Turtles as Hopeful Monsters – Future book and past essay by O. Rieppel

“Early theories attempted to explain the evolution of the turtle shell in the context of a step-wise, hence gradual process of transformation. The distant ancestor of turtles was hypothesized to have had a body loosely covered by osteoderms. Within the evolutionary lineage leading to turtles, the number of osteoderms would have gradually increased, until the bony plates would eventually have provided a complete covering of the trunk, thus forming an epitheca. Thecal ossifications would have developed below the epi- theca at later stages in the evolution of the turtle body plan, while epithecal ossifications would subsequently be lost at even more advanced stages of turtle evolution. This theory met with various difficulties, however, such as the fact that the earliest fossil turtle (Proganochelys) from the Upper Triassic of Europe (215 Mio years) has a complete theca. Furthermore, epithecal ossifications appear later than ossifications of the theca in development and, in modern turtles, epithecal ossifications tend to form in evolutionarily relatively advanced forms only.

“Turtles are unique among tetrapods, however, in that the shoulder blade (scapula) lies inside the rib cage (Fig. 3). The reason for this inverse relationship of the scapula is the close association of the ribs with the costal plates of the theca. The scapula of turtles comes to lie inside the rib cage because of a deflection of rib growth to a more superficial position. Recent developmental work has identified inductive interaction generated by the carapacial ridge as probable cause of this deflection of rib growth.

“A recent theory proposed the evolution of turtles from Paleozoic Pareiasaurs by a process of “correlated progression”. Correlated key elements of this progressive transformation are an increase in the number of osteoderms until they form a closed dorsal shield (carapace), the broadening of the ribs below this dorsal hield, the shortening of the trunk, the immobilization of the dorsal vertebral column and a backwards shift of the pectoral girdle.”

In the phylogenetic analysis provided by the LRT
turtles have two parallel origins, both from pareiasaurs: One for the domed, hard-shell clade (Bunostegos > Elginia > Meiolania) and one for the flattened soft-shell clade (Sclerosaurus > Odontochelys > Trionyx). In both these cases it appears that the tall scapula extended to either side of the narrow, cervical-like dorsal ribs. Then the anterior dorsal ribs rotated anteriorly over the tall scapulae, paralleling the rotation of the elbow anteriorly. You’ll note that turtles have more cervicals and fewer dorsals than pareiasaurs. The posterior cervicals in turtles appear to be the former dorsals of pareiasaurs. So, the pectoral girdle did not shift, but the posterior cervicals and anterior dorsals transformed around them. And this occurred during phylogenetic miniaturization.

Distinct from other reptiles
turtles do not have lateral movement in the torso with precursors in the short-torsoed, heavily ribbed pareiasaurs. “This repositioning of the vertebrae relative to the primary body segments is achieved by resegmentation of the somites. Each somite splits in half, and the posterior part of one somite recombines with the anterior part of the succeeding somite to form a vertebra,” reports Rieppel.

More from Dr. Rieppel
“As a turtle embryo grows and develops, the contours of the future carapace are soon mapped out by an accelerated growth and a thickening of the skin on its back.”

“The ribs of turtles are unique among vertebrates in that they chondrify within the deep layers of the thickened dermis of the carapacial disk.”

It has been known for over 100 years that in turtles, the neural arches of the dorsal vertebrae shift forward by half a segment, carrying the ribs with them, again a unique condition in amniotes.”

Embryological studies
(Ruckes 1929) indicate, “the scapula of turtles comes to lie inside the rib cage because of a deflection of rib growth to a more superficial position. The ribs come to lie lateral to the no longer functional intervertebral joints. The functional reason for this anterior shift of the neural arches is not clear, other than that it may contribute to the mechanical strength of the carapace, as the neural plates come to alternate with the costal plates. Neural and costal plates are endoskeletal components of the turtle carapace, and cannot be derived from a hypothetical ancestral condition by fusion of exoskeletal osteoderms. All other parts of the turtle carapace are exoskeletal.” Rieppel reports, “The turtle body plan is evidently highly derived, indeed unique among tetrapods.”

I’m going to say ‘not true’ here…
Several turtle-like forms developed among placodonts and the two turtle clades developed independently in parallel. Glyptodon had a turtle-like carapace, but no plastron. Even Minimi, the phytodinosaur, developed a club tail, convergent with meiolaniids.

Moreover, only the shells of the two clades of turtles are unique unto themselves, as most other of their body parts are microevolutionary adjustments from their separate micro pareiasaur bauplans. And, based on current fossil chronology, they had 25 to 45 million years to develop their respective shells from their proximal outgroup sisters.

References
Gilbert SF, Loredo GA, Brukman A, Burke AC. 2001.  Morphogenesis of the turtle shell: the development of a novel structure in tetrapod evolution. Evol Dev 2001;3:47 ± 58.
Götte A 1899. Über die Entwicklung des knoÈ chernen RuÈ ckenschildes (Carapax) der SchildkroÈten. Z wiss Zool 1899;66:407±434.
Lee MSY 1996. Correlated progression and the origin of turtles. Nature 1996;379:811- 815.
Rieppel O 1996. Turtles as diapsid reptiles. Nature 384 (6608), 453-455
Rieppel O 2001. Turtles as hopeful monsters. BioEssays 23:987-991.|
Rieppel O 2012. The Evolution of the Turtle Shell. Morphology and Evolution of Turtles. Part 2, 51-61. DOI: 10.1007/978-94-007-4309-0_5
Rieppel O 2017. Turtles as Hopeful Monsters. Origins and Evolution. Indiana University Press. 212 pp.. Online here.
Ruckes H. 1929 (12) Studies in chelonian osteology. Part II. The morphological relationships between girdles, ribs and carapace. Ann NY Acad Sci 1929;31:81-120.

Rieppel book summary online