Spinoaequalis and the origin of two clades of Younginiformes

Unless there is more breaking news, like Yi qi, the next few sessions will cover some news at the base of the younginiformes and archosauriformes.

Spinoaequalis schultzei (deBraga and Reisz 1995; Late Carboniferous ~306 mya; ~30 cm in length; Fig. 1), was originally considered a basal diapsid close to Petrolacosaurus and Araeoscelis, but with a deep, sculling tail. Here that nesting is confirmed with a closer relationships to Eudibamus.

 

More importantly,
Spinoaequalis nests at the base of the marine younginiformes, like Galesphyrus and the terrestrial younginformes, like Youngina SAM K7710 (see links below and Fig. 1). In other words these are all the taxa higher than Carboniferous diapsids (araeoscelids).

The deep tail is an autapomorphy of Spinoaeaqualis not expressed in descendant taxa, except, perhaps and separated by several nodes, Hovasaurus.

Figure 1. Spinoaequalis (above) to scale with a member of the juvenile den specimen of Youngina SAM K1770. These are sister taxa. The SAM specimens are not juveniles, but nest at the base of all terrestrial Younginisofmes + Protorosauria + Archosauriformes.

Figure 1. Spinoaequalis (above) to scale with a member of the purported juvenile den specimen of Youngina, SAM K1770. These are sister taxa. The SAM specimens are not juveniles, but nest at the base of all terrestrial Younginisofmes + Protorosauria + Archosauriformes. Another sister, Galesphyrus, nests at the base of all aquatic younginformes.

Spinoaequalis is much more important than deBraga and Reisz ever realized due to its nesting at the base of all terrestrial younginiformes (including archosauriformesdinosaurs including birds) …

Figure 2. Spinoaequalis and a number of basal terrestrial yonginiformes, many of which are represented by skulls only.

Figure 2. Spinoaequalis and a number of basal terrestrial yonginiformes, many of which are represented by skulls only. Click to enlarge. One other mislabeled Youngina specimen nests among the marine clade (Fig. 3).

…and at the base of all marine younginformes (including ichthyosaurs and sauropterygians).

Figure 3. Spinoaequalis and descendant marine younginiformes.

Figure 3. Spinoaequalis and descendant marine younginiformes. These give rise to plesiosaurs, placodonts, mesosaurs, ichthyosaurs and thalattosuchians. Click to enlarge. Note the nesting of one purported Youngina specimen separate from all others listed here.

And here (Fig. 4) is the tree and taxa we’ll be looking at over the next few blogposts. Note this is not the tree topology promoted by prior published work. The entire reptile tree can be viewed here.

Figure 3. Subset of the large reptile tree focusing on the Protodiapsida, the Diapsida, Marine Younginiformes and Terrestrial Younginiformes, including Protorosaurs and Archosauriformes. Click to enlarge.

Figure 4. Subset of the large reptile tree focusing on the Protodiapsida, the Diapsida, Marine Younginiformes and Terrestrial Younginiformes, including Protorosaurs and Archosauriformes.
Click to enlarge.

References
deBraga M and Reisz RR 1995. A new diapsid reptile from the uppermost Carboniferous (Stephanian) of Kansas. Palaeontology 38 (1): 199–212. palass-pub.pdf

wiki/Spinoaequalis

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