Phylogenetic origin of the turtle plastron and hypoischium

Several prior workers
have attempted to explain the origin of the turtle carapace. By contrast, the plastron has been largely ignored (please let me know if otherwise), except by Rice et al. 2016, who looked at developing embryos of the pond slider, Trachemys, rather than extinct taxa. They found that condensates for each plastron bone [form] at the lateral edges of the ventral mesenchyme,” like sternal cartilage development in chicks and mice, but with the suppression of cartilage and a bias toward bone development.

Unfortunately,
Rice et al. bought into the invalid hypothesis that Pappochelys (misspelled ‘Pappachelys‘ in their paper) was related to turtles. They also mention the undocumented ‘gastralia hypothesis’ of plastron origin. However Rice et al. report, “whereas plastron bones start to mineralize from the periphery of the ventrum in a slight anterior-to-posterior preference, gastralia mineralize in a posterior-to-anterior sequence….”

The plastron in most modern turtles
is composed of nine bones (listed below) that develop between the visceral organs and ectodermal scutes. Four more appear only in the basal soft-shell turtle, Odontochelys (Fig. 1, discussed below).

In the large reptile tree (LRT, 1042 taxa) the proximal ancestors of both soft shell and hard shell turtles lack gastralia or a plastron. By contrast, all turtles from both clades have a plastron. (Yes, it is odd that so many traits developed in parallel in the two clades, but attests to the authority of the LRT that it is able to lump and separate the two clades.)

The soft shell turtle plastron
first appears in the fossil record in lake deposit specimens of the Late Triassic Odontochelys (Fig. 1). Its current proximal ancestor, Middle Triassic Sclerosaurus (Fig. 9) has no gastralia or plastron, but it does appear to have a hypoischium (novel ventral bone posterior to the ischium).

Typically the turtle plastron consists of
four sets of bones.

  1.  Anteriorly the former clavicles and interclavicle appear beneath the neck where they are renamed the epiplastra and entoplastron.
  2. Further back the hyoplastron rims the forelimbs.
  3. Posteriorly the hypoplastron rims the hind limbs.
  4. Approaching and sometimes beneath the pelvis are the xiphiplastra.

Odontochelys has two extra sets of plastra not found in extant taxa. The two mesoplastron sets are located between the hyoplastra and hypoplastra. They appear to be new structures unique to this genus given that no other known turtles have them.

FIgure 1. GIF animation of the plastron of Odontochelys. Note it only extends to the anterior pelvis. Following the pelvis is another new ventral plate, the hypoischium.

FIgure 1. GIF animation of the plastron of Odontochelys. Note it only extends to the anterior pelvis (Pu + Is). Following the pelvis is another new ventral plate, the hypoischium.

The most primitive (but not the oldest) hard shell plastron
appears in late-surviving Meiolania (Fig. 2). Proximal outgroup taxa from the Late Permian, either don’t preserve a post-crania (Elginia) or lack belly bones (Bunostegos). In the more derived Late Triassic Proganochelys and Proterochersis, the central hole is filled with bone.

Figure 2. The plastron from two specimens of Meiolania. Note the large hole in the center and the nearly complete lack of any bone shape in common with the plastron bones of Odontochelys (Fig. 1).

Figure 2. The plastron from two specimens of Meiolania. Note the large hole in the center and the nearly complete lack of any bone shape in common with the plastron bones of Odontochelys (Fig. 1).

The plastron of hard shell turtles
apparently developed in convergence with the plastron of soft shell turtles (no last common ancestor has a plastron). In basal taxa the structures are distinct from one another (Figs. 1, 2), but derived taxa converge on one another.

The soft shell plastron bones in Odontochelys
(Fig. 1) appears to radiate from the center extending to fragile lateral connections to the carapace. Note: Rice et al. did not observe any developing soft-shell turtle embryos so what they learned from Trachemys (see above) may or may not be applicable to soft shell clade.

By contrast the hard shell plastron
of Meiolania has a strong lateral connection to the carapace, underlaps the pectoral and pelvic girdles, and avoids the center. So each plastron essentially rims each limb opening. The plastra of Meiolania appear to be fused to one another, but that is not the case with other hard shell taxa (see below).

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

Figure 3. Meiolania, the most primitive of known hard shell  turtles, has lateral forelimbs, like non turtles. The plastron covers most of the pelvis. The neck could not be withdrawn beneath the carapace. The plastron had a large central fenestra lacking in the plastron of Odontochelys (Fig. 1). Remember, this is a model, not the actual bones.

Triassic Proganochelys
(Fig. 4) fills the central hole in the plastron and it has a hypoischium posterior to its pelvis, as seen in Odontochelys and Sclerosaurus. It’s too bad Elginia and Bunostegos preserve the post-crania so poorly. We should be able to find a hypoischium in their remains, too. Since Meiolania has never been described with a hypoischium, we should go look for it (see below).

Figure 3. The plastron of Proganochelys is solid, and is solidly connected laterally, but it also has a hypoischium posterior to the ischium and the plastron barely underlaps the pelvis.

Figure 4. The plastron of Proganochelys is solid, and is solidly connected laterally, but it also has a hypoischium posterior to the ischium and the plastron barely underlaps the pelvis.

And now, just to make things more confusing…
Compared to Odontochelys, the extant soft shell turtle, Trionyx (Fig. 5), has a reduced plastron with central fenestrae. The two midplastra are absent here. So is any ossification along the midline, convergent with hard shell turtles. The interclavicle and clavicles are not co-ossified. It’s as if ossification ceased at a certain point in the development of the plastron here.

Figure 2. Some parts of the soft-shell turtle plastron have their origins in the interclavicle and clavicle of other tetrapods. Other parts are not modified gastralia because outgroups do not have gastralia.

Figure 5. Some parts of the soft-shell turtle plastron have their origins in the interclavicle and clavicle of other tetrapods. The carapace is also shown here.

Likewise,
hard shell sea turtles, like Chelonia, do not fully ossify the plastron. Here (Fig. 6) none of the plastron elements are co-ossified. The hyoplastra and hypoplastra appear to radiate from four centers. The radiations likely point to their origins in the center of each plate. The posterior xiphiplastra likewise radiate but in a narrower pattern.

FIgure 7. Sea turtle plastron. This looks like a soft shell turtle plastron.

FIgure 6. Sea turtle plastron. Bone development ceased prior to suturing.

The predecessor to soft shell turtles, Sclerosaurus,
is known from a nearly complete and articulated skeleton (Fig. 7) that appears to preserve no plastron, but has the genesis of a hypoischium. The flexible spine composed on more than ten dorsal vertebrae and ribs was probably stiffened and reduced prior to the invention of the plastron, but some dorsal osteoderms are present along the midline.

Figure 8. Sclerosaurus insitu. This turtle ancestor still bas a flexible spine, but the pectoral girdle has migrated anterior to the dorsal ribs. A hypoischiuum is present.

Figure 7. Sclerosaurus insitu. This turtle ancestor still bas a flexible spine, but the pectoral girdle has migrated anterior to the dorsal ribs. A hypoischiuum is present.

A reconstruction of Sclerosaurus
(Fig. 8) shows the migration of the much shorter scapula anterior to the dorsal ribs and the first appearance of the hypoischium. The scapula shift is the first step toward tucking the pectoral girdle beneath the anterior dorsal ribs.

Figure 9. Sclerosaurus reconstructed. Note the placement of the narrow pectoral girdle anterior to the wide dorsal ribs.

Figure 8. Sclerosaurus reconstructed. Note the placement of the narrow pectoral girdle anterior to the wide dorsal ribs. The supratemporal horns are homologous with those of Elginia and Meiolania.

FIgure 10. Trachemys plastron and diagram. The scutes overlap the bones. The bones are impossible to understand without the diagram because they retain the impressions of the scutes.

FIgure 9. Trachemys plastron and diagram. The scutes overlap the bones. The bones are impossible to understand from photos such as this one without the diagram because the bones retain the impressions of the scutes.

Figure 10. The unidentified bone from Gaffney 19xx here imagined as the half of hypoischium attached to the posterior ischium.

Figure 10. The unidentified bone from Gaffney 1996 here imagined as the half of hypoischium attached to the posterior ischium.

Did Meiolania have a hypoischium?
Gaffney 1996 did a fantastic job of reconstructing Meiolania (Fig. 3) from bits and pieces, including a xiphiplastron from over a dozen broken bits. He also published what he called an ‘unidentified bone’ (Fig. 10). If turtle expert Gaffney was not able to identify it, I wonder if it was an unexpected bone, like a hypoischium? Let’s leave that as a big maybe for now…

References
Gaffney ES 1996. The postcranial morphology of Meiolania platyceps and a review of the Meiolaniidae. Bulletin of the AMNH no. 229.
Rice R et al. 2016. Development of the turtle plastron, the order-defining skeletal structure. PNAS 113(19):5317–5322.
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