In a while it will be a crocodile: the early evolution of Crocodylomorpha in Europe

Crocodylomorpha, the crocodylian stem-lineage, is the only pseudosuchian clade that survived into the Jurassic. Its earliest members, the non-crocodyliform crocodylomorphs or ‘sphenosuchians’, were terrestrial and mostly small-bodied (<2 m long). A redescription of both known European ‘sphenosuchian’ taxa is provided, Terrestrisuchus gracilis from the Late Triassic (Norian-Rhaetian?) of southern Wales, and Saltoposuchus connectens from the Norian of southwestern Germany. Terrestrisuchus and Saltoposuchus can clearly be distinguished based on many character states, contrary to some previous hypotheses. A new phylogenetic analysis finds that both taxa form a clade of gracile, long-legged crocodylomorphs, identified as Saltoposuchidae, together with Litargosuchus leptorhynchus. Analysis of a µCT-scan provides a virtually complete threedimensional reconstruction of the Terrestrisuchus braincase. The quadrate only forms a small, unfused contact with the prootic, contrary to later crocodylomorphs in which the braincase is heavily fused to surrounding cranial elements. The posterior skull region is extensively pneumatised by, among others, large pre- and postcarotid recesses on the parabasisphenoid and a large pneumatic cavity within the articular of the mandible, revealing extensive braincase pneumatisation occurred early within Crocodylomorpha. Terrestrisuchus preserves an ossified basihyal and scleral ring, the latter representing the first occurrence among non-bird-line archosaurs. Based on phylogenetic flexible discriminant analysis (pFDA) of the relative dimensions of the sclerotic ring and orbit, Terrestrisuchus was likely active in a range of light levels. Histological long bone sections of both Terrestrisuchus and Saltoposuchus reveal highly vascularized fibrolamellar tissue, indicating sustained high growth rates and thus high resting metabolic rates and active lifestyles for saltoposuchids.