Late Jurassic basal sea turtle, Thalassemys, enters the LRT

Joyce, Mäuser and Evers 2021
document two new specimens of a basal sea turtle from the Late Jurassic Solnhofen Formation, Thalasssemys bruntrutana (Fig. 1. NKMB Watt18/211; Rütimeyer 1873). One fossil preserves soft tissue around the hind pes. The shell is disc-like with lateral ribs and the skull is small with large orbits. Manual digit 5 is the longest and metacarpal 4 is the longest.

Other Solnhofen turtles include
Eurysternum, Solnhofia and Palaeomedusa.

Figure 1. Thalassemys bruntrutana in situ with diagram from Joyce, Mäuser and Evers 2021 who nested Thalassemys at the base of their monophyletic clade of sea turtles (Fig. 2).

The Joyce, Mäuser and Evans cladogram
nests Thalassemys at the base of several Late Jurassic fossil sea turtles (Fig. 2, red arrow).

Figure 2. Turtle cladogram from Joyce, Mäuser and Evans excludes taxa that would change this tree topology.
Figure 2. Turtle cladogram from Joyce, Mäuser and Evans excludes taxa that would change this tree topology.

Similarly,
in the large reptile tree (LRT, 1867+ taxa; Fig. 3) Thalassemys nests at the base of one sea turtle clade among several.

Figure 3. Subset of the LRT focusing on turtle evolution. The outgroup includes pareaiasaurs and Stephanospondylus..

Taxon exclusion in the Joyce, Mäuser and Evans cladogram
prevents these turtle experts from understanding 1) the dual origin of turtles from pareiasaurs, 2) softshell sea turtles, like Ocepecephalon, are not related to hardshell sea turtles; 3) the extant pig-nosed turtle, Carettochelys is related to the extant leatherback, Dermochelys; and 4) the giant Cretaceous sea turtle, Archelon, is derived from Macrochelys, the snapping turtle (video below) and thus is not related to the clade of other ‘hardshell’ sea turtles. Adding taxa solves all these problems.

Noticeable by its absence,
the basal softshell turtle, Odontochelys, is not mentioned in the text nor listed among the turtle taxa in Joyce, Mäuser and Evans 2021 (Fig. 2). Also absent from the Joyce, Mäuser and Evans cladogram are any turtle outgroup taxa. These guys are turtle experts and they don’t know where turtles came from? C’mon! Add taxa to find out (see Fig. 3 for the current solution).

The generic evolution of sea turtles
is more or less documented in the following three videos. Note the transition from typical asymmetrical left-right-left-right tetrapod locomotion to symmetrical flapping in river turtles and sea turtles. Turtles are essentially weightless in water, so they didn’t have to develop extended coracoids and mighty muscles like birds and pterosaurs, or extended clavicles like bats to power their flaps against the pull of gravity. Instead, sea turtles seem to have simply and slowly learned to swim by increasingly symmetrical flapping over generations as their forelimbs gradually became longer, flatter, more flexible and evolved into flippers.

Evidently turtles were in no hurry to learn how to flap while swimming.
Several clades did it (Fig. 3), so this must have come  about quite naturally, if slowly.

References
Joyce WG 2003. A new Late Jurassic turtle specimen and the taxonomy of Palaeomedusa testa and Eurysternum wagleri. PaleoBios. 23 (3): 1–8.
Joyce WG, Mäuser M and Evers SW 2021. Two turtles with soft tissue preservation from the platy limestones of Germany provide evidence for marine flipper adaptations in Late Jurassic thalassochelydians. PLoS ONE 16(6): e0252355. https://doi.org/10.1371/journal.pone.0252355
Rütimeyer L 1873. Die fossilen Schildkröten von Solothurn und der übrigen Juraformation. Neue Denkschriften der allgemeinen Schweizerischen Gesellschaft für die gesamten Naturwissenschaften 25:1–185.

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