The 30 cm (1 ft) long creature, known from fossils found in Kansas, United States, was one of the first diapsids, along with Petrolacosaurus.

Spinoaequalis was not fully aquatic, frequently returning to dry land.

Its name means "symmetrical spine" referring to its deep, laterally compressed tail.

Spinoaequalis has been found along with beautifully preserved marine fish, suggesting it occasionally left fresh water streams for the sea.

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