Hovasaurus tarsus ontogeny animation

Caldwell 1995
provided a series of growth stages of the tarsus of Hovasaurus that chronicle the appearance of the ankle bones. Here is an animation of the same (Fig. 1).

Figure 1. From Caldwell 1995, an ontogenetic series showing the growth of the carpus in the basal diapsid Hovasaurus.

Figure 1. From Caldwell 1995, an ontogenetic series showing the growth of the  tarsus in the basal diapsid Hovasaurus. Scale bar = 1 cm.  Since these specimens were not found as part of a family assemblage, there are some specimens that appear to diverge from others in terms of carpal element shapes. Plus, we see here a certain amount of individual variation, the driving force behind evolution. There is a large discontinuity between K and L due to a lack of fossils at that stage of growth. C = calcaneum. A = Astragalus. c = centralia.

Unfortunately
Caldwell was under the impression that the basal diapsid Hovasaurus was close to the ancestry of extant lepidosaurs. The large reptile tree (LRT, 1028 taxa) invalidates that hypothesis with the addition and inclusion of more taxa.

According to the LRT
Hovasaurus
is a marine younginiform, basal to those diapsids that ultimately produced members of the Enaliosauria, a large clade of marine (new) archosauromorphs. Lepidosaurs had a separate origin going back to basalmost amniotes (= reptiles) like Gephyrostegus.

Figure 1. Tangasaurus, Hovasaurus and Thadeosaurus, three marine younginiformes, apparently have no scapula.

Figure 2 Tangasaurus, Hovasaurus and Thadeosaurus, three marine younginiformes, apparently have no scapula.

Hovasaurus is interesting
in that it developed a plesiosaur-style pectoral girdle without being directly related to plesiosaurs. Hovasaurus and Tangasaurus (Fig. 2) look like  they are missing a scapula. In related Thadeosaurus the scapula has been reported only on juvenile taxa (gray box).

References
Caldwell MW 1995. Developmental constraints and limb evolution in Permian and extant  lepidosauromorph diapsids.

What?? No scapula??

In a few marine younginiforms,
Tangasaurus (Haughton 1924, Currie 1982), Hovasaurus (Piveteau 1926, Currie 1981) and Thadeosaurus (Carroll 1981, Currie 1984) the scapula cannot be found (Fig. 1). But in a young thadeosaur (if conspecific), a scapula is present (in gray). These are all currently sisters in their own clade in the large reptile tree, The lack of a scapula is not currently a scored trait in the large reptile tree.

Figure 1. Tangasaurus, Hovasaurus and Thadeosaurus, three marine younginiformes, apparently have no scapula.

Figure 1. Tangasaurus, Hovasaurus and some specimens of Thadeosaurus, three marine younginiformes, apparently have no scapula. Click to enlarge. The young Thadeosaurus, if that is indeed what it is (in gray box) shows what a scapula should look like.

When you first encounter these specimens
you scratch your head and search, looking for the scapulae to no avail. Then, when you realize these three sisters share this trait — it still is difficult to accept. The coracoids and sternae + interclavicle form a chest plate. What holds that pectoral girdle in place? What locks the humerus down?  It is hard to look at those naked anterior ribs. Usually something is there to cover them~ Maybe I just missed it…

It is at this node in the evolution of marine younginiforms
that they were moving from a terrestrial niche into an aquatic one. From such Late Permian taxa we get plesiosaurs, placodonts, mesosaurs, thalattosaurs and ichthyosaurs, along with the widely varied sinosaurosphargids including Atopodentatus. So the change in niche is echoed and sometimes amplified in the morphology of descendant taxa, starting with these three (Fig. 1).

References
Carroll RL 1981. Plesiosaur ancestors from the Upper Permian of Madagascar. Philosophical Transactions of the Royal Society London B 293: 315-383
Currie PJ 1984. Ontogenetic changes in the eosuchian reptile Thadeosaurus. Journal of Vertebrate Paleontology 4(1 ): 68-84.
Currie PJ 1981. Hovasaurus boulei, an aquatic eosuchian from the Upper Permian of Madagascar. Palaeontologica Africana, 24:99-163.
Currie P 1982. The osteology and relationships of Tangasaurus mennelli Haughton. Annals of The South African Museum 86:247-265. http://biostor.org/reference/111508
Haughton SH 1924. On Reptilian Remains from the Karroo Beds of East Africa. Quarterly Journal of the Geological Society 80 (317): 1–11.
Piveteau J 1926. Paleontologie de Madagascar XIII. Amphibiens et reptiles permiens. Annls Paleont. 15: 53-180.
Reisz RR, Modesto SP and Scott DM 2011. A new Early Permian reptile and its significance in early diapsid evolution. Proceedings of the Royal Society B 278 (1725): 3731–3737.

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