Earlier here and here we looked at the ancestry of turtles and changes to the labels on the turtle skull. In the next few posts we’ll take a deeper look at Elginia, Meiolania, Odontochelys and Proganochelys, basal turtles that illuminate relationships and bone labels. I encourage you to flip back and forth to compare images or drag an image or two to your desktop for ready comparison.
Apparently the loss of teeth in turtles occurred at least twice, once in the clade of living turtles and once in this clade of horned turtles.
Perhaps the key to understanding turtle skull morphology is the placement of the squamosal, which retains its ancestral shape and placement in horned turtles while the greatly enlarged supratemporal provided a new posterior rim to the skull. I don’t think this has been recognized before. Once that identification is made, the rest of the bones fall into place.
Side-by-side comparisons of Elginia and Meiolania (Fig. 4) make the case for homology and close relationship.
Sclerosaurus is another horned reptile, now nesting at the base of the all turtles (Fig. 5). It had a low wide body without a carapace and plastron, but with a pattern of dermal ossicles. Apparently supratemporal horns were primitive and later lost in the clade of living turtles and their prehistoric ancestors.
Ironically several ancestors of turtles
have been known for quite some time. They just have not been included in phylogenetic analyses together and in a large enough analysis to eliminate all other possibilities.
Gaffney ES 1983. The cranial morphology of the extinct horned turtle, Meiolania platyceps, from the Pleistocene of Lord Howe Island, Australia. Bulletin of the AMNH 175, article 4: 361-480.
Gaffney ES 1985. The cervical and caudal vertebrae of the cryptodiran turtle, Meiolania platyceps, form the Pleistocene of Lord Howe Island, Australia. American Museum Novitates 2805:1-29.
Gaffney ES 1996. The postcranial morphology of Meiolania platyceps and a review of the Meiolaniidae. Bulletin of the AMNH no. 229.
Newton ET 1893. On some new reptiles from the Elgin Sandstone: Philosophical Transactions of the Royal Society of London, series B 184:473-489.
Owen R 1882. Description of some remains of the gigantic land-lizard (Megalania prisca
Owen), from Australia. Part III.Philosophical Transactions of the Royal Society London, series B, 172:547-556.
Owen R 1888. On parts of the skeleton of Meiolania platyceps (Owen). Philosophical Transactions of the Royal Society London, series B, 179: 181-191.