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A Short-Snouted, Middle Triassic Phytosaur and its Implications for the Morphological Evolution and Biogeography of Phytosauria

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ABSTRACT

Following the end-Permian extinction, terrestrial vertebrate diversity recovered by the Middle Triassic, and that diversity was now dominated by reptiles. However, those reptilian clades, including archosaurs and their closest relatives, are not commonly found until ~30 million years post-extinction in Late Triassic deposits despite time-calibrated phylogenetic analyses predicting an Early Triassic divergence for those clades. One of these groups from the Late Triassic, Phytosauria, is well known from a near-Pangean distribution, and this easily recognized clade bears an elongated rostrum with posteriorly retracted nares and numerous postcranial synapomorphies that are unique compared with all other contemporary reptiles. Here, we recognize the exquisitely preserved, nearly complete skeleton of Diandongosuchus fuyuanensis from the Middle Triassic of China as the oldest and basalmost phytosaur. The Middle Triassic age and lack of the characteristically-elongated rostrum fill a critical morphological and temporal gap in phytosaur evolution, indicating that the characteristic elongated rostrum of phytosaurs appeared subsequent to cranial and postcranial modifications associated with enhanced prey capture, predating that general trend of morphological evolution observed within Crocodyliformes. Additionally, Diandongosuchus supports that the clade was present across Pangea, suggesting early ecosystem exploration for Archosauriformes through nearshore environments and leading to ease of dispersal across the Tethys.

No MeSH data available.


Related in: MedlinePlus

Holotype specimen of Diandongosuchus fuyuanensis (ZMNH M8770), showing relevant cranial features shared with Phytosauria.Line drawings of the skull in dorsal (above) and ventral (below) views. (a) Anterior portion of rostrum in left lateral view, showing elongated premaxillae and interdigitating premaxilla-maxilla suture; (b) left supratemporal fenestra in dorsal view, showing narrow parietal-squamosal bar and fossa in dorsal surface of postorbital-squamosal bar; (c) region of left mandibular articulation in left lateral view, showing short retroarticular process well ventral to the distal end of the quadrate; (d) skull roof in dorsal view, showing frontal depressions and cranial ornamentation; (e) mandibles in right lateral view, showing splenials separated for their length but visible in lateral view along ventral margin of mandibular ramus; (f) last maxillary tooth in lateral view, showing spade-shaped morphology; (g) region of antorbital fenestra in left lateral view, showing extensive maxillary and lacrimal components to the antorbital fossa. Scale bar for line drawing = 5 cm; scale bars for all other images = 1 cm.
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f1: Holotype specimen of Diandongosuchus fuyuanensis (ZMNH M8770), showing relevant cranial features shared with Phytosauria.Line drawings of the skull in dorsal (above) and ventral (below) views. (a) Anterior portion of rostrum in left lateral view, showing elongated premaxillae and interdigitating premaxilla-maxilla suture; (b) left supratemporal fenestra in dorsal view, showing narrow parietal-squamosal bar and fossa in dorsal surface of postorbital-squamosal bar; (c) region of left mandibular articulation in left lateral view, showing short retroarticular process well ventral to the distal end of the quadrate; (d) skull roof in dorsal view, showing frontal depressions and cranial ornamentation; (e) mandibles in right lateral view, showing splenials separated for their length but visible in lateral view along ventral margin of mandibular ramus; (f) last maxillary tooth in lateral view, showing spade-shaped morphology; (g) region of antorbital fenestra in left lateral view, showing extensive maxillary and lacrimal components to the antorbital fossa. Scale bar for line drawing = 5 cm; scale bars for all other images = 1 cm.

Mentions: Using a holistic approach by examining the cranium and postcranium, we targeted the Middle Triassic (Ladinian2425) taxon Diandongosuchus fuyuanensis26 (Figs 1 and 2), recently described as the basalmost poposauroid and well nested within crocodylian-line archosaurs. This taxon lacks a number of suchian character states (e.g., a posteriorly-directed calcaneal tuber, long pelvic elements) that should be present if the taxon does represent a poposauroid. Instead, we detail a number of character states from the cranium and postcranium that are only present in phytosaurs and lead us to reinterpret this taxon not as a poposauroid, but as the sister taxon to all other known phytosaurs. This is a major step in linking the crania-heavy analyses of phytosaur ingroup relationships with postcrania-heavy analyses of archosauriform relationships, and, indeed, fills the temporal gap predicted by phylogenies of Archosauriformes and informs our knowledge of body size and morphological changes during this critical span of phytosaur evolution.


A Short-Snouted, Middle Triassic Phytosaur and its Implications for the Morphological Evolution and Biogeography of Phytosauria
Holotype specimen of Diandongosuchus fuyuanensis (ZMNH M8770), showing relevant cranial features shared with Phytosauria.Line drawings of the skull in dorsal (above) and ventral (below) views. (a) Anterior portion of rostrum in left lateral view, showing elongated premaxillae and interdigitating premaxilla-maxilla suture; (b) left supratemporal fenestra in dorsal view, showing narrow parietal-squamosal bar and fossa in dorsal surface of postorbital-squamosal bar; (c) region of left mandibular articulation in left lateral view, showing short retroarticular process well ventral to the distal end of the quadrate; (d) skull roof in dorsal view, showing frontal depressions and cranial ornamentation; (e) mandibles in right lateral view, showing splenials separated for their length but visible in lateral view along ventral margin of mandibular ramus; (f) last maxillary tooth in lateral view, showing spade-shaped morphology; (g) region of antorbital fenestra in left lateral view, showing extensive maxillary and lacrimal components to the antorbital fossa. Scale bar for line drawing = 5 cm; scale bars for all other images = 1 cm.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5385495&req=5

f1: Holotype specimen of Diandongosuchus fuyuanensis (ZMNH M8770), showing relevant cranial features shared with Phytosauria.Line drawings of the skull in dorsal (above) and ventral (below) views. (a) Anterior portion of rostrum in left lateral view, showing elongated premaxillae and interdigitating premaxilla-maxilla suture; (b) left supratemporal fenestra in dorsal view, showing narrow parietal-squamosal bar and fossa in dorsal surface of postorbital-squamosal bar; (c) region of left mandibular articulation in left lateral view, showing short retroarticular process well ventral to the distal end of the quadrate; (d) skull roof in dorsal view, showing frontal depressions and cranial ornamentation; (e) mandibles in right lateral view, showing splenials separated for their length but visible in lateral view along ventral margin of mandibular ramus; (f) last maxillary tooth in lateral view, showing spade-shaped morphology; (g) region of antorbital fenestra in left lateral view, showing extensive maxillary and lacrimal components to the antorbital fossa. Scale bar for line drawing = 5 cm; scale bars for all other images = 1 cm.
Mentions: Using a holistic approach by examining the cranium and postcranium, we targeted the Middle Triassic (Ladinian2425) taxon Diandongosuchus fuyuanensis26 (Figs 1 and 2), recently described as the basalmost poposauroid and well nested within crocodylian-line archosaurs. This taxon lacks a number of suchian character states (e.g., a posteriorly-directed calcaneal tuber, long pelvic elements) that should be present if the taxon does represent a poposauroid. Instead, we detail a number of character states from the cranium and postcranium that are only present in phytosaurs and lead us to reinterpret this taxon not as a poposauroid, but as the sister taxon to all other known phytosaurs. This is a major step in linking the crania-heavy analyses of phytosaur ingroup relationships with postcrania-heavy analyses of archosauriform relationships, and, indeed, fills the temporal gap predicted by phylogenies of Archosauriformes and informs our knowledge of body size and morphological changes during this critical span of phytosaur evolution.

View Article: PubMed Central - PubMed

ABSTRACT

Following the end-Permian extinction, terrestrial vertebrate diversity recovered by the Middle Triassic, and that diversity was now dominated by reptiles. However, those reptilian clades, including archosaurs and their closest relatives, are not commonly found until ~30 million years post-extinction in Late Triassic deposits despite time-calibrated phylogenetic analyses predicting an Early Triassic divergence for those clades. One of these groups from the Late Triassic, Phytosauria, is well known from a near-Pangean distribution, and this easily recognized clade bears an elongated rostrum with posteriorly retracted nares and numerous postcranial synapomorphies that are unique compared with all other contemporary reptiles. Here, we recognize the exquisitely preserved, nearly complete skeleton of Diandongosuchus fuyuanensis from the Middle Triassic of China as the oldest and basalmost phytosaur. The Middle Triassic age and lack of the characteristically-elongated rostrum fill a critical morphological and temporal gap in phytosaur evolution, indicating that the characteristic elongated rostrum of phytosaurs appeared subsequent to cranial and postcranial modifications associated with enhanced prey capture, predating that general trend of morphological evolution observed within Crocodyliformes. Additionally, Diandongosuchus supports that the clade was present across Pangea, suggesting early ecosystem exploration for Archosauriformes through nearshore environments and leading to ease of dispersal across the Tethys.

No MeSH data available.


Related in: MedlinePlus