Major evolutionary transition: sensory adaptations of Mesozoic thalattosuchians and their extant crocodylian relatives
Item statusRestricted Access
Embargo end date04/05/2023
Schwab, Julia A.
A prime example of a group that underwent a major evolutionary transition are thalattosuchian crocodylomorphs, extinct relatives of modern crocodiles. Known from the Early Jurassic to the Early Cretaceous (ca. 191–125 Ma), they evolved from terrestrial ancestors into the first radiation of marine crocodylomorphs. One thalattosuchians subgroup, teleosauroids, originated in semiaquatic environments with a gharial–like bodyplan, and are known from freshwater, brackish and coastal marine environments. However, the metriorhynchid subgroup radically modified their bauplan during their transition to an obligately pelagic lifestyle, including paddle–shaped limbs, a vertically orientated tail fluke, and a smooth body lacking bony osteoderms. Additionally modifications of their pelvis allowed them to give birth to live young. While these osteological changes are well understood, little is known about how their neurosensory systems adapted during this transition. Cranial sensory organs recognisable in fossils, such as the bony labyrinth of the inner ear and brain endocasts, are powerful ecological proxies that can be correlated with animal behaviour and lifestyles. Underpinning this thesis is a wealth of new data from computed tomography (CT) scanning of extinct and extant crocodylomorphs, which allows a better insight and understanding of cranial sensory systems and how their anatomy, morphology and physiology changed during a major evolutionary transition. This thesis consists of two parts, first how sensory systems changed and adapted during the transition of thalattosuchians into the open ocean (Chapters II, III and IV), and secondly on the ontogeny of the bony labyrinth and otoliths of modern crocodylians (Chapters V and VI) in relation to thalattosuchians. Chapter II, describes the braincase and endocranial anatomy of the Middle Jurassic metriorhynchid, ‘Metriorhynchus’ cf. ‘M.’ brachyrhynchus (NHMUK PV OR 32617). Three–dimensionally rendered neuroanatomical features, such as the bony labyrinth, brain endocast, sinuses and cranial nerves provide an insight into how thalattosuchians adapted their sensory systems to their new ocean environment. Neuroanatomical adaptations of the specimen include a unique brain flexure, enlarged cerebral hemispheres, rounded pituitary fossa, enlarged internal carotid arteries, hypertrophied venous sinus and pelagic labyrinth morphology. Some of these features evolved early in thalattosuchians and helped them adapt to their new ocean environment. Chapters III and IV focus on the ecomorphological changes of two endocranial systems, the bony endocast of the inner ear and of the brain. Both represent crucial sensory system that can give insights into the lifestyle and behaviour of ancient animals. Based on CT–scans and three–dimensional models, multivariate analysis were used to quantitatively describe morphological changes. Bony labyrinths shows three different morphologies in crocodylomorphs (terrestrial, semiaquatic and pelagic), with pelagic metriorhynchids having a dorsoventrally shorter and more compact labyrinth, and larger canal diameters. Similarly, metriorhynchids also modified their brain endocast, and developed an expanded cerebrum, rounded hindbrain, compressed medulla, larger/rounded pituitary fossa and most importantly a smaller optic lobe, compared to semiaquatic taxa. This is somewhat similar to changes observed in other marine reptiles, however those sensory adaptations only occurred after they already modified their bodyplan and were likely a response to their new ocean habitat and not leading the transition. Chapter V, focuses on ontogenetic differences in the vestibular system of modern crocodylians. Body size, skull dimensions, bite force and neurosensory systems change drastically during ontogeny. Based on CT–scans and three–dimensional geometric morphometrics of 30 crocodylian labyrinths the size and shape changes throughout ontogeny were investigated, across four stages, hatchling, juvenile, subadult, and adult. There are two major shifts happening in the crocodylian labyrinth. First, it changes in size during ontogeny with negative allometry in relation to skull size. Second, a morphological change occurs, with hatchlings having shorter semicircular canal radii, thicker canal diameters, and an overall dorsoventrally shorter labyrinth than those of more mature individuals. It can be proposed that those changes are likely driven by constraints imposed by skull growth, due to the morphological change of the braincase (e.g. verticalization of the basicranium) rather than changes in locomotion, diet, or behaviour. Chapter VI, describes the presence of ‘ear stones’ or otoliths in the vestibule of crocodylians. Those are biomineralised structures that are well known from fish, but have not been studied in detail in most tetrapods. This chapter describes for the first time in detail the otoliths of crocodylians. They are monocrystalline structures present in all three major crocodylian lineages (Alligatoridae, Crocodylidae, and Gavialidae) and grow with positive allometry in relation to skull size. This suggests that they play an important role in sensory detection (e.g. hearing, balance and behaviour) and need to be examined in further detail across all tetrapods. Overall the results of this thesis show that endocranial sensory systems can help answer key questions on major evolutionary transitions and played an important role for these unique ancient crocodylomorphs to adapt to their ocean realm.