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Smilosuchus gregorii is an extinct species of phytosaur. Phytosaurs look like crocodiles, but evolved independently. S. gregorii is an early relative of the archosaur clade. Archosauria is the group that contains birds, crocodiles, and the evolutionary line leading back to their common ancestor (1). S. gregorii is among the largest phytosaurs, with a skull length of approximately 1.5 m (2) and an estimated total length of 12 meters. It lived 221.5-205.6 million years ago in the Late Triassic and is found in the Chinle Formation of Arizona (3). S. gregorii was a semi-aquatic predator with a long snout, armored form, and sprawling posture similar to modern crocodiles.
Though S. gregorii has many physical and lifestyle similarities with modern crocodiles, their relationship is currently thought to be an example of convergent or parallel evolution rather than ancestry. That is, S. gregorii has many features similar to a modern crocodile because it filled a similar niche and evolved those characteristics independently, rather than because it is closely related to crocodiles. S. gregorii has been traditionally classified as an archosaur and a member of the Pseudosuchia clade ("false crocodiles"), but according to Nesbitt (4), S. gregorii evolved before the common ancestor of crocodiles and birds, and is better classified in Archosauriformes.