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SVP 17 Asilisaurus notes
Nesbitt et al. 2015 (svp abstract)
describe the poposaur, Asilisaurus, as highly convergent with dinosaurs, yet distinct.
True.
But there are still problems here. See the (*) asterisks.
From the abstract:
“Asilisaurus kongwe, from the Middle Triassic Manda beds of southwestern Tanzania, is one of the oldest known ornithodirans* and provides new insights into early dinosauriform evolution**. Originally represented by disarticulated bones and one semi articulated partial skeleton from a single locality, recent fieldwork has yielded six sites at approximately the same stratigraphic level, with specimens representing nearly the entire skeleton. A new, exquisitely preserved skeleton of a single individual includes much of the skull, pectoral girdle, forelimbs, pelvis, hind limbs, and tail, and confirms that all of the original, largely disarticulated material belongs to Asilisaurus. The skeleton of Asilisaurus shares several plesiomorphic character states with other dinosauriforms, including a well-developed metatarsal I with a distinct articulation surface for a phalanx, a calcaneum with a clear tuber, a closed acetabulum, and a short deltopectoral crest. Additionally, Asilisaurus shares several features with Silesaurus and other putative silesaurids (e.g., Sacisaurus), such as teeth ankylosed with the jaw, a distinctive scar on the proximal surface of the femur, a bony pointed beak at the rostral tip of the lower jaw, and a notch at the base of the femoral head. Classic ‘dinosaurian’ character states, such as the upper temporal musculature extending onto the frontal and epipophyses on the cervical vertebrae, must be considered plesiomorphies given the phylogenetic position of Asilisaurus. The nearly complete osteology of Asilisaurus allows re-evaluation of other problematic taxa; Agnosphitys*** and Lewisuchus share apomorphies with Asilisaurus and thus represent silesaurids with a carnivorous dentition****. Our comprehensive phylogenetic analysis demonstrates that silesaurids***** are monophyletic and fall just outside Dinosauria (which is diagnosed by very few character states). The most derived silesaurids (e.g., Silesaurus, Sacisaurus) share features with both ornithischian and theropod dinosaurs, illustrating the high degree of convergence among dinosauriforms in the Triassic. In addition, the results of our cladistic analysis imply at least three independent transformations towards quadrupedality and herbivory. The frequency of dietary (carnivorous vs. herbivorous) and locomotor (quadrupedal vs. bipedal) shifts within Dinosauriformes appears unique within Amniota.”
*a diphyletic clade with pterosaurs in the lepidosauromorpha and dinos in the archosauromorpha.
**actually a poposaur, just outside of the Archosauria/Dinosauria in the large reptile tree.
***no mystery, Agnosphitys is a theropod sister to Marasuchus. Lewisuchus is a basal archosaur, a little closer to dinos than to crocs in the large reptile tree.
****Nesbitt et al are confused by convergence, which is resolved by phylogenetic analysis.
*****or do they mean poposaurs? Silesaurus and Sacisaurus are poposaurs. Actually Gracilisuchus and the Crocodylomorpha ‘fall’ (= nest) just outside the Dinosauria with poposaurs the outgroup according to the large reptile tree.
References
Nesbitt SJ et al. 2015. The anatomy of Asilisaurus kongwe (Dinosauriformes: silesauridae) and closely related taxa provides new insights into the anatomical and chronological evolution of dinosauriforms. Journal of Vertebrate Paleontology abstracts.
Source: https://pterosaurheresies.wordpress.com/2015/11/19/svp-17-asilisaurus-notes/
