An exceptional fossil skull from South America and the origins of the archosauriform radiation

Abstract

Birds, dinosaurs, crocodilians, pterosaurs and their close relatives form the highly diverse clade Archosauriformes. Archosauriforms have a deep evolutionary history, originating in the late Permian, prior to the end-Permian mass extinction, and radiating in the Triassic to dominate Mesozoic ecosystems. However, the origins of this clade and its extraordinarily successful body plan remain obscure. Here, we describe an exceptionally preserved fossil skull from the Lower Triassic of Brazil, representing a new species, Teyujagua paradoxa, transitional in morphology between archosauriforms and more primitive reptiles. This skull reveals for the first time the mosaic assembly of key features of the archosauriform skull, including the antorbital and mandibular fenestrae, serrated teeth, and closed lower temporal bar. Phylogenetic analysis recovers Teyujagua as the sister taxon to Archosauriformes, and is congruent with a two-phase model of early archosauriform evolution, in response to two mass extinctions occurring at the end of the Guadalupian and the Permian.

First direct archosauromorph remains from the Early-Middle Triassic transition of the Iberian Peninsula

“…By contrast, other archosauromorphs lack finely serrated crowns (e.g., tanystropheids, prolacertids, trilophosaurids, rhynchosaurs; Benton and Clark, 1988;Juul, 1994;Dilkes, 1998;Nesbitt et al, 2015;Ezcurra, 2016;Pinheiro et al, 2016) and, although azendohsaurids possess denticles in the crown, they are considerably larger and coarser (with a lower density; Flynn et al, 2010;Nesbitt et al, 2015). As a result, recent quantitative phylogenetic analyses found compressed and finely serrated crowns as synapomorphies of lesser inclusive clades within Crocopoda (Ezcurra, 2016;Pinheiro et al, 2016) and support the assignment of the three teeth from the late Early to Middle Triassic of Catalonia to this clade. The teeth described here also share the absence of mesial denticles in the crown.…”

“…Compressed crowns are present in several crocopodan archosauromorphs (sensu Ezcurra, 2016), namely azendohsaurids (e.g., Azendohsaurus madagaskarensis: UA 10603, 10604, 8-29-97-160), prolacertids (e.g., Prolacerta broomi: BP/1/2675), Teyujagua (Pinheiro et al, 2016), Tasmaniosaurus (Ezcurra, 2014) and archosauriforms (e.g., Proterosuchus fergusi: RC 846; Erythrosuchus afri- (Ezcurra, 2016;Pinheiro et al, 2016). By contrast, other archosauromorphs lack finely serrated crowns (e.g., tanystropheids, prolacertids, trilophosaurids, rhynchosaurs; Benton and Clark, 1988;Juul, 1994;Dilkes, 1998;Nesbitt et al, 2015;Ezcurra, 2016;Pinheiro et al, 2016) and, although azendohsaurids possess denticles in the crown, they are considerably larger and coarser (with a lower density; Flynn et al, 2010;Nesbitt et al, 2015). As a result, recent quantitative phylogenetic analyses found compressed and finely serrated crowns as synapomorphies of lesser inclusive clades within Crocopoda (Ezcurra, 2016;Pinheiro et al, 2016) and support the assignment of the three teeth from the late Early to Middle Triassic of Catalonia to this clade.…”

“…The teeth described here also share the absence of mesial denticles in the crown. The occurrence of apically restricted, incipient, or absent mesial denticles has been scored for proterosuchids (e.g., Proterosuchus fergusi, "Chasmatosaurus" yuani) (Ezcurra, 2016) and more recently Teyujagua (Pinheiro et al, 2016) among basal archosauromorphs, whereas Kalisuchus rewanensis, Sarmatosuchus otschevi, erythrosuchids, and the vast majority of Triassic eucrocopodan archosauriforms possess densely serrated mesial carinae (Ezcurra, 2016). However, the presence of crowns with a serrated distal margin and lacking serrations in the mesial one convergently occur in disparate archosauriform groups, such as in some proterochampsids (e.g., Chanaresuchus bonapartei: MCZ 4686) and early dinosauriforms (MDE, personal observation).…”

Should the terms 'basal taxon' and 'transitional taxon' be extinguished from cladistic studies with extinct organisms?

“…The terms 'basal taxon/form' (e.g., Martinez et al, 2011;Sues et al, 2011;Xu et al, 2014) and 'transitional taxon/form' (e.g., Daeschler et al, 2006;Longrich et al, 2012;Pinheiro et al, 2016) have been widely adopted in cladistic studies in vertebrate palaeontology. These terms are taken as informative and sometimes applied in an attempt to increase the putative impact of research when communicating the importance of specimens or taxa.…”

“…These terms are taken as informative and sometimes applied in an attempt to increase the putative impact of research when communicating the importance of specimens or taxa. For example, "Coniophis therefore represents a transitional snake, combining a snake-like body and a lizard-like head" (Longrich et al, 2012); "new discoveries of transitional fossils such as Tiktaalik make the distinction 2 between fish and the earliest tetrapods increasingly difficult to draw" (Daeschler et al, 2006); "Teyujagua paradoxa, transitional in morphology between archosauriforms and more primitive reptiles" (Pinheiro et al, 2016); "the result of analysis of that second matrix places Chongmingia as the most basal bird other than the iconic 'Urvogel' Archaeopteryx (Wang et al, 2016); "despite its basal position in early avian evolution, the advanced features of the pectoral girdle and the carpal trochlea of the carpometacarpus of Jeholornis indicate the capability of powerful flight" (Zhou and Zhang, 2002); represent only some of several uses of the terms in order to tentatively capture the phylogenetic context of particular taxa. As such, it can be seen that these are often highly attractive expressions to journals and for broader science communication activities.…”