Reassessment of Scleromochlus: Bennett 2020

SC Bennett 2020
followed Benton 1999 and others (citations below) in giving us a closer look at Scleromochlus taylori (Woodward 1907; Late Carnian, Late Triassic ~217 mya, 18 cm long; Figs. 1, 2), a tiny biped crocodylomorph derived from a sister to Gracilisuchus and Saltopus according to the large reptile tree (LRT, 1650+ taxa; Fig. 3).

Bipeds of the Triassic

Figure 1. Bipeds of the Triassic. Top to bottom: Cosesaurus, Scleromochlus, Marasuchus and Tropidosuchus. Each represents a distinct lineage of bipeds with bipedal sister taxa. This version of Scleromochlus was published in Peters 2002, based on Benton 1999.

despite the firsthand examination of this taxon, Bennett ignored sister taxa recovered by the LRT (Figs. 3, 4). His cladograms failed to recover a single node on which to nest Scleromochlus. In essence, he still doesn’t know what Scleromochlus is, despite his best efforts (see below for Bennett’s self assessment).

From Bennett’s 2020 introduction
“The first specimens were briefly described and named by Woodward (1907), who interpreted Scleromochlus as a small bipedal running or leaping dinosaur. Huene (1914) described the specimens more thoroughly and interpreted Scleromochlus as an arboreal climbing and leaping pseudosuchian close to the origin of pterosaurs. Swinton (1960), Brodkorb (1971) and Martin (1983) discussed Scleromochlus in relation to the origin of birds, whereas Padian (1984) suggested that Huene had it only half right and interpreted Scleromochlus as a digitigrade bipedal cursor close to the origin of pterosaurs and dinosaurs, a view that has gained general acceptance (Gauthier, 1986; Sereno, 1991; Benton, 1999; Fraser, 2006; Brusatte et al., 2010). Despite that, Bennett (1996, 1997) argued that Huene had only the other half right and Padian had it all wrong and that Scleromochlus was an arboreal leaper not close to pterosaurs.”

True to Bennett’s curse,
“You will never be published, and if you are published, you will not be cited,” Bennett 2020 did not cite Peters 2002, who wrote, “Among recent workers, Padian (1984), Sereno (1991) and Benton (1999) noted pterosaur similarities in the bipedal diapsid, Scleromochlus. The homoplasy is striking (Table I). However, figures by Benton (1999), which are reconstructed here (Fig. 8D), show that this archosauriform had a low, wide skull, a deep antorbital fossa, a terminal naris, a short neck of only six or seven cervicals, a long lumbar region, a small manus, a broadly separated pubis and ischium, a fibular flange, a calcaneal heel and a spike-like, digit-less metatarsal V. These characters are not found in pterosaurs. They are synapomorphies of basal bipedal crocodylomorphs, such as Gracilisuchus (Romer, 1972) and Saltoposuchus (Huene, 1921; Sereno and Wild, 1992).”

Bennett 2020 failed to mention
or include Junggarsuchus, Pseudhesperosuchus, Gracilisuchus and Saltposuchus in his taxon list. He only mentioned Saltopus as a coeval predator. These are all bipedal basal crocodylomorpha, a clade  ignored by Bennett 2020.

Figure 2. From Bennett 2020 showing in dorsal view the skull of Scleromochlus with DGS overlays colorizing the bones. At right, Bennett's drawing of same.

Figure 2. From Bennett 2020 showing in dorsal view the skull of Scleromochlus with DGS overlays colorizing the bones. At right, Bennett’s drawing of same. A compression crack across the fragile frontal was identified as the only suture in the skull, between the nasal and frontal, by Bennett 2020.

Back to Bennett’s 2020 introduction
“In 2013 I came to suspect that Bennett (1997), too, had it at least half wrong. By happy coincidence, I had shortly before perfected my technique for studying small slab specimens, so I took another look at the evidence and after several years of study gained some confidence in interpreting the specimens. This article is not a thorough redescription of the osteology of Scleromochlus but rather is a reassessment of the osteological evidence that has been used to interpret Scleromochlus’s mode of life, locomotion, and phylogenetic relationships.”

Again, the major shortcoming
in Bennett’s phylogenetic analysis is taxon exclusion. And Bennett’s “perfected technique” is not perfect (Fig. 2). His outmoded freehand technique overlooks many bones and sutures.

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

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

Concluding Bennett’s 2020 introduction
“A principal component analysis of skeletal measurements of Scleromochlus and other vertebrates of known locomotor type was done to examine the locomotion of Scleromochlus, and it was found to plot with frogs. Based on osteological evidence, including previously overlooked evidence from the specimens, and the principal component analysis, Scleromochlus is interpreted as a sprawling quadrupedal hopper analogous to frogs. Phylogenetic analyses found that Scleromochlus was not an ornithodiran, but rather either within the Doswelliidae or outside the clade consisting of the most recent common ancestor of the Erythrosuchidae and Archosauria and all its descendants.”

Pretty vague…
If the best Bennett can do is nest tiny bipedal Scleromochlus with giant, quadrupedal  Doswellia OR Erythrosuchus, then Bennett should have added taxa to his cladogram. If Bennett would have just added archosaur taxa that were small, bipedal and with flat skulls and osteoderms, he would have nailed it.

Other than the proportions, size, skeletal details and osteoderms
of Scleromochlus, the anterior lean of the long quadrate is also a crocodylomorph trait overlooked by all prior workers, except Peters 2002 (Figs. 1,2). Bennett traced the quadrate in stereo, but identified it with a question mark (Fig. 2). Whenever that happens, the technique has not been, as Bennet reported, “perfected.’

Bennett SC 1996. The phylogenetic position of the Pterosauria within the Archosauromorpha. Zoolological Journal of the Linnean Society 118: 261–308.
Bennett SC 1997. The arboreal leaping theory of the origin of pterosaur flight. Historical Biology 12(3–4):265–290
Bennett SC 2020. Reassessment of the Triassic archosauriform Scleromochlus taylori: neither runner nor biped, but hopper. PeerJ 8:e8418 DOI 10.7717/peerj.8418
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
Clark JM 2011. A new shartegosuchid crocodyliform from the Upper Jurassic Morrison Formation of western Colorado. Zoological Journal of the Linnean Society. 163 (s1): S152–S172.
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.
Padian K. 1984. The Origin of Pterosaurs. Proceedings, Third Symposium on Mesozoic Terrestrial Ecosystems, Tubingen 1984. Online pdf
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing – with a twist. Hist Bio 15: 277–301.
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.


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