Pol et al. 2021: Anachronistic look at the origin of the Sauropodomorpha

From the Pol et al. 2021 abstract
“Sauropodomorpha is the first major dinosaurian group that radiated during the Triassic.”

No. That’s not how it works in phylogenetic analysis. Clades don’t radiate alone without a second clade also radiating.

Here’s how it works: From a basal clade, llike Dinosauria (represented by Herrerasaurus), there is always a dichotomy where one branch, like Theropoda (represented by Tawa), splits from the another branch, like Phytodinosauria (represented by Buriolestes). This happens in all phylogenetic analyses as it does in the large reptile tree (LRT 1793+ taxa). Rarely three branches arise to produce an unresolved node. That usually means a mistake in scoring. Such a node is not present in this subset  of the LRT focusing on Sauropodomorpha (Fig. 1).

Figure 2. Subset of the LRT focusing on basal phytodinosauria. Aardonyx nests with Saturnalia here.

Figure 1. Subset of the LRT focusing on basal phytodinosauria. Aardonyx nests with Saturnalia here. The Sauropodomorpha is the sister clade to Ornithischia (cropped off the bottom of the graphic).

From the abstract:
“Sauropodomorpha is one of the three major groups of Dinosauria, along with Theropoda and Ornithischia (Benton, 1983; Novas, 1996), and became the most conspicuous herbivores of terrestrial ecosystems of the Mesozoic.”

Again, this is anachronistic paleontology. There are never three major groups of any vertebrate. There should always be dichotomies. Pol et al. need to add taxa to understand the first dichotomy in the Dinosauria splits the Theropoda from the Phytodinosauria. Several nodes later a dichotomy splits Sauropodomorpha from Ornithischia. Both are plant eaters.

Figure 2. Plateosaurus skeleton digitized.

Figure 2. Plateosaurus skeleton digitized.

From the abstract
“These early lineages are currently referred as basal or early sauropodomorphs (Bronzati, 2017) or more traditionally grouped in Prosauropoda (Sereno, 1999; Galton and Upchurch, 2004), a term now in disuse as most phylogenetic studies in the last decade agree in the paraphyly of this group.”

The first dichotomy in Sauropodomorpha in the LRT splits taxa leading to Plateosaurus (Plateosauridae Marsh 1895 = Prosauropoda Huene 1920, Sereno 1998, Fig. 2) from taxa leading to Brachiosaurus.

Recently Baron, Norman and Barrett 2017 were unable to recover basal Phytodinosauria due to taxon exclusion. They also mixed up basal sauropods with basal plateosaurs.

Figure 1. Tiny forelimbs with three sharp-clawed fingers indicate that Guaibasaurus is a theropod, not a sauropodomorph. Shown to scale with related theropods Marasuchus and Procompsognathus.

Figure 3. Tiny forelimbs with three sharp-clawed fingers indicate that Guaibasaurus is a theropod, not a sauropodomorph. Shown to scale with related theropods Marasuchus and Procompsognathus.

Pol et al. report, 
“Guaibasaurus is included in the table although many studies depicted this taxon as an early theropod or saurischian (see text).”

In the LRT Guaibasaurus (Fig. 3) is indeed a basal theropod, so should not have been included in a study focused on phytodinosaurs or sauropodomorphs. That would be like adding pterosaurs to a study focused on archosaurs. Hah! Who would do THAT? (Everyone else, sadly, each time by excluding pertinent taxa).


References
Baron MG, Norman DB and Barrett PM 2017. A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature, 543: 501–506.
Pol D, Otero A, Apaldetti CA and Martinez RJ 2021.
Triassic sauropodomorph dinosaurs from South America: the origin and diversification of dinosaur dominated herbivorous faunas. Journal of South American Earth Sciences. https://doi.org/10.1016/j.jsames.2020.103145

wiki/Plateosauria
https://pterosaurheresies.wordpress.com/2020/11/24/plateosaurus-enters-the-lrt/

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