The origin of the amniote astragalus – Piñeiro et al. 2016

A new PeerJ paper
by Piñeiro, et al. (2016) attempts to shed light on the origin of the amniote astragalus by comparison to the ontogenetic development of the mesosaur tarsus. They claim that Mesosaurus is a very primitive amniote, following the thinking of traditional paleontologists.  By contrast, the  large reptile tree nests mesosaurs with highly derived thalattosaurs and ichthyosaurs, following basal pachypleurosaurs (sauropterygians) in the revived clade Enaliosauria, derived from marine younginiforms, diapsids, prodiapsids and basal archosauromorphs, all arising during the preceding Carboniferous.

In any case
The new paper seeks to answer the question, ‘Did the astragalus arise from one bone (the intermedium) or the fusion of several bones, including the tibiale, centralia and intermedium?’ Piñeiro et al. found that among embryo mesosaurs, a single four-part bone, creates the astragalus.

Two false paradigms affect the Piñeiro et al study 
1: They follow traditional beliefs that amniote skeletal structures should be present in basalmost amniotes. In reality, as we all know, amniotes are defined ONLY by the way they protect their embryos, with an amniotic sac, a structure lacking in amphibians. As we learned earlier, the basalmost amniote in the large reptile tree is Gephyrostegus bohemicus, a late-surviving member of a an earlier Viséan radiation. It has no traditional amniote traits. But a revised list of amniote traits can be seen here and in the six blogs that follow.

2: They believe that Mesosaurus is a basal amniote. The Early Permian is indeed early, but with basal reptiles already diversifying in the Viséan, some 40 million years earlier, the Permian is not early enough. Moreover a basal nesting is not supported in the large reptile tree. Along the same lines they do not understand that Diadectes, Tseajaia, Westlothiana and others nest within the Amniota (= Reptilia) in the only study that tests their relationships in a large gamut study of other tetrapods and amniotes, the large reptile tree.

Piñeiro et al. used an interesting graphic technique
of presenting the tarsal elements of several taxa as a series of interlocking hexagons. That’s fine on one level, but does not let us see the actual elements or reconstructions of the same, which is an unfortunate loss. They also mix up left and right, dorsal and ventral views of skeletal elements. It would be helpful to flip certain elements in order to present all the elements consistency for ready comparison. This should be standard operating procedure.

In the large reptile tree the intermedium remains a separate element
from the tibiae in stem (pre) amniotes like Proterogyrinus, Seymouria and the basal amniote Gephyrostegus (Fig. 1).

Figure 1. The intermedium remains separate from the tibiale in Proterogyrinus, Seymouria and Gephyrostegus.

Figure 1. The intermedium remains separate from the tibiale in Proterogyrinus, Seymouria and Gephyrostegus.

BTW, while researching Seymouria
I came across a bizarre reconstruction in Berman et al. 2000 (Fig. 1 under red circle) that did not match the bone tracings and added another row of central tarsals that no other tetrapods have.

Essentially
the Reptilia is, in reality, two clades, the Lepidosauromorpha and Archosauromorpha. Let’s take the former first. The tibiale does not fuse to the intermedium in all reptiles. So the astragalus is not present in every amniote.

Figure 2. Sample lepidosauromorph tarsi compared to Gephyrostegus. Here are Captorhinus, Emeroleter and Tjubina, a basal tritosaur lepidosaur.

Figure 2. Sample lepidosauromorph tarsi compared to Gephyrostegus. Here are Captorhinus,Orobates, Emeroleter and Tjubina, a basal tritosaur lepidosaur. Note the separate tibiae in Gephyrostegus, Orobates and Emeroleter. So an astragalus appears most of the time in this clade, not all of the time.

The appearance
and or fusion of tarsal elements varies within the Reptilia. And it varies with ontogeny within certain taxa, like mesosaurs. Older individuals often have more bones and more sharply defined bones. In the Archosauromorpha (Fig. 2), perhaps eight taxa precede the first appearance of the astragalus (fusion of tibiae and intermedium) in Casineria.

Figure 2. Comparison of archosauromorph tarsi, including Mesosaurus, the latter from Piñeiro et al

Figure 2. Comparison of archosauromorph tarsi, including Mesosaurus, the latter from Piñeiro et al Not to scale. Note the generally conservative pattern here, despite the liberal changes in relative bone sizes.

Here
(Figs. 1, 2) the astragalus (yellow/orange element) is only composed of the tibiale and intermedium in these taxa and a small perforation marks the division. The elements of the centralia may fuse together, but not with other elements in the above listed taxa. These fusion patterns occur by convergence in the two basal reptile clades.

According to Piñeiro et al
the astragalus changes greatly during the ontogeny of Mesosaurus. They interpret (“with doubts”) the embryo astragalus as the fusion of the tibiae, intermedum and two centralia. I don’t see any more than three centralia in the above illustrated taxa, and sometimes they fuse together, but not with the tibiale or intermedium (Fig. 2).

As a final note
I find it odd that workers are eager to change the names of some fused bones, like the astragalus and navicular, but are not interested in renaming other fused bones (like the postfrontal + postorbital). Instead, one bone is typically said to be present while the other is said to be absent. And that doesn’t make sense when both are present, just fused.

Let’s fix that in consensus.

References
Berman DS, Henrici AC, Sumida SS, Martens T. 2000. Redescription of Seymouria
sanjuanensis (Seymouriamorpha) from the Lower Permian of Germany based on complete mature specimens with a discussion of paleoecology of the Bromacker locality assemblage. Journal of Vertebrate Paleontology 20(2):253268
Piñeiro et al. 2016. The ontogenetic transformation of the mesosaurid tarsus: a contribution to the origin of the primitive amniotic astragalus. PeerJ 4:e2036; DOI 10.7717/peerj.2036

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