Archelon enters the LRT with snapping turtles

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. ” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2021/02/archelon_ventral_dorsal588.jpg?w=166″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2021/02/archelon_ventral_dorsal588.jpg?w=568″ class=”size-full wp-image-53226″ src=”https://pterosaurheresies.files.wordpress.com/2021/02/archelon_ventral_dorsal588.jpg?w=584&h=1053″ alt=”Figure 1. Classic photos of Archelon in ventral and dorsal views. ” width=”584″ height=”1053″ srcset=”https://pterosaurheresies.files.wordpress.com/2021/02/archelon_ventral_dorsal588.jpg?w=584&h=1053 584w, https://pterosaurheresies.files.wordpress.com/2021/02/archelon_ventral_dorsal588.jpg?w=83&h=150 83w, https://pterosaurheresies.files.wordpress.com/2021/02/archelon_ventral_dorsal588.jpg?w=166&h=300 166w, https://pterosaurheresies.files.wordpress.com/2021/02/archelon_ventral_dorsal588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />

Figure 1. Classic photos of Archelon in ventral and dorsal views.

After testing
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. ” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2021/02/archelon_skull2lateral588.gif?w=300″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2021/02/archelon_skull2lateral588.gif?w=584″ class=”size-full wp-image-53228″ src=”https://pterosaurheresies.files.wordpress.com/2021/02/archelon_skull2lateral588.gif?w=584&h=318″ alt=”Figure 2. Skull of Archelon with colors identifying bones. Compare to Macrochelys in figure 3. ” width=”584″ height=”318″ />

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. ” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2021/02/macrochelys-skull-colors.gif?w=196″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2021/02/macrochelys-skull-colors.gif?w=584″ class=”size-full wp-image-53230″ src=”https://pterosaurheresies.files.wordpress.com/2021/02/macrochelys-skull-colors.gif?w=584&h=894″ alt=”Figure 3. Macrochelys skull in three views with colors added to bones. Compare to Archelon in figure 2. ” width=”584″ height=”894″ />

Figure 3. Macrochelys skull in three views with colors added to bones. Compare to Archelon in figure 2.

Archelon ischyros
 (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.

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). ” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2021/02/dermochelys_skull588.gif?w=300″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2021/02/dermochelys_skull588.gif?w=584″ class=”size-full wp-image-53234″ src=”https://pterosaurheresies.files.wordpress.com/2021/02/dermochelys_skull588.gif?w=584&h=322″ alt=”Figure 4. Skulls of Dermochelys, the extant leatherback turtle. The skull pattern here is distinct from patterns in Archelon and other snapping turtles (above).” width=”584″ height=”322″ />

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.

References
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.

wiki/Macrochelys
wiki/Archelon