Valentin Fischer’s research while affiliated with University of Liège and other places

The iconic marine raptorial predators Ichthyosauria and Eosauropterygia co-existed in the same ecosystems throughout most of the Mesozoic Era, facing similar evolutionary pressures and environmental perturbations. Both groups seemingly went through a massive macroevolutionary bottleneck across the Triassic–Jurassic (T/J) transition that greatly reduced their morphological diversity, leaving pelagic lineages as the only survivors. However, analyses of marine reptile disparity across the T/J transition have usually employed coarse morphological and temporal data. We comprehensively compare the evolution of ichthyosaurian and eosauropterygian morphology and body size across the Middle Triassic to Early Jurassic interval and find contrasting macroevolutionary patterns. The ecomorphospace of eosauropterygians predominantly reflects a strong phylogenetic signal, resulting in the clustering of three clades with clearly distinct craniodental phenotypes, suggesting ‘leaps’ towards novel feeding ecologies. Ichthyosaurian diversification lacks a discernible evolutionary trend, as we find evidence for a wide overlap of craniodental morphologies between Triassic and Early Jurassic forms. The temporal evolution of ecomorphological disparity, fin shape and body size of eosauropterygians and ichthyosaurians during the Late Triassic does not support the hypothesis of an abrupt macroevolutionary bottleneck near the T/J transition. Rather, an important turnover event should be sought earlier, during times of rapid sea level falls.

The evolution of organisms can be studied through the lens of developmental systems, as the timing of development of morphological features is an important aspect to consider when studying a phenotype. Such data can be challenging to obtain in fossil amniotes owing to the scarcity of their fossil record. However, the numerous remains of Rancho La Brea allow a detailed study of the postnatal changes in an extinct sabertoothed felid: Smilodon fatalis. Despite numerous previous studies on the ontogeny of Smilodon, an important question remained open: how did the cubs of Smilodon acquire and process food? By applying 3D geometric morphometrics and finite element analyses to 49 mandibles at various developmental stages (22 of S. fatalis, 23 of Panthera leo, and 4 of early diverging felids), we assess the changes in mandibular shape and performance during growth. Both lions and sabertooths exhibit a shift in mandibular shape, aligning with eruption of the lower carnassial. This marks the end of weaning in lions and suggests a prolonged weaning period in S. fatalis owing to its delayed eruption sequence. We also highlight distinct ontogenetic trajectories, with S. fatalis undergoing more postnatal mandibular shape changes. Finally, although S. fatalis appears more efficient than P. leo at performing an anchor bite, this efficiency is acquired through ontogeny and at a quite late age. The delayed shape change compared with P. leo and the low biting efficiency during the growth in Smilodon could indicate an extended duration of the parental care compared with P. leo.

The initial radiation of Eosauropterygia during the Triassic biotic recovery represents a key event in the dominance of reptiles secondarily adapted to marine environments. Recent studies on Mesozoic marine reptile disparity highlighted that eosauropterygians had their greatest morphological diversity during the Middle Triassic, with the co-occurrence of Pachypleurosauroidea, Nothosauroidea and Pistosauroidea, mostly along the margins of the Tethys Ocean. However, these previous studies quantitatively analysed the disparity of Eosauropterygia as a whole without focussing on Triassic taxa, thus limiting our understanding of their diversification and morphospace occupation during the Middle Triassic. Our multivariate morphometric analyses highlight a clearly distinct colonization of the ecomorphospace by the three clades, with no evidence of whole-body convergent evolution with the exception of the peculiar pistosauroid Wangosaurus brevirostris , which appears phenotypically much more similar to nothosauroids. This global pattern is mostly driven by craniodental differences and inferred feeding specializations. We also reveal noticeable regional differences among nothosauroids and pachypleurosauroids of which the latter likely experienced a remarkable diversification in the eastern Tethys during the Pelsonian. Our results demonstrate that the high phenotypic plasticity characterizing the evolution of the pelagic plesiosaurians was already present in their Triassic ancestors, casting eosauropterygians as particularly adaptable animals.

Marine amniotes have played many crucial roles in ocean ecosystems since the Triassic, including predation at the highest trophic levels. One genus often placed into this guild is the large Early Jurassic neoichthyosaurian Temnodontosaurus, the only post-Triassic ichthyosaurian known with teeth which bear a distinct cutting edge or carina. This taxonomically problematic genus is currently composed of seven species which show a wide variety of skull and tooth morphologies. Here we assess the craniodental disparity in Temnodontosaurus using a series of functionally informative traits. We describe the range of tooth morphologies in the genus in detail, including the first examples of serrated carinae in ichthyosaurians. These consist of false denticles created by the interaction of enamel ridgelets with the carinal keel, as well as possible cryptic true denticles only visible using scanning electron microscopy. We also find evidence for heterodonty in the species T. platyodon, with unicarinate mesial teeth likely playing a role in prey capture and labiolingually compressed, bicarinate distal teeth likely involved in prey processing. This type of heterodonty appears to be convergent with a series of other marine amniotes including early cetaceans. Overall, the species currently referred to as the genus Temnodontosaurus show a range of craniodental configurations allowing prey to be captured and processed in different ways - for example, T. eurycephalus has a deep snout and relatively small bicarinate teeth likely specialised for increased wound infliction and grip-and-tear feeding, whereas T. platyodon has a more elongate yet robust snout and larger teeth and may be more adapted for grip-and-shear feeding. These results suggest the existence of niche partitioning at higher trophic levels in Early Jurassic ichthyosaurians and have implications for future work on the taxonomy of this wastebasket genus, as well as for research into the ecology of other extinct megapredatory marine tetrapods.

Pliosaurid plesiosaurians are iconic marine reptiles that regulated marine trophic chains from the Middle Jurassic to the early Late Cretaceous. However, their evolution during the Cretaceous remains poorly documented. Recent discoveries from the Hauterivian–Aptian interval suggest that the radiation of brachaucheniine pliosaurids produced a wide disparity of forms following the Pliosaurus-dominated assemblages of the Late Jurassic. Among the most bizarre of these early brachaucheniines is Luskhan itilensis, from the Hauterivian of Russia. We describe the osteology of this tusked, longirostrine pliosaurid and discuss its possible behaviour by drawing comparisons with other marine amniotes possessing forward-pointing teeth. We take this opportunity to make extensive anatomical comparisons among Cretaceous pliosaurids, including previously overlooked cranial features. Bayesian inference of phylogenetic relationships of plesiosaurians reveals that the internal branches in Late Jurassic–Late Cretaceous pliosaurids have generally low rates of morphological evolution, indicating that the recently described Early Cretaceous pliosaurids have effectively bisected the long branch leading to the ‘classical’ brachaucheniines of the middle Cretaceous (Brachauchenius, Kronosaurus and Megacephalosaurus). Pliosaurids exhibit low evolutionary rates and a dwindling disparity before their extinction, mirroring the events seen, roughly at the same time, for ichthyosaurians.

Finite element analysis (FEA) is a computational method used to predict the behaviour (stresses, strains and deformation) of a structure under predefined loading conditions. It can be applied to different biological structures, such as bone, to study defined muscle‐driven scenarios. However, as muscle is an extremely complex structure to model, evolutionary biologists usually model muscle forces indirectly. In 2007, the BONELOAD MATLAB routine was developed to distribute muscle forces on a surface defined by the user. This routine then had to be coupled with a pre‐existing FEA software (e.g. Strand7) to perform the analyses and has been widely used ever since. In this manuscript, we present a new method to run muscle‐driven finite element simulations on a bone by distributing muscle forces on their insertions area, all within a single environment. We apply this protocol in three different situations: two biting simulations (unilateral and bilateral) and a shoulder flexion simulation. We demonstrate how to prepare the mesh, delineate the muscle origins and insertions, define the constraints, adjust material properties, choose a loading scenario (uniform, tangential or tangential‐plus‐normal), and extract the results. Our automated script meshes the 3D model, defines the constraints and distributes muscle forces within a single simulation software: ‘Metafor’ (nonlinear solver, owned and distributed by Gesval S.A) or ‘Fossils’ (a new open‐source linear static solver developed in the frame of this work). ‘Metafor’ and ‘Fossils’ can perform the entire protocol (from the meshing to the muscle‐induced simulations) on high‐resolution volumetric meshes (millions of tetrahedra) and rapidly, exceeding the processing time of other widely used software protocols by up to four times. We demonstrate that the results obtained from our protocol are highly congruent with brands such as Strand7. Thus, our protocol opens up the possibility to routinely and rapidly simulate the behaviour of high‐precision muscle‐driven FE models containing millions of tetrahedra.

Cat-like carnivorans are a textbook example of convergent evolution with distinct morphological differences between taxa with short or elongated upper canines, the latest being often interpreted as an adaptation to bite at large angles and subdue large prey. This interpretation of the sabretooth condition is reinforced by a reduced taxonomic sampling in some studies, often focusing on highly derived taxa or using simplified morphological models. Moreover, most biomechanical analyses focus on biting scenarios at small gapes, ideal for modern carnivora but ill-suited to test for subduction of large prey by sabre-toothed taxa. In this contribution we present the largest 3D collection-based muscle-induced biting simulations on cat like carnivorans by running a total of 1,074 analyses on 17 different taxa at three different biting angles (30°, 60° and 90°) including both morphologies. While our results show a clear adaptation of extreme sabre-toothed taxa to bite at larger angles in terms of stress distribution, other performance variables display surprising similarities between all forms at the different angles tested, highlighting a continuous rather than bipolar spectrum of hunting methods in cat-like carnivorans and demonstrating a wide functional disparity and nuances of the sabretooth condition that cannot simply be characterized by specialized feeding biomechanics.