SVP abstracts 8: µCT studies on Scleromochlus reveal it is a ‘reptile’

Foffa et al. 2020 apply µCT scanning technology
to the tiny Late Triassic Scleromochlus (Fig. 1). This bipedal crocodylomoph with tiny fingers was Michael Benton’s (1999) and Chris Bennett’s (1996) cherry-picked choice to be the taxon closest to pterosaurs (whenever the actual sisters (Peters 2000) were omitted).

Faxinalipterus matched to Scleromochlus. The former is more primitive, like Gracilisuchus, in having shorter hind limbs and more robust fore limbs. The maxilla with fenestra and fossa, plus the teeth, are a good match.

Figure 1. Faxinalipterus matched to Scleromochlus. The former is more primitive, like Gracilisuchus, in having shorter hind limbs and more robust fore limbs. The maxilla with fenestra and fossa, plus the teeth, are a good match.

Scleromochlus has become more popular lately.
Earlier this year, Bennett 2020 provided new drawings, but not much new insight. His cladograms failed to recover a single node on which to nest Scleromochlus.

From the Foffa et al. 2020 abstract:
“The herpetofauna of the Lossiemouth Sandstone Formation (Late Triassic) of Elgin (Moray, Scotland) includes several close relatives of key groups such as dinosaurs, pterosaurs, crocodilians and lepidosaurs, although the affinities of some taxa within this assemblage are contentious.”

How contentious?

  • Pterosaurs? No.
  • Dinosaurs? No.
  • Crocodilians (= Crocodylomporha)? Yes: Saltopus and Scleromochlus.
  • Lepidosaurs? No, according to Wikipedia and the LRT.

Continuing from the Foffa et al. 2020 abstract:
“The specimens of this assemblage are notoriously challenging to study because of their preservation as voids in sandstone. Historically, the ‘Elgin reptiles’ have been studied primarily using physical molds, which only provide incomplete, and potentially distorted information – an issue that particularly affects small-bodied taxa. Here we use microcomputed tomographic (μCT) techniques as an alternative method to study these important specimens, and access hidden parts of their skeletons.”

“Scleromochlus taylori is one of the most controversial taxa within the assemblage. It is an enigmatic, small-bodied, bipedal reptile that was long hypothesised to be closely related to dinosaurs and pterosaurs, and is represented by several specimens of varying completeness.”

Not an enigma. In the large reptile tree (LRT, 1749+ taxa; subset Fig. 2), and earlier (Peters 2002) nested Scleromochlus as a basal bipedal crocodylomorph. Add pertinent sister taxa (and let’s see your reconstructions and tracings to make sure interpretations are correct) to confirm or refute.

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

Figure 2. Subset of the LRT focusing on the Crocodylomorpha, dorsal scutes, elongate proximal carpals, bipedality and clades.

Continuing from the Foffa et al. 2020 abstract:
“It was recently reinterpreted as a quadrupedal ‘hopper’, (Bennett 2020) positioned phylogenetically either within doswelliid archosauriforms, or outside of the Archosauria + Erythrosuchidae clade. Neither of these interpretations has been universally accepted, and other aspects of the biology of Scleromochlus are also contentious.

Taxon exclusion is the universal problem with prior studies. Given the proportions of Scleromochlus (Fig. 1) and the proportions of its phylogenetic sisters (Fig. 2), why force it into an awkward quadrupedal posture?

Figure 1. Taxa from the croc subset of the LRT to scale. Click to enlarge.

Figure 1. Taxa from the croc subset of the LRT to scale. Click to enlarge.

Continuing from the Foffa et al. 2020 abstract:
“Here we analyse the first μCT scan data collected for Scleromochlus, using all available specimens, and show that historic molding incompletely captured its anatomy. We access and describe previously inaccessible (and thus unaltered) portions of its skeleton including a complete
[unintentionally left blank], as well as new details of already described regions. Overall, we clarify previous ambiguous features such as vertebral count, dorsal rib length and curvature, and reveal new details from the neck, tail, girdles, fore and hindlimb (particularly manus, femur and pes). We use this information, alongside that from multiple generations of molds, to shed light on some of the most controversial aspects of its anatomy, phylogenetic relationships, taphonomy, and ecology.”

Well,  that’s a lot of teasing without telling readers what Scleromochlus is. The title of the abstract only refers to Scleromochlus as a ‘reptile/’. No other conclusions are presented.


References
Foffa D, Barrett P, Butler R, Nesbitt S, Walsh S, Brusatte S, Fraser N 2020. New Information on the Late Triassic reptile Scleromochlus taylori from µCT data. SVP abstracts 2020.

http://reptileevolution.com/scleromochlus.htm

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 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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