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.
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.
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.
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.
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.
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.
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.
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.