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Richard J Butler - One of the best experts on this subject based on the ideXlab platform.
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The anatomy of Teleocrater Rhadinus, an early avemetatarsalian from the lower portion of the Lifua Member of the Manda Beds (Middle Triassic)
2018Co-Authors: Sterling J. Nesbitt, Martín D. Ezcurra, Richard J Butler, Alan J. Charig, Paul M. BarrettAbstract:Bird-line Archosaurs (= Avemetatarsalia, the clade containing birds, dinosaurs, pterosaurs, and their kin) originated in the Triassic Period. However, the earliest evolution of this group is poorly documented because fossils are extremely rare and consist mostly of postcrania. Here, we document the osteology of Teleocrater rhadinus, an early avemetatarsalian from the lower portion of the Middle Triassic Lifua Member of the Manda Beds of the Ruhuhu Basin, southwestern Tanzania. Material of Teleocrater rhadinus includes the holotype partial skeleton comprising a single individual, including cervical, trunk, and caudal vertebrae, pectoral, pelvic, forelimb, and hind limb material, and referred specimens representing parts (skull elements, vertebrae, pectoral, pelvic, and limb elements) of at least three other individuals collected from a bonebed. Character states of the skull elements, vertebrae, girdles, and limbs indicate that Teleocrater rhadinus represents the first documented non-ornithodiran avemetatarsalian known from well-preserved, associated material. Furthermore, Teleocrater rhadinus forms part of a newly recognized clade, Aphanosauria, which also contains formerly enigmatic Archosaur taxa from across Pangea, including Dongusuchus efremovi from the Middle Triassic of Russia, Yarasuchus deccanensis from the Middle Triassic of India, and Spondylosoma absconditum from the ?Middle Triassic of Brazil. This new clade and other new discoveries from the Middle to Late Triassic elucidate the sequence of character acquisitions at the base of Avemetatarsalia and fill a crucial gap in the understanding of the anatomical transformations that enabled dinosaurs to flourish later in the Mesozoic. SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP Citation for this article: Nesbitt, S. J., R. J. Butler, M. D. Ezcurra, A. J. Charig, and P. M. Barrett. 2018. The anatomy of Teleocrater rhadinus, an early avemetatarsalian from the lower portion of the Lifua Member of the Manda Beds (Middle Triassic); pp. 142–177 in C. A. Sidor and S. J. Nesbitt (eds.), Vertebrate and Climatic Evolution in the Triassic Rift Basins of Tanzania and Zambia. Society of Vertebrate Paleontology Memoir 17. Journal of Vertebrate Paleontology 37(6, Supplement).
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Mandasuchus tanyauchen, gen. et sp. nov., a pseudosuchian Archosaur from the Manda Beds (?Middle Triassic) of Tanzania
Journal of Vertebrate Paleontology, 2017Co-Authors: Richard J Butler, Sterling J. Nesbitt, David J. Gower, Alan J. Charig, Paul M. BarrettAbstract:ABSTRACTThe diverse assemblage of extinct Archosaur species known from the Manda Beds of Tanzania has provided key insights into the timing and tempo of the early part of the Archosaur radiation during the Middle Triassic. Several Archosaur specimens were collected from the Manda Beds in 1933 by F. R. Parrington, and three of these were subsequently described and made the basis of a new genus, ‘Mandasuchus,’ in a 1956 doctoral dissertation. However, this important fossil material was never formally published, and >60 years later ‘Mandasuchus’ and ‘Mandasuchus tanyauchen’ remain nomina nuda, despite frequent references to them in the literature. Here, we provide a detailed description of this material, provide the first formal diagnosis of Mandasuchus tanyauchen, gen. et sp. nov., and assess its phylogenetic position. The holotype of M. tanyauchen includes a well-preserved partial postcranial skeleton and fragmentary cranial remains. Four referred specimens include two partial skeletons, consisting primary...
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The earliest bird-line Archosaurs and the assembly of the dinosaur body plan
Nature, 2017Co-Authors: Sterling J. Nesbitt, Martín D. Ezcurra, Michelle R. Stocker, Richard J Butler, Paul M. Barrett, Kenneth D. Angielczyk, Roger M. H. Smith, Christian A. Sidor, Grzegorz Niedźwiedzki, A. G. SennikovAbstract:The Archosaur species Teleocrater rhadinus, part of the new clade Aphanosauria, is an example of the earliest divergence of the avian stem lineage (Avemetatarsalia), the lineage that contains dinosaurs (including birds). The early history of the bird-line Archosaurs, a group including dinosaurs, birds and pterosaurs, but excluding crocodilians, is not well defined. This is due in part to a fragmentary fossil record, but the distinctive morphology of pterosaurs has also obscured their ancestry. Sterling Nesbitt and colleagues describe a new species, Teleocraterrhadinus, from the Middle Triassic of Tanzania, that represents the most primitive known member of the bird-line Archosaurs. Teleocrater provides the best guide so far to the ancestral bird-line condition. It was a lightly built, quadrupedal carnivore, so more like a crocodile than the small bipeds often depicted at this point in Archosaur evolution. These are long-awaited findings on Teleocrater, which was undergoing study by the late Alan Charig of the Natural History Museum in London, and remained unpublished on his death in 1997. The relationship between dinosaurs and other reptiles is well established1,2,3,4, but the sequence of acquisition of dinosaurian features has been obscured by the scarcity of fossils with transitional morphologies. The closest extinct relatives of dinosaurs either have highly derived morphologies5,6,7 or are known from poorly preserved8,9 or incomplete material10,11. Here we describe one of the stratigraphically lowest and phylogenetically earliest members of the avian stem lineage (Avemetatarsalia), Teleocrater rhadinus gen. et sp. nov., from the Middle Triassic epoch. The anatomy of T. rhadinus provides key information that unites several enigmatic taxa from across Pangaea into a previously unrecognized clade, Aphanosauria. This clade is the sister taxon of Ornithodira (pterosaurs and birds) and shortens the ghost lineage inferred at the base of Avemetatarsalia. We demonstrate that several anatomical features long thought to characterize Dinosauria and dinosauriforms evolved much earlier, soon after the bird–crocodylian split, and that the earliest avemetatarsalians retained the crocodylian-like ankle morphology and hindlimb proportions of stem Archosaurs and early pseudosuchians. Early avemetatarsalians were substantially more species-rich, widely geographically distributed and morphologically diverse than previously recognized. Moreover, several early dinosauromorphs that were previously used as models to understand dinosaur origins may represent specialized forms rather than the ancestral avemetatarsalian morphology.
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‘Proterosuchia’: the origin and early history of Archosauriformes
2016Co-Authors: Martín D. Ezcurra, Richard J Butler, David J. GowerAbstract:Abstract: The earliest history of Archosauriformes is mainly represented by members of Proter-osuchidae and Erythrosuchidae, which are known worldwide from latest Permian to Middle Trias-sic beds. These two groups were historically combined within ‘Proterosuchia’, with approximately 30 nominal species. Two morphotypes have been recognized among proterosuchians: proterosu-chids with a generally more sprawling gait and elongated and low skulls with an overhanging pre-maxilla, and the more heavily built erythrosuchids, with a probably less sprawling gait and large, presumably hypercarnivorous, skulls. The systematics of ‘Proterosuchia ’ was relatively chaotic throughout most of the twentieth century, but currently there exists consensus regarding the non-monophyly of proterosuchians and their phylogenetic position outside all other Archosauri-forms. In contrast, the delimitation and taxonomic content of Proterosuchidae and Erythrosuchidae remain unstable. Few studies of proterosuchian palaeobiology have been carried out. Current lines of evidence favour a predominantly terrestrial lifestyle for proterosuchians. Limb bone histology indicates rapid continuous growth rates in Proterosuchus and Erythrosuchus before reaching sexual maturity. A better knowledge of proterosuchian anatomy, systematics, evolution and ecology is important for advancing understanding of the origin and early radiation of Archosaur
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Unappreciated diversification of stem Archosaurs during the Middle Triassic predated the dominance of dinosaurs
BMC evolutionary biology, 2016Co-Authors: Christian Foth, Martín D. Ezcurra, Stephen L. Brusatte, Roland B. Sookias, Richard J ButlerAbstract:Background Archosauromorpha originated in the middle–late Permian, radiated during the Triassic, and gave rise to the crown group Archosauria, a highly successful clade of reptiles in terrestrial ecosystems over the last 250 million years. However, scientific attention has mainly focused on the diversification of Archosaurs, while their stem lineage (i.e. non-Archosaurian Archosauromorphs) has often been overlooked in discussions of the evolutionary success of Archosauria. Here, we analyse the cranial disparity of late Permian to Early Jurassic Archosauromorphs and make comparisons between non-Archosaurian Archosauromorphs and Archosaurs (including Pseudosuchia and Ornithodira) on the basis of two-dimensional geometric morphometrics.
David J. Gower - One of the best experts on this subject based on the ideXlab platform.
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Mandasuchus tanyauchen, gen. et sp. nov., a pseudosuchian Archosaur from the Manda Beds (?Middle Triassic) of Tanzania
Journal of Vertebrate Paleontology, 2017Co-Authors: Richard J Butler, Sterling J. Nesbitt, David J. Gower, Alan J. Charig, Paul M. BarrettAbstract:ABSTRACTThe diverse assemblage of extinct Archosaur species known from the Manda Beds of Tanzania has provided key insights into the timing and tempo of the early part of the Archosaur radiation during the Middle Triassic. Several Archosaur specimens were collected from the Manda Beds in 1933 by F. R. Parrington, and three of these were subsequently described and made the basis of a new genus, ‘Mandasuchus,’ in a 1956 doctoral dissertation. However, this important fossil material was never formally published, and >60 years later ‘Mandasuchus’ and ‘Mandasuchus tanyauchen’ remain nomina nuda, despite frequent references to them in the literature. Here, we provide a detailed description of this material, provide the first formal diagnosis of Mandasuchus tanyauchen, gen. et sp. nov., and assess its phylogenetic position. The holotype of M. tanyauchen includes a well-preserved partial postcranial skeleton and fragmentary cranial remains. Four referred specimens include two partial skeletons, consisting primary...
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‘Proterosuchia’: the origin and early history of Archosauriformes
2016Co-Authors: Martín D. Ezcurra, Richard J Butler, David J. GowerAbstract:Abstract: The earliest history of Archosauriformes is mainly represented by members of Proter-osuchidae and Erythrosuchidae, which are known worldwide from latest Permian to Middle Trias-sic beds. These two groups were historically combined within ‘Proterosuchia’, with approximately 30 nominal species. Two morphotypes have been recognized among proterosuchians: proterosu-chids with a generally more sprawling gait and elongated and low skulls with an overhanging pre-maxilla, and the more heavily built erythrosuchids, with a probably less sprawling gait and large, presumably hypercarnivorous, skulls. The systematics of ‘Proterosuchia ’ was relatively chaotic throughout most of the twentieth century, but currently there exists consensus regarding the non-monophyly of proterosuchians and their phylogenetic position outside all other Archosauri-forms. In contrast, the delimitation and taxonomic content of Proterosuchidae and Erythrosuchidae remain unstable. Few studies of proterosuchian palaeobiology have been carried out. Current lines of evidence favour a predominantly terrestrial lifestyle for proterosuchians. Limb bone histology indicates rapid continuous growth rates in Proterosuchus and Erythrosuchus before reaching sexual maturity. A better knowledge of proterosuchian anatomy, systematics, evolution and ecology is important for advancing understanding of the origin and early radiation of Archosaur
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Manda Beds (Middle Triassic) of Southwestern Tanzania
2016Co-Authors: Sterling J. Nesbitt, Richard J Butler, David J. GowerAbstract:Background: Archosauria and their closest relatives, the non-Archosaurian Archosauriforms, diversified in the Early and Middle Triassic, soon after the end-Permian extinction. This diversification is poorly documented in most Lower and Middle Triassic rock sequences because fossils of early groups of Archosauriforms are relatively rare compared to those of other amniotes. The early Middle Triassic (? late Anisian) Manda beds of southwestern Tanzania form an exception, with early Archosaur skeletons being relatively common and preserved as articulated or associated specimens. The Manda Archosaur assemblage is exceptionally diverse for the Middle Triassic. However, to date, no non-Archosaurian Archosauriforms have been reported from these rocks. Methodology/Principal Findings: Here, we name a new taxon, Asperoris mnyama gen. et sp. nov., from the Manda beds and thoroughly describe the only known specimen. The specimen consists of a well-preserved partial skull including tooth-bearing elements (premaxilla, maxilla), the nasal, partial skull roof, and several incomplete elements. All skull elements are covered in an autapomorphic highly rugose sculpturing. A unique combination of character states indicates that A. mnyama lies just outside Archosauria as a stem Archosaur within Archosauriformes, but more precise relationships of A. mnyama relative to other early Archosauriform clades (e.g., Erythrosuchidae) cannot be determined currently. Conclusions/Significance: Asperoris mnyama is the first confirmed non-Archosaurian Archosauriform from the Manda bed
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Reassessment of the Evidence for Postcranial Skeletal Pneumaticity in Triassic Archosaurs, and the Early Evolution of the Avian Respiratory System
2015Co-Authors: Richard J Butler, Paul M. Barrett, David J. GowerAbstract:Uniquely among extant vertebrates, birds possess complex respiratory systems characterised by the combination of small, rigid lungs, extensive pulmonary air sacs that possess diverticula that invade (pneumatise) the postcranial skeleton, unidirectional ventilation of the lungs, and efficient crosscurrent gas exchange. Crocodilians, the only other living Archosaurs, also possess unidirectional lung ventilation, but lack true air sacs and postcranial skeletal pneumaticity (PSP). PSP can be used to infer the presence of avian-like pulmonary air sacs in several extinct Archosaur clades (non-avian theropod dinosaurs, sauropod dinosaurs and pterosaurs). However, the evolution of respiratory systems in other Archosaurs, especially in the lineage leading to crocodilians, is poorly documented. Here, we use mCT-scanning to investigate the vertebral anatomy of Triassic Archosaur taxa, from both the avian and crocodilian lineages as well as non-Archosaurian diapsid outgroups. Our results confirm previous suggestions that unambiguous evidence of PSP (presence of internal pneumatic cavities linked to the exterior by foramina) is found only in bird-line (ornithodiran) Archosaurs. We propose that pulmonary air sacs were present in the common ancestor of Ornithodira and may have been subsequently lost or reduced in some members of the clade (notably in ornithischian dinosaurs). The development of these avian-like respiratory features might have been linked to inferred increases in activity levels among ornithodirans. By contrast, no crocodile-line Archosaur (pseudosuchian) exhibits evidence for unambiguous PSP, but many of these taxa possess the complex array of vertebra
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reassessment of the evidence for postcranial skeletal pneumaticity in triassic Archosaurs and the early evolution of the avian respiratory system
PLOS ONE, 2012Co-Authors: Richard J Butler, Paul M. Barrett, David J. GowerAbstract:Uniquely among extant vertebrates, birds possess complex respiratory systems characterised by the combination of small, rigid lungs, extensive pulmonary air sacs that possess diverticula that invade (pneumatise) the postcranial skeleton, unidirectional ventilation of the lungs, and efficient crosscurrent gas exchange. Crocodilians, the only other living Archosaurs, also possess unidirectional lung ventilation, but lack true air sacs and postcranial skeletal pneumaticity (PSP). PSP can be used to infer the presence of avian-like pulmonary air sacs in several extinct Archosaur clades (non-avian theropod dinosaurs, sauropod dinosaurs and pterosaurs). However, the evolution of respiratory systems in other Archosaurs, especially in the lineage leading to crocodilians, is poorly documented. Here, we use µCT-scanning to investigate the vertebral anatomy of Triassic Archosaur taxa, from both the avian and crocodilian lineages as well as non-Archosaurian diapsid outgroups. Our results confirm previous suggestions that unambiguous evidence of PSP (presence of internal pneumatic cavities linked to the exterior by foramina) is found only in bird-line (ornithodiran) Archosaurs. We propose that pulmonary air sacs were present in the common ancestor of Ornithodira and may have been subsequently lost or reduced in some members of the clade (notably in ornithischian dinosaurs). The development of these avian-like respiratory features might have been linked to inferred increases in activity levels among ornithodirans. By contrast, no crocodile-line Archosaur (pseudosuchian) exhibits evidence for unambiguous PSP, but many of these taxa possess the complex array of vertebral laminae and fossae that always accompany the presence of air sacs in ornithodirans. These laminae and fossae are likely homologous with those in ornithodirans, which suggests the need for further investigation of the hypothesis that a reduced, or non-invasive, system of pulmonary air sacs may be have been present in these taxa (and secondarily lost in extant crocodilians) and was potentially primitive for Archosauria as a whole.
Sterling J. Nesbitt - One of the best experts on this subject based on the ideXlab platform.
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osteology of the late triassic bipedal Archosaur poposaurus gracilis Archosauria pseudosuchia from western north america
Anatomical Record-advances in Integrative Anatomy and Evolutionary Biology, 2020Co-Authors: Emma R. Schachner, Randall B. Irmis, Adam K Huttenlocker, Kent Sanders, Robert L Cieri, Sterling J. NesbittAbstract:Poposaurus gracilis is a bipedal pseudosuchian Archosaur that has been poorly understood since the discovery of the holotype fragmentary partial postcranial skeleton in 1915. Poposaurus. gracilis is a member of Poposauroidea, an unusually morphologically divergent clade of pseudosuchians containing taxa that are bipedal, quadrupedal, toothed, edentulous, and some individuals with elongated thoracic neural spines (i.e., sails). In 2003, a well preserved, fully articulated, and nearly complete postcranial skeleton of P. gracilis was discovered with some fragmentary cranial elements from the Upper Triassic Chinle Formation of Grand Staircase-Escalante National Monument of southern Utah, USA. The aim of this work is to describe the osteology of this specimen in detail and compare P. gracilis to other closely related pseudosuchian Archosaurs. The open neurocentral sutures throughout the majority of the vertebral column, the small size of this individual, and the presence of seven evenly spaced cyclic growth marks in the histologically sectioned femur indicate that this specimen was a skeletally immature juvenile, or subadult when it died. The pes of P. gracilis contains multiple skeletal adaptations and osteological correlates for soft tissue structures that support a hypothesis of digitigrady for this taxon. When coupled with the numerous postcranial characters associated with cursoriality, and the many anatomical traits convergent with theropod dinosaurs, this animal likely occupied a similar ecological niche with contemporaneous theropods during the Late Triassic Period. Anat Rec, 303:874-917, 2020. © 2019 American Association for Anatomy.
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A Detailed description of Rugarhynchos sixmilensis, gen. et comb. nov. (Archosauriformes, Proterochampsia), and cranial convergence in snout elongation across stem and crown Archosaurs
Journal of Vertebrate Paleontology, 2019Co-Authors: Brenen M. Wynd, Sterling J. Nesbitt, Michelle R. Stocker, Andrew B. HeckertAbstract:The Triassic Period (252–201.5 Ma) records a great expansion of saurian diversity and disparity, particularly in skull morphology. Stem Archosaurs exhibit substantial cranial disparity, especially ...
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Integrating gross morphology and bone histology to assess skeletal maturity in early dinosauromorphs: new insights from Dromomeron (Archosauria: Dinosauromorpha).
PeerJ, 2019Co-Authors: Christopher T. Griffin, Lauren S. Bano, Alan H Turner, Nathan D. Smith, Randall B. Irmis, Sterling J. NesbittAbstract:: Understanding growth patterns is central to properly interpreting paleobiological signals in tetrapods, but assessing skeletal maturity in some extinct clades may be difficult when growth patterns are poorly constrained by a lack of ontogenetic series. To overcome this difficulty in assessing the maturity of extinct Archosaurian reptiles-crocodylians, birds and their extinct relatives-many studies employ bone histology to observe indicators of the developmental stage reached by a given individual. However, the relationship between gross morphological and histological indicators of maturity has not been examined in most Archosaurian groups. In this study, we examined the gross morphology of a hypothesized growth series of Dromomeron romeri femora (96.6-144.4 mm long), the first series of a non-dinosauriform dinosauromorph available for such a study. We also histologically sampled several individuals in this growth series. Previous studies reported that D. romeri lacks well-developed rugose muscle scars that appear during ontogeny in closely related dinosauromorph taxa, so integrating gross morphology and histological signal is needed to determine reliable maturity indicators for early bird-line Archosaurs. We found that, although there are small, linear scars indicating muscle attachment sites across the femur, the only rugose muscle scar that appears during ontogeny is the attachment of the M. caudofemoralis longus, and only in the largest-sampled individual. This individual is also the only femur with histological indicators that asymptotic size had been reached, although smaller individuals possess some signal of decreasing growth rates (e.g., decreasing vascular density). The overall femoral bone histology of D. romeri is similar to that of other early bird-line Archosaurs (e.g., woven-bone tissue, moderately to well-vascularized, longitudinal vascular canals). All these data indicate that the lack of well-developed femoral scars is autapomorphic for this species, not simply an indication of skeletal immaturity. We found no evidence of the high intraspecific variation present in early dinosaurs and other dinosauriforms, but a limited sample size of other early bird-line Archosaur growth series make this tentative. The evolutionary history and phylogenetic signal of gross morphological features must be considered when assessing maturity in extinct Archosaurs and their close relatives, and in some groups corroboration with bone histology or with better-known morphological characters is necessary.
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The anatomy of Teleocrater Rhadinus, an early avemetatarsalian from the lower portion of the Lifua Member of the Manda Beds (Middle Triassic)
2018Co-Authors: Sterling J. Nesbitt, Martín D. Ezcurra, Richard J Butler, Alan J. Charig, Paul M. BarrettAbstract:Bird-line Archosaurs (= Avemetatarsalia, the clade containing birds, dinosaurs, pterosaurs, and their kin) originated in the Triassic Period. However, the earliest evolution of this group is poorly documented because fossils are extremely rare and consist mostly of postcrania. Here, we document the osteology of Teleocrater rhadinus, an early avemetatarsalian from the lower portion of the Middle Triassic Lifua Member of the Manda Beds of the Ruhuhu Basin, southwestern Tanzania. Material of Teleocrater rhadinus includes the holotype partial skeleton comprising a single individual, including cervical, trunk, and caudal vertebrae, pectoral, pelvic, forelimb, and hind limb material, and referred specimens representing parts (skull elements, vertebrae, pectoral, pelvic, and limb elements) of at least three other individuals collected from a bonebed. Character states of the skull elements, vertebrae, girdles, and limbs indicate that Teleocrater rhadinus represents the first documented non-ornithodiran avemetatarsalian known from well-preserved, associated material. Furthermore, Teleocrater rhadinus forms part of a newly recognized clade, Aphanosauria, which also contains formerly enigmatic Archosaur taxa from across Pangea, including Dongusuchus efremovi from the Middle Triassic of Russia, Yarasuchus deccanensis from the Middle Triassic of India, and Spondylosoma absconditum from the ?Middle Triassic of Brazil. This new clade and other new discoveries from the Middle to Late Triassic elucidate the sequence of character acquisitions at the base of Avemetatarsalia and fill a crucial gap in the understanding of the anatomical transformations that enabled dinosaurs to flourish later in the Mesozoic. SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP Citation for this article: Nesbitt, S. J., R. J. Butler, M. D. Ezcurra, A. J. Charig, and P. M. Barrett. 2018. The anatomy of Teleocrater rhadinus, an early avemetatarsalian from the lower portion of the Lifua Member of the Manda Beds (Middle Triassic); pp. 142–177 in C. A. Sidor and S. J. Nesbitt (eds.), Vertebrate and Climatic Evolution in the Triassic Rift Basins of Tanzania and Zambia. Society of Vertebrate Paleontology Memoir 17. Journal of Vertebrate Paleontology 37(6, Supplement).
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Mandasuchus tanyauchen, gen. et sp. nov., a pseudosuchian Archosaur from the Manda Beds (?Middle Triassic) of Tanzania
Journal of Vertebrate Paleontology, 2017Co-Authors: Richard J Butler, Sterling J. Nesbitt, David J. Gower, Alan J. Charig, Paul M. BarrettAbstract:ABSTRACTThe diverse assemblage of extinct Archosaur species known from the Manda Beds of Tanzania has provided key insights into the timing and tempo of the early part of the Archosaur radiation during the Middle Triassic. Several Archosaur specimens were collected from the Manda Beds in 1933 by F. R. Parrington, and three of these were subsequently described and made the basis of a new genus, ‘Mandasuchus,’ in a 1956 doctoral dissertation. However, this important fossil material was never formally published, and >60 years later ‘Mandasuchus’ and ‘Mandasuchus tanyauchen’ remain nomina nuda, despite frequent references to them in the literature. Here, we provide a detailed description of this material, provide the first formal diagnosis of Mandasuchus tanyauchen, gen. et sp. nov., and assess its phylogenetic position. The holotype of M. tanyauchen includes a well-preserved partial postcranial skeleton and fragmentary cranial remains. Four referred specimens include two partial skeletons, consisting primary...
Paul M. Barrett - One of the best experts on this subject based on the ideXlab platform.
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The anatomy of Teleocrater Rhadinus, an early avemetatarsalian from the lower portion of the Lifua Member of the Manda Beds (Middle Triassic)
2018Co-Authors: Sterling J. Nesbitt, Martín D. Ezcurra, Richard J Butler, Alan J. Charig, Paul M. BarrettAbstract:Bird-line Archosaurs (= Avemetatarsalia, the clade containing birds, dinosaurs, pterosaurs, and their kin) originated in the Triassic Period. However, the earliest evolution of this group is poorly documented because fossils are extremely rare and consist mostly of postcrania. Here, we document the osteology of Teleocrater rhadinus, an early avemetatarsalian from the lower portion of the Middle Triassic Lifua Member of the Manda Beds of the Ruhuhu Basin, southwestern Tanzania. Material of Teleocrater rhadinus includes the holotype partial skeleton comprising a single individual, including cervical, trunk, and caudal vertebrae, pectoral, pelvic, forelimb, and hind limb material, and referred specimens representing parts (skull elements, vertebrae, pectoral, pelvic, and limb elements) of at least three other individuals collected from a bonebed. Character states of the skull elements, vertebrae, girdles, and limbs indicate that Teleocrater rhadinus represents the first documented non-ornithodiran avemetatarsalian known from well-preserved, associated material. Furthermore, Teleocrater rhadinus forms part of a newly recognized clade, Aphanosauria, which also contains formerly enigmatic Archosaur taxa from across Pangea, including Dongusuchus efremovi from the Middle Triassic of Russia, Yarasuchus deccanensis from the Middle Triassic of India, and Spondylosoma absconditum from the ?Middle Triassic of Brazil. This new clade and other new discoveries from the Middle to Late Triassic elucidate the sequence of character acquisitions at the base of Avemetatarsalia and fill a crucial gap in the understanding of the anatomical transformations that enabled dinosaurs to flourish later in the Mesozoic. SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP Citation for this article: Nesbitt, S. J., R. J. Butler, M. D. Ezcurra, A. J. Charig, and P. M. Barrett. 2018. The anatomy of Teleocrater rhadinus, an early avemetatarsalian from the lower portion of the Lifua Member of the Manda Beds (Middle Triassic); pp. 142–177 in C. A. Sidor and S. J. Nesbitt (eds.), Vertebrate and Climatic Evolution in the Triassic Rift Basins of Tanzania and Zambia. Society of Vertebrate Paleontology Memoir 17. Journal of Vertebrate Paleontology 37(6, Supplement).
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Mandasuchus tanyauchen, gen. et sp. nov., a pseudosuchian Archosaur from the Manda Beds (?Middle Triassic) of Tanzania
Journal of Vertebrate Paleontology, 2017Co-Authors: Richard J Butler, Sterling J. Nesbitt, David J. Gower, Alan J. Charig, Paul M. BarrettAbstract:ABSTRACTThe diverse assemblage of extinct Archosaur species known from the Manda Beds of Tanzania has provided key insights into the timing and tempo of the early part of the Archosaur radiation during the Middle Triassic. Several Archosaur specimens were collected from the Manda Beds in 1933 by F. R. Parrington, and three of these were subsequently described and made the basis of a new genus, ‘Mandasuchus,’ in a 1956 doctoral dissertation. However, this important fossil material was never formally published, and >60 years later ‘Mandasuchus’ and ‘Mandasuchus tanyauchen’ remain nomina nuda, despite frequent references to them in the literature. Here, we provide a detailed description of this material, provide the first formal diagnosis of Mandasuchus tanyauchen, gen. et sp. nov., and assess its phylogenetic position. The holotype of M. tanyauchen includes a well-preserved partial postcranial skeleton and fragmentary cranial remains. Four referred specimens include two partial skeletons, consisting primary...
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The earliest bird-line Archosaurs and the assembly of the dinosaur body plan
Nature, 2017Co-Authors: Sterling J. Nesbitt, Martín D. Ezcurra, Michelle R. Stocker, Richard J Butler, Paul M. Barrett, Kenneth D. Angielczyk, Roger M. H. Smith, Christian A. Sidor, Grzegorz Niedźwiedzki, A. G. SennikovAbstract:The Archosaur species Teleocrater rhadinus, part of the new clade Aphanosauria, is an example of the earliest divergence of the avian stem lineage (Avemetatarsalia), the lineage that contains dinosaurs (including birds). The early history of the bird-line Archosaurs, a group including dinosaurs, birds and pterosaurs, but excluding crocodilians, is not well defined. This is due in part to a fragmentary fossil record, but the distinctive morphology of pterosaurs has also obscured their ancestry. Sterling Nesbitt and colleagues describe a new species, Teleocraterrhadinus, from the Middle Triassic of Tanzania, that represents the most primitive known member of the bird-line Archosaurs. Teleocrater provides the best guide so far to the ancestral bird-line condition. It was a lightly built, quadrupedal carnivore, so more like a crocodile than the small bipeds often depicted at this point in Archosaur evolution. These are long-awaited findings on Teleocrater, which was undergoing study by the late Alan Charig of the Natural History Museum in London, and remained unpublished on his death in 1997. The relationship between dinosaurs and other reptiles is well established1,2,3,4, but the sequence of acquisition of dinosaurian features has been obscured by the scarcity of fossils with transitional morphologies. The closest extinct relatives of dinosaurs either have highly derived morphologies5,6,7 or are known from poorly preserved8,9 or incomplete material10,11. Here we describe one of the stratigraphically lowest and phylogenetically earliest members of the avian stem lineage (Avemetatarsalia), Teleocrater rhadinus gen. et sp. nov., from the Middle Triassic epoch. The anatomy of T. rhadinus provides key information that unites several enigmatic taxa from across Pangaea into a previously unrecognized clade, Aphanosauria. This clade is the sister taxon of Ornithodira (pterosaurs and birds) and shortens the ghost lineage inferred at the base of Avemetatarsalia. We demonstrate that several anatomical features long thought to characterize Dinosauria and dinosauriforms evolved much earlier, soon after the bird–crocodylian split, and that the earliest avemetatarsalians retained the crocodylian-like ankle morphology and hindlimb proportions of stem Archosaurs and early pseudosuchians. Early avemetatarsalians were substantially more species-rich, widely geographically distributed and morphologically diverse than previously recognized. Moreover, several early dinosauromorphs that were previously used as models to understand dinosaur origins may represent specialized forms rather than the ancestral avemetatarsalian morphology.
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Reassessment of the Evidence for Postcranial Skeletal Pneumaticity in Triassic Archosaurs, and the Early Evolution of the Avian Respiratory System
2015Co-Authors: Richard J Butler, Paul M. Barrett, David J. GowerAbstract:Uniquely among extant vertebrates, birds possess complex respiratory systems characterised by the combination of small, rigid lungs, extensive pulmonary air sacs that possess diverticula that invade (pneumatise) the postcranial skeleton, unidirectional ventilation of the lungs, and efficient crosscurrent gas exchange. Crocodilians, the only other living Archosaurs, also possess unidirectional lung ventilation, but lack true air sacs and postcranial skeletal pneumaticity (PSP). PSP can be used to infer the presence of avian-like pulmonary air sacs in several extinct Archosaur clades (non-avian theropod dinosaurs, sauropod dinosaurs and pterosaurs). However, the evolution of respiratory systems in other Archosaurs, especially in the lineage leading to crocodilians, is poorly documented. Here, we use mCT-scanning to investigate the vertebral anatomy of Triassic Archosaur taxa, from both the avian and crocodilian lineages as well as non-Archosaurian diapsid outgroups. Our results confirm previous suggestions that unambiguous evidence of PSP (presence of internal pneumatic cavities linked to the exterior by foramina) is found only in bird-line (ornithodiran) Archosaurs. We propose that pulmonary air sacs were present in the common ancestor of Ornithodira and may have been subsequently lost or reduced in some members of the clade (notably in ornithischian dinosaurs). The development of these avian-like respiratory features might have been linked to inferred increases in activity levels among ornithodirans. By contrast, no crocodile-line Archosaur (pseudosuchian) exhibits evidence for unambiguous PSP, but many of these taxa possess the complex array of vertebra
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Comments and corrections on 3D modeling studies of locomotor muscle moment arms in Archosaurs.
PeerJ, 2015Co-Authors: Karl T. Bates, Emma R. Schachner, Susannah C. R. Maidment, Paul M. BarrettAbstract:In a number of recent studies we used computer modeling to investigate the evolution of muscle leverage (moment arms) and function in extant and extinct Archosaur lineages (crocodilians, dinosaurs including birds and pterosaurs). These studies sought to quantify the level of disparity and convergence in muscle moment arms during the evolution of bipedal and quadrupedal posture in various independent Archosaur lineages, and in doing so further our understanding of changes in anatomy, locomotion and ecology during the group’s >250 million year evolutionary history. Subsequent work by others has led us to re-evaluate our models, which revealed a methodological error that impacted on the results obtained from the abduction–adduction and long-axis rotation moment arms in our published studies. In this paper we present corrected abduction–adduction and long axis rotation moment arms for all our models, and evaluate the impact of this new data on the conclusions of our previous studies. We find that, in general, our newly corrected data differed only slightly from that previously published, with very few qualitative changes in muscle moments (e.g., muscles originally identified as abductors remained abductors). As a result the majority of our previous conclusions regarding the functional evolution of key muscles in these Archosaur groups are upheld.
Michael J. Benton - One of the best experts on this subject based on the ideXlab platform.
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A new crurotarsan Archosaur from the Late Triassic of South Wales
Journal of Vertebrate Paleontology, 2019Co-Authors: Erin L. Patrick, David I. Whiteside, Michael J. BentonAbstract:ABSTRACTWe report a third Archosaur from the Late Triassic cavity-fill sediments of Pant-y-ffynnon Quarry in South Wales, in addition to the basal crocodylomorph Terrestrisuchus and the basal sauro...
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Macroevolutionary patterns in the evolutionary radiation of Archosaurs (Tetrapoda
2016Co-Authors: Stephen L. Brusatte, Michael J. Benton, Marcello Ruta, Graeme T. Lloyd, Steve C. WangAbstract:The rise of Archosaurs during the Triassic and Early Jurassic has been treated as a classic example of an evolutionary radiation in the fossil record. We review published studies and provide new data on Archosaur lineage origination, diversity and lineage evolution, morphological disparity, rates of morphological character change, and faunal abundance during the Triassic-Early Jurassic. The fundamental Archosaur lineages originated early in the Triassic, in concert with the highest rates of character change. Disparity and diversity peaked later, during the Norian, but the most significant increase in disparity occurred before maximum diversity. Archosaurs were rare components of Early-Middle Triassic faunas, but were more abundant in the Late Triassic and preeminent globally by the Early Jurassic. The Archosaur radiation was a drawn-out event and major components such as diversity and abundance were discordant from each other. Crurotarsans (crocodile-line Archosaurs) were more disparate, diverse, and abundant than avemetatarsalians (bird-line Archosaurs, including dinosaurs) during the Late Triassic, but these roles were reversed in the Early Jurassic. There is no strong evidence that dinosaurs outcompeted or gradually eclipsed crurotarsans during the Late Triassic. Instead, crurotarsans diversity decreased precipitously by the end Triassic extinction, which helped usher in the age of dinosaurian dominance.
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Archosaur remains from the Otter Sandstone Formation (Middle Triassic, late Anisian) of Devon, southern UK
Proceedings of the Geologists' Association, 2011Co-Authors: Michael J. BentonAbstract:A new jaw from the Middle Triassic (Anisian) Otter Sandstone Formation of Devon confirms the existence of a derived Archosaur (avesuchian). Numerous isolated teeth and vertebrae had already suggested the presence of Archosaurs in the Otter Sandstone Formation, presumed predators on the fauna of temnospondyls, procolophonids, and rhynchosaurs, but the new fossil is the first to show some diagnostic characters. Other elements in the same block as the jaw, but not necessarily from the same animal, include a possible skull or pelvic bone, a slender long bone, a small tooth (perhaps prolacertiform), and two presumed Archosaur dermal scutes. An additional scute is present, as well as the probable distal end of a pubis, perhaps from a large poposauroid Archosaur like the Anisian-age Bromsgroveia or Arizonasaurus. The jaw and pubis represent animals of very different sizes, some 0.8 m and 3 m long in estimated body length respectively.
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Macroevolutionary patterns in the evolutionary radiation of Archosaurs (Tetrapoda: Diapsida)
Earth and Environmental Science Transactions of The Royal Society of Edinburgh, 2010Co-Authors: Stephen L. Brusatte, Michael J. Benton, Marcello Ruta, Graeme T. Lloyd, Steve C. WangAbstract:The rise of Archosaurs during the Triassic and Early Jurassic has been treated as a classic example of an evolutionary radiation in the fossil record. This paper reviews published studies and provides new data on Archosaur lineage origination, diversity and lineage evolution, morpho- logical disparity, rates of morphological character change, and faunal abundance during the Triassic-Early Jurassic. The fundamental Archosaur lineages originated early in the Triassic, in concert with the highest rates of character change. Disparity and diversity peaked later, during the Norian, but the most significant increase in disparity occurred before maximum diversity. Archo- saurs were rare components of Early-Middle Triassic faunas, but were more abundant in the Late Triassic and pre-eminent globally by the Early Jurassic. The Archosaur radiation was a drawn-out event and major components such as diversity and abundance were discordant from each other. Crurotarsans (crocodile-line Archosaurs) were more disparate, diverse, and abundant than avemeta- tarsalians (bird-line Archosaurs, including dinosaurs) during the Late Triassic, but these roles were reversed in the Early Jurassic. There is no strong evidence that dinosaurs outcompeted or gradually eclipsed crurotarsans during the Late Triassic. Instead, crurotarsan diversity decreased precipitously by the end-Triassic extinction, which helped usher in the age of dinosaurian dominance.
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Superiority, Competition, and Opportunism in the Evolutionary Radiation of Dinosaurs
Science, 2008Co-Authors: Stephen L. Brusatte, Michael J. Benton, Marcello Ruta, Graeme T. LloydAbstract:The rise and diversification of the dinosaurs in the Late Triassic, from 230 to 200 million years ago, is a classic example of an evolutionary radiation with supposed competitive replacement. A comparison of evolutionary rates and morphological disparity of basal dinosaurs and their chief "competitors," the crurotarsan Archosaurs, shows that dinosaurs exhibited lower disparity and an indistinguishable rate of character evolution. The radiation of Triassic Archosaurs as a whole is characterized by declining evolutionary rates and increasing disparity, suggesting a decoupling of character evolution from body plan variety. The results strongly suggest that historical contingency, rather than prolonged competition or general "superiority," was the primary factor in the rise of dinosaurs.
