The Early Evolution of Rhynchosaurs

Abstract

The rhynchosaurian archosauromorphs are an important and diverse group of fossil tetrapods that first appeared during the Early Triassic and probably became extinct during the early Late Triassic (early Norian). Here, the early evolution of rhynchosaurs during the Early and early Middle Triassic (Induan-Anisian: 252.2-242 Mya) is reviewed based on new anatomical observations and their implications for the taxonomy, phylogenetic relationships and macroevolutionary history of the group. A quantitative phylogenetic analysis recovered a paraphyletic genus Rhynchosaurus, with "Rhynchosaurus" brodiei more closely related to hyperodapedontines than to Rhynchosaurus articeps. Therefore, a new genus is erected, resulting in the new combination Langeronyx brodiei. A body size analysis found two independent increases in size in the evolutionary history of rhynchosaurs, one among stenaulorhynchines and the other in the hyperodapedontine lineage. Maximum likelihood fitting of phenotypic evolution models to body size data found ambiguous results, with body size evolution potentially interpreted as fitting either a non-directional Brownian motion model or a stasis model. A Dispersal-ExtinctionCladogenesis analysis reconstructed the areas that are now South Africa and Europe as the ancestral areas of Rhynchosauria and Rhynchosauridae, respectively. The reconstruction of dispersal events between geographic areas that are broadly separated paleolatitudinally implies that barriers to the dispersal of rhynchosaurs from either side of the paleo-Equator during the Middle Triassic were either absent or permeable.

Supradapedon revisited: geological explorations in the Triassic of southern Tanzania

“…It stresses a point previously hinted by authors working with mid Triassic rhynchosaurs (Nesbitt & Whatley, 2004; Langer et al, 2010; Butler et al, 2015; Ezcurra, Montefeltro & Butler, 2015; Schultz, Langer & Montefeltro, 2016), the convergent acquisition of a jaw apparatus composed of two maxillary grooves and dentary blades, instead of a single loss of such arrangement along rhynchosaur evolution. The latter hypothesis was first advocated in a phylogenetic context by Benton (1983) and Benton (1990), suggesting that the plesiomorphic condition for Rhynchosauridae was a set of two mandibular blades and maxillary grooves.…”

“…The latter hypothesis was first advocated in a phylogenetic context by Benton (1983) and Benton (1990), suggesting that the plesiomorphic condition for Rhynchosauridae was a set of two mandibular blades and maxillary grooves. This relied on a different understanding of the dental anatomy of some English Middle Triassic forms (Ezcurra, Montefeltro & Butler, 2015), as well as on incomplete knowledge of rhynchosaur diversity (forms such as Ammorhynchus navajoi , Hyperodapedon huenei , Brasinorhynchus mariantensis , Teyumbaita sulcognathus , as well as better preserved specimens of Isalorhynchus genovefae , were unknown at the time). As such, the only known Late Triassic rhynchosaur at that time with two mandibular blades and maxillary grooves, T. sulcognathus , was considered “transitional” between the plesiomorphic Middle Triassic taxa and the other Late Triassic forms (Langer & Schultz, 2000b; Whatley, 2005; Mukherjee & Ray, 2014; Ezcurra, Montefeltro & Butler, 2015).…”

“…This relied on a different understanding of the dental anatomy of some English Middle Triassic forms (Ezcurra, Montefeltro & Butler, 2015), as well as on incomplete knowledge of rhynchosaur diversity (forms such as Ammorhynchus navajoi , Hyperodapedon huenei , Brasinorhynchus mariantensis , Teyumbaita sulcognathus , as well as better preserved specimens of Isalorhynchus genovefae , were unknown at the time). As such, the only known Late Triassic rhynchosaur at that time with two mandibular blades and maxillary grooves, T. sulcognathus , was considered “transitional” between the plesiomorphic Middle Triassic taxa and the other Late Triassic forms (Langer & Schultz, 2000b; Whatley, 2005; Mukherjee & Ray, 2014; Ezcurra, Montefeltro & Butler, 2015). Yet it became clear that various Middle Triassic rhynchosaurs either have an uncertain condition regarding this character ( Bentonyx sidensis , Rhynchosaurus articeps ; Langer et al, 2010; Butler et al, 2015; Ezcurra, Montefeltro & Butler, 2015) or actually bear jaws with a single blade/groove ( Ammorhynchus navajoi , Langeronyx brodiei ; Nesbitt & Whatley, 2004; Ezcurra, Montefeltro & Butler, 2015).…”

“…As such, the only known Late Triassic rhynchosaur at that time with two mandibular blades and maxillary grooves, T. sulcognathus , was considered “transitional” between the plesiomorphic Middle Triassic taxa and the other Late Triassic forms (Langer & Schultz, 2000b; Whatley, 2005; Mukherjee & Ray, 2014; Ezcurra, Montefeltro & Butler, 2015). Yet it became clear that various Middle Triassic rhynchosaurs either have an uncertain condition regarding this character ( Bentonyx sidensis , Rhynchosaurus articeps ; Langer et al, 2010; Butler et al, 2015; Ezcurra, Montefeltro & Butler, 2015) or actually bear jaws with a single blade/groove ( Ammorhynchus navajoi , Langeronyx brodiei ; Nesbitt & Whatley, 2004; Ezcurra, Montefeltro & Butler, 2015). Indeed, more parsimonious reconstructions now show the presence of a single blade/groove jaw apparatus as plesiomorphic for Rhynchosauridae, with the independent acquisition of the double blade/groove apparatus in core stenaulorhynchines (Schultz, Langer & Montefeltro, 2016) and some members of the Hyperodapedon -line such as Fodonyx spenceri and T. sulcognathus .…”

Peer Review #3 of "The braincase of Mesosuchus browni (Reptilia, Archosauromorpha) with information on the inner ear and description of a pneumatic sinus (v0.2)"

“…Derived taxa show unique adaptations to herbivory such as bony beaks formed by the premaxilla and tooth plates with complex medio-lateral replacement suitable for processing hard plant material (Benton, 1984). Rhynchosaurs originated in the Early Triassic, in the aftermath of the end-Permian mass extinction (Ezcurra, Montefeltro & Butler, 2016), and became important faunistic elements of Middle and Late Triassic ecosystems (Butler et al, 2015). Most of the rhynchosaur diversity suddenly disappeared at around the Carnian-Norian boundary, playing an important role in pointing an extinction event at about this time (Brusatte et al, 2010).…”