Palaeopathology Between Marine Reptiles And Non-Avian Dinosaurs

An introduction to palaeopathology

The study of ancient, traumatic injuries and diseases is known as Palaeopatholgy. Through examining the fossil record, palaeopathologists can gather information on the behaviour, lifestyle and general paleobiology of extinct animals, which can be used to explain similar physiological or morphological components of modern, extant animals. Palaeopathologists are usually restricted to examining the skeletal diseases, as soft tissue rarely survives the preservation process. However, as bone diseases are rare, palaeopathologists scarcely discover the cause of death when examining remains. The skeletal system that vertebrates possess is complex, making it prone to trauma and disease, and hence, the fossil record sometimes shows bones that possess unusual structural abnormalities that can often be described as pathologies.

Infections, degenerative disorders, tumours, fractures and osteopetrosis (marble bone disease), are all examples of osseous pathologies that been observed in both saurischian and ornithischian taxa. Fractures and amputations are the most common traumatic bone injuries. If the injury is not responsible for the direct death of the animal, then it is measured by it’s healing responses. This generally takes the form of a callus. Usually, the callus will surround the perimeter of an injured bone, forming a growth of non-mineralized connective tissue to stabilise the wound.

Saurischia vs Ornithischia: The Hips Don’t Lie

Saurischia, and their sister-clade, Ornithischia, split during the late Middle to early Late Triassic; confirmed through phylogenetic analysis, and have been recognized scientifically as two different clades for the last 130 years. Saurischia can be broken into two main groups, Theropoda and Sauropodomorpha, and Ornithischia consists of a multitude of herbivorous dinosaurs. Examples of Sauropods are: Diplodocus, Brachiosaurus, Argentinosaurus and Brontosaurus. Examples of Theropods are: Allosaurus, Velociraptor, Tyrannosaurus and Spinosaurus. Lastly, some examples of Ornithischians are armoured dinosaurs like horned dinosaurs like Triceratops, armoured dinosaurs like Stegosaurus and Anklyosaurus, Hadrosaurids like Parasaurolophus and Pachycephalosaurs like Stygimoloch.

The main difference between Saurischia and Ornithischia is the structure of their hips. Saurischia have been dubbed ‘lizard-hipped’, Ornithischia being dubbed ‘bird-hipped’. Whilst both groups have their ilium at the top of their hips and their ischium at the rear, Ornithischians have their pubis facing backwards whilst Saurischians have their pubis pointing towards the front. Ironically, Ornithischians were named as such because of their bird-like hips, even though they go extinct. Saurischians are the group that evolved to create modern birds, with their pubis slowing moving towards the rear over time; an example of convergent evolution

Ornithischia: Extinction to the Front and the Pubis to the Rear

Dinosauria had split into three main groups by the Late Triassic: Ornithischia, Sauropodomorpha and Theropoda. Ornithischia is renowned for the wide variety of display structures expressed throughout the different dinosaurs. Specific physiological alterations such as horns, frills, armour, crests and domes are all seen throughout the ornithischian group. Ornithischian dinosaurs had bipedal ancestors, later evolving into quadrupedal animals. Through limb-bone scaling, it was determined that throughout ornithischians, there was a large diversity of stances and gaits. However, this is likely due to the horns, dermal armour and other display structures which affected the centre of mass of the organism, resulting in differences in limb proportion and scaling. Armoured dinosaurs, like Anklyosaurus, had osteoderms; bone-like structures that formed in its skin’s dermis layer, providing protection from the claws and teeth possessed by the Theropods. Parasaurolophus, a duck billed dinosaur, had a modified nasal passage, hypothesized to be responsible for creating a low frequency sound to communicate with other members. Triceratops, one of the largest ornithischians (the other being Shantungosaurus), had metre long horns that could easily pierce through the predator’s rip cage.

In palaeobiology, the consensus is that function can be inferred from the structural elements of an organism’s remains.

Saurischia: King Gizzard and the Lizard-Hipped Dinosaurs

When examining the remains of Tyrannosaurus rex specimen FMNH PR2081, multiple legions were found, leading scientists to think that it was due to an infection of some kind. It was hypothesized that these Tyrannosaurids were commonly infected by a protozoan like Trichomonas gallinae, an avian disease. Large Theropods like Tyrannosaurus, Allosaurus and Mapusaurus, were usually found with traumatic injuries, likely due to the risk undertaken when preying on other dinosaurs. The prominence of this disease in non-avian dinosaurs suggests that it may have spread through intraspecific behavior such as consuming another infected individual. Furthermore, the severe level of lesions suggest that Tyrannosaurids may have died because of this disease or through starvation caused by the disease. Interestingly, the modern-day avian diseases, seems to evolve from a disease carried in non-avian dinosaurs, showing that it wasn’t just the large organisms evolving. A & D provide a lateral angle of view, B & E provide a medial view, C shows an interpretation of the reconstructed skull with red outlining a lateral view of the mandibular ramus.

When understanding Mesozoic ecology, sauropods are an important consideration. Sauropods were giant dinosaurs, weighing between 50 to 80 metric tons, spanning over 40 metres in body length and were a dominant megaherbivore for approximately 135 million years; most of the Jurassic and Cretaceous. It was found that the articular cartilage found in Sauropods was quite thick. This may indicate an adaptation to the large amounts of pressure that would be on the Sauropod’s bone joints, largely due to their gigantism.

Pathology of Mesozoic Marine Reptiles: Everything isn’t better under the sea

Avascular necrosis is a disease in which bones don’t receive enough blood and start to die, a pathology observed in Ichthyosaurus, Sauropterygians, Mosasaurs and even turtles, from the Middle Jurassic to late Cretaceous. This disease, being observed in marine reptiles, has been attributed to be because of decompression syndrome. Decompression syndrome induced avascular necrosis was found in more than 15% of Late Middle Jurassic to Cretaceous Ichthyosaurs.

Bone diseases are more common in Crocodilians and Varanids when compared to other lizards. This indicates that examining the families Crocodylae and Varanidae would be an ideal starting place for epidemiologic analysis.

There were few locomotory adaptations in ancestral Mosasaurs for aquatic habitats; but over time, the dinosaurs body optimised for effective aquatic locomotion due to selective pressure.