Refining the marine reptile turnover at the Early–Middle Jurassic transition

Abstract

Even though a handful of long-lived reptilian clades dominated Mesozoic marine ecosystems, several biotic turnovers drastically changed the taxonomic composition of these communities. A seemingly slow paced, within-geological period turnover took place across the Early–Middle Jurassic transition. This turnover saw the demise of early neoichthyosaurians, rhomaleosaurid plesiosaurians and early plesiosauroids in favour of ophthalmosaurid ichthyosaurians and cryptoclidid and pliosaurid plesiosaurians, clades that will dominate the Late Jurassic and, for two of them, the entire Early Cretaceous as well. The fossil record of this turnover is however extremely poor and this change of dominance appears to be spread across the entire middle Toarcian–Bathonian interval. We describe a series of ichthyosaurian and plesiosaurian specimens from successive geological formations in Luxembourg and Belgium that detail the evolution of marine reptile assemblages across the Early–Middle Jurassic transition within a single area, the Belgo–Luxembourgian sub-basin. These fossils reveal the continuing dominance of large rhomaleosaurid plesiosaurians, microcleidid plesiosaurians and Temnodontosaurus-like ichthyosaurians up to the latest Toarcian, indicating that the structuration of the upper tier of Western Europe marine ecosystems remained essentially constant up to the very end of the Early Jurassic. These fossils also suddenly record ophthalmosaurid ichthyosaurians and cryptoclidid plesiosaurians by the early Bajocian. These results from a geographically-restricted area provide a clearer picture of the shape of the marine reptile turnover occurring at the early–Middle Jurassic transition. This event appears restricted to the sole Aalenian stage, reducing the uncertainty of its duration, at least for ichthyosaurians and plesiosaurians, to 4 instead of 14 million years.

Anatomy and phylogenetic relationships of Temnodontosaurus zetlandicus (Reptilia: Ichthyosauria)

“…The specimen MNHNL TU885 was found at Schouweiler near Bascharage and Sanem, Grand Duchy of Luxembourg (Fig. 1B), a region well known for its marine reptile fossil richness (Streitz, 1983;Godefroit, 1994;Vincent et al, 2017;Johnson et al, 2019;Fischer et al, 2021).…”

A fragmentary leptonectid ichthyosaurian from the lower Pliensbachian of Luxembourg

“…A novelty of the Pliensbachian is to record the slightly more derived taxon Hauffiopteryx (Maxwell and Cortés, 2020). Thanks to the Cloche d'Or locality, all stages of the Early Jurassic are now known to contain large early neoichthyosaurians (with centra diameter > 10 cm) in many places of the European Archipelago (Godefroit, 1993;McGowan, 1996;Martin et al, 2012;Maxwell and Vincent, 2015;Fischer et al, 2021). By contrast, baracromians dominate ichthyosaurian assemblages in the European Archipelago by the early Toarcian with the ubiquitous presence of multiple species of Stenopterygius (Huene, 1931b;Hauff, 1953;Godefroit, 1994;Maisch, 2010;Maxwell, 2012;Dick et al, 2016) and are the only ichthyosaurians found after the Bajocian, with the exception of Malawania anachronus (Fernández, 1994;Fischer et al, 2013Fischer et al, , 2021.…”

“…Thanks to the Cloche d'Or locality, all stages of the Early Jurassic are now known to contain large early neoichthyosaurians (with centra diameter > 10 cm) in many places of the European Archipelago (Godefroit, 1993;McGowan, 1996;Martin et al, 2012;Maxwell and Vincent, 2015;Fischer et al, 2021). By contrast, baracromians dominate ichthyosaurian assemblages in the European Archipelago by the early Toarcian with the ubiquitous presence of multiple species of Stenopterygius (Huene, 1931b;Hauff, 1953;Godefroit, 1994;Maisch, 2010;Maxwell, 2012;Dick et al, 2016) and are the only ichthyosaurians found after the Bajocian, with the exception of Malawania anachronus (Fernández, 1994;Fischer et al, 2013Fischer et al, , 2021. The Pliensbachian gap in the fossil record of ichthyosaurians might contain this change of dominance; in any case, these remains further highlight the importance of the Luxembourg-Trier basin deposits in understanding the diversity dynamics of Jurassic marine reptiles.…”

“…However, the quality of their fossil record strongly fluctuates with time and space (Benson et al, 2010;Benson and Butler, 2011;Butler et al, 2011), and possibly less so with phylogeny (Tutin and Butler, 2017). This fluctuation appears fractal, being also present within Early Jurassic ichthyosaurians: the early Toarcian shales have provided thousands of fossils in western Europe, with most specimens from Germany and England (Hauff, 1953;Godefroit, 1994;Benson et al, 2010), while the Pliensbachian and late Toarcian assemblages are extremely poorly sampled (Bardet et al, 2008;Fernández et al, 2018;Lomax and Massare, 2018a;Fischer et al, 2021). Pliensbachian ichthyosaurian faunas generally resemble those from the Sinemurian, with the presence of Leptonectes (McGowan and Milner, 1999;Fernández et al, 2018;Lomax and Massare, 2018a), Ichthyosaurus (Lomax, 2010;Lomax and Massare, 2015;Massare and Lomax, 2016), and Temnodontosaurus (Huene, 1931a;Hungerbühler and Sachs, 1996;Maisch and Hungerbühler, 1997), but also Hauffiopteryx (Maisch and Reisdorf, 2006;Maxwell and Cortés, 2020).…”

Geoheritage in the Minett UNESCO Biosphere (Southern Luxembourg): Inventory, Evaluation, and Conservation Aspects of Representative Geosites

“…Four lithostratigraphic members are outcropping at the different levels of the quarry (Fig. 5d), notably the 'Calcaire d'Audun-le-Tiche' with coral patch reefs (Lucius 1945;Hary 1970;Geister 1984Geister , 1986Lathuilière 2000a,b) and the 'Marnes sableuses d'Audun-le-Tiche' with namely large cephalopods (Weis 2006), fish teeth (Delsate & Felten 2015;Popov et al 2019), echinids (Thuy 2003(Thuy , 2010b, and marine reptile remains (Fischer et al 2021), as well as a single dinosaur osteoderm (Delsate et al 2018). The site has high educational potential and access is possible to professionals, under certain conditions and with a special permit.…”