Lee 1993 – An Important Contribution to Turtle Origins

Earlier we looked at several convergent turtle-like taxa. Today we’ll take a good look at the pareiasaurs, the second* closest taxon to the turtles themselves.

The origin of turtles
is one of the most hotly debated topics in paleontology. Unique among living amniotes, turtles have a carapace, plastron and a shoulder girdle within the rib cage. Some DNA studies point to an archosaur link. Other studies link turtles to lizards. Only the large reptile tree looked at over 335 possible nesting sites for turtles and came up with one.

Dr. Michael S. Y. Lee (1993) provided the best published morphological report on turtle origins to date. This paper precedes the discovery of Odontochelys (Li et al. 2008), overlooks Stephanospondylus (Geinitz and Deichmuller 1882) and nests turtles with pareiasaurs like Deltavjatia (Hartmann-Weinberg 1937).

In Lee (1993) pareiasaurs were found to share 16 derived traits with turtles. These traits are identified with an “A“.

Cranial traits synapomorphies:
A1 – (Fig. 2) Choana (internal nares) located far medially.
A2 – (Figs. 1,2) Foramen palatinum medially located (similar to the suborbital fenestra).
A3 – (Fig. 2) Massive horizontal paroccipital process sutured to squamosal.
A4 – (Fig. 2) Long lateral flange of the exoccipital on the posterior face of the paroccipital process.
A5 – (Fig. 2) Basisphenoid and basioccipital ossified together.
A6 – (Fig. 2) Ossified medial wall of prootic.
A7 – (Fig. 2) Transverse flange of pterygoid reduced and forwardly directed.
A8 – (Fig. 1`) Supraoccipital forms a long, high, narrow and median ridge sutured to the skull roof along its entire length.
A9 - The entire palate is raised well above the ventral margin of the maxilla.

Postcranial Trait Synapomorphies
A10 – (Fig. 3) Prominent lateral projections on at least the first 14 caudal vertebrae.
A11 – (Fig. 3) Chevrons not wedged between adjacent centra.
A12 – (Figs. 1, 3) Scapula with acromion process on the anterior margin
A13 – (Fig. 1) Humerus with ectepicondylar foramen.
A14 – (Fig. 3) Femur with a major trochanteron the posterior margin.
A15 – (Fig. 3) Reduced pedal digit 5.
A16 – Prominent dorsal buttress, V-shaped in ventral view, overhanging the acetabulum.

Sclerosaurus
Nine more traits are shared by Proganochelys, pareiasaurs and Sclerosaurus, the smaller, flatter, pareiasaur sister. These are identified with a “B” by Lee (1993).

B1 – (Fig. 1) Twenty or fewer presacral vertebrae.
B2 – (Figs. 1, 3) Tall and narrow scapula (4x higher than wide).
B3 – (Figs. 1, 3) Shoulder glenoid not screw-shaped, but bipartite.
B4 – (Figs, 1-3) Scapula oriented anterodorsally, not horizontally.
B5 – Reduced manual phalangeal formula (23332)
B6 – (Fig. 3) Astragalus and calcaneum fused
B7 – (Fig. 3) Reduced pedal phalangeal formula (23343)
B8 – (Fig. 1) Thick dermal armor over the dorsal region.
B9 – Loss of gastralia.

The large reptile tree found Sclerosaurus to be a derived pareiasaur, not closer to turtles. Chronologically Stephanospondylus preceded turtles and Sclerosaurus by 70 million years. Stephanspondylus preceded pareiasaurus by 35 million years, plenty of time for these radiations to occur. Look for primitive turtles in the mid to late Permian, concurrent with pareiasaurs.

But wait, there’s more…
The large reptile tree used only a few of the above traits yet to likewise nest turtles with pareiasaurs and Sclerosaurus. Stephanospondylus does not preserve any palate, tail, manus femur, pes or armor data.

The scapula question
Lee notes that pareiasaurs and Sclerosaurus possess 5 cervicals and 14-15 dorsals for a total of 19 to 20. Turtles possess 8 cervicals and 10 dorsals, meaning that 3 turtle cervicals are former dorsals. This change was accompanied by a posterior shift of the pectoral girdle (Watson 1914) that is recapitulated during turtle ontogeny (embryogenesis).

All known pareiasaurs are too pareiasaur-y to be ancestral to turtles
*Stephanospondylus is a key taxon linking diadectids to pareiasaurs and turtles that avoids being to “pareiasaur-y.” No known archosaur shares so many turtle traits. No known sauropterygian comes close either. Out of 335+ taxa, Stephanospondylus remains the best candidate I’ve found. But, sans that taxon, turtles would nest just outside the Pareiasauria.

Hats off to Dr. Lee for doing a great job.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Geinitz HB and Deichmüller JV 1882. Die Saurier der unteren Dyas von Sachsen. Paleontographica, N. F. 9:1-46.
Hartmann-Weinberg AP 1933. Evolution der Pareiasauriden: Trudy Palaeontological institute Academe Nauk, SSSR, 1933, n. 3, p. 1-66.
Lee MSY 1993. The Origin of the Turtle Body Plan: Bridging a Famous Morphological Gap. Science 264:1716-17-1719.
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.
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.
Watson DMS 1914. Eunotosaurus africanus Seeley and the ancestors of the Chelonia. Proceedings of the Zoological Society of London 11:1011.

Palaos discussion