The turtle – placodont/pareiasaur wars

Before the pterosaur – archosaur/lepidosaur wars…
There were the turtle – placodont/pareiasaur wars.

And they’re still ongoing.
Well-meaning and thoughtful scientists have supported the notion that turtles are placodont-sauropterygians kin. Some thought flippered sea turtles were close to flippered plesiosaurs. More recently, placodonts, some of which have a plastron and carapace, have been favored. Those in this group include: Baur, Buckland, Jaeckel, Broom, Rieppel, Reisz and deBraga,

Gregory wrote about this old argument
in his 1946 paper (available online). Gregory favored nesting diadectids and pareiasaurs with turtles, then compared turtles unfavorably to captorhinomorphs, seymouriamorphs and placodonts. He correctly noted the earlier more basal placodonts were far less turtle-like and show more affinities with plesiosaurs and nothosaurs.

Gregory (1946) wrote, “When I turned my attention to the pareiasaurs, I was surprised to find that apart from their gigantic size they seemed on the whole to afford an excellent starting point for the chelonian line, both in their general construction and in many features of the skull, vertebrae, ribs, girdles, limb bones, hands, and feet.” Gregory found it odd that neither Seeley nor Haughton and Boonstra compared turtles to the pareiasaurs they knew about.

Oddly and on the same note,
Gregory never compared turtles to Stephanospondylus, a taxon studied by his student Romer (1925) and earlier by Stappenbeck (1905). Neither paper made it to his bibliography.

Gregory notes Eunotosaurus, but not millerosaurs, among the suprageneric “cotylosauria” from whence turtles most likely arose. We looked at this false lead earlier.

Of sideline interest, Gregory reports homologies between Procolophon and Diadectes. I’m glad someone else noticed that 60 years ago. That relationship is firmly established in the large reptile tree.

Rieppel and Reisz (1991)
These two did not consider diadectids to be amniotes, which is falsified by the large reptile tree results. Reisz and Laurin 1991 discussed turtles originating out of procolophonids in their paper on Owenetta, which does not nest anywhere near Procolophon in the large reptile tree.

Lee 1993 supported a pareiasaur affinity with turtles.

DeBraga and Rieppel (1997) expanded the taxon list and found turtles nested with diapsids as a sister group to the Sauropterygia.

Rieppel and Reisz (1999)
renewed their interest in this problem. Lee  examined the data matrix of deBraga & Rieppel (1991) and argued that many of the characters were incorrectly coded. Rieppel and Reisz (1999) agreed with the majority and made the changes. The new results  duplicated their original results, nesting turtles at the base of the sauropterygia (placodonts + pachypleurosaurs). Lepidosauriformes nested as the proximal outgroup. Choristoderes and Prolacertiformes was that clade’s outgroup. Millerettids, lanthanoschids, Macroleter, Procolophon and pareiasaurs nested in a separate clade in a broad radiation at  the base of the Amniota (standard for that day). Rieppel and Reisz (1999) pruned turtles and shifted them to pareiasaurs, adding 5 steps to a tree length of 793 steps.This study could have benefitted by fewer suprageneric taxa and more genus-based taxa.

Professional disagreements
Rieppel and Reisz (1999) reported, “we disagree with several of the proposed corrections, and for these retain the original coding. They include: [65] distinct basal tubera are absent in pareiasaurs, which show a secondarily derived condition, i.e., basal tubera on the parasphenoid; [82] the position of the mandibular joint relative to occiput is polymorphic in turtles; [I031 cervical centra of pareiasaurs are ridged but not keeled; [120, 1211 one coracoid ossification is present, and the coracoid foramen is enclosed by the coracoid and scapula, in pareiasaurs; [I 271 the presence of the ectepicondylar foramen in the humerus is polymorphic for turtles; [I401 a weak 4th trochanter is present in pareiasaurs and shifted to the edge of the femur; [I521 the first distal tarsal is retained as polymorphic in turtles.”

See, even the pros see the “facts” in different lights. And that’s okay. We’ll work it out. I don’t see anyone railing against these workers because they see things differently.

Lately
DNA has entered the fray, but to no avail. At the very least it has added to the confusion. Unfortunately DNA still can’t deal with Permian and Triassic taxa and we’ll always need to find closest kin in fossil forms from those eras.

The origin of turtles is one of the Holy Grails of paleontology.
Outmoded reliance on terms like “Diapsid” and “Anapsid” have only added to the confusion. In the large reptile tree, no suprageneric taxa are used. Among 341 possible nesting sites turtles nest with Millerosaurs / Pareiasaurs / Stephanospondylus with the near turtle, Odontochelys arising more directly and by convergence from Milleretta.

The whole point of this post is: Attitude and Bias
Once a paleontologist sinks his teeth into a particular hypothesis, no amount of verifiable data is going to shake him loose, whether you’re talking about pterosaurs or turtles. Many rely on suprageneric taxa to cherry pick traits favorable to their position. In that way it doesn’t matter whether basal taxa have a carapace and plastron or not.

It is so important to not use suprageneric taxa. It is also important to establish a large gamut study of generic taxa first so one has confidence that the correct taxa have been chosen for smaller, more focused studies. In the large reptile tree the traits carapace and plastron are not used, but could have appeared up to 8 times by convergence.

By the way,
in an earlier experiment, pterosaurs actually did nest more closely with turtles than with any archosaur. And absent all pareisaurs and millerettids, etc. etc. turtles and pterosaurs nested exactly at the base of the sauropterygia, as the earlier Rieppel studies indicated.

Now THAT’S the power of phylogenetic analysis!!!

References
Broom R 1924. On the classification of the reptiles. Bulletin of the American Museum of Natural History 51:39-45.
Carroll  RL 2012. Problems of the Ancestry of Turtles. Morphology and Evolution of Turtles. Part 2, 19-36. DOI: 10.1007/978-94-007-4309-0_3
deBraga M, Rieppel 0  1997. Reptile phylogeny and the interrelationships of turtles. Zool. J. Linn. Soc. 120:281-354
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
Layson TR, Bever GS, Bhullar B-AS, Joyce WG and Gauthier JA 2010. Transitional fossils and the origin of turtles. Biology Letters June 9 2010. doi: 10.1098/rsbl.2010.0371
Lee MSY 1993. The origin of the turtle body plan: bridging a famous morphological gap. Science 261: 1716-20
Li C, Wu X-C, Rieppel O, Wang L-T and Zhao L-J 2008. An ancestral turtle from the Late Triassic of southwestern China. Nature 456: 497-501.
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
Nagashima H, Sugahara F, Takechi M, Ericsson R,Kawashima-Ohya Y, Narita Y and Kuratani S 2009. Evolution of the Turtle Body Plan by the Folding and Creation of New Muscle Connections.Science 10 July 2009: 193-196.
Reisz RR, Laurin M. 1991. Owenetta and the origin of turtles. Nature 349:324-26.
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 2012. The Evolution of the Turtle Shell. Morphology and Evolution of Turtles. Part 2, 51-61. DOI: 10.1007/978-94-007-4309-0_5.
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/Odontochelys
wiki/Stephanospondylus

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