Two basal turtles added to the large reptile tree

Today
we add Niolamia (Fig. 2) and Mongolochelys (Fig. 4) to the large reptile tree (Fig. 3).

Figure 3. Meiolania is another club-tailed, short-toed turtle like Proganochelys.

Figure 3. Meiolania is a club-tailed, late-surviving basalmost turtle  related to Niolamia.

One of the basalmost hard shell turtles,
Meiolania, has a more bizarre yet older sister. Niolamia (Fig. 2) has even larger supratemporal and tabular horns, that originated with taxa like toothy Elginia from the Late Permian (Fig. 2), All known meiolanids were late-surviving members of the most basal hard-shell turtle clade with probable origins in the Early Triassic. The age of Niolamia can only be estimated (Late Cretaceous to Eocene) due to purposeful loss of formation data by its 19th century collectors. Prior turtle workers have either judiciously or accidentally avoided putting these three taxa together in the same phylogenetic analysis, despite their obvious similarities.

Figure 1. The basal turtle, Niolamia, compared to the toothed pareiasaur/turtle?, Elginia. We have no post-crania for Elginia. Figure 1. The basal turtle, Niolamia, compared to the toothed pareiasaur/turtle?, Elginia. We have no post-crania for Elginia.

Figure 2. The basal turtle, Niolamia, compared to the toothed pareiasaur/turtle?, Elginia. We have no post-crania for Elginia.

Phylogenetic analysis
confirms what any first look suggests: Niolamia and Meiolania nest with each other. In the large reptile tree these two nest as basalmost hard shell turtles. If only we knew that Elginia has a carapace and plastron (current status unknown), then it would also be the basalmost hard shell turtle, despite the teeth.

Figure 1. The origin and evolution of turtles. Here Meiolania and Niolamia nest as the most basal hard shell turtles. Odontochelys is the most basal soft shell turtle.

Figure 3 The origin and evolution of turtles. Here Nolamia and Mongolochelys have been added to the previous tree. Meiolania and Niolamia nest as the most basal hard shell turtles. Odontochelys is the most basal soft shell turtle. Mongolochelys and Chubutemyx (Fig. 4) do not nest with meiolanids, but as more derived turtles lacking skull ornamentation.

Sterli and de la Fuente 2011
produced the latest published literature on Niolamia and this following a thorough cleaning of the fossil. Many of the bones colored above (Fig. 1) match similar bones in Elginia, but are considered ossified scales by Sterli and de la Fuente. They also considered the shelf-like tabulars to be neo-ossifications. Their emended diagnosis notes, “an extensive contribution of the supraoccipital to the dorsal skull roof.” THAT would be very odd to see any supraoccipital as an extensive dorsal element. On more derived turtles, the supra occipital extends posteriorly as a narrow element and its dorsal contribution is minimal. Sterli and de la Fuente do not list tabulars on their list of bone abbreviations. They’re not making homologies with pareiasaurs. The supratemporal horns are considered to be ‘horns’ arising from the squamosal. We looked at the traditional misidentification of the suptratemporal and squamosal in turtles earlier here. That concept has to be adopted universally in order to make further progress on turtle systematics.

In phylogenetic analysis
Sterli and de la Fuente 2011 nest meiolanids with  Mongolochelys (Late Cretaceous) and Chubutemys (Early Cretaceous), two late-surviving basal turtles with no trace of horns (Fig. 3). These two start the process of bone fusion seen in all later turtles. In the large reptile tree (subset Fig. 2) these two taxa nest more derived than Proganochelys and Proterochersis.

Figure 3. Mongolocheys and Cubutemys nest together near, but not at, the base of the hard shell turtles. Both were considered sisters to Meiolania and Niolamia by prior workers who did not include Elginia in phylogenetic analysis.

Figure 4. Mongolocheys and Cubutemys to scale nest together near, but not at, the base of the hard shell turtles. Both were considered sisters to Meiolania and Niolamia by prior workers who did not include Elginia in phylogenetic analysis. Both have the genesis of the large post temporal fenestra that contains large jaw muscles.

The horns of meiolanids
were lost in more derived hard-shell turtles. At the same time, turtles began to become sea turtles while others gained the ability to withdraw their skull beneath the carapace in one of two ways, vertical and sideways. So what look like derived traits in an apparently aberrant late-surviving clade are actually primitive and not quite as aberrant as previously thought.  Elginia provides the blueprint or bauplan for hard-shell turtle skulls, which retain a large supratemporal, contra all prior studies. As noted earlier, meiolanids are the last turtles to retain laterally splayed forelimbs. In all other known turtles, the elbows are oriented anteriorly. The club tail is also be primitive for turtles, but we don’t have the data on known stem turtle pareiasaurs yet…

Paralleling the situation in pterosaur ancestors
like Sharovipteryx and Longisquama, the exotic, difficult to nest turtle and stem turtle taxa (Fig. 2) end up nesting together in a large gamut phylogenetic analysis.

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.
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.
Sterli J and de la Fuente M 2011. Re-Description and Evolutionary Remarks on the Patagonian Horned Turtle Niolamia argentina Ameghino, 1899 (Testudinata, Meiolaniidae). Journal of Vertebrate Paleontology 31 (6): 1210–1229. doi:10.1080/039.031.0618.

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