Crocodyliforms have a much richer evolutionary history than represented by their extant
descendants, including several independent marine and terrestrial radiations during the Mesozoic.
However, heterogeneous sampling of their fossil record has obscured their macroevolutionary
dynamics, and obfuscated attempts to reconcile external drivers of these patterns. Here, we present
a comprehensive analysis of crocodyliform biodiversity through the Jurassic/Cretaceous (J/K)
transition using subsampling and phylogenetic approaches, and apply maximum likelihood methods
to fit models of extrinsic variables to assess what mediated these patterns. A combination of
fluctuations in sea level and episodic perturbations to the carbon and sulphur cycles was primarily
responsible for both a marine and non-marine crocodyliform biodiversity decline through the J/K
boundary, primarily documented in Europe. This was tracked by high extinction rates at the
boundary and suppressed origination rates throughout the Early Cretaceous. The diversification of
Eusuchia and Notosuchia likely emanated from the easing of ecological pressure resulting from the
biodiversity decline, which also culminated in the extinction of the marine thalattosuchians in the
late Early Cretaceous. Through application of rigorous techniques for estimating biodiversity, our
results demonstrate that it is possible to tease apart the complex array of controls on diversification
patterns in major archosaur clades.