The Crocodylomorpha at and between geological boundaries: the Baden-Powell approach to change?

Abstract

Although the Crocodylomorpha spans a time period of over 200 million years, none of its notable morphological transitions appear 'to coincide with major geological boundaries. The Crocodylomorpha originated before the extinction event at the end of the Carnian Stage (Late Triassic), concomitant with a shift of locomotor pattern from bipedal erect posture to quadrupedal erect posture. With the successive divergence of the Sphenosuchia and Protosuchia the majority of features of the extant Crocodylia became established before the end of the Triassic. The Mesoeucrocodylia may have arisen in the early part of the Early Jurassic. The adoption of a more sprawling posture in the Longirostrine Taxa of the Mesoeucrocodylia was associated with a change of way of life from terrestrial to aquatic, probably immediately after the extinction event at the end of the Pliensbachian Stage (Jurassic). The Eusuchia arose in the Early Cretaceous, being characterized mainly by a complete bony secondary palate and the procoely of all vertebrae. The three extant clades most probably all arose before the Cretaceous-Tertiary boundary. Thus, the patterns of cladogenesis and diversification within the Crocodylomorpha indicate that, at least as currently understood, these taxa were for the major environmental and biotic events that occurred during their history. Baden-Powell's motto of prepared (Maynard 1946), taken from his own initials, appears to be an adequate descriptor of the history of the Crocodylomorpha,