Ichthyosaurs: their discovery, evolution, and adaptation to aquatic lifestyles
Ryosuke Motani
UC Davis
From: 11:00 AM To: 12:00 PM
Thursday, Apr 24, 2014
Burns Lecture Hall, Keck Science Center
Ichthyosaurs are a group of Mesozoic marine reptiles that lived alongside with dinosaurs and plesiosaurs. They are most famous for being the first tetrapod group in which a fish-shaped body profile evolved, about 200 million years before the same occurred in whales. Probably more notably, the largest eyeballs and highest manual digital count among vertebrates also belong to this group. The first substantial ichthyosaur fossil was collected by Mary Anning, a charismatic fossil collector who was born in 1799 in Dorset, England. Since then, many fossils have been found from Europe and North America. More recently, a large proportion of new findings are being made in China, where many complete specimens of ichthyosaurs have been collected during the last 15 years, including the most “primitive” forms. It took at least a few evolutionary steps for ichthyosaurs to invade the large part of the ocean, starting from a terrestrial ancestor. First, they invaded the coastal water, where they probably adjusted their physiology and reproductive biology according to the surrounding aquatic environment. Second, they invaded the open sea, thanks to the cruising ability that they acquired later. Finally, they invaded the deep sea after tuning their senses and stamina. New fossils from China allow us to scrutinize the evolutionary timing of these three steps. The earliest Ichthyosaurs, which appeared about 248 million years ago, had an appearance of a lizard-shaped animal with flippers. These earliest forms were already adapted to basic aquatic locomotion but the degree of physiological adaptation is unknown. Their reproductive biology was of terrestrial style, according to a new fossil find of neonates being born. The same fossil suggests that they most likely inherited viviparity from their land ancestor, allowing them to become fully aquatic without having to lay eggs on land from the beginning. The first fish-shaped ichthyosaurs, which probably were capable of cruising, most likely appeared about 239 million years ago, less than 10 million years after the initial marine invasion. This timing is comparable to, or slightly shorter than, the duration that was taken by cetaceans to give rise to a fish-shaped body profile. Deep-diving probably did not evolve until about 166 million years ago. This late timing is probably because the deep-sea organisms were not sufficiently available until that time.
Seminar Registered by: Velda Yount


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