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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.

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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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Anatomy of Gracilisuchus stipanicicorum Romer [8]. A. Skull in right lateral view (reversed). B. Skull in dorsal view. C. Close-up of the right premaxilla and anterior ends of right maxilla and nasal in lateral view (reversed). D. Close-up of the left antorbital fossa above the antorbital fenestra in lateral view. E. Posterodorsal process of the posterior end of the right maxilla in lateral view (reversed). F. Left infratemporal region in lateral view. G. Braincase and posterior end of the palate in ventral view. Abbreviations: afo, antorbital fossa; avna, anteroventral process of the nasal; bptp, basipterygoid process; exna, external naris; itf, infratemporal fenestra; jg, jugal; la, lacrimal; max, maxilla; na, nasal; orb, orbit; pbs, parabasisphenoid; pdpm, posterodorsal process of the posterior end of the maxilla; pdpp, posterodorsal process of the premaxilla; po, postorbital; pre, premaxilla; stf, supratemporal fenestra; sq, squamosal; vppo, ventral process of the postorbital; vsq, ventral process of the squamosal. A-F. MCZ 4117. G. cast of PULR 08.
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Figure 1: Anatomy of Gracilisuchus stipanicicorum Romer [8]. A. Skull in right lateral view (reversed). B. Skull in dorsal view. C. Close-up of the right premaxilla and anterior ends of right maxilla and nasal in lateral view (reversed). D. Close-up of the left antorbital fossa above the antorbital fenestra in lateral view. E. Posterodorsal process of the posterior end of the right maxilla in lateral view (reversed). F. Left infratemporal region in lateral view. G. Braincase and posterior end of the palate in ventral view. Abbreviations: afo, antorbital fossa; avna, anteroventral process of the nasal; bptp, basipterygoid process; exna, external naris; itf, infratemporal fenestra; jg, jugal; la, lacrimal; max, maxilla; na, nasal; orb, orbit; pbs, parabasisphenoid; pdpm, posterodorsal process of the posterior end of the maxilla; pdpp, posterodorsal process of the premaxilla; po, postorbital; pre, premaxilla; stf, supratemporal fenestra; sq, squamosal; vppo, ventral process of the postorbital; vsq, ventral process of the squamosal. A-F. MCZ 4117. G. cast of PULR 08.

Mentions: Gracilisuchus stipanicicorum is known from at least six specimens (PULR 08, holotype; MCZ 4116A, 4117, 4118; PVL 4597, 4612) from the Chañares Formation of La Rioja Province, Argentina (Figure 1; [8-10]). Between these six specimens, the majority of the cranial and postcranial osteology of G. stipanicicorum is known. The Chañares Formation is generally considered to be of Ladinian to early Carnian age [24,25]. Because the anatomy of G. stipanicicorum has recently been comprehensively revised [10], and is in the process of publication by one the authors (AL), we do not provide new detailed descriptive comments here. Nevertheless, we highlight some new observations on the anatomy of G. stipanicicorum below in drawing comparisons to Turfanosuchus dabanensis and Yonghesuchus sangbiensis.


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)

Anatomy of Gracilisuchus stipanicicorum Romer [8]. A. Skull in right lateral view (reversed). B. Skull in dorsal view. C. Close-up of the right premaxilla and anterior ends of right maxilla and nasal in lateral view (reversed). D. Close-up of the left antorbital fossa above the antorbital fenestra in lateral view. E. Posterodorsal process of the posterior end of the right maxilla in lateral view (reversed). F. Left infratemporal region in lateral view. G. Braincase and posterior end of the palate in ventral view. Abbreviations: afo, antorbital fossa; avna, anteroventral process of the nasal; bptp, basipterygoid process; exna, external naris; itf, infratemporal fenestra; jg, jugal; la, lacrimal; max, maxilla; na, nasal; orb, orbit; pbs, parabasisphenoid; pdpm, posterodorsal process of the posterior end of the maxilla; pdpp, posterodorsal process of the premaxilla; po, postorbital; pre, premaxilla; stf, supratemporal fenestra; sq, squamosal; vppo, ventral process of the postorbital; vsq, ventral process of the squamosal. A-F. MCZ 4117. G. cast of PULR 08.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 1: Anatomy of Gracilisuchus stipanicicorum Romer [8]. A. Skull in right lateral view (reversed). B. Skull in dorsal view. C. Close-up of the right premaxilla and anterior ends of right maxilla and nasal in lateral view (reversed). D. Close-up of the left antorbital fossa above the antorbital fenestra in lateral view. E. Posterodorsal process of the posterior end of the right maxilla in lateral view (reversed). F. Left infratemporal region in lateral view. G. Braincase and posterior end of the palate in ventral view. Abbreviations: afo, antorbital fossa; avna, anteroventral process of the nasal; bptp, basipterygoid process; exna, external naris; itf, infratemporal fenestra; jg, jugal; la, lacrimal; max, maxilla; na, nasal; orb, orbit; pbs, parabasisphenoid; pdpm, posterodorsal process of the posterior end of the maxilla; pdpp, posterodorsal process of the premaxilla; po, postorbital; pre, premaxilla; stf, supratemporal fenestra; sq, squamosal; vppo, ventral process of the postorbital; vsq, ventral process of the squamosal. A-F. MCZ 4117. G. cast of PULR 08.
Mentions: Gracilisuchus stipanicicorum is known from at least six specimens (PULR 08, holotype; MCZ 4116A, 4117, 4118; PVL 4597, 4612) from the Chañares Formation of La Rioja Province, Argentina (Figure 1; [8-10]). Between these six specimens, the majority of the cranial and postcranial osteology of G. stipanicicorum is known. The Chañares Formation is generally considered to be of Ladinian to early Carnian age [24,25]. Because the anatomy of G. stipanicicorum has recently been comprehensively revised [10], and is in the process of publication by one the authors (AL), we do not provide new detailed descriptive comments here. Nevertheless, we highlight some new observations on the anatomy of G. stipanicicorum below in drawing comparisons to Turfanosuchus dabanensis and Yonghesuchus sangbiensis.

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