14 research outputs found
An archaic crested plesiosaur in opal from the Lower Cretaceous high-latitude deposits of Australia
Umoonasaurus demoscyllus gen. et sp. nov. is a new small-bodied (approx. 2.5âm) pliosauroid plesiosaur from the Lower Cretaceous (AptianâAlbian) of southern Australia. It is represented by several partial skeletons (one with a near complete skull is the most complete opalized vertebrate fossil yet known), and is unique in having large crests on the skull midline and above the orbits. Umoonasaurus is surprisingly archaic despite its relatively late age (approx. 115âMyr ago)âbeing simultaneously the most basal (primitive) and last surviving rhomaleosaurid. Notably, it lacks the âpliosauromorphâ features (large head, short neck, gigantism) typically characterizing many more derived Jurassic rhomaleosaurids; thus, reinforcing the suspected convergent evolution of the âpliosauromorphâ hypercarnivore body plan. Umoonasaurus inhabited an Early Cretaceous high-latitude (approx. 70°âS) inland seaway subject to seasonally near-freezing climatic conditions. This extreme environment supported a diverse range of plesiosaur taxa, suggesting that these marine reptiles might have possessed adaptations (e.g. heightened metabolic levels) to cope with cold-water temperatures. Indeed, survival of ancient endemic lineages such as Umoonasaurus is a common phenomenon in Australian Cretaceous vertebrate assemblages and might have been facilitated by isolation in low-temperature high-latitude regions