Novas et al. 2021 review South American early dinosaurs, but omit the only valid outgroup: South American early crocodylomorphs

This is becoming a repeating pattern.
Taxon exclusion mars an otherwise wonderful detailed study of Triassic dinosaur interrelations.

From the Novas et al. 2021 abstract:
“Triassic beds from Argentina and Brazil provide the most relevant fossil record of early dinosauriforms in terms of numerical abundance and taxonomic diversity.”

Correction: When more taxa are added, as in the large reptile tree (LRT, 1839+ taxa) Crocodylomorpha is the outgroup for the Dinosauria (Fig. 1). These two clades combine to create the clade Archosauria. Therefore there will never be such thing as a dinosauriform, except herrerasaurids, which are outside the Passer + Triceratops definition.

“This record currently represents the best source to understand the origin and early evolutionary radiation of dinosaurs.”

That’s true. And by the way, some of the best basal bipedal crocodylomorphs (e.g. Pseudhesperosuchus, Fig. 1) are also from South America. They were omitted from Novas et al. 2021… AND their last common ancestor, the phylogenetically miniaturized PVL 4597 specimen (Fig. 1) wrongly attributed to Gracilisuchus (Fig. 1), is also from South America.

Figure 1. The origin of dinosaurs in the LRT to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.  Note the phylogenetic miniaturization at the origin of Archosauria (Crocs + Dinos).
Figure 1. The origin of dinosaurs in the LRT to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.  Note the phylogenetic miniaturization at the origin of Archosauria (Crocs + Dinos).

The abstract continues:
“In the present paper we offer an updated review focused on the available evidence of Carnian dinosaurs from this continent, but we also discuss the record of Triassic dinosaur precursors and the evolution of Triassic dinosaurs in other continents.”

Unfortunately Novas et al. delete the valid proximal Triassic dinosaur precursors. By putting herbivorous silesaurids basal to dinosaurs, herbivorous ornithischians were attracted to the base of the Dinosauria. Now we have a problem. An herbivorous clade precedes carnivorous basal dinosaurs (= herrerasaurids). In the LRT once some dinosaurs became herbivorous, they stayed herbivorous while carnivorous basal dinos arose from carnivorous bipedal crocs.

“It is clear that, aside the agreed taxonomic composition of some particular dinosaurian subclades (e.g., Herrerasauridae, Neotheropoda), there is no consensus about early dinosaur phylogeny, and our paper is not the exception.”

Why is there no consensus? Because all prior workers also omitted basal bipedal crocs.

“Recent years witnessed the discovery of several new early dinosaurian taxa, as well as reviews of the taxonomic allocation of several renowned forms such as Lagerpeton, Lewisuchus, Pisanosaurus, and Eorpator. New analyses demonstrate that evidence supporting the taxonomic referrals of pre-Norian dinosaurs to Theropoda, Sauropodomorpha and Ornithischia are tenuous, at best.”

Why tenuous at best? Because all prior workers also omitted basal bipedal crocs.

“Here we present new anatomical observations and comparisons for each of these South American early dinosauriforms with the aim to test previous phylogenetic interpretations.”

Their testing was flawed by taxon exclusion as Novas et al. omit basal bipedal crocs.

Figure 2. Cladogram from Novas et al. 2021 omits members of Crocodylomorpha, the outgroup for the Dinosauria in the LRT. Instead Novas et al. nest herbivorous Silesauridae outside the Dinosaria, which mistakenly attracts herbivorous dinosaurs to the base of the clade.

The abstract continues:
“Evidence from South America allows reviewing the phylogenetic relationships of taxa from other continents, including Tawa, Chindesaurus, and Daemonosaurus, which are here suggested to nest within Herrerasauria.”

Here suggested”??? Let’s do better than that. In the LRT Tawa (Fig. 1) is THE basal theropod. Daemonosaurus is a basal member of the Ornithischia, a clade within Phytodinosauria (= Ornithischia + Sauropodomorpha and their ancestors, includng Eoraptor, Fig. 1). Novas et al. fail to find these relationships due to including Silesauridae (herviorous derived members of the dinosaur mimic clade, Poposauridae) as the dinosaur outgroup due to excluding carnivorous basal bipedal crocs. Two basal members of the Poposauridae, the dinosaur mimics Turfanosuchus and Poposaurus, were also omitted from Novas et al.

“Evidence at hand indicates that herrerasaurs were a successful clade of archaic predatory saurischians that inhabited both South and North America, and probably also India and Europe.”

This is true. So, why do workers continue to omit basal bipedal crocodylomrophs, the most similar taxa to Herrerasaurus (Fig. 1) and basal bipedal dinosaurs? I wonder, too.

We talked about
paleontologists wearing blinders several years ago. A few days ago we looked a similar situation in which paleontologists refused to include tiny tanystropheids within their study of Tanystropheidae.

Don’t waste your time
and your readers’ time with detailed studies of any clade or taxon without first setting the stage within a valid phylogenetic context. It never hurts to add taxa. It always hurts to omit taxa. And once you have it, you can use it forever with authority, updating it whenever you please. Cherry-picking taxa is something you can say you did when you were young and foolish.

References
Novas FE, Agnolin FL, Ezucrra MD, Müller RT, Martinelli A and Langer M 2021. Review of the fossil record of early dinosaurs from South America, and its phylogenetic implications. Journal of South American Earth Sciences xxx (xxxx) 103341.

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