The skull of PVL 4597 joins its post-crania in the LRT

The PhD thesis of Agustina Lecuona 2013
on the several specimens attributed to the Middle Triassic Gracilisuchus (Fig. 2) is online (PDF). It includes the previously unpublished skull of PVL 4597 (Figs. 1, 2), which the large reptile tree (LRT, 1592 taxa) nests apart from Gracilisuchus, as the last common ancestor of all archosaurs (crocs + dinos only) with or without its skull. We reviewed Gracilisuchus yesterday, so this addition to the LRT is timely.

Figure 1. The skull of PVL 4597 in several views from the 2013 PhD thesis of A. Leucona. Colors added.

Figure 1. The skull of PVL 4597 in several views from the 2013 PhD thesis of A. Lecuona. Colors added.

The differences between PVL 4597 and Gracilisuchus are few (Fig. 2).
So, it is not a surprise that Lecuona considered them congeneric.

However,
the differences are fewer between PVL 4597 and its ancestor, Turfanosuchus (Fig. 4), and  its descendant, Herrerasaurus, the last common ancestor of dinosaurs traditionally and in the LRT. 19 additional steps are added when PVL 4597 is forced to nest with Gracilisuchus in the LRT.

FIgure 2. Comparing PVL 4597 to Gracilisuchus. Despite their many similarities, these two do not nest together in the LRT.

FIgure 2. Comparing PVL 4597 to Gracilisuchus. Despite their many similarities, these two do not nest together in the LRT. Taxon exclusion is the issue with the PhD dissertation and the use of an invalidated analysis from Nesbitt 2011.

Basal members of large clades
are sisters to basal members of sister clades (Fig. 3). We compare those taxa with one another, ignoring the more derived members.

Figure 3. Subset of the LRT focusing on basal archosaurs and their immediate ancestors.

Figure 3. Subset of the LRT focusing on basal archosaurs and their immediate ancestors.

Here (Fig. 4) are the skulls of
Turfanosuchus and Herrerasaurus, taxa closer to PVL 4597 than PVL 4597 is to Gracilisuchus is in the LRT. The long-awaited skull confirms the nesting of the post-crania.

Figure 2. Skull of Turfanosuchus compared to Herrerasaurus, the basalmost dinosaur.

Figure 4. Skull of Turfanosuchus compared to Herrerasaurus, the basalmost dinosaur.

Without PVL 4597, 
the LRT still nests Turfanosuchus and basal bipedal crocs close to the base of the Dinosauria, contra the results of other studies that generally do not include those taxa.

Unfortunately,
Lecuona’s PhD thesis employed a borrowed and flawed cladogram on which she mistakenly trusted in: Nesbitt 2011. Even though Lecuona’s revised cladogram includes the basal bipedal crocs (which nest at derived nodes in her thesis), earlier we dismantled Nesbitt 2011 in a 7-part series ending here. Rescored Nesbitt 2011 resembles the LRT.


References
Lecuona A 2013. Anatomía y relaciones filogenéticas de Gracilisuchus stipanicorum y sus implicancias en el origen de Crocodylomorpha. PhD thesis. PDF
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.

wiki/Gracilisuchus

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