The plate and counterplate of Sclerosaurus

Earlier the large reptile tree nested the small pareisaur Sclerosaurus armatus (von Meyer 1857; Early to Middle Triassic; 30 cm long; Fig.1) at the base of the soft shell turtle clade (Fig. 2). This is at odds with current thinking (see below). Here the software program Adobe Photoshop enables researchers to superimpose the fossil plate upon the counterplate to provide a more complete set of data. This is the DGS method, a tried and true method for identifying bones to aid in interpretation as a prelude to creating a reconstruction. It’s much better than simply putting a label or arrow somewhere on the unoutlined bone. The only limitations are in the data available and the expertise of the interpreter.

Figure1. The plate and counter plate of Sclerosaurus, an ancestral taxon to soft shell turtled. Girdles and extremities are reconstructed.

Figure1. Click to enlarge. The plate and counter plate of Sclerosaurus, an ancestral taxon to soft shell turtled. Girdles and extremities are reconstructed. Frames change every 5 seconds. Here imposing the  plate and counter plate upon one another in Photoshop helps to reconstruct the specimen. The humerus has been rotated about 180 degrees during taphonomy.

Sclerosaurus armatus (Meyer 1859, Sues and Reisz 2008; Middle Triassic; ~50 cm in length), was originally considered a procolophonid, then a pareiasaurid, then back and forth again and again, with a complete account in Sues and Reisz (2008) who considered it a procolophonid. After Procolophon, Sues and Reisz (2008) considered TichvinskiaHypsognathus, Leptopleuron and Scoloparia sister taxa to Sclerosaurus. These all nest with Diadectes in the large reptile tree, not pareiasaurs.

Wikipedia also reports that Sclerosaurus is a procolophonid. Shifting Sclerosaurus to the procolophonids in the large reptile tree adds 55 steps.

Figure 1. New cladogram of turtle systematics. Note the separation of soft shell turtles with orbits visible in dorsal view from domed hard shell turtles with laterally oriented orbits here.

Figure 2. New cladogram of turtle systematics. Note the separation of soft shell turtles with orbits visible in dorsal view from domed hard shell turtles with laterally oriented orbits here.

Here, based on data from Sues and Reisz (2008), Sclerosaurus nests between pareiasaurs and basal soft-shell turtles like Odontochelys and Trionyx. It is a sister to Arganaceras, but was smaller with larger supratemporal horns.

Figure 3. Soft shell turtle evolution featuring Arganaceras, Sclerosaurus, Odontochelys and Trionyx.

Figure 3. Soft shell turtle evolution featuring Arganaceras, Sclerosaurus, Odontochelys and Trionyx. Sclerosaurus is currently transitional between pareiasaurs and soft shell turtles.

References
Meyer H von 1859. Sclerosaurus armatus aus dem bunten Sandestein von Rheinfelsen. Palaeontographica 7:35-40.
Sues H-D and Reisz RR 2008. Anatomy and Phylogenetic Relationships of Sclerosaurus armatus (Amniota: Parareptilia) from the Buntsandstein (Triassic) of Europe. Journal of Vertebrate Paleontology 28(4):1031-1042. doi: 10.1671/0272-4634-28.4.1031 online

wiki/Sclerosaurus

 

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