[Paleontology • 2023] Lorrainosaurus keileni • The Rise of Macropredatory Pliosaurids near the Early-Middle Jurassic Transition

 Lorrainosaurus keileni   

 in Sachs, Madzia, Thuy & Kear, 2023.

 DOI: 10.1038/s41598-023-43015-y 

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artwork: Joschua Knüppe

Abstract

The emergence of gigantic pliosaurid plesiosaurs reshaped the trophic structure of Mesozoic marine ecosystems, and established an  ~ 80 million-year (Ma) dynasty of macropredatory marine reptiles. However, the timescale of their ‘defining’ trait evolution is incompletely understood because the fossil record of gigantic pliosaurids is scarce prior to the late-Middle Jurassic (Callovian),  ~ 165.3 Ma. Here, we pinpoint the appearance of large body size and robust dentitions to early-Middle Jurassic (Bajocian) pliosaurids from northeastern France and Switzerland. These specimens include a new genus that sheds light on the nascent diversification of macropredatory pliosaurids occurring shortly after the Early-Middle Jurassic transition, around  ~ 171 Ma. Furthermore, our multivariate assessment of dental character states shows that the first gigantic pliosaurids occupied different morphospace from coeval large-bodied rhomaleosaurid plesiosaurs, which were dominant in the Early Jurassic but declined during the mid-Jurassic, possibly facilitating the radiation and subsequent ecomorph acme of pliosaurids. Finally, we posit that while the emergence of macropredatory pliosaurids was apparently coordinated with regional faunal turnover in the epeiric basins of Europe, it paralleled a globally protracted extinction of other higher trophic-level marine reptiles that was not completed until after the earliest-Late Jurassic,  ~ 161.5 Ma.

Skeletal remains of the holotype (MNHNL BU159) of Lorrainosaurus keileni.
(a) Reconstruction in lateral view showing recovered elements. (b) Tooth crown with root. (c) Posterior section of mandible in lateral view. (d) Glenoid section of mandible in articular view. (e) Complete mandible in ventral view. (f) Enlargement of the mandibular symphysis. (g) Coracoid in dorsal view.

  

Plesiosauria de Blainville, 1835 

Pliosauridae Seeley, 1874 

Thalassophonea Benson & Druckenmiller, 2014 

Lorrainosaurus gen. nov.

Etymology: Derived from ‘Lorraine’, for the type locality; and ‘σαῦρος’ (sauros), Greek for ‘reptile’.

Type species: Lorrainosaurus keileni (Godefroit, 1994) 

Type locality and stage: A temporary road cutting between Montois-la-Montagne and Sainte-Marie-aux-Chênes ~ 18 km northeast of Metz in Lorraine, northeastern France. These deposits form part of the Marnes de Gravelotte regional lithostratigraphical unit correlated with the upper Bajocian (mid-Middle Jurassic) Parkinsonia parkinsoni Zone.

Diagnosis: Large-bodied thalassophonean pliosaurid autapomorphically distinguished by a transversely broad, ‘wedge-shaped’ splenial contact that extends anteriorly to the level of the fourth mandibular alveolus. Lorrainosaurus keileni also displays a unique character state combination: (1) laterally expanded and posteriorly constricted ‘spatulate’ symphyseal section of the mandible bearing five to six alveoli; (2) lateral trough on the mandible anterior to the glenoid fossa; (3) a retroarticular process that is shorter than the glenoid fossa; (4) retroarticular process with posteroventrally oriented dorsoventral long axis and slightly posteromedially inflected mediolateral long axis; (5) wide posteromedial seperation of the coracoids; (6) posterolateral edge of the coracoid (cornu) projecting beyond the level of the glenoid fossa (Fig. 1).

 

 Sven Sachs, Daniel Madzia, Ben Thuy and Benjamin P. Kear. 2023. The Rise of Macropredatory Pliosaurids near the Early-Middle Jurassic Transition. Scientific Reports. 13: 17558. DOI: 10.1038/s41598-023-43015-y

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