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Chaohusaurus
Temporal range: Lower Triassic ~248 mya
Chaohusaurus.png
Specimen AGM CHS-5
Scientific classification
Kingdom:
Animalia
Class:
Superorder:
Order:
Family:
Genus:
Chaohusaurus
Chaohusaurus BW
C. geishanensis restoration

Chaohusaurus is an extinct genus of basal ichthyopterygian, or ichthyosaur, from the Lower Triassic of Chaohu and Yuanan, China.

It was closer in time and form to Cymbospondylus and Mixosaurus than to more advanced genera like Ichthyosaurus. It did not have the dolphin-like form of later ichthyosaurs, but it was fully aquatic. The tail fin is wide-based and short. The neck is rather long, whereas later ichthyosaurs have no neck. There is no dorsal fin.

It was one of the smallest ichthyosaurs, about 70 to 180 cm long. Its weight was about 10 kg.

Reproduction

Maternal Chaohusaurus
Diagram from the PLoS paper, showing key features of the fossil Chaohusaurus
The maternal Chaohusaurus specimen with three embryos. Color coding shows: black, maternal vertebral column, including neural and haemal spines; blue, maternal pelvis and hind flipper; green, maternal ribs and gastralia. Embryos 1 and 2 are in orange and yellow, and neonate 1 is in red. Scale bar = 1 cm

A recent study of Chaohusaurus shows an early example of live birth. A remarkable feature was a baby close to emerging head-first. The authors comment:

"Its headfirst birth posture... strongly indicates a terrestrial origin of viviparity, in contrast to the traditional view. The tail-first birth posture in derived ichthyopterygians, convergent with the conditions in whales and sea cows, therefore is a secondary feature". They comment "obligate marine amniotes appear to have evolved almost exclusively from viviparous land ancestors".

In other words, ichthyosaurs gave live birth when they were still land dwellers. Then, later, they adapted to an aquatic life.

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