Nesbitt, S. J. and M. A. Norell. Extreme convergence in the body plans of an early suchian (Archosauria) and ornithomimid dinosaurs (Theropoda). Proceedings of the Royal Society of London B Biological Sciences

Columbia University, New York, New York, United States
Proceedings of the Royal Society B: Biological Sciences (Impact Factor: 5.05). 06/2006; 273(1590):1045-8. DOI: 10.1098/rspb.2005.3426
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Living archosaurs comprise birds (dinosaurs) and crocodylians (suchians). The morphological diversity of birds and stem group dinosaurs is tremendous and well-documented. Suchia, the archosaurian group including crocodylians, is generally considered more conservative. Here, we report a new Late Triassic suchian archosaur with unusual, highly specialized features that are convergent with ornithomimid dinosaurs. Several derived features of the skull and postcranial skeleton are identical to conditions in ornithomimids. Such cases of extreme convergence in multiple regions of the skeleton in two distantly related vertebrate taxa are rare. This suggests that these archosaurs show iterative patterns of morphological evolution. It also suggests that this group of suchians occupied the adaptive zone that was occupied by ornithomimosaurs later in the Mesozoic.

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Available from: Sterling Nesbitt, Feb 06, 2014
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    • "M. B. VON BACZKO & M. D. EZCURRA 10.1144/SP379.4 Geological Society, London, Special Publications published online January 24, 2013 as doi: 1994; Nesbitt & Norell 2006; Brusatte et al. 2010 "
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    ABSTRACT: The ornithosuchids were a group of archosaurs with body lengths ranging from 2 to 4 m recorded from Upper Triassic beds in Argentina and Scotland. The group was defined as a node-based clade including Ornithosuchus longidens, Riojasuchus tenuisceps, Venaticosuchus rusconii and all descendants of their most recent common ancestor. The ornithosuchids are diagnosed by the following apomorphies observed in the three known species of the clade: downturned premaxilla; premaxilla-maxilla contact with a diastema in the alveolar margin equal in length to two teeth; palatine-pterygoid fenestra; and orbit with a distinct ventral point surrounded by 'V'-shaped dorsal processes of the jugal. The most remarkable postcranial apomorphy of the group is the presence of the so-called crocodile reversed ankle joint, a condition that seems to be unique for the ornithosuchids among amniotans. The systematic history of Ornithosuchidae is complex and Ornithosuchus was allied with dinosaurs or phytosaurs prior to the implementation of numerical phylogenetic analyses. Currently, there is consensus that Ornithosuchidae is positioned within Pseudosuchia, but their phylogenetic position within the group remains strongly debated. Nevertheless, all hypotheses agree in inferring an extremely long ghost lineage at the base of the clade. The presence of derived pseudosuchians in the late Olenekian produces a ghost lineage of c. 16-18 millions of years for Ornithosuchidae, indicating that only the late evolutionary history of the clade is currently sampled in the fossil record.
    Geological Society London Special Publications 08/2013; 379(1):187-202. DOI:10.1144/SP379.4 · 2.58 Impact Factor
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    • "; Barrett et al . 2011 ) . Mace - lognathus vagans has a dorsoventrally flattened , spatulate and anteriorly edentulous mandibular symphysis as well as tooth crowns without serrated carinae ( Göhlich et al . 2005 ) . There exists intriguing evidence that the hun - ters occasionally became the hunted . Based on gross morphology and osteohistology , Nesbitt et al . ( 2006 ) demonstrated that bones preserved in the abdominal region of the neotype of the latest Trias - sic theropod Coelophysis bauri are not those of a juvenile conspecific ( as argued by Colbert 1989 ) but actually belong to an early crocodylomorph ."
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    ABSTRACT: Non-crocodyliform crocodylomorphs, often called 'sphenosuchians', were the earliest-diverging lineages of Crocodylomorpha, and document the stepwise acquisition of many of the features that characterize extant crocodylians. The first crocodylomorph fossils are approximately 230 million years old (upper Carnian, Late Triassic), and at least one of these early lineages persisted until at least 150 million years ago (Late Jurassic). These taxa occupied a wide variety of terrestrial environments from equatorial regions to high-paleolatitudes during the early Mesozoic. Despite a quarter-century of quantitative phylogenetic work, the interrelationships of early crocodylomorphs remain in a state of flux, though recent studies suggest that these lineages are paraphyletic with respect to Crocodyliformes, rather than forming a monophyletic early offshoot of Crocodylomorpha as some previously hypothesized. Nearly all early crocodylomorphs were upright quadrupedal small-bodied taxa, but lumping them all together as small cursorial faunivores masks ecological and morphological disparity in diet and limb functional morphology. With the accelerated pace of recent discovery of new specimens and taxa, future consensus on early crocodylomorph phylogeny will provide a solid framework for understanding their change in diversity and disparity through time, potential biogeographic patterns, and the morphological transformation leading to Crocodyliformes.
    Geological Society London Special Publications 08/2013; 379(1):275-302. DOI:10.1144/SP379.24 · 2.58 Impact Factor
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    • "The premaxilla is much larger than in other archosauriforms except for phytosaurs (Chatterjee, 1978; Hungerbühler , 2002) and the pterosauromorph Eudimorphodon. It is most similar to that of Qianosuchus, the proterochampsian Chanaresuchus, and the pseudosuchian Effigia (Nesbitt and Norell, 2006) in that the portion anterior to the external naris is much longer than the posteroventral process. Its anterodorsal (nasal) process is much larger than the posterodorsal (maxillary) process and extends posteriorly well beyond the external naris as in Eudimorphodon; this appears to be unique among the Pseudosuchia . "
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    ABSTRACT: A new Middle Triassic archosaur, Diandongosuchus fuyuanensis, gen. et sp. nov., is described on the basis of a skeleton from the Zhuganpo Member (Ladinian) of the Falang Formation, eastern Yunnan Province, China. It is primarily characterized by the nasal process of the premaxilla extending posteriorly well beyond the external naris, the super-sized coracoid foramen laterally bordered by the scapula, the ischium with a strongly expanded medial portion anteroposteriorly longer than the proximodistal height of the bone, and anteriorly notched cervical osteoderms. D. fuyuanensis is a pseudosuchian on the basis of the crocodile-normal tarsal joint and other features, such as the distal end of the ulna in posterolateral view squared off, osteoderms with a distinct anterior process, the presacral vertebrae dorsally covered by more than one osteoderm, dorsal osteoderm alignment dorsal to presacrals 10–24 staggered, the pubis-ischium contact reduced to a thin proximal contact, and the medial contact of the ischia extensive but the dorsal margins separate. It is from a marine deposit but shows few morphological adaptations of the postcranial skeleton for a semiaquatic way of life when compared with Qianosuchus from the Anisian limestone of the same area. A phylogenetic analysis derived from an existing data matrix suggests that the new archosaur occupies the basal-most position in Poposauroidea and further confirms the poposauroid status of Qianosuchus. On the basis of current information, the discovery of Diandongosuchus does not firmly underscore the affinity of the semiterrestrial vertebrate faunas between the eastern and western regions along the northern coastline of the Tethys.
    Journal of Vertebrate Paleontology 09/2012; 32(5):1064-1081. DOI:10.1080/02724634.2012.694383 · 1.98 Impact Factor
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