Basal turtles with a lateral temporal fenestra

Today let’s look at
Glyptops plicatulus (Marsh 1890; AMNH 336; Late Jurassic), an associated skull, shell, and partial skeleton (Fig. 1). Gaffney (1979) reported, “The poor preservation of the skulls precludes a detailed study of the skull roof.” That may be true. Or not. Bones appear to be lost from the temporal regions, but every temporal bone can be identified, just smaller.

Apparently Glyptops had large skull openings
like other turtles. Here the temporal bones were reduced, leaving lateral and suparaoccipital openings, like other turtles. A DGS tracing (Fig. 1) and reconstruction (Fig. 2) provide one solution. Perhaps not the only solution, but one worth considering because no bones are missing here (contra Gaffney 1979).

Figure 1. Glyptops, a basal hard-shell turtle in several views. All data from Gaffney 1979 except the color overlays, which are applied here and used to make the reconstruction in figure 2.

Figure 1. Glyptops, a basal hard-shell turtle in several views. All data from Gaffney 1979 except the color overlays, which are applied here and used to make the reconstruction in figure 2.

According to Gaffney (1979), “their sole unique feature an elongate basisphenoid extending the length of and completely separating the pterygoids.”

Figure 2. Glyptops skull reconstructed from color overlays in figure 1. Note the semi-fenestrated skull mimicking the diapsid configuration.

Figure 2. Glyptops skull reconstructed from color overlays in figure 1. Note the semi-fenestrated skull mimicking the diapsid configuration that Gaffney considered poorly preserved. Gray areas are restored based on sister taxa.

Many traits presage the appearance of traits
in derived turtles, like Terrapene, the Eastern box turtle, by convergence. The two are not directly related to one another, despite sharing several traits. In Glyptops the frontals (lavender) were separated from the parietals (amber) by intervening postfrontals (orange) and postorbitals (aqua) that meet at the midline.

Figure 3. Subset of the large reptile tree (LRT, 1199 taxa) with the addition of three basal turtles

Figure 3. Subset of the large reptile tree (LRT, 1300 taxa) with the addition of three basal turtles

Other turtles that have lateral temporal fenestrae
include the leatherback sea turtle, Dermochelys (Fig. 3, we looked at yesterday), and Meiolania (Fig. 4, now basal to Proganochelys) by convergence.

Figure 2. Skull of Dermochelys adult and juvenile demonstrating the lengthening of the temporal region during maturity. The lateral temporal fenestra appears between the squamosal and quadrate.

Figure 4. From yesterday’s blogpost, the skull of Dermochelys adult and juvenile demonstrating the lengthening of the temporal region during maturity. The lateral temporal fenestra appears between the squamosal and quadrate.

So some turtles are anapsids,
(reptiles that lack temporal openings). Others are not. None are phylogenetic diapsids, despite having large skull openings (Fig. 1) from the top and the sides.

These exceptions remind us
not to define reptiles by their traits (although most of the time this method works well), but rather by their phylogenetic placement (Fig. 3), a method that always works.

Figure 1. Meiolania has a lateral temporal fenestra created by more bone encircling the tympanum (ear drum) at the quadrate. It could be that the top of the qj is actually the fused sq.

Figure 5. Meiolania has a lateral temporal fenestra created by more bone encircling the tympanum (ear drum) at the quadrate, not related to the other temporal openings that start at the back or the bottom of the skull.

Tomorrow,
more laterally fenestrated turtles.

References
Gaffney ES 1979. The Jurassic Turtles of North America. Bulletin of the American Museum of Natural History 162(3):91-136.
Marsh OC 1890. Notice of some extinct Testudinata. American Journal of Science ser. 3, vol. 40, art. 21: 177–179.

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