predation. Raptorial specialist dentition encompasses simplistic conidont to derived ziphodont tooth morphologies, with the ziphodont teeth appearing more commonly in sphenacodontids 42 , 43 , illustrating their growing efficiency for shearing flesh and specialisation as tetrapod predators 44 .…”

“…Growing tooth size and shape variation through basal synapsid evolution 42 supports a shift towards more complex jaw use and feeding behaviour associated with tetrapod-on-tetrapod predation. Raptorial specialist dentition encompasses simplistic conidont to derived ziphodont tooth morphologies, with the ziphodont teeth appearing more commonly in sphenacodontids 42 , 43 , illustrating their growing efficiency for shearing flesh and specialisation as tetrapod predators 44 . Raptorial specialists somewhat echo the jaw functionality of some sauropsid reptiles, and such similarity may extend to prey capture/killing behaviour (Fig.…”

Halgaitosaurus gregarius, a New Upper Carboniferous Araeoscelidian (Reptilia: Diapsida) from the Halgaito Formation, Bears Ears National Monument, Utah, USA

“…The specimens were preserved in a massive siltstone unit about 6.4 cm thick and topped by mudcracks in the middle of a discrete, roughly 80 cm thick sedimentologic deposit representing a minor stream channel that was a tributary to a nearby major channel. Isolated and fragmentary bones have been found in the underlying conglomeratic units (e.g., Huttenlocker 2021). Huttenlocker et al (2018) concluded that the Birthday bonebed fossils were deposited during a flooding event affecting both the tributary and main channels in which the tributary channel stream flow was dammed by the higher velocity flow of the main channel.…”