Casting a blind eye toward: Scleromochlus

Figure 1. Scleromochlus by Witton 2013, via Benton 1999. Now promoted as a fuzzy pterosaur precursor.

Figure 1. Scleromochlus by Witton 2013, via Benton 1999. Now promoted as a fuzzy pterosaur precursor. Note the quadrate articulating with the surangular and the giant retroarticular process. Lean that quadrate the other way and these problems go away — and suddenly Scleromochlus is a croc! The fossil, found in dorsal and ventral views, does not expose the quadrate, except in one specimen, which is conveniently ignored. Witton gives Scleromochlus one extra cervical than Benton illustrated and a few extra fingers.

We’ve seen this sort of thing before.
Mark Witton is good at cherry-picking his references and keeping his audience in the dark about alternate and more viable realities in paleontology. In a recent blog, M. Witton uncritically takes on the seven specimens of Scleromochlus, first described by Woodward (1907) and later covered by Benton (1999, Fig. 1). He lists the phylogenetic analyses and other papers that posit Scleromochlus as a pterosaur precursor, but not the ones that indicate Scleromochlus was a basal bipedal crocodylomorph, like Terrestrisuchus and Saltoposuchus (Peters 2000, 2002, Fig. 2). These also demonstrate that pterosaurs were something entirely distinct from archosaurs.

From the Witton blog:
“For 100 years Scleromochlus has been implicated as a relative of pterosaurs (e.g. Huene 1914; Padian 1984; Gauthier 1986; Sereno 1991; Bennett 1996; Hone and Benton 2008; Brusatte et al. 2010, Nesbitt 2010) or, at very least, an ornithodiran representing a very early stage of stem-bird evolution (Benton 1999; Hone and Benton 2008).”

And yet Scleromochlus has several traits that rule it out as a pterosaur precursor: 1) flat, wide skull; 2) deep antorbital fossa; 3) broad triangular pterygoids; 4) no sternum; 5) no interclavicle; 6) no elongate coracoid; 9) deep chevrons; 10) diverging pubis and ischium; 11) no laterally increasing fingers 1-4); 12) no pedal digit 5. Click here and here for more info. And click here for the evolution of the pterosaur wing.

All the above traits are shared with basal bipedal crocs. And we already have a long list of taxa that show an increasing number of pterosaurian traits, listed here.

Witton mentions:
*You can’t mention Scleromochlus on the internet without someone pointing out that its status as an ornithodrian has not been tested in analyses containing non-archosaur archosauromorphs. This is true enough, but – at least within the current limits of testing – its ornithodiran status is not controversial, having been recovered in at least six different analyses (e.g. Gauthier 1986; Sereno 1991; Bennett 1996; Hone and Benton 2008; Brusatte et al. 2010) and sharing several unique characteristics with Pterosauria (Padian 1984). Hence, we’re following convention here.”

Again, forgetting Peters 2000 and the large reptile tree at reptileevolution.com.

Basal Crocodylomorpha

Figure 2. Basal Crocodylomorpha, including Gracilisuchus, Saltopus, Scleromochlus and Terrestrisuchus. Here the deep antorbital fenestra, deep chevrons, tiny hands and lack of a fifth toe feel right at home.

After describing the vestigial hand and lack of a fifth toe in Scleromochlus, Witton writes:Scleromochlus hindlimb arthrology betrays a parasagittal posture akin to that of dinosaurs and pterosaurs – the suite of characteristics associated with this is one clue that Scleromochlus is closely related to these clades (Bennett 1996; Benton 1999; Hone and Benton 2008).” Hey, wait a minute Mark! You’re forgetting the bipedal crocs (Fig.2)! And pterosaurs have giant hands and a long fifth toe!

The meat of the blog, it’s raison d’être
After listing the transversely-banded scales on the dorsal surface of the Scleromochlus torso (think croc skin), Witton let’s his imagination out of the barn, “Scleromochlus may have been covered in scales, but it is equally likely that it had fuzz-like filaments in places. There are several reasons for this. Firstly, it belongs within a phylogenetic bracket where filaments are the ancestral condition or, at very least, scales were prone to developing filamentous morphologies. Secondly, virtually all models of archosaur evolution recover Scleromochlus as sister taxon to a fuzzy clade – pterosaurs, so there is good ‘phylogenetic proximity’ for fuzz. Thirdly, insulating integuments are common – if not ubiquitous – in small, active (see below) desert-dwelling animals.” 

Mark, it’s those analyses that expand the inclusion set, that nest Scleromochlus with bipedal basal crocs, you should be keeping an eye on. This is what I don’t understand about paleontologists. Sometimes the obvious (Fig. 2) becomes taboo territory because I’m associated with it.

Leaping pterosaur precursor?
It’s not a stretch to imagine Scleromochlus as a leaping archosaur. But then Witton takes it one step further, “Indeed, the powerful leaping and bounding abilities of early ornithodirans has been tied to the evolution of pterosaur flight (Bennett 1997; Witton 2013).” Unfortunately such hind limb leaping gives the forelimbs nothing to do and no reason to begin flapping. And flapping is key to pterosaurian evolution. Read more on the origin of pterosaur flapping here.

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
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 2000. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
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.

wiki/Scleromochlus

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