Patellos et al. 2019 brings us
news of the earliest archosaur in the lineage of birds (rather than crocs).
Okay. That’s already wrong. In the large reptile tree (LRT, 1592 taxa) only crocs and dinos make up the Archosauria. Nesbitt et al. does not understand that hypothesis of interrelationships due to taxon exclusion and poor scoring going back to Nesbitt 2011. The purported clade, ‘Avemetatarsalia’ (= Ornithodira) was invalidated by the LRT.
From their abstract:
“Understanding of the evolution of the earliest avemetatarsalian (bird-line) archosaurs and the morphology of the hypothetical common ancestor of Archosauria is hampered by a poor fossil record.”
The common ancestor of Archosauria has been identified in the LRT as the PVL 4597
specimen wrongly attributed to Gracilisuchus
. After that: Turfanosuchus
Figure 1. Skull of Turfanosuchus compared to Herrerasaurus, the basalmost dinosaur.
Patellos et al. 2019 continue:
“The earliest-diverging avemetatarsalians known, such as Teleocrater, are separated from the earliest diverging pseudosuchian (crocodylian-line) archosaurs, and the closest outgroups of Archosauria by a clear morphological gap.”
The LRT invalidates the traditional clade, ‘Pseudosuchia.’ Crocodylian-line archosaurs are Crocodylomorphs, distinct from bird-line archosaurs, dinosaurs. Remember, these authors consider the lepidosaurian pterosaurs to be closely related to dinosaurs, a theory with as much evidence as tail-dragging dinosaurs.
“Here we describe a potential early-diverging avemetatarsalian from the Middle Triassic (~ 230 Ma) “Basal Isalo II” beds of Madagascar, which appears to bridge these gaps. This new taxon is represented by a well-preserved partial skeleton including articulated cervical
vertebrae with articulated osteoderms; a scapulocoracoid; a partial femur; isolated trunk, sacral, and caudal vertebrae; and an ilium.”
“Noteworthy features of the neck region include: anteroposteriorly elongated vertebrae with laterally expanded dorsal ends of the neural spines, and an articulated set of osteoderms dorsal to the vertebrae. The cervical osteoderms, three pairs per vertebra, arranged in paramedian row, and bear tapering anterior processes.”
“Potential synapomorphies of this specimen with avemetatarsalians include: femur with an incipient anterior trochanter, 1st sacral vertebra with a dorsoventrally expanded sacral rib, and ilium possessing a notch on the articulation surface with the ischium. This combination of features places the new taxon represented by this specimen at the base of Avemetatarsalia, outside aphanosaurs + dinosaurs, but this position is poorly supported.”
The best known members
of the invalid Aphanosauria include Yarasuchus
(Fig. 2), taxa nested with a long line of non-Aphanosauria by the LRT between Rauisuchia and Archosauria.
Figure 2. Yarasuchus, Qianosuchus and Turfanosuchus nest together in Nesbitt et al. 2017 after rescoring.
Patellos et al. 2019 continue:
“More broadly, this new specimen indicates that cervical osteoderms were present in the earliest avemetatarsalians and were soon lost in the lineage.”
There’s no need for such phylogenetic gymnastics in the LRT.
“The generally plesiomorphic morphology of the new taxon also underscores the difficulty of identifying early avemetatarsalians from incomplete skeletons. Presence of an early diverging avemetatarsalian together with a lagerpetid and silesaurid in the “Basal Isalo II” beds of Madagascar documents the co-occurrence of multiple avemetatarsalian subgroups in Gondwana during the Triassic.”
They wish. The LRT resolves all such problems with high resolution. Blame S. Nesbitt for relying on his own poorly scored cladogram, inventing the ‘Aphanosauria’ and supporting the ‘Avemetatarsalia.’ Blame M. Benton for inventing the clade ‘Avemetatarsalia’.
Don’t trust those clades. Don’t trust the LRT. Run your own tests so you’ll know. In science this is the first, last and best option to resolve all such disagreements.
Patellos E et al. 2019. A new reptile from the ?Middle Triassic of Madagascar may represent the earliest-diverging avemetatarsalian (Archosauria). Journal of Vertebrate Paleontology abstracts.