The Terrestrial Younginiformes

When Alfred Romer proposed the term ‘Younginiformes‘ in 1947 as a replacement name for the taxon, Eosuchia, he had no idea this clade was diphyletic (marine and terrestrial clades). He was not yet aware of Spinoaequalis (Bickelmann, et al. 2009), which both clades share as a last common ancestor, (Fig. 2). As we learned earlier, Bickelmann, et al. (2009) also found terrestrial and marine branches for the younginiformes, but they included many unrelated taxa and did not include several pertinent taxa.

Earlier we looked at the clade of basal marine (aquatic) younginiformes. Today we’ll examine the clade of basal terrestrial younginiformes (Fig. 1). From Romer’s original list, only Youngina is included (Kenyasaurus is off the list now as we learned yesterday).

This presentation will take several blog posts
as we shed new light on a new tree topology for the base of the Archosauriformes. There is a lot to cover, many mysteries will be solved and many paradigms will be overturned, as you’ll soon see. Today: an overview (Fig. 1):

Figure 1. Terrestrial Yonginiformes + Galesphyrus representing the marine clade, all to scale except the toned area containing protorosaurs, which have their own scale.

Figure 1. Terrestrial Yonginiformes + Galesphyrus representing the marine clade, all to scale except the toned area containing protorosaurs, which have their own scale.

As before
we start with the basal diapsid, Spinoaequalis (Fig.1).

Almost right from the start
two clades diverge (Fig. 3), the Protorosauria and the Archosauriformes: The both start off with little lizard-like taxa, Youngina and Prolacerta. The following taxa are in phylogenetic order within the Protorosauria as recovered by the large reptile tree.

  1. The SAM K 1770 specimen(s) attributed to Youngina (the several den specimens)
  2. The BPI 375 specimen attributed to Youngina
  3. Prolacerta AMNH 9520
  4. Prolacerta BPI/ I/475
  5. Protorosaurus
  6. Jaxtasuchus
  7. Boreopricea
  8. Azendohsaurus
  9. Pamelaria

The following taxa are in phylogenetic order within the basal Archosauriformes as recovered by the large reptile tree.

  1. The TM 3603 specimen attributed to Youngina
  2. The RC90 specimen of Youngopsis rubidgei
  3. The  TM 1490 specimen of Youngopsis kitchingi 
  4. The RC91 specimen of Youngoides minor
  5. Youngina capensis holotype, AMNH 5661
  6. Youngoides romeri holotype, FMNH UC 1528
  7. The BPI I 4016 specimen attributed to Proterosuchus
  8. Euparkeria and Osmolskina.

Note that
the BPI 3859 specimen attributed to Youngina is not related to the others in the terrestrial clade, but nests within the marine clade (Fig. 2).

Figure 2. 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 2. Subset of the large reptile tree focusing on the Protodiapsida, the Diapsida, Marine Younginiformes and Terrestrial Younginiformes, including Protorosaurs and Archosauriformes.
Click to enlarge.

Key to this discussion
is the basal position of various Youngina/Youngopsis/Youngoides specimens closer to Protorosaurs and Archosauriformes than traditionally considered, basal to lepidosaurs and archosaurs.

According to Wikipedia,
the bastion of traditional thinking in paleontology, “The [Eosuchia] has almost been treated as a dustbin for diapsids that are not obviously lepidosaurian or archosaurian. One consequence has been Romer’s suggestion of the alternative order Younginiformes to be applied strictly to those forms with the primitive diapsid form, in particular, a complete lowermost arch as the quadratojugal and jugal bones of the skull meet.”

Unfortunately,
the large reptile tree has put the division between lepidosaurs and archosaurs clades back to the Viséan, near the origin of the Amniota (= Reptilia). Now Younginiformes are basal to the taxa listed above (Fig. 2). This new insight arises from increasing taxon inclusion. These results require a major paradigm shift for most paleontologists.

Youngina and kin
The various specimens attributed to Youngina need to be updated. Some of the latest figures go back 40 years. Others go back 80 years. Only a few of the above figures were traced from recent photos, some taken after viewing the specimen.

A den of Youngina specimens (Smith and Evans 1996)
(SAM K7710) were considered juveniles because they were smaller than other known Youngina specimens, otherwise only known from skulls. Unfortunately, Smith and Evans did not include Spinoaequalis in their study. Here Spinoaequalis nests as an outgroup sister to the den of Youngina specimens, and it is slightly larger than the den specimens are. And the den specimens are indeed smaller than the rest of the Youngina specimens are. Thus the origin of the terrestrial younginiformes also experienced a slight amount of phylogenetic miniaturization. The den specimens are older than the rest of the Youngina specimens, and, according to the large reptile tree (Fig. 2) they are also more primitive.

Tomorrow we’ll look at the many faces of Proterosuchus. The one shown above (Fig. 1) has been considered a juvenile, but it is also the one closest in morphology to the outgroup taxa among basal Youngoides and Youngina specimens.

References
Bickelmann C, Müller J and Reisz RR 2009. The enigmatic diapsid Acerosodontosaurus piveteaui (Reptilia: Neodiapsida) from the Upper Permian of Madagascar and the paraphyly of “younginiform” reptiles. Canadian Journal of Earth Sciences 46:651-661.
Smith, RMH and Evans SE 1996. New material of Youngina: evidence of juvenile aggregation in Permian diapsid reptiles. Palaeontology, 39 (2):289–303.

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