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New clade of enigmatic early archosaurs yields insights into early pseudosuchian phylogeny and the biogeography of the archosaur radiation.

Butler RJ, Sullivan C, Ezcurra MD, Liu J, Lecuona A, Sookias RB - BMC Evol. Biol. (2014)

Bottom Line: Our results indicate that these three previously enigmatic taxa form a well-supported clade of Middle Triassic archosaurs that we refer to as Gracilisuchidae.The recognition of Gracilisuchidae provides further support for a rapid phylogenetic diversification of crown archosaurs by the Middle Triassic.The disjunct distribution of the gracilisuchid clade in China and Argentina demonstrates that early archosaurs were distributed over much or all of Pangaea although they may have initially been relatively rare members of faunal assemblages.

View Article: PubMed Central - HTML - PubMed

Affiliation: School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. r.butler.1@bham.ac.uk.

ABSTRACT

Background: The origin and early radiation of archosaurs and closely related taxa (Archosauriformes) during the Triassic was a critical event in the evolutionary history of tetrapods. This radiation led to the dinosaur-dominated ecosystems of the Jurassic and Cretaceous, and the high present-day archosaur diversity that includes around 10,000 bird and crocodylian species. The timing and dynamics of this evolutionary radiation are currently obscured by the poorly constrained phylogenetic positions of several key early archosauriform taxa, including several species from the Middle Triassic of Argentina (Gracilisuchus stipanicicorum) and China (Turfanosuchus dabanensis, Yonghesuchus sangbiensis). These species act as unstable 'wildcards' in morphological phylogenetic analyses, reducing phylogenetic resolution.

Results: We present new anatomical data for the type specimens of G. stipanicicorum, T. dabanensis, and Y. sangbiensis, and carry out a new morphological phylogenetic analysis of early archosaur relationships. Our results indicate that these three previously enigmatic taxa form a well-supported clade of Middle Triassic archosaurs that we refer to as Gracilisuchidae. Gracilisuchidae is placed basally within Suchia, among the pseudosuchian (crocodile-line) archosaurs. The approximately contemporaneous and morphologically similar G. stipanicicorum and Y. sangbiensis may be sister taxa within Gracilisuchidae.

Conclusions: Our results provide increased resolution of the previously poorly constrained relationships of early archosaurs, with increased levels of phylogenetic support for several key early pseudosuchian clades. Moreover, they falsify previous hypotheses suggesting that T. dabanensis and Y. sangbiensis are not members of the archosaur crown group. The recognition of Gracilisuchidae provides further support for a rapid phylogenetic diversification of crown archosaurs by the Middle Triassic. The disjunct distribution of the gracilisuchid clade in China and Argentina demonstrates that early archosaurs were distributed over much or all of Pangaea although they may have initially been relatively rare members of faunal assemblages.

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Simplified version of the strict consensus of 90 most parsimonious trees. Figure shows known stratigraphic ranges for major clades and stratigraphic uncertainty for gracilisuchid taxa, which are all known from point occurences.
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Figure 5: Simplified version of the strict consensus of 90 most parsimonious trees. Figure shows known stratigraphic ranges for major clades and stratigraphic uncertainty for gracilisuchid taxa, which are all known from point occurences.

Mentions: The search recovered 90 most parsimonious trees of 1313 steps, with a consistency index (CI) of 0.3679, a retention index (RI) of 0.7706, and the best score hit in all the replications. The topology of the strict consensus tree (Figure 5; Additional file 4) is almost identical to that originally obtained by Nesbitt [2], differing only in that phylogenetic interrelationships among early suchians are completely resolved. Nesbitt [2] recovered a polytomy among Gracilisuchus stipanicicorum, Turfanosuchus dabanensis, Revueltosaurus callenderi + Aetosauria, and Ticinosuchus ferox + Paracrocodylomorpha. By contrast, in the present analysis the clade composed of R. callenderi + Aetosauria was recovered at the base of Suchia, and a novel clade formed by G. stipanicicorum, T. dabanensis and Y. sangbiensis was recovered as the sister-taxon of T. ferox + Paracrocodylomorpha. This novel clade is termed here Gracilisuchidae (see Systematic Palaeontology, below). The monophyly of Gracilisuchidae is supported by six unambiguous synapomorphies (see Diagnosis in the Systematic Palaeontology section, below). Within Gracilisuchidae, G. stipanicicorum was recovered as more closely related to Y. sangbiensis than to T. dabanensis on the basis of the following three synapomorphies: (1) postorbital-squamosal contact continues ventrally for much or most of the ventral length of the squamosal (character 66: 0 → 1); (2) jugal with a posterior termination posterior to the infratemporal fenestra (character 72: 0 → 1); (3) parabasisphenoid between basal tubera and basipterygoid processes significantly elongated, at least 1.5 times longer than wide (character 103: 0 → 1).


New clade of enigmatic early archosaurs yields insights into early pseudosuchian phylogeny and the biogeography of the archosaur radiation.

Butler RJ, Sullivan C, Ezcurra MD, Liu J, Lecuona A, Sookias RB - BMC Evol. Biol. (2014)

Simplified version of the strict consensus of 90 most parsimonious trees. Figure shows known stratigraphic ranges for major clades and stratigraphic uncertainty for gracilisuchid taxa, which are all known from point occurences.
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4061117&req=5

Figure 5: Simplified version of the strict consensus of 90 most parsimonious trees. Figure shows known stratigraphic ranges for major clades and stratigraphic uncertainty for gracilisuchid taxa, which are all known from point occurences.
Mentions: The search recovered 90 most parsimonious trees of 1313 steps, with a consistency index (CI) of 0.3679, a retention index (RI) of 0.7706, and the best score hit in all the replications. The topology of the strict consensus tree (Figure 5; Additional file 4) is almost identical to that originally obtained by Nesbitt [2], differing only in that phylogenetic interrelationships among early suchians are completely resolved. Nesbitt [2] recovered a polytomy among Gracilisuchus stipanicicorum, Turfanosuchus dabanensis, Revueltosaurus callenderi + Aetosauria, and Ticinosuchus ferox + Paracrocodylomorpha. By contrast, in the present analysis the clade composed of R. callenderi + Aetosauria was recovered at the base of Suchia, and a novel clade formed by G. stipanicicorum, T. dabanensis and Y. sangbiensis was recovered as the sister-taxon of T. ferox + Paracrocodylomorpha. This novel clade is termed here Gracilisuchidae (see Systematic Palaeontology, below). The monophyly of Gracilisuchidae is supported by six unambiguous synapomorphies (see Diagnosis in the Systematic Palaeontology section, below). Within Gracilisuchidae, G. stipanicicorum was recovered as more closely related to Y. sangbiensis than to T. dabanensis on the basis of the following three synapomorphies: (1) postorbital-squamosal contact continues ventrally for much or most of the ventral length of the squamosal (character 66: 0 → 1); (2) jugal with a posterior termination posterior to the infratemporal fenestra (character 72: 0 → 1); (3) parabasisphenoid between basal tubera and basipterygoid processes significantly elongated, at least 1.5 times longer than wide (character 103: 0 → 1).

Bottom Line: Our results indicate that these three previously enigmatic taxa form a well-supported clade of Middle Triassic archosaurs that we refer to as Gracilisuchidae.The recognition of Gracilisuchidae provides further support for a rapid phylogenetic diversification of crown archosaurs by the Middle Triassic.The disjunct distribution of the gracilisuchid clade in China and Argentina demonstrates that early archosaurs were distributed over much or all of Pangaea although they may have initially been relatively rare members of faunal assemblages.

View Article: PubMed Central - HTML - PubMed

Affiliation: School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. r.butler.1@bham.ac.uk.

ABSTRACT

Background: The origin and early radiation of archosaurs and closely related taxa (Archosauriformes) during the Triassic was a critical event in the evolutionary history of tetrapods. This radiation led to the dinosaur-dominated ecosystems of the Jurassic and Cretaceous, and the high present-day archosaur diversity that includes around 10,000 bird and crocodylian species. The timing and dynamics of this evolutionary radiation are currently obscured by the poorly constrained phylogenetic positions of several key early archosauriform taxa, including several species from the Middle Triassic of Argentina (Gracilisuchus stipanicicorum) and China (Turfanosuchus dabanensis, Yonghesuchus sangbiensis). These species act as unstable 'wildcards' in morphological phylogenetic analyses, reducing phylogenetic resolution.

Results: We present new anatomical data for the type specimens of G. stipanicicorum, T. dabanensis, and Y. sangbiensis, and carry out a new morphological phylogenetic analysis of early archosaur relationships. Our results indicate that these three previously enigmatic taxa form a well-supported clade of Middle Triassic archosaurs that we refer to as Gracilisuchidae. Gracilisuchidae is placed basally within Suchia, among the pseudosuchian (crocodile-line) archosaurs. The approximately contemporaneous and morphologically similar G. stipanicicorum and Y. sangbiensis may be sister taxa within Gracilisuchidae.

Conclusions: Our results provide increased resolution of the previously poorly constrained relationships of early archosaurs, with increased levels of phylogenetic support for several key early pseudosuchian clades. Moreover, they falsify previous hypotheses suggesting that T. dabanensis and Y. sangbiensis are not members of the archosaur crown group. The recognition of Gracilisuchidae provides further support for a rapid phylogenetic diversification of crown archosaurs by the Middle Triassic. The disjunct distribution of the gracilisuchid clade in China and Argentina demonstrates that early archosaurs were distributed over much or all of Pangaea although they may have initially been relatively rare members of faunal assemblages.

Show MeSH
Related in: MedlinePlus