We first and last looked at Gracilisuchus (Romer 1872)
a few years ago here and here. According to a recent paper by Lecuona et al. 2017, six specimens have been attributed to Gracilisuchus (Fig. 1). However, of three tested, only two are congeneric in the large reptile tree (LRT, 1394 taxa, subset Fig. 6), where all three Gracilisuchus specimens nest at or close to the base of the Archosauria (crocs + dinos only). However, that’s not how Lecuouna et al. see it (Figs. 3–5), based on Nesbitt 2011.
Key to the present discussion
is figuring out what is and is not an archosaur.
‘Archosauria’ is defined as crocs + birds, their last common ancestor and all descendants. The archosaur taxon list in Lecuona et al. (Fig. 3) is much broader than in the LRT (Fig. 6), where the clade Archosauria is restricted to just crocs + dinos. The last common ancestor of all known archosaurs in the LRT is one of the specimens Lecuona et al. assigned to Gracilisuchus, PVL 4597 (Fig. 2.
Inappropriate taxon inclusion
Lecuona et al. mistakenly recover pterosaurs with archosaurs. That’s because Lecuona et al. do not include the tested, but ignored pterosaur sisters in the clade Fenestrasauria. Pterosaurs are lepidosaurs, as their elongate wing fingers (digit 4) tell us. All archosaurs have a relatively small finger 4 and Scleromochlus (Fig. 1), a putative pterosauromorph (according to Benton 1999, Lecuona et al. 2017, and many others), has tiny hands! So Scleromochlus is not the taxon you want to nest with pterosaurs (contra Benton 1999). Inappropriate taxon inclusion and omission makes current archosaur cladograms not only fictional, but verging on ridiculous. No one, it seems, is checking their results.
The Lecuona et al. 2017 cladograms
(Figs, 3–5) suffer from taxon exclusion and inappropriate taxon inclusion.
The Lecuona et al cladogram of archosaurs
(Fig. 3) includes several taxa and clades that are not archosaurs in the LRT. Note how Lecuona et al. split pterosaurs from ornithosuchids at the base of the Archosauria. These two clades share very few traits, as everyone knows.
It only gets worse for Lecuona et al.
(Fig. 4) when they add phytosaurs, nesting as the last common ancestors of pterosaurs, dinosaurs and ornithosuchids. In the LRT these four clades are not closely related to one another. One wonders how Nesbitt 2011 and Lecuona et al. 2017 were able to get their work published with such results.
Above is the Lecuona et al. cladogram
(Fig. 5) that encouraged study of the CM 73372 specimen we looked at yesterday. In the Lecuona et al. cladogram CM 73372 and tiny Hesperosuchus are sisters. In the LRT (Fig. 6) the two are not related to one another despite their many convergent traits, including a bipedal stance and short fingers.
In the LRT
(Fig. 6) two specimens of Gracilisuchus nests with similarly sized and shaped, Saltopus and Scleromochlus. That clade was derived from similar Lewisuchus and these are sisters to the Junggarsuchus clade, which also includes bipedal Pseudhesperosuchus. Hesperosuchus nests in the middle of the Crocodylomorpha (Fig. 6), not at the base. We looked at taxon exclusion in the Crocodylomorpha recently here.
You’ll know a good cladogram
when enough candidate taxa are included that all sister taxa actually resemble one another, producing a gradual accumulation of derived traits. This is how evolution works, so this process should be accurately reflected in cladograms. If they don’t: add more taxa until they do.
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