Early evolution of archosaurs – SVP abstract 2016

Nesbitt et al. 2016 discuss the early evolution of bird-line archosaurs.

From the Nesbitt et al. abstract:
“Bird-line archosaurs (= Avemetatarsalia, the clade containing dinosaurs, pterosaurs, and their kin) (1) had their origin in the Triassic Period. However, that origin is poorly documented (2) as fossils from their early evolutionary history are extremely rare and  consist mostly of postcrania. (3) Here, we report the discovery of a new reptile (femoral length = 17 cm) (4) from the lower portion of the Middle Triassic Lifua Member (Manda beds) of the Ruhuhu Basin, southwestern Tanzania. Material referred to the new taxon includes a partial skeleton of a single individual including cervical, trunk, and caudal vertebrae, pectoral, pelvic, forelimb, and hind limb material (= ‘Teleocrater’ of A. Charig), and parts (skull elements, vertebrae, pectoral, pelvic, and limb elements) of a minimum of three individuals collected from a bonebed discovered in 2015 very close to Charig’s original partial skeleton. Character states of the limbs, vertebrae, and ilium indicate a close relationship with early dinosauromorphs including: elongated cervical vertebrae, an ilium with a slightly concave ischial peduncle and clear anterior crest, a weakly developed anterior trochanter of the femur, an anteriorly compressed fibula with long strap-like iliofibularis crest, and absence of osteoderms (5). Many character states suggest that the new reptile taxon falls outside of the pterosaur-dinosaur clade (= Ornithodira). (6) However, the distributions of some of these character states at the base of Archosauria are unclear and some character states of the new taxon suggest a more basal relationship outside Archosauria (e.g., absence of two medial tubera of the proximal femur). No matter the position within or outside Archosauria, the new Lifua taxon shares seemingly unique character states with the poorly known Dongusuchus from the Middle Triassic of Russia (known from femora) and Yarasuchus from the Middle Triassic of India (known from partial skeletons), rather than with other archosauriforms. As a result, these forms appear to represent a globally distributed clade of early diverging avemetatarsalians. (7) The larger body size of the Manda form and its potential phylogenetic position outside of pterosaurs and dinosauromorphs indicates that there was a size decrease at the origin of Ornithodira. (8) This new taxon, and other new discoveries from the Middle to Late Triassic, are elucidating the sequence of character acquisitions in Avemetatarsalia and fill a crucial gap in the evolutionary history that led to the flourishing of dinosaurs later in the Mesozoic”. (9)

Notes

  1. Avemetatarsalia is a parphyletic clade or a junior synonym of the clade Reptilia because pterosaurs do not nest with dinosaurs and archosaurs in the LRT. Evidently taxon exclusion bias keeps Nesbitt et al. from considering fenestrasaur, tritosaur lepidosaurs as pterosaur ancestors. Only crocs and dinos comprise the Archosauria in the LRT. Like Dr. Naish, this clade of paleontologists are not exploring all the possibilities offered by a large gamut analysis, but opting to stay with traditional untested taxon lists.
  2. In the LRT the origins of all reptilian clades are well-documented going all the way back to basal tetrapods.
  3. Bogus claim, probably based on the mistaken assumption that headless Lagerpeton is basal to dinos. The origin of dinos is documented here.
  4. Similar in size to Lewisuchus, a pro-dinosaur.
  5. Some ‘dinosauromorph traits’ are in contention based on the inclusion set and recovered taxa basal to dinos.
  6. The ‘Ornithodira‘ is an invalid clade, or at the least, a junior synonym for ‘Reptilia’ in the LRT.
  7. Yarasuchus nests with a variety of genera in a clade between Rauisuchidae and Archosauria in the LRT.
  8. Indeed there was a size decrease at the origin of pterosaurs and at the origin of dinosaurs, but they had separate origins.
  9. If related to Yarasuchus, the new taxon is indeed distantly related to the origin of archosaurs, which included Gracilisuchus and the PVL specimen attributed to Gracilisuchus, both derived from sisters to the much larger Decuriasuchus and Turfanosuchus. The purported ‘gap’ reported by Nesbitt et al. appears to be because they are looking at the wrong outgroup taxa due to taxon exclusion in their phylogenetic analysis.
References
Nesbitt SJ, Butler RJ, Barrett PM, Stocker MR, Sidor CA, Angielczyk KD, Ezcurra MD and Smith RM 2016. The early evolution of bird-line archosaurs: a possible new clade of globally distributed avemetatarsalians just outside the dinosaur-pterosaur split. Abstract from the 2016 meeting of the Society of Vertebrate Paleontology.

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