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A Note on Anningasaura lymense, a Basal Pre-Plesiosaurian

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A recent paper by Vincent and Benson (2012) redescribed a plesiosaur specimen NHMUK OR 49202 from the Lias. Considered a juvenile with a skull 34(!) cm long, this specimen was originally (Lydekker 1889) named Plesiosaurus macrocephalus, but it is taxonomically distinct in several regards and very primitive.

Figure 1. Anningsaura (formerly Plesiosaurus macrocephalus) in three views. This specimen nests between Simosaurus and Pistosaurus (Fig. 2) at the base of the Pistosauria at the base of the Plesiosauria. The supplementary parietal foramen was omitted here. 

Autapomorphies Described by Vincent and Benson (2012)
1. Posteromedial processes of the premaxillae (or possible anterior portion of the frontal) forming a dorsoventrally thick, mediolaterally expanded platform [unfortunately Vincent and Benson failed to note the presence of nasals and this area actually represents the nasals on either side of the ascending process of the premaxilla, see Fig. 1]; 2. supplementary foramen penetrating the parietal sagittal crest; 3. absence of a pterygoid-vomerine contact [this may not be so, see Fig. 1]; 4. absence of a contact between the pterygoids in palatal aspect [this may not be so, see Fig. 1]; 5. cultriform process of the parasphenoid wider mediolaterally than the combined posterior interpterygoid vacuities; 6. and two closely spaced foramina in the lateral surface of the exoccipital.

That Unfused Palate
Vincent and Benson (2012) thought the palatal elements did not contact one another and the nasals were not identified. In their conclusion Vincent and Benson (2012) emphasized the lack of contact between the pterygoids and considered this the most reduced ossification among plesiosaurians. If true, this would be an autapomorphy in a clade that otherwise has a solid palate (Fig. 2). However, lack of skull fusion is an ontogenetic feature of juvenile skulls in plesiosaurs, according to Vincent and Benson (2012) and I’m out of my league here judging ontogeny in plesiosaurs. However, the large size of the skull, relative to the relatives of Anningasaura argues against a juvenile assignment, I would think (see below).

Palate closure? Maybe not.
In Anningasaura a new reconstruction (Fig. 1)  - can – put the pterygoids back together again, along with the palatines. Even so, this may not be the natural configuration. According to Vincent and Benson (2012) “Plesiosaurus dolichodeirus possesses pterygoids that meet each other for only a short distance anterior to the anterior interpterygoid vacuity, and therefore shows the condition most similar to that of NHMUK OR49202. O’Keefe (2006) suggested that basal plesiosaurs are neotenic in their pterygoid ossification, and that the secondary closure of the palate is an evolutionary trend within the Plesiosauria.” Vincent and Benson (2012) also mentioned certain taxa with an interpterygoid vacuity that I am unfamiliar with, but did not illustrate these.

Juvenile? Maybe not.
With a skull three times the size of the skull of Simosaurus and half again as large as Pistosaurus, the skull of Anningasaura likely does not represent a juvenile.

Figure 2. The sisters of Anningasaura, Simosaurus and Pistosaurus. These provide the only clues as to the post-crania of Anningasaura, of which only the first eight cervicals are known. The palate of Simosaurus is interesting in this context, because it does not show a suture between the pterygoids and the medial parasphenoid, a trait also seen in pachypleurosaurs and nothosaurs.

Nesting
Vincent and Benson (2012) reported, “In general morphology, NHMUK OR49202 does not resemble any known plesiosaurian taxon.” Anningasaura represents a completely ‘new’ branch of the plesiosauria in which the orbits virtually cannot be seen in dorsal view and the jugals bend down posteriorly to produce an angled temporal arch. Vincent and Benson (2012) did not mention Simosaurus or Pistosaurus in their text. Earlier Benson (2012) created a phylogenetic analysis that nested Anningsasaura at the base of the pliosaur/plesiosaur split with Bobosaurus as the outgroup.  The large reptile tree recovers pretty much the same nesting, but nested Anningsaura between Pistosaurus and Simosaurus, just off the taxon list of Benson et al. (2012). However, the large reptile tree does not include Bobosaurus (Dalla Vecchia 2006), a pistosaurid taxon without a known skull. I wonder how Anningasaurus would have nested in Vincent and Benson’s analysis after adding Simosaurus and a few other basal sauropterygians?

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

Reference
Benson RBJ, Evans M, Druckenmiller PS 2012. Lalueza-Fox, Carles. ed. ”High Diversity, Low Disparity and Small Body Size in Plesiosaurs (Reptilia, Sauropterygia) from the Triassic–Jurassic Boundary”. PLoS ONE 7 (3): e31838. doi:10.1371/journal.pone.0031838
Dalla Vecchia FM 2006. A new sauropterygian reptile with plesiosaurian affinity from the Late Triassic of Italy. Rivista Italiana di Paleontologia e Stratigrafia 112 (2): 207–225.
Vincent P and Benson RBJ 2012. Anningasaura, a basal plesiosaurian (Reptilia, Plesiosauria) from the Lower Jurassic of Lyme Regis, United Kingdom, Journal of Vertebrate Paleontology, 32:5, 1049-1063.

wiki/Anningasaura



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