Largocephalosaurus and a new marine reptile clade, the Saurosphargidae

A new paper by Li et al. 2013 adds more data to what was known of the aquatic enaliosaur, Largocephalosaurus polycarpon (Cheng et al. 2012a, Middle Triassic, Fig. 1).

Cheng et al. 2012a note, “Compared to Sinosaurosphargis, Largocephalosaurus is morphologically less modified from a typical diapsid reptile; it still retains an elongate body shape, a well-developed supratemporal fenestra, a suborbital fenestra (although reduced in size) and an incomplete osteoderm covering.”

Largocephalosaurus nests between long-necked tiny Claudiosaurus and short-necked turtle-like Sinosaurosphargis. These represent an entirely new clade of marine reptiles.

Figure 1. Click to enlarge. Largocephalosaurus nests between long-necked tiny Claudiosaurus and short-necked turtle-like Sinosaurosphargis. These represent an entirely new clade of marine reptiles. Like Hovasaurus, the tail of Largocephalosaurus is vertically expanded to create better propulsion underwater. This is the first lateral view reconstruction of Largocephalosaurus.

Largocephalosaurus and Sinosaurophargis are members of the Saurosphargidae. Since they are so disparate in their morphologies, expect more transitional and distinct taxa to be discovered as time goes by as this clade welcomes new members.

Saurosphargidae Described by Cheng et al. 2012a
“Aquatic diapsids characterized by the following combination of apomorphies:

  1. dorsal ribs forming a closed basket
  2. presence of dorsal osteoderms
  3. external naris retracted, much closer to orbit than rostral tip [shared with all later enaliosaurs]
  4. median elements of gastral ribs often with a two-pronged lateral process on one side
  5. lateralmost elements of gastral ribs broadened and contacting each other
  6. supratemporal extensively contacting quadrate shaft
  7. posterior margin of skull roof deeply emarginated
  8. jugal–squamosal contact
  9. presence of ectopterygoid [plesiomorphic with Claudiosaurus].
  10. presence of interpterygoid vacuity and open braincase – palatal articulation
  11. leaf-shaped tooth crown with convex labial surface and concave lingual surface
  12. dorsal vertebrae with elongate transverse process and a very low neural spine
  13. tip of neural spines table-like, covered by osteoderm(s)
  14. large interclavicle boomerang-like or atypical T-shaped, with a small and sharp posterior process
  15. humerus not expanded at both ends [expanded distally in Sinosaurosphargis]
  16. nine carpals [evidently at least nine]
  17. four tarsals [at least seven, probably nine].

Published phylogenetic analysis 
Li et al. 2013 nested Saurosphargidae between the suprageneric taxa, Thalattosauria and Sauropterygia (placodonts + plesiosaurs and their kin). Outgroups included Helveticosaurus + Eusaurosphargis, which nest within the Thalattosauria in the large reptile tree. Further outgroups include the suprageneric taxa, “Turtles” and “Lepidosauriformes.” Earlier we discussed the dangers of using suprageneric taxa and those dangers have surfaced here.  In Li et al. 2013 Claudiosaurus nested basal to Archosauromorpha, Younginiformes and all the above taxa in the Cheng et al. 2013 tree. That’s another problem brought on by the use of suprageneric taxa.

Reptile tree phylogenetic analysis
According to the large reptile tree Largocephalosaurus is one of many descendants to a sister of Claudiosaurus. A more derived but still contemporary sister is Sinosaurophargis (Fig. 1), a turtle-like basal saurosphargid and enaliosaur from the Middle Triassic.

Other Claudiosaurus sisters and descendants
In the large reptile tree the topology is different from Cheng et al. (2013).

  1. Hovasaurus
  2. Largocephalosaurus + Sinosaurosphargis [recent additions!] = Saurosphargidae
  3. Mesosaurs + Wumengosaurus + Ichthyosaurs + Thalattosaurs
  4. Palatodonta + Placodontia
  5. Pachypleurosaurus + Nothosaurus + the Plesiosauria.

So, distinct from other enaliosaurs and derived from Claudiosaurus, the Saurosphargidae rise to equal importance with Mesosauria, Ichthyosauria and Sauropterygia.

As an aside, the similarly named, Eusaurosphargis, is an unrelated thalattosaur close to Helveticosaurus in both trees.

Phylogenetic history
Largocephalosaurus was originally considered to be a sauropterygian, phylogenetically nested in a clade including typical pachypleurosaurs and nothosaurs (Cheng et al. 2012a). This result was based mainly on skull characters because the postcranial skeleton was not then available for study, and details of Sinosaurosphargis, had not yet been published for comparison. When that happened (Li et al. 2011) the Saurosphargidae nested with thalattosaurs.

Such different sisters
Sinosaurosphargis differs radically from Claudiosaurus, yet the two nested as sisters in the large reptile tree without the help of Largocephalosaurus. Though distinct in many ways, Largocephalosaurus nests between them, demonstrating the acquisition of osteoderms, a larger skull, more roubust forelimbs, a wider torso, transeverse processes, shorter neck, etc. in the evolution of a sister to Claudiosaurus into Sinosaurosphargis.

Changes from Claudiosaurus to Largocephalosaurus
1. Longer premaxilla; 2. Tiny naris; 3. Loss of quadratojugal-jugal contact by reduction of the quadratojugal. 4. Leaf-shaped teeth; 5. Relatively shorter neck; 6. Long transverse  processes on dorsal vertebrae; 7. Dorsal ribs angled and robust laterally; 8. Anterior caudal neural spines much taller; 9. Scapula reduced to posterior rim, separate from clavicle and no longer fused to coracoid; 10. Coracoid circular; 11. Interclavicle loses posterior process; 12. Forelimb larger than hind limb; 13. Loss of intermedium.

References
Cheng L, Chen X, Zeng X and Cai Y 2012a. A new eosauropterygian (Diapsida: Sauropterygia) from the Middle Triassic of Luoping, Yunnan Province. Journal of Earth Science 23, 33–40.
Li C, Rieppel O, Wu X-C, Zhao L-J and Wang LT 2011. A new Triassic marine reptile from southwestern China. Journal of Vertebrate Paleontology 31 (2): 303-312. doi:10.1080/02724634.2011.550368.
Li C, Jiang D-Y, Cheng L, Wu X-C and Rieppel O 2013. A new species of Largocephalosaurus (Diapsida:Saurosphargidae), with implications for the morphological diversity and phylogeny of the group.

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