The plesiosaur: remarkable morphology, histology, and physiology

Abstract

In this dissertation, contributions to understanding the morphology, histology, and physiology of plesiosaurs, a major group of marine reptiles from the Age of Dinosaurs, are presented. By using comparative methods and new investigations on recent and fossil taxa, new insights of general importance for amniotes (true land animals) are developed. Furthermore, the results demonstrate the potential of interdisciplinary research, combining conventional vertebrate paleontological approaches, human anatomy, developmental biology, and medical biomechanics. <br /> Plesiosaurs are one of the first described vertebrate fossils and raised many questions over the last 300 years in vertebrate paleontology. However, from the beginnings of paleontology as a science, it was also believed that plesiosaurs evolved in the Early Jurassic, and only few studies discussed a Triassic radiation. After an extraordinary discovery in a German clay pit near Warburg in North Rhine-Westphalia, the first Triassic plesiosaur was found and described in this thesis. The nearly complete and articulated skeleton of <em>Rhaeticosaurus mertensi</em> showed that already in the Triassic the plesiosaur body plan, unique histology, and physiology had evolved. This means that the evolution of plesiosaurs took place in the Triassic and furthermore, that plesiosaurs crossed the Triassic-Jurassic boundary, surviving the end-Triassic extinction event, unlike several other marine and terrestrial tetrapod groups. <br /> In general, plesiosaurs show two major body plans, the plesiosauromorph body plan and the pliosauromorph body plan. The main difference between the two is that plesiosauromorphs have a small head and a long neck (up to 76 cervical vertebrae) and the pliosauromorphs have a huge head and a short neck. However, both body plans evolved convergently in different lineages in Plesiosauria. Nevertheless, the basal condition is a small head and a relative long neck built up by a high number of cervical vertebrae compared to the ancestor. <br /> Plesiosaur cervical vertebrae, independently of taxon, show a special character: two large and symmetrical foramina piercing the ventral surface of the centra. These foramina are here renamed “intersegmental artery foramina” because they do not represent nutrient foramina, as previously believed, but the entry of intersegmental arteries. Retention of these arteries is a strongly paedomorphic character because intersegmental arteries are an embryonic feature that is normally resorbed in a very early ontogenetic stage during the process of resegmentation. μCt investigations and comparison to the development in modern amniotes showed that plesiosaurs did not resorb the arteries during the resegmentation process. The reason is not understood, but it is probably linked to the fact that the plesiosaur count of cervical vertebrae is the highest seen among all amniotes, suggesting that the processes involved in cervical vertebra development must have been faster than in any other amniote group. <br /> However, not only the intervertebral artery foramina raise intriguing questions but also the function and mobility of the long neck of plesiosauromorphs, which had been the subject of many studies. The plesiosaur neck, even in those taxa where it is longest relative to the body length, surprisingly is quite immobile. The neural spine of the cervical vertebrae is high, the zygapophyses are medially inclined, and the distance between the segments is very small. All of these features in the end must have resulted in an only slightly mobile neck. Hence, the mobility of the neck was tested in this thesis using an innovative approach from human anatomy, finite element modeling. The model indicates very limited mobility of only a few degrees in the intervertebral joint, allowing for the calculation of the total mobility of the neck of less than 180º. <br /> Nevertheless, the long, immobile neck must have been useful. Plesiosaurs, especially the plesiosauromorph forms with the small head and the long neck, were hunting in school of fish. Fish in general are very sensitive to hydrodynamic waves. The small head and the long neck caused a hydrodynamic and optical camouflage effect, preventing the fish or school of fish from recognizing the plesiosaur as a large predator. This would have been the case if the plesiosaur would have had a shorter neck but also a small head. The long neck in plesiosaurs thus represent, a special adaptation to prey acquisition in the aquatic environment. <br /> The studies on the plesiosaur neck led to the formulation of the hypothesis that the intervertebral joints in plesiosaurs and some other fossil reptiles had a proper intervertebral disc (IVD) in the dorsal vertebral column, similar to the IVD of mammals. To test this hypothesis, a comprehensive sample of amniote dorsal vertebrae, fossil and extant, was acquired. Particular emphasis was placed on articulated segments of vertebral columns from black shales. Morphology of the centrum, histology of the articular surface, and preserved soft tissues, such as different kinds of cartilage, confirm that the IVD is not only restricted to mammals. <br /> Ancestral character state reconstruction showed that the IVD evolved convergently at least twice in the phylogeny of amniotes, once in synapsids and once in diapsids, but possibly more often. Furthermore, the reptile synovial joint (of recent snakes and crocodiles) shows also convergent evolution, which had been mentioned briefly in different studies but not shown comprehensively. In general, we see here that the evolution and development of the amniote axial skeleton follows similar rules, independently of whether it is from a mammal or a reptile.