Coelacanthiform fishes of the British Rhaetian

Abstract

Coelacanth fishes are well represented in the fossil record, ranging in age from the Early Devonian to the present day, though now represented by only two species of the genus Latimeria. After some diversification of form in the Early Devonian, the morphology of coelacanths remained relatively stable throughout the Mesozoic, with distinctive mawsoniid and latimeriid coelacanth morphologies established by the Late Jurassic. Coelacanth remains are well known from Paleozoic and Cretaceous rocks in Britain but poorly known from the Late Triassic. Recently, it was shown that many isolated coelacanth bones from the Rhaetian (latest Triassic) bonebeds had been misidentified as elements of the enigmatic thalattosaur Pachystropheus rhaeticus. We further identify a number of coelacanth bones, primarily from the skull, providing descriptions of each for the first time. For some bones, we describe numerous morphotypes, suggesting more than one taxon was present. Most specimens pertain to Mawsoniidae, while others may be assignable to Latimeriidae or are indeterminate coelacanths. We highlight taxonomic misidentifications to show that coelacanth specimens from the British Rhaetian have been confused as far back as the late 1800s. Numerous morphotypes of many bones in the sample hint at a complex community structure, with individuals of varying age, size, and likely several species, supported within the ecosystem. The fossil occurrences suggest these Triassic coelacanths preferred shallow water environments, matching our observation that many specimens pertain to Mawsoniidae, a clade elsewhere reported from brackish, near-shore conditions. In light of recent findings, the new specimens reinforce a European Triassic diversification for Mawsoniidae, which probably influenced their subsequent diversification and dispersion during the Jurassic.