Showing posts with label poposauroids. Show all posts
Showing posts with label poposauroids. Show all posts
A New Ctenosauriscid from Eastern Europe, Bystrowisuchus flerovi May Represent the Oldest Known Crown-Group Archosaur
This paper describes a new ctenosauriscid archosaur from the Early Triassic of Europe. Bystrowisuchus flerovi is based upon a series of fragmentary cervical vertebrae and a partial right ilium. It differs from other ctenosauriscids in the presence of expanded spine tables on the apices of the neural spines, and by possessing generally shorter neural spines. Based on these characters Sennikov considers Bystrowisuchus to be transitional between rauisuchids and ctenosauriscids. Note that Sennikov prefers to use traditional Linnaean systematics rather than cladistics (and still recognizes Thecodontia as a taxonomic entity). A new family Lotosauridae is erected and considered distinct from other ctenosauriscids [Ctenosauriscidae]. The type locality is dated latest Olenekian, which means that Bystrowisuchus is probably older than Xilousuchus which is ambiguously from the latest Olenekian-early Anisian of China and therefore could represent the oldest record of a ctenosauriscid and crown-group archosaur.
Sennikov, A. G. 2012. The first ctenosauriscid (Reptilia: Archosauromorpha) from the Lower Triassic of Eastern Europe. Paleontological Journal [Paleontologicheskii Zhurnal] 46:499-511.
Abstract - A new Early Triassic thecodont from the Donskaya Luka locality is described. A new species and genus of Rauisuchidae, Bystrowisuchus flerovi gen. et sp. nov., the first East European and earliest known member of the family Ctenosauriscidae is established. The taxonomy and phylogeny of Rauisuchidae and their stratigraphical and geographical distribution in connection with new finds are discussed.
Disparity and Convergence in Bipedal Archosaur Locomotion
Bates, K. T., and E. R. Schachner. 2011. Disparity and convergence in bipedal archosaur locomotion. Journal of the Royal Society Interface (advance online publication) doi: 10.1098/rsif.2011.0687
Abstract - This study aims to investigate functional disparity in the locomotor apparatus of bipedal archosaurs. We use reconstructions of hindlimb myology of extant and extinct archosaurs to generate musculoskeletal biomechanical models to test hypothesized convergence between bipedal crocodile-line archosaurs and dinosaurs. Quantitative comparison of muscle leverage supports the inference that bipedal crocodile-line archosaurs and non-avian theropods had highly convergent hindlimb myology, suggesting similar muscular mechanics and neuromuscular control of locomotion. While these groups independently evolved similar musculoskeletal solutions to the challenges of parasagittally erect bipedalism, differences also clearly exist, particularly the distinct hip and crurotarsal ankle morphology characteristic of many pseudosuchian archosaurs. Furthermore, comparative analyses of muscle design in extant archosaurs reveal that muscular parameters such as size and architecture are more highly adapted or optimized for habitual locomotion than moment arms. The importance of these aspects of muscle design, which are not directly retrievable from fossils, warns against over-extrapolating the functional significance of anatomical convergences. Nevertheless, links identified between posture, muscle moments and neural control in archosaur locomotion suggest that functional interpretations of osteological changes in limb anatomy traditionally linked to postural evolution in Late Triassic archosaurs could be constrained through musculoskeletal modelling.
Redescription of the Sail-backed Poposauroid Ctenosauriscus from the Early Triassic of Germany
Available now at PLoSONE is a redescription of this important historic Triassic taxon which is the namesake of a clade of sail-backed poposauroid archosaurs that have come into recent prominence given the discovery of a well-preserved specimen of Arizonasaurus babbitti in 2002.
Butler, R. J., Brusatte, S. L., Reich, M., Nesbitt, S. J., Schoch, R. R., and J. J. Hornung. 2011. The sail-backed reptile Ctenosauriscus from the latest Early Triassic of Germany and the timing and biogeography of the early archosaur radiation. PLoS ONE 6(10): e25693. doi:10.1371/journal.pone.0025693
Background
Archosaurs (birds, crocodilians and their extinct relatives including dinosaurs) dominated Mesozoic continental ecosystems from the Late Triassic onwards, and still form a major component of modern ecosystems (>10,000 species). The earliest diverse archosaur faunal assemblages are known from the Middle Triassic (c. 244 Ma), implying that the archosaur radiation began in the Early Triassic (252.3–247.2 Ma). Understanding of this radiation is currently limited by the poor early fossil record of the group in terms of skeletal remains.
Methodology/Principal Findings
We redescribe the anatomy and stratigraphic position of the type specimen of Ctenosauriscus koeneni (Huene), a sail-backed reptile from the Early Triassic (late Olenekian) Solling Formation of northern Germany that potentially represents the oldest known archosaur. We critically discuss previous biomechanical work on the ‘sail’ of Ctenosauriscus, which is formed by a series of elongated neural spines. In addition, we describe Ctenosauriscus-like postcranial material from the earliest Middle Triassic (early Anisian) Röt Formation of Waldhaus, southwestern Germany. Finally, we review the spatial and temporal distribution of the earliest archosaur fossils and their implications for understanding the dynamics of the archosaur radiation.
Conclusions/Significance
Comprehensive numerical phylogenetic analyses demonstrate that both Ctenosauriscus and the Waldhaus taxon are members of a monophyletic grouping of poposauroid archosaurs, Ctenosauriscidae, characterised by greatly elongated neural spines in the posterior cervical to anterior caudal vertebrae. The earliest archosaurs, including Ctenosauriscus, appear in the body fossil record just prior to the Olenekian/Anisian boundary (c. 248 Ma), less than 5 million years after the Permian–Triassic mass extinction. These earliest archosaur assemblages are dominated by ctenosauriscids, which were broadly distributed across northern Pangea and which appear to have been the first global radiation of archosaurs.
China Fossil Shows Bird, Crocodile Family Trees Split Earlier than Thought
From the University of Washington:
A fossil unearthed in China in the 1970s of a creature that died about 247 million years ago, originally thought to be a distant relative of both birds and crocodiles, turns out to have come from the crocodile family tree after it had already split from the bird family tree, according to research led by a University of Washington paleontologist.
The only known specimen of Xilousuchus sapingensis has been reexamined and is now classified as an archosaur. Archosaurs, characterized by skulls with long, narrow snouts and teeth set in sockets, include dinosaurs as well as crocodiles and birds.
The new examination dates the X. sapingensis specimen to the early Triassic period, 247 million to 252 million years ago, said Sterling Nesbitt, a UW postdoctoral researcher in biology. That means the creature lived just a short geological time after the largest mass extinction in Earth's history, 252 million years ago at the end of the Permian period, when as much as 95 percent of marine life and 70 percent of land creatures perished. The evidence, he said, places X. sapingensis on the crocodile side of the archosaur family tree.
"We're marching closer and closer to the Permian-Triassic boundary with the origin of archosaurs," Nesbitt said. "And today the archosaurs are still the dominant land vertebrate, when you look at the diversity of birds."
The work could sharpen debate among paleontologists about whether archosaurs existed before the Permian period and survived the extinction event, or if only archosaur precursors were on the scene before the end of the Permian.
"Archosaurs might have survived the extinction or they might have been a product of the recovery from the extinction," Nesbitt said.
The research is published May 17 online in Earth and Environmental Science Transactions of the Royal Society of Edinburgh, a journal of Cambridge University in the United Kingdom.
Co-authors are Jun Liu of the American Museum of Natural History in New York and Chun Li of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, China. Nesbitt did most of his work on the project while a postdoctoral researcher at the University of Texas at Austin.
The X. sapingensis specimen – a skull and 10 vertebrae – was found in the Heshanggou Formation in northern China, an area with deposits that date from the early and mid-Triassic period, from 252 million to 230 million years ago, and further back, before the mass extinction.
The fossil was originally classified as an archosauriform, a "cousin" of archosaurs, rather than a true archosaur, but that was before the discovery of more complete early archosaur specimens from other parts of the Triassic period. The researchers examined bones from the specimen in detail, comparing them to those from the closest relatives of archosaurs, and discovered that X. sapingensis differed from virtually every archosauriform.
Among their findings was that bones at the tip of the jaw that bear the teeth likely were not downturned as much as originally thought when the specimen was first described in the 1980s. They also found that neural spines of the neck formed the forward part of a sail similar to that found on another ancient archosaur called Arizonasaurus, a very close relative of Xilousuchus found in Arizona.
The family trees of birds and crocodiles meet somewhere in the early Triassic and archosauriforms are the closest cousin to those archosaurs, Nesbitt said. But the new research places X. sapingensis firmly within the archosaur family tree, providing evidence that the early members of the crocodile and bird family trees evolved earlier than previously thought.
"This animal is closer to a crocodile, but it's not a crocodile. If you saw it today you wouldn't think it was a crocodile, especially not with a sail on its back," he said.
REFERENCE
Nesbitt, S. J., Liu, J., and C. Li. 2011. A sail-backed suchian from the Heshanggou Formation (Early Triassic: Olenekian) of China. Earth and Environmental Science Transactions of the Royal Society of Edinburgh 101 (for 2010):271-284.
A fossil unearthed in China in the 1970s of a creature that died about 247 million years ago, originally thought to be a distant relative of both birds and crocodiles, turns out to have come from the crocodile family tree after it had already split from the bird family tree, according to research led by a University of Washington paleontologist.
The only known specimen of Xilousuchus sapingensis has been reexamined and is now classified as an archosaur. Archosaurs, characterized by skulls with long, narrow snouts and teeth set in sockets, include dinosaurs as well as crocodiles and birds.
The new examination dates the X. sapingensis specimen to the early Triassic period, 247 million to 252 million years ago, said Sterling Nesbitt, a UW postdoctoral researcher in biology. That means the creature lived just a short geological time after the largest mass extinction in Earth's history, 252 million years ago at the end of the Permian period, when as much as 95 percent of marine life and 70 percent of land creatures perished. The evidence, he said, places X. sapingensis on the crocodile side of the archosaur family tree.
"We're marching closer and closer to the Permian-Triassic boundary with the origin of archosaurs," Nesbitt said. "And today the archosaurs are still the dominant land vertebrate, when you look at the diversity of birds."
The work could sharpen debate among paleontologists about whether archosaurs existed before the Permian period and survived the extinction event, or if only archosaur precursors were on the scene before the end of the Permian.
"Archosaurs might have survived the extinction or they might have been a product of the recovery from the extinction," Nesbitt said.
The research is published May 17 online in Earth and Environmental Science Transactions of the Royal Society of Edinburgh, a journal of Cambridge University in the United Kingdom.
Co-authors are Jun Liu of the American Museum of Natural History in New York and Chun Li of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, China. Nesbitt did most of his work on the project while a postdoctoral researcher at the University of Texas at Austin.
The X. sapingensis specimen – a skull and 10 vertebrae – was found in the Heshanggou Formation in northern China, an area with deposits that date from the early and mid-Triassic period, from 252 million to 230 million years ago, and further back, before the mass extinction.
The fossil was originally classified as an archosauriform, a "cousin" of archosaurs, rather than a true archosaur, but that was before the discovery of more complete early archosaur specimens from other parts of the Triassic period. The researchers examined bones from the specimen in detail, comparing them to those from the closest relatives of archosaurs, and discovered that X. sapingensis differed from virtually every archosauriform.
Among their findings was that bones at the tip of the jaw that bear the teeth likely were not downturned as much as originally thought when the specimen was first described in the 1980s. They also found that neural spines of the neck formed the forward part of a sail similar to that found on another ancient archosaur called Arizonasaurus, a very close relative of Xilousuchus found in Arizona.
The family trees of birds and crocodiles meet somewhere in the early Triassic and archosauriforms are the closest cousin to those archosaurs, Nesbitt said. But the new research places X. sapingensis firmly within the archosaur family tree, providing evidence that the early members of the crocodile and bird family trees evolved earlier than previously thought.
"This animal is closer to a crocodile, but it's not a crocodile. If you saw it today you wouldn't think it was a crocodile, especially not with a sail on its back," he said.
REFERENCE
Nesbitt, S. J., Liu, J., and C. Li. 2011. A sail-backed suchian from the Heshanggou Formation (Early Triassic: Olenekian) of China. Earth and Environmental Science Transactions of the Royal Society of Edinburgh 101 (for 2010):271-284.
A New Nearly Complete Articulated Skeleton of the Bipedal Pseudosuchian Poposaurus gracilis from the Upper Triassic of Utah
Back in 2003 I was fortunate to be part of a research team that discovered a mostly complete articulated skeleton of Poposaurus gracilis from the Chinle Formation of Utah. Uncovering such an amazing specimen is definitely one of the highlights of my career, especially the moment when we realized how amazingly complete and articulated the specimen was. Below is a photo of the articulated hind limb and pelvis at that moment during the excavation.
I've seen the specimen a couple of times at Yale in the process of preparation and the specimen is definitely a beauty. Furthermore is is extremely important because prior to this find P. gracilis was only known from the pelvis, hind limbs, and some vertebrae. Unfortunately, the skull of this new specimen was unrecoverable, but the rest provides much information including an articulated forelimb and the pes (foot). I should also add that we met a lot of good friends on this trip.
The first paper out on this specimen is not a detailed osteological study but rather introduces the specimen and provides a detailed determination of the proposed locomotor abilities of this taxon, concluding from numerous lines of evidence (phylogenetic, extant phylogenetic bracketing, morphological) that Poposaurus was an obligatory biped. Morphological support for bipedality in P. gracilis includes a fully erect posture, relatively short forelimbs, elongate pes, five sacral vertebrae, partially open acetabulum, and an elongate tail. Gauthier et al. also provide an in-depth argument that the development of bipedality in Archosauria represents a major evolutionary innovation in vertebrate history.
Finally this paper gives a preview of Sterling Nesbitt's upcoming (in press, out very very shortly) extremely detailed phylogenetic analysis of the Archosauria (from his PhD dissertation). Sterling's analysis provides some pretty surprising (and well-supported) placements of various taxa that will turn some heads and cause a good bit of discussion, especially regarding the revised definitions of some beloved (by some) clade names.
Although I am told that the Poposaurus paper is officially out, it is not up yet on either the Yale Peabody Museum website or the BioOne site, but should be soon. I'll post the link as soon as it does. In the meantime you can read more about the initial find here. I'm also looking forward to the complete osteological description of this amazng specimen.
Gauthier, J. A., Nesbitt, S. J., Schachner, E. R., Bever, G. S., and W. G. Joyce. 2011. The bipedal stem crocodilian Poposaurus gracilis: inferring function in fossils and innovation in archosaur locomotion. Bulletin of the Peabody Museum of Natural History 52:107-126.
Abstract - We introduce a spectacular new specimen of a Late Triassic stem crocodilian identified as Poposaurus gracilis. It is part of a poorly known group, Poposauroidea, that, because of its striking similarities with contemporaneous stem avians (“dinosaurs”), has long puzzled archosaur paleontologists. Observed vertebrate locomotor behaviors, together with exceptional preservation of distinctive anatomical clues in this fossil, enable us to examine locomotor evolution in light of new advances in phylogenetic relationships among Triassic archosaurs. Because this stem crocodilian is unambiguously an archosaur, a diapsid, a tetrapod and a choanate sarcopterygian, we can safely infer major components of its locomotor behavior. These inferences, together with form-function constraints, suggest that P. gracilis was a fleet-footed, obligately erect-postured, striding biped. That behavior seems to have been superimposed on the ancestral archosaur’s innovative locomotor repertoire, which includes the capacity to “high walk.” These novelties persist in a recognizable form in archosaurs for at least 245 million years and are widely distributed across Earth’s surface in diverse ecological settings. They thus qualify as evolutionary innovations regardless of significant differences in diversification rates among extant diapsid reptiles.
I've seen the specimen a couple of times at Yale in the process of preparation and the specimen is definitely a beauty. Furthermore is is extremely important because prior to this find P. gracilis was only known from the pelvis, hind limbs, and some vertebrae. Unfortunately, the skull of this new specimen was unrecoverable, but the rest provides much information including an articulated forelimb and the pes (foot). I should also add that we met a lot of good friends on this trip.
The first paper out on this specimen is not a detailed osteological study but rather introduces the specimen and provides a detailed determination of the proposed locomotor abilities of this taxon, concluding from numerous lines of evidence (phylogenetic, extant phylogenetic bracketing, morphological) that Poposaurus was an obligatory biped. Morphological support for bipedality in P. gracilis includes a fully erect posture, relatively short forelimbs, elongate pes, five sacral vertebrae, partially open acetabulum, and an elongate tail. Gauthier et al. also provide an in-depth argument that the development of bipedality in Archosauria represents a major evolutionary innovation in vertebrate history.
Finally this paper gives a preview of Sterling Nesbitt's upcoming (in press, out very very shortly) extremely detailed phylogenetic analysis of the Archosauria (from his PhD dissertation). Sterling's analysis provides some pretty surprising (and well-supported) placements of various taxa that will turn some heads and cause a good bit of discussion, especially regarding the revised definitions of some beloved (by some) clade names.
Although I am told that the Poposaurus paper is officially out, it is not up yet on either the Yale Peabody Museum website or the BioOne site, but should be soon. I'll post the link as soon as it does. In the meantime you can read more about the initial find here. I'm also looking forward to the complete osteological description of this amazng specimen.
Gauthier, J. A., Nesbitt, S. J., Schachner, E. R., Bever, G. S., and W. G. Joyce. 2011. The bipedal stem crocodilian Poposaurus gracilis: inferring function in fossils and innovation in archosaur locomotion. Bulletin of the Peabody Museum of Natural History 52:107-126.
Abstract - We introduce a spectacular new specimen of a Late Triassic stem crocodilian identified as Poposaurus gracilis. It is part of a poorly known group, Poposauroidea, that, because of its striking similarities with contemporaneous stem avians (“dinosaurs”), has long puzzled archosaur paleontologists. Observed vertebrate locomotor behaviors, together with exceptional preservation of distinctive anatomical clues in this fossil, enable us to examine locomotor evolution in light of new advances in phylogenetic relationships among Triassic archosaurs. Because this stem crocodilian is unambiguously an archosaur, a diapsid, a tetrapod and a choanate sarcopterygian, we can safely infer major components of its locomotor behavior. These inferences, together with form-function constraints, suggest that P. gracilis was a fleet-footed, obligately erect-postured, striding biped. That behavior seems to have been superimposed on the ancestral archosaur’s innovative locomotor repertoire, which includes the capacity to “high walk.” These novelties persist in a recognizable form in archosaurs for at least 245 million years and are widely distributed across Earth’s surface in diverse ecological settings. They thus qualify as evolutionary innovations regardless of significant differences in diversification rates among extant diapsid reptiles.
New From Jeff Martz - Poposaurus in Color; and the Question of Bipedality in Animals with a Crocodile-normal Ankle Type
Jeff Martz has recently colo(u)rized his Poposaurus gracilis reconstruction and here it is in its new glory.
When I see reconstructions of pseudosuchians in a bipedal stance I always reflect upon the question of whether or not it is functionally possible for an animal with a crocodile-normal ankle type to actually be bipedal, especially an obligate biped. I've perused the literature but have found very little to address this problem. I'm certain that Emma Schachner dealt with this in her recently completed PhD on the new equisitely preserved Poposaurus specimen from the Chinle Formation in Utah, but unfortunately I've not had a chance to borrow and read a copy. Her 2009 SVP abstract suggests that myological developments differ between bipedal pseudosuchians and bipedal dinosaurs, suggesting that these myological changes compensate for the alternate ankle structures.
When I see reconstructions of pseudosuchians in a bipedal stance I always reflect upon the question of whether or not it is functionally possible for an animal with a crocodile-normal ankle type to actually be bipedal, especially an obligate biped. I've perused the literature but have found very little to address this problem. I'm certain that Emma Schachner dealt with this in her recently completed PhD on the new equisitely preserved Poposaurus specimen from the Chinle Formation in Utah, but unfortunately I've not had a chance to borrow and read a copy. Her 2009 SVP abstract suggests that myological developments differ between bipedal pseudosuchians and bipedal dinosaurs, suggesting that these myological changes compensate for the alternate ankle structures.
Reptile Assemblage from the Middle Triassic Moenkopi Formation of New Mexico
A very well-written and detailed paper demonstrating the importance of apomophy based identifications when assigning scrappy material to taxa for accurate determination of faunal assemblages. Further demonstrates the abundance of archosaurs, including primitive poposauroids and a possible shuvosaurid, in the Early to Middle Triassic rocks of the American Southwest.
Schoch, R. R., Nesbitt, S. J., Mueller, J. M., Lucas, S. G., and J. A. Boy, J. A. 2009, The reptile assemblage from the Moenkopi Formation (Middle Triassic) of New Mexico. Neues Jahrbuch für Geologie und Paläontologie Abhandlungen, DOI: 10.1127/0077-7749/2009/0030; Stuttgart.
Abstract: Focused collecting and excavation in the Moenkopi Formation (Anton Chico Member)of north-central New Mexico yielded a large quantity of tetrapod bones. Most of the finds were collected from intraformational conglomerates, and consist of isolated bones or bone fragments. The most abundant large members of the assemblage, the archosaurs, include at least three taxa: (1) a primitive suchian or archosauriform, (2) a primitive poposauroid (Arizonasaurus babbitti), and (3) a taxon similar to shuvosaurids. Less abundant remains are tentatively referred to archosauromorphs with rhynchosaur affinities. An analysis of the tetrapod Lagerstätten reveals that primary deposits formed in lakes that were located on floodplains. In these lakes, autochthonous conchostracans, actinopterygians, coelacanths, and temnospondyls were evidently preserved. Fluvial reworking of lacustrine deposits resulted in a secondary deposition of bones, teeth, coprolites, and wood in channel-borne conglomerates. However, the large amount of elements from terrestrial tetrapods indicates that these conglomerates acquired bones from additional primary deposits (?channels, paleosols?) that are still unknown.
Schoch, R. R., Nesbitt, S. J., Mueller, J. M., Lucas, S. G., and J. A. Boy, J. A. 2009, The reptile assemblage from the Moenkopi Formation (Middle Triassic) of New Mexico. Neues Jahrbuch für Geologie und Paläontologie Abhandlungen, DOI: 10.1127/0077-7749/2009/0030; Stuttgart.
Abstract: Focused collecting and excavation in the Moenkopi Formation (Anton Chico Member)of north-central New Mexico yielded a large quantity of tetrapod bones. Most of the finds were collected from intraformational conglomerates, and consist of isolated bones or bone fragments. The most abundant large members of the assemblage, the archosaurs, include at least three taxa: (1) a primitive suchian or archosauriform, (2) a primitive poposauroid (Arizonasaurus babbitti), and (3) a taxon similar to shuvosaurids. Less abundant remains are tentatively referred to archosauromorphs with rhynchosaur affinities. An analysis of the tetrapod Lagerstätten reveals that primary deposits formed in lakes that were located on floodplains. In these lakes, autochthonous conchostracans, actinopterygians, coelacanths, and temnospondyls were evidently preserved. Fluvial reworking of lacustrine deposits resulted in a secondary deposition of bones, teeth, coprolites, and wood in channel-borne conglomerates. However, the large amount of elements from terrestrial tetrapods indicates that these conglomerates acquired bones from additional primary deposits (?channels, paleosols?) that are still unknown.
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