Possible postcranial pneumaticity in the last common ancestor of birds andcrocodilians: evidence from Erythrosuchus and other Mesozoic archosaurs

Birds and crocodilians (extant archosaurs) have differing, distinctive morp hologies. Birds have respiratory airsacs with diverticula that pneumatize t he postcranial skeleton, a feature absent in crocodilians. Bony correlates of pneumatic sinuses are known in the vertebrae of some non-avian dinosaurs and in pterosaurs - taxa more closely related to birds than crocodilians. This and the apparent absence of pneumatic postcranial bones in fossil arch osaurs more closely related to crocodilians than to birds, has been interpr eted as evidence that postcranial pneumaticity is a derived character of bi rds and their nearest fossil relatives. The presence of apparent osteologic al correlates of postcranial pneumaticity is here reported in some non-crow n-group archosaurs, and some of the fossil taxa more closely related to cro codilians than to birds. This suggests that the last common ancestor of bir ds and crocodilians might have had a pneumatized postcranium, and that the absence of this feature in crocodilians might be derived.