Antarctanax: a late-surviving basal synapsid, not a dino ancestor

Peecock, Smith and Sidor 2019
bring us news of a Early Triassic amniote from the Transantarctic Mountains, Antarctanax shackletoni (Figs. 1, 2), “known from a partial postcranial skeleton including cervical and dorsal vertebrae, a humerus, and both pedes.” 

Unfortunately,
if the scale bars are correct, and they seem to be, the smaller ‘pes’, the one surrounded by cervicals, is really a manus (Figs. 1, 2). Furthermore, the small manus matches the small humerus and radius.

The authors mislabeled
the robust, displaced metatarsal 5 as metatarsal 1, which lies beneath it (colored orange, Figs. 1, 2). Perhaps a reconstruction would have helped expose this error before submission.

The authors report,
“Our inclusion of A. shackletoni in phylogenetic analyses of early amniotes finds it as an archosauriform archosauromorph.” Their cladogram based on Ezcurra et al. 2014 nested Antarctanax in an unresolved polytomy with the basal archosauriforms, Proterosuchus, Erythrosuchus and Euparkeria. Their cladogram based on Ezcurra 2016 nested Antarctanax in an unresolved polytomy with other basal archosauriforms, Fugusuchus, Sarmatosuchus. I am not aware of a manus or pes preserved for these two taxa. Of the above listed taxa, Proterosuchus (Fig. 3) comes closest, but has a hooked metatarsal 5 and metacarpal 3 is the longest, distinct from Antarctanax.

This time it is not taxon exclusion, but bad timing.
When the manus and pes of Antarctanax are added to the large reptile tree (LRT, 1395 taxa), Antarctanax nests with basalmost synapsids, like Cabarzia (Figs. 2, 4) and Aerosaurus (Fig. 2). Aerosaurus was included in Ezcurra et al. 2014 and tested by Peecock, Smith and Sidor 2019. You’ll have to ask the authors why Antarctanax did not nest closer to Aerosaurus. Cabarzia trostheidei (Spindler, Werneberg and Schneider 2019, Fig. 3) could have influenced their thinking and scoring, but it was published only a few weeks ago, too late to include in their submission.

Peecock, Smith and Sidor did not provide a reconstruction
of Antarctanax, but online Discover magazine provided an in vivo painting and crowned it, “Dinosaur Relative Antarctanax.” According to the LRT, Antarctanax was a late-surviving (Early Triassic) basal member of our own lineage, the Synapsida, with a late Carboniferous genesis.

Therapsid synapsids were plentiful in Antarctica in the Early Triassic.
The headline should have focused on the unexpected presence of this sprawling, pre-pelycosaur, basal synapsid in the Mesozoic, surviving the Permian extinction event in this Antarctic refuge, alongside a closer relative of mammals, Thrinaxodon.

References
Ezcurra MD, Scheyer TM and Butler RJ 2014. The origin and early evolution of Sauria: reassessing the Permian saurian fossil record and the timing of the crocodile-lizard divergence. PLoS ONE 9:e89165.
Ezcurra MD 2016. The phylogenetic relationships of basal archosauromorphs, with an emphasis on the systematics of proterosuchian archosauriforms. PeerJ 4:e1778.
Peecock BR, Smith RMH and Sidor C 2019. A novel archosauromorph from Antarctica and an updated review of a high-latitude vertebrate assemblage in the wake of the end-Permian mass extinction. Journal of Vertebrate Paleontology e1536664 (16 pages) DOI: 10.1080/02724634.2018.1536664
Spindler F, Werneberg R and Schneider JW 2019. A new mesenosaurine from the lower Permian of Germany and the postcrania of Mesenosaurus: implications for early amniote comparative osteology. PalZ Paläontologische Gesellschaf