A lower jaw of <i>Palaeoxonodon</i> from the Middle Jurassic of the Isle of Skye, Scotland, sheds new light on the diversity of British stem therians

Abstract

Abstract The M iddle J urassic was a key interval of mammalian evolutionary history that witnessed the diversification of the therian stem group. G reat B ritain has yielded a significant record of mammalian fossils from this interval, represented by numerous isolated jaws and teeth from the B athonian of O xfordshire and the I sle of S kye. This record captures a key period in early cladotherian evolution, with amphitheriids, peramurans and ‘stem zatherians’ displaying intermediate talonid morphologies that document the evolutionary assembly of tribosphenic molars. We present a mandible with near‐complete dentition from the late B athonian ( c . 167.4–166.5 Ma) K ilmaluag F ormation, near E lgol, S kye, representing the amphitheriid P alaeoxonodon ooliticus , previously known only from isolated teeth. The specimen sheds new light on the taxonomic diversity of B ritish M iddle J urassic stem therians, as the morphological variation within the preserved tooth row encompasses that previously ascribed to three distinct species within two genera: P alaeoxonodon ooliticus , P . freemani and K ennetheridium leesi . Thus, both P . freemani and K . leesi are subjective junior synonyms of P . ooliticus . The dental formula of P . ooliticus (i4:c1:p5:m5) is intermediate between the primitively larger postcanine count (p5:m6–7) of A mphitherium and the reduced number in peramurans and tribosphenidans (p5:m3). Phylogenetic analyses of P . ooliticus generally confirm a close affinity with A mphitherium , but highlight the lack of strong empirical support for hypothesized patterns of divergences among early cladotherians.