A hidden diversity of ornithischian dinosaurs: U.K. Middle Jurassic microvertebrate faunas shed light on a poorly represented period

Abstract

ABSTRACT Current research suggests that ornithischians originated in the Middle–Late Triassic and achieved a global distribution by the Early Jurassic, but the Middle Jurassic was a pivotal period in which the clade underwent rapid diversification and radiation. However, Middle Jurassic ornithischian fossils are rare, with few named taxa and numerous occurrences of isolated teeth with disputed identifications. Here, we apply detailed morphological comparisons to a suite of isolated ornithischian teeth from Bathonian microvertebrate sites in the U.K., to assess their taxonomic affinities. These reveal a hitherto unknown, highly diverse ornithischian fauna that significantly increases the known diversity of ornithischians from this time period in the U.K. Comparisons indicate the presence of six ornithischian morphotypes: an indeterminate ornithischian, a heterodontosaurid, two indeterminate thyreophorans, a stegosaur, and an ankylosaur. These results confirm the predictions made by phylogenetic studies that Ornithischia rapidly diversified in the Middle Jurassic, fill in temporal gaps within lineages, and also include recognition of one of the oldest global occurrences of stegosaurs. In addition, the mixture of non-eurypodan and eurypodan morphotypes identified suggests that not only did non-eurypodans survive until at least the Middle Jurassic but they also co-existed with early eurypodans.