Reconstructing the hand of Terrestrisuchus

Basal Crocodylomorpha

Figure 1. Basal Crocodylomorpha, including Gracilisuchus, Saltopus, Scleromochlus and Terrestrisuchus. Wikipedia and other references insist that these were all quadrupeds. Possible in some. Improbable in others, IMHO.

Paleontologists don’t like to think of Scleromochlus (Woodward 1907) as a bipedal crocodylomorph. Not sure why not. It’s in good company (Fig. 1). They’d rather see it as pterosaur closest cousin (Benton 1999, Bennett 1996, Hone and Benton 2008, Senter 2003, Sereno 1991). Unlikely, considering those tiny hands.

There are quite a few bipedal basal crocs and protoarchosaurs. You can see them all here to scale.

Another fairly well known bipedal croc is Terrestrisuchus (Fig. 1). Funny thing, you almost never see these two taxa mentioned together in the same sentence or see them on the same phylogenetic analysis. But you can see them together here and here and in figure 1.

Figure 2. Basal crocs hands with a focus on a new reconstruction of the hand of Terrestrisuchus that more closely follows the patterns of sister taxa.

Figure 2. Basal crocs hands with a focus on a new reconstruction of the hand of Terrestrisuchus that more closely follows the patterns of sister taxa.

Focus on the hand
Crush (1984) introduced us to the complete skeleton of a basal bipedal crocodylomorph, Terrestrisuchus (Late Triassic) and did a great job! The elements came from a fissure fill, so had to be reassembled as if they were puzzle pieces. Some were easy. Others were difficult. Crush (Fig. 2) put the hand together with a small thumb, similar to a human hand. However, reassembling the pieces to more closely mimic or echo sister taxa provides a revised reconstruction (Fig. 2) with continuous PILs. I did this with Photoshop to maintain precision and avoid freehand unconscious bias.

The revised hand (Fig. 2) aligns metacarpals 1-3 as in sister taxa. Metacarpal 4 is slightly shorter than mc3. Metacarpal 5 was probably very short as in Erpetosuchus and Hesperosuchus. The digits are easy. They taper distally and are shorter distally terminating in tiny unguals. The proximal phalanges are subequal.

Basal croc hands, as you can see, are notoriously incomplete. So we have to glean clues from all of them, despite their morphological disparity and wonderful variety.

Once again, phylogenetic bracketing saves the day!

And yes those incredibly long wrist bones are proximal carpals, the radiale and ulnare. Modern crocs retain those.

References
Bennett SC 1996. The phylogenetic position of the Pterosauria within the Archosauromorpha. Zoolological Journal of the Linnean Society 118: 261–308.
Benton MJ 1999. Scleromochlus taylori and the origin of the pterosaurs. Philosophical Transactions of the Royal Society London, Series B 354 1423-1446. Online pdf
Crush PJ 1984. A late upper Triassic sphenosuchid crocodilian from Wales. Palaeontology 27: 131-157.
Hone DWE and Benton MJ 2007. An evaluation of the phylogenetic relationships of the pterosaurs to the archosauromorph reptiles. Journal of Systematic Palaeontology 5:465–469.
Hone DWE and Benton MJ 2008. Contrasting supertree and total evidence methods: the origin of the pterosaurs. Zitteliana B28:35–60.
Huene F von 1921. Neue Pseudosuchier und Coelurosaurier aus dem Wurttembergischen Keuper: Acta Zoologica, v. 2, p. 329-403.
Padian K. 1984. The Origin of Pterosaurs. Proceedings, Third Symposium on Mesozoic Terrestrial Ecosystems, Tubingen 1984. Online pdf
Senter P 2003. Taxon Sampling Artifacts and the Phylogenetic Position of Aves. PhD dissertation. Northern Illinois University, 1-279.
Sereno PC 1991. Basal archosaurs: phylogenetic relationships and functional implications. Journal of Vertebrate Paleontology 11 (Supplement) Memoire 2: 1–53.
Woodward AS 1907. On a new dinosaurian reptile (Scleromochlus taylori, gen. et sp. nov.) from the Trias of Lossiemouth, Elgin. Quarterly Journal of the Geological Society 1907 63:140-144.

wiki/Scleromochlus
wiki/Terrestrisuchus

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