Turtle elbows: extreme pronation of the humerus

Pronation and Supination
Typically when we humans pronate our forelimbs, our elbows tuck into our sides and our palms open skyward. Supination is the opposite, useful when separating stuck elevator doors by hand. In pronation (palms up, thumbs out) the radius and ulna are parallel. In supination (palms down, thumbs in) the radius crosses over the ulna.

Figure 1. Galapagos turtles. Note the anterior direction of the elbows, very odd for a tetrapod.

Figure 1. Galapagos turtles. Note the anterior direction of the elbows, very odd for a tetrapod.

In birds, bats and pterosaurs
neither pronation nor supination of the forelimb is possible. That restriction makes for a more stable wing in flight.

In the standard tetrapod
in terrestrial locomotion, the elbows are held out laterally to posteriorly and the palms of the hand face ventrally, in contact with the substrate, toes anteriorly. This involves mild supination of the forelimb in which the radius crosses over the ulna in quadrupedal mammals.

In living turtles
the forelimbs are extremely pronated (Owen 1866, Figs 1, 2), with elbows that extend anteriorly while the toes continue to extend anteriorly. Don’t even try to do this yourself. It is an impossible task for anyone with a normal human anatomy.

Figure 2. Box turtle (Terrapene) ventral view with humerus, in red. Elbows anterior here, hands extremely pronated.

Figure 2. Box turtle (Terrapene) ventral view with humerus, in red. Elbows anterior here, hands extremely pronated.

The question is:
Do we have fossils that show this transition from lateral elbows in non-turtles to anterior elbows in turtles?

Apparently so… if
we follow the cladogram in the large reptile tree.

Sclerosaurus (Fig. 3) is the most proximal pre-turtle. It had lateral elbows and no shell. We don’t have post-crania for a sister taxon, Elginia, yet, but when we do it will be big news.

Figure 3. Soft shell turtle evolution featuring Arganaceras, Sclerosaurus, Odontochelys and Trionyx.

Figure 3. Soft shell turtle evolution featuring Arganaceras, Sclerosaurus, Odontochelys and Trionyx.

Odontochelys is a basal soft-shelled turtle with teeth and anterolateral elbows (Fig. 3). You can see the in situ fossil here.

Nesting between Sclerosaurus and Odontochelys are two other basal turtle taxa.

Proganochelys (Baur 1887)
is like Odontochelys and living turtles with anterior elbows (Fig. 3). Proganochelys is often considered the most primitive turtle next to Odontochelys.

Figure 4. Dorsal view of Proganochelys in situ showing the anterior elbows emerging from its shell.

Figure 5. GIF animation, 2 frames, each 5 seconds in length. Dorsal view of Proganochelys in situ showing the anterior elbows emerging from its shell. Images from Gaffney 1990.

Meiolania (Owen 1882)
is more primitive and has more lateral elbows, like those of Sclerosaurus (Fig. 6). BTW, this humeral orientation supports a heretical nesting of Meiolania basal to all other known turtles in the large reptile tree.

Figure 5. Meiolania, the most primitive of known turtles, has lateral forelimbs, like non turtles.

Figure 6. Meiolania, the most primitive of known turtles, has lateral forelimbs, like non turtles. Large plastron openings permit the movement of the short forelimbs. Later turtles would rotate the elbows forward and seal off the plastron with additional armor. Images from Gaffney 1996 with red areas added.

Meiolania is a sister to the most primitive (as yet undiscovered) turtle, more primitive than the toothed turtle Odontochelys. So teeth probably disappeared in turtles in more than one lineage. Large open areas in the plastron of Meiolania provided room for the small limbs to move beneath the shell, not out in front. Later turtles sealed up the plastron and rotated their large limbs and elbows forward and beyond the carapace.

Figure 6. GIF animation of the humerus of Proganochelys and Meiolania. Note the angle between the proximal and distal condyles changes.

Figure 7. GIF animation of the humerus of Proganochelys and Meiolania. Note the angle between the proximal and distal condyles changes. Scene changes every 5 seconds for two frames.

Comparing the humerus of Proganochelys to Meiolania (Fig. 7) shows the proximo-distal angle of condyles changes. The former had more anterior elbows than the latter.

We’re used to having elbows in the back. Here’s what a turtle skeleton looks like when the elbow is anterolateral (Fig. 8), as it is anytime the forelimb is in extreme pronation. Compare figure 7 to figure 1 (Sclerosaurus) and note the positions of the manus, radius and ulna do not change during the rotation of the humerus from lateral to anterior.

Figure 8. Galapagos turtle humerus painted red. Note the position of the anteriolateral elbow.

Figure 8. Galapagos turtle, humerus painted red. Note the position of the chiefly lateral elbow on a humerus that is anteriorly oriented. The positions of the manus, radius and ulna are no different here than in the lateral fore limb of Sclerosaurus.

References
Baur G 1887. On the phylogenetic arrangement of the Sauropsida: Journal of Morphology, v. 1, n. 1:93-104.
Gaffney ES 1990. The comparative osteology of the Triassic turtle Proganochelys, Bull. Am. Mus. Nat. Hist. 194: 1–263.
Gaffney ES 1996. The postcranial morphology of Meiolania platyceps and a review of the Meiolaniidae. Bulletin of the AMNH no. 229.
Owen R. 1866. On the anatomy of vertebrates, volume 1. pp. 172.
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.

wiki/Meiolania
wiki/Proganochelys

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