An enigmatic large mawsoniid coelacanth (Sarcopterygii, Actinistia) from the Upper Jurassic Kimmeridge Clay Formation of England

Abstract

A large mawsoniid coelacanth from the lower part of the marine Kimmeridge Clay Formation of England (Kimmeridgian, Upper Jurassic) is studied here. The material is constituted by a group of bones from the head and shoulder girdle of a considerably large individual (estimated length ca. 1.5 m), including the left angular, left dentary, left prearticular, left palatoquadrate complex, both ceratohyals and right cleithrum. Characters such as the coarse external ornamentation of the angular, and the robustness of the quadrate and the cleithrum allow classification of the individual as a member of the Mawsoniidae; whereas the configuration of external bones of the lower jaw (ornamentation of the angular constituted mainly by longitudinal ridges, the presence of a lateral swelling in the dentary) indicates stronger Gondwanan affinities than previously expected (i.e., with the genus Mawsonia, up to now only recorded in South America by the end of the Jurassic). Considering the above, two alternative evolutionary, paleobiogeographic, and taxonomic scenarios are discussed: (1) the new individual can be referred to the European mawsoniid genus Trachymetopon (Lower–Middle Jurassic), in which case it should be assumed this genus reached the Upper Jurassic, and with a morphological variability higher than previously suspected (including some characters previously assumed as diagnostic for Mawsonia). Or (2) an unknown Mawsonia-like form was present in the Upper Jurassic of Europe. The last scenario puts the identification of isolated elements of European Jurassic giant mawsoniids in a new complex taxonomic and paleobiogeographic context, which will deserve further research.

Reconstructing an ancient fish: Three‐dimensional skeletal restoration of the head of Mawsonia (Sarcopterygii, Actinistia) using CT scan, and an adjusted model for body size estimation in fossil coelacanths

“…It also typifies the Mawsoniidae, one of the two families of latimerioid coelacanths (the other being Latimeriidae, the family which contains the living Latimeria) (e.g., Cavin et al, 2013Cavin et al, , 2017Cavin & Grădinaru, 2014;Dutel et al, 2012;Forey, 1998;Schultze, 2004;Toriño, Soto, & Perea, 2021). The interest of mawsoniid coelacanths has increased during the last years due to several reasons: its key role in the phylogenetic definitions of the clades Latimerioidei, Mawsoniidae and Latimeriidae (see Dutel et al, 2012;Toriño, Soto, & Perea, 2021); new occurrences outside Gondwana and a low morphological variability throughout its biochron (e.g., Cavin et al, 2023;Cavin, Piuz, et al, 2021;Cavin, Toriño, et al, 2021;Dutel et al, 2012;Toriño et al, 2022;Toriño, Soto, & Perea, 2021;Toriño, Soto, Perea, & de Carvalho, 2021); besides its possible evolutionary significance in the acquisition of long generation times (as it happens with L. chalumnae, see Cavin & Alvarez, 2022;Mahé et al, 2021).…”

“…But when the general proportions of the head in latimeriid and mawsoniid coelacanths are compared in detail, the latter seem to be characterized by relatively more shallower and elongated skulls than the former (being the head relatively deeper and shorter in latimeriids) (e.g., Dutel et al, 2012Dutel et al, , 2014Dutel et al, , 2019Dutel, Herbin, Clément, & Herrel, 2015;Toriño et al, 2022). This unique cranial anatomy in mawsoniids is already present in Triassic basal forms, such as Chinlea (e.g., Elliott, 1987;Schaeffer, 1967), but it is more pronounced in Jurassic and Cretaceous-derived forms, such as Axelrodichthys and very especially Mawsonia (e.g., Maisey, , 1991aMaisey, , 1991b.…”