Another turtle with teeth, Elginia – part 3

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.

Figure 1. The skull of Elginia in four views. The basipterygoid and basiocciput are missing. While most skull bones are fused, colors are applied here that delineate the bones. Note the squamosal (lavender) in the central cheek is surrounded by other bones, especially the supratemporal that descends to the quadratojugal as in Meiolania.

Figure 1. The skull of Elginia in four views. The basipterygoid and basiocciput are missing. While most skull bones are fused, colors are applied here that delineate the bones. Note the squamosal (lavender) in the central cheek is surrounded by other bones, especially the supratemporal that descends to the quadratojugal as in Meiolania (Fig. 2).

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.

Figure 2. The skull of Meiolania platyceps as  restored by Gaffney 1983. Color overlays delineate skull bones identified here. Note the squamosal retains its concave posterior rim while the supratemporal descends to articulate with the quadratojugal, increasing the armor coverage of the neck while retaining an eardrum aperture.

Figure 2. The skull of Meiolania platyceps as restored by Gaffney 1983 (in black). Color overlays delineate skull bones identified here. Note the squamosal retains its concave posterior rim while the supratemporal descends to articulate with the quadratojugal, increasing the armor coverage of the neck while retaining an eardrum aperture. Distinct from living turtles, the lacrimal contacts the naris (Fig. 3).

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.

Figure 3. The skull of Meiolania platy ceps in several views from Gaffney 1983. Color overlays identify bones. Note the separation of the postfrontal and postorbital along with the lacrimal naris contact.

Figure 3. The skull of Meiolania platy ceps in several views from Gaffney 1983. Color overlays identify bones. Note the separation of the postfrontal and postorbital along with the lacrimal naris contact.

Side-by-side comparisons of Elginia and Meiolania (Fig. 4) make the case for homology and close relationship.

Figure 1. Elginia is a toothed turtle, basal to the giant horned toothless turtle, Meiolania.

Figure 4. Elginia is a toothed turtle, basal to the giant horned toothless turtle, Meiolania. The former has teeth. The latter does not. The former is known from a skull only. The latter is known from several complete skeletons complete with a carapace, plastron and armored, club tail.

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.

Figure 5. Sclerosaurus now nests at the base of all turtles, both horned and hornless. Note the dermal ossicles.

Figure 5. Traditionally considered a horned procolophonid, Sclerosaurus now nests at the base of all turtles, both horned and hornless. Note the dermal ossicles, unknown in other procolophonids. Evidently horns were lost in the lineage of living turtles.

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.

References
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.

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