This post was set in motion by a recent PBS Eons YouTube video
all about the biggest fossil turtle ever described, Archelon (Figs. 1, 2). Click to play.
The narrator reported
that Archelon (Figs. 1, 2) was not related to living sea turtles, not even to Dermochelys, the living leatherback (Fig. 4). Well that mystery sounds like a job for the LRT. Maybe it can do some good. And it’s good to get back to reptiles for an evening. It’s been awhile…
Figure 1. Classic photos of Archelon in ventral and dorsal views.
in the large reptile tree (LRT, 1802+ taxa) Archelon (Figs. 1, 2) nests firmly with Macrochelys, the alligator snapping turtle (Fig. 3). That’s why Archelon is not related to living sea turtles and perhaps why it’s terrestrial origin has remained a mystery until now.
Once again, testing taxa together that have never been tested together before sometimes recovers such unexpected, but inevitable results.
When you see the skulls together
(Figs. 2, 3), the relationship seems obvious. Most turtles do not extend their premaxilla like a hawk beak, but Archelon and snapping turtles do. The skull suture patterns are also distinct from other turtles and shared between only these two of all other turtles tested in the LRT.
Figure 2. Skull of Archelon with colors identifying bones. Compare to Macrochelys in figure 3.
In the ancient and dangerous Niobrara Sea covering much of North America,
it took a giant, mean-old snapping turtle with flippers to survive in a seaway full of other giant monster reptiles.
Figure 3. Macrochelys skull in three views with colors added to bones. Compare to Archelon in figure 2. Image from Catalogue of shield reptiles in the collection of the British Museum.
(Wieland 1896; Late Cretaceous; 4.6m or 15 feet in length; Figs 1,2) is the largest turtle ever documented. Along with Protostegus, Archelon is traditionally considered a member of the Protostegidae. In the LRT Archelon nests with Macrochelys, the alligator snapping turtle (Fig. 3). Distinct from Macrochelys, the naris opens dorsally in Archelon.
Figure 4. Macrochelys skeleton documenting the origin of the open ribs with small fenestrations.
Archelon is distinct from and parrallel to
other sea turtles, all of which have a shorter, transverse premaxilla and different skull bone patterns (e.g. Fig. 4). Previous workers had already removed protostegids from other sea turtles, but then stopped there. The Archelon relationship to snapping turtles was not tested or known until now. If proposed previously, please send a citation so I can promote it here.
A leathery carapace,
like that of Dermochelys, covered the similarly open ribs of Archelon (Fig. 1), but the two tax are not related. Dermochelys is closer to sea turtles with a traditional hard-shelled carapace.
Figure 4. Skulls of Dermochelys, the extant leatherback turtle. The skull pattern here is distinct from patterns in Archelon and other snapping turtles (above).
Not sure why snapping turtles and Archelon
were never shown to be related to one another before. It seems obvious in hindsight. This struck me as low-hanging fruit left by PhDs for armchair amateurs to deduce. It just took one evening to nest this enigma. Let me know if there are any more enigmas lurking out there that need a good nesting. This is the fun part.
Postscript Feb. 19, 2021
Readers have reported that I might have colorized osteoderms or scales instead of bone sutures. Jura sent the images on the left, which I desaturated and burned to bring out details. Those seem to show scalation. The colored images appear to show sutures. Right? Or wrong?
Jura replied: top = sutures, bottom = welded osteoderms. Compare the top image with figure 4 from Sheil 2005′
The Shiel 2005 image of Macrochelys (= Macroclemys) is a diagram drawing from Gaffney 1979. The Gaffney 1979 image is a diagram drawing from Gaffney 1975e.
Figure x. Osteoderms on the left don’t always align with bones on the right in these images of Macrochelys.
Figure y. Macrochelys skull with traditional labels (b&w) and LRT labels (color). The LRT prefrontal rims the orbit, as in all other tetrapods.
It seems to me,
and let me know if this is an error, that everybody recognizes the pair of bones over the naris. Traditionally these are labeled prefrontals (Fig. y), even though they don’t touch the orbit. Other bones have different traditional labels, too. My labels come from pareiasaur and Elginia homologs so those labels come from a valid phylogenetic context. Traditional labels are wrong because the pareiasaur ancestry is not yet widely, if at all, recognized. All other turtle ancestor candidates are tested in the LRT.
Gaffney ES 1975e. Phylogeny of the chelydrid turtles: a study of shared derived characters in the skull. Fieldiana:Geol., vol. 33, pp. 157-178.
Gaffney ES 1979. Comparative cranial morphology of recent and fossil turtles. Bulletin of the American Museum of Natural History 164(2):65–376.
Sheil CA 2005. Skeletal development of Macrochelys terrminckii (Reptilia: Testudines: Chelydridae) Journal of Morphology 263:71–106.
Wieland GR 1896. Archelon ischyros: a new gigantic cryptodire testudinate from the Fort Pierre Cretaceous of South Dakota. American Journal of Science. 4th series. 2 (12): 399–412.