A sail-backed suchian from the Heshanggou Formation (Early Triassic: Olenekian) of China

Abstract

The two major lineages of extant archosaurs, crocodylians and avians, diverged in the Triassic, but the details and timing of this event are incompletely understood. Fragmentary and phylogenetically uninformative specimens, in addition to poor temporal constraints on rock units from the Early and Middle Triassic, typify obstacles in identifying early archosaurs. This paper re-describes the partial skeleton of the only known specimen of Xilousuchus sapingensis Wu, 1981 from the Early Triassic Heshanggou Formation in north-central China. Originally assigned to the non-archosaurian archosauriform clade Proterosuchidae, an extensive phylogenetic analysis posits X. sapingensis as a crown-group archosaur within Suchia, thus making this taxon the unequivocally oldest known member of Archosauria. The age and phylogenetic position of X. sapingensis indicate that many archosaurs, including all major clades of non-archosaurian archosauriforms, the avianline, ornithosuchids, aetosaurs and paracrocodylomorph lineages, must have diverged by the end of the Early Triassic. X. sapingensis is part of a possible clade of sail-backed poposauroids that were common components of archosaur assemblages during the Early to Middle Triassic.

The Origin and Early Radiation of Archosauriforms: Integrating the Skeletal and Footprint Record

“…The striking absence of crown archosaurs in the Early Triassic, with the exception of poposauroids, has led to the hypothesis that early archosaurs originated or, at least began to diversify, in areas that have a bias towards the non-preservation of body fossil remains or are not well sampled, such as the tropics [117,119]. The geographic palaeodistribution of archosauriform skeletal remains during the Early Triassic shows that most of them occur outside the tropical belt, with the sole exception of the three European taxa: Ctenosauriscus [117], Osmolskina , and Collilongus [137,160,166].…”

A new archosaur from the Manda beds (Anisian, Middle Triassic) of southern Tanzania and its implications for character state optimizations at Archosauria and Pseudosuchia

“…It has long been viewed as an example of an adaptive radiation because of the seemingly rapid diversification of forms in the Middle Triassic (Benton, 1983(Benton, , 2010Brusatte et al, 2008Brusatte et al, , 2010Nesbitt, 2011). More recently, this evolutionary event has been constrained temporally by the discovery of Early Triassic crown group archosaurs (Gower and Sennikov, 2000;Nesbitt et al, 2011;Butler et al, 2011), an extensively revised Triassic timescale (Muttoni et al, 2004;Furin et al, 2006;Mundil et al, 2010;Irmis et al, 2011), the discovery of new Middle Triassic taxa (e.g., Gower, 1999;Sen, 2005), and refined phylogenetic hypotheses (Benton, 1999;Irmis et al, 2007;Nesbitt et al, 2009aNesbitt et al, , 2009bNesbitt et al, , 2010Brusatte et al, 2010;Nesbitt, 2011). These advances confirm that the diversification of archosauriforms (particularly archosaurs) was rapid and likely to have occurred during the Early Triassic, raising the possibility that it was an integral part of the recovery from the Permo-Triassic mass extinction (Sidor et al, 2013).…”

“…The timing of divergence of Archosauria and Pseudosuchia is constrained by the Olenekian (late Early Triassic) appearance of Ctenosauriscidae Nesbitt et al, 2011), a clade that is well nested within Pseudosuchia, whereas the timing of divergence of avemetatarsalian groups is constrained by the late Anisian appearance of Silesauridae, a clade well nested within Avemetatarsalia (Nesbitt et al, 2010). Yet, the crownward placement of these early-occurring clades implies that much of the early fossil record of the archosaur radiation is almost completely missing.…”

Supplemental Material for 'Postcranial osteology of Azendohsaurus madagaskarensis (?Middle to Upper Triassic, Isalo Group, Madagascar) and its systematic position among stem archosaur reptiles. (Bulletin of the American Museum of Natural History, no. 398)'

“…The initial diversification of Archosauria during the Triassic has been the focus of voluminous research (e.g., Romer, 1956;Charig, 1980;Benton, 1983;Gauthier, 1986;Benton and Clark, 1988;Sereno, 1991;Parrish, 1993;Juul, 1994;Nesbitt, 2003Nesbitt et al, 2011;Brusatte et al, 2008;Butler et al, 2011) because this clade: (1) includes two important extant groups, crocodylians and birds; (2) is a classic example of an adaptive radiation; and (3) has recently been the focus of many groundbreaking macroevolutionary studies. Archosauria is just one component of a larger radiation of Archosauromorpha (stem archosaurs and the crown clade Archosauria) that arose either just prior to the end of the Permian or soon after the end-Permian extinction (Ezcurra et al, 2014).…”