What is Gracilisuchus? Add more taxa to find out.

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.

Figure 1. The ancestry of Scleromochlus going back to Lewisuchus, Saltoposuchus, Terrestrisuchus, SMNS 12591 and Gracilisuchus.

Figure 1. The ancestry of Scleromochlus going back to Lewisuchus, Saltoposuchus, Terrestrisuchus, SMNS 12591 and Gracilisuchus. The limbs of Gracilisuchus are unknown, despite Romer’s reconstruction traced here.

Key to the present discussion
is figuring out what is and is not an archosaur.

Definition
‘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.

Figure 2. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.

Figure 2. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.

The Lecuona et al. 2017 cladograms
(Figs, 3–5) suffer from taxon exclusion and inappropriate taxon inclusion.

Figure 3. This is Figure 19 from Lecuona et al. 2017. All taxa are archosaurs in the Lecuona et al. cladogram. Red taxa are not archosaurs in the LRT.

Figure 3. From Lecuona et al. 2017. Red taxa are not archosaurs in the LRT (subset figure 6).

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.

Figure 4. Figure 17 from Lecuona et al. 2017 with colors added to taxa that are not eu-archosauriforms in the LRT.

Figure 4. Figure 17 from Lecuona et al. 2017 listing archosauriformes from Nesbitt 2011. Colors added archosaurs (green(, pararchosauriforms (yellow), and non-archosauriformes (red).

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.

Figure 5. Figure 18 from Lecuona et al. 2017 with colors and reconstructions added. Here the giant, derived CM 73373 specimen nests basal to Hesperosuchus and taxa leading to Crocodiliformes. The LRT (subset Fig. 6) does not recover this topology.

Figure 5. Figure 18 from Lecuona et al. 2017 with colors and reconstructions added. Here the giant, derived CM 73373 specimen nests basal to Hesperosuchus and taxa leading to Crocodiliformes. The LRT (subset Fig. 6) does not recover this topology and finds that bipedal locomotion developed convergently in these two taxa.

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.

FIgure 1. Subset of the LRT focusing on the Crocodylomorpha, dorsal scutes, elongate proximal carpals, bipedality and clades.

FIgure 6. Subset of the LRT focusing on the Crocodylomorpha, dorsal scutes, elongate proximal carpals, bipedality and clades.

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.

References
Benton MJ and Clark JM 1988. Archosaur phylogeny and the relationships of the Crocodilia in MJ Benton (ed.), The Phylogeny and Classification of the Tetrapods 1: 295-338. Oxford, The Systematics Association.
Brinkman D 1981. The origin of the crocodiloid tarsi and the interrelationships of thecodontian archosaurs. Breviora 464: 1–23.
Butler RJ, Sullivan C, Ezcurra MD, Liu J, Lecuona A and Sookias RB (2014. New clade of enigmatic early archosaurs yields insights into early pseudosuchian phylogeny and
the biogeography of the archosaur radiation. BMC Evolutionary Biology 14:1-16.
Juul L 1994. The phylogeny of basal archosaurs. Palaeontographica africana 1994: 1-38.
Lecuona A and Desojo, JB 2011. Hind limb osteology of Gracilisuchus stipanicicorum (Archosauria: Pseudosuchia). Earth and Environmental Science Transactions of the Royal Society of Edinburgh 102 (2): 105–128.
Lecuona A, Desojo JB and Pol D 2017. New information on the postcranial skeleton of
Gracilisuchus stipanicicorum (Archosauria: Suchia) and reappraisal of its phylogenetic position. Zoological Journal of the Linnean Society, 2017, XX, 1–40.
Parrish JM 1993. Phylogeny of the Crocodylotarsi, with reference to archosaurian and crurotarsan monophyly. Journal of Vertebrate Paleontology 13(3):287-308.
Romer AS 1972. The Chañares (Argentina) Triassic reptile fauna. An early ornithosuchid pseudosuchian, Gracilisuchus stipanicicorum, gen. et sp. nov. Breviora 389:1-24.

wiki/Gracilisuchus

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