Stephanospondylus had a proto-carapace! (and probably a plastron)

Updated April 08, 2014

Stappenbeck (1905) illustrated several parts of a disarticulated Early Permian fossil, Stephanospondylusthat he considered a diadectid. So did Kissel (2010) in his much more recent PhD dissertation. Stappenbeck (1905) also created an odd reconstruction which Romer (1925) thoroughly dismissed because Stappenbeck included bones from a nearby.

This weekend
I reassembled the individual parts illustrated in Stappenbeck (1905) and discovered that the clavicle is really a costal plate on a dorsal rib (Fig. 1), as in Odontochelys. That’s exciting news! That solidifies Stephanospondylus to turtles. We took a recent look at Odontochelys here.

The Stephanospondylus parts are reassembled here (Fig. 1). Not sure how many vertebrae separated the scapula and pelvis. Not sure how long the neck or tail were.

Cementing an earlier nesting
Earlier we found that Stephanospondylus was a sister to the ancestor of turtles based on the results of the large reptile tree. These new bone identifications further cement that relationship and, for the first time, provide it with a proto-carapace and strongly hint at the presence of a plastron.

Figure 1. Click to enlarge. Stephanospondylus was considered a type of diadectid, but it nests with turtles and pareiasaurs, all derived from millerettids.

Figure 1. Click to enlarge. Stephanospondylus was considered a type of diadectid, but it nests with turtles and pareiasaurs, all derived from millerettids.

The origin of turtles is one of the biggest enigmas in paleontology. Hopefully these new insights will help resolve the problem. 65 million years separate Stephanospondylus from Odontochelys, and 80 million years to Proganochelys (Fig.1), so there’s plenty of time to evolve more turtle-like traits and a variety of turtles world-wide, even at a turtle-like pace.

Figure 2. Costal plate rib of Stephanospondylus, originally identified as a clavicle by Stappenbeck 1905.

Figure 2. Costal plate rib of Stephanospondylus, originally identified as a clavicle by Stappenbeck 1905. Note the resemblance to similar ribs on Odontochelys, expanded from the narrow ribs on Milleretta.

I had one of those “discovery” moments
At first I accepted Stappenbeck’s identification of the spade-shape bone (Fig. 2) as a clavicle. But it just didn’t fit. And it didn’t resemble any clavicles on sister taxa.  I realized that the bone more closely resembled the expanded ribs of Odontochelys, which happened to be on the same figure (Fig. 1). It also resembles the anteriormost dorsal ribs of Diadectes, but those do not have the texture seen above.

The take-away:
it pays to create reconstructions. Odd-balls become known. Comparisons provide more parsimonious answers.

The same sort of magic moments
occurred earlier with the prepubis and coracoid of Cosesaurus and with the entire back end of Longisquama. And there are several other examples, from the vampire pterosaur to the flightless pterosaur. Sometimes it just takes insight. And insight comes randomly and with increasing momentum with more experience, which I am slowly gaining with turtles.

Back in 1905,
Stappenbeck did not have the example of Odontochelys, nor any of that century’s discoveries to compare his fossil to. So making those errors does not make him a bad scientist. This is just an example of the progress we’re all here to advance, especially on rarely studied fossils.

The history of the fossil(s) and Romer’s 1925 take
According to Wikipedia, “Stephanospondylus is known only from several vertebrae and fragments of the upper and lower jaws. It was named in 1882 on the basis of two slabs, the fossils in which were thought to represent two individuals. With the erection of a new genus in 1905, the fossils were considered to be part of a single individual.[1] In 1925, Alfred Romer determined that only parts of the jaws and some vertebrae belonged to Stephanospondylus; the other material belonged to the temnospondyl amphibian Onchiodon.” (Fig. 3).

Figure 3. Click to enlarge. Onchiodon is a temnospondyl amphibian known since 1866. Romer (1925) was not thinking of basal turtles when he saw the costal ribs. Moreover, as a diadectid, Stephanospondylus was considered an amphibian, not a reptile. Note the colorized parts do bear some resemblance to those in Stephanospondylus, but a closer match to Stephanospondylus is with Odontochelys.

Figure 3. Click to enlarge. Onchiodon is a temnospondyl amphibian known since 1866. Romer (1925) was not thinking of basal turtles when he saw the costal ribs. Moreover, as a diadectid, Stephanospondylus was considered an amphibian, not a reptile. Note the colorized parts do bear some resemblance to those in Stephanospondylus, but a closer match to Stephanospondylus is with Odontochelys.

Romer made an important contribution in 1925. He found four scapulae and two different kinds of vertebrae in the scattered slab materials. Romer did not attempt a reconstruction. That might have helped as he had all the data necessary to reconstruct a basal turtle.

References
Broom R 1924. On the classification of the reptiles. Bulletin of the American Museum of Natural History 51:39-45.
Geinitz HB and Deichmüller JV 1882. Die Saurier der unteren Dyas von Sachsen. Paleontographica, N. F. 9:1-46.
Gregory WK 1946. Pareiasaurs versus placodonts as near ancestors to turtles. Bulletin of the American Museum of Natural History 86:275-326
Kissel R 2010. Morphology, Phylogeny, and Evolution of Diadectidae (Cotylosauria: Diadectomorpha). Toronto: University of Toronto Press. pp. 185. online pdf
Li C, Wu X-C, Rieppel O, Wang L-T, Zhao L-J 2008. An ancestral turtle from the Late Triassic of southwestern China. Nature 456: 497-501.
Lyson TR, Sperling EA, Heimberg AM, GauthierJA, King BL, and Peterson KJ 2011.MicroRNAs support a turtle + lizard clade. Biol Lett 2011 : rsbl.2011.0477v1-rsbl20110477.abstract – online news story
Reisz RR and Head JJ 2008. Turtle origins out to sea. Nature 456, 450–451.
Rieppel O and deBraga M 1996. Turtles as diapsid reptiles. Nature 384:453-454.
Rieppel O and Reisz RR 1999. The Origin and Early Evolution of Turtles. Annual Review of Ecology and Systematics 30: 1-22.
Romer AS 1925. Permian amphibian and reptilian remains described as Stephanospondylus. Journal of Geololgy 33: 447-463.
Stappenbeck R 1905. Uber Stephanospondylus n. g. und Phanerosaurus H. v. Meyer: Zeitschrift der Deutschen Geologischen Gesellschaft, v. 57, p. 380-437.
Williston SW 1917. The phylogeny and classification of Reptilies. Journal of Geology 28: 41-421.

wiki/Stephanospondylus

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