Diandongosaurus – pachypleurosaur/placodont transitional taxon

Just found another paper on Diandongosaurus the day after, August 30, 2015. See below.

Diandongosaurus acutidentatus (Shang et al. 2011, IVPP V 17761) was originally considered, “neither a pachpleurosau nor a nothosauroid; it might be the sister group of the clade consisting of Wumengosaurus, the nothosauroid and those taxa traditionally considered as pachypleurosaurs.”

Shang et all are almost correct.
Despite its very pachypleurosaur-ish overall appeance (Fig. 2), Diandongosaurus nested at the base of the Placodontia in the large reptile tree.

Figure 1. Diandongosaurus skull. The DGS method shows the dorsal and palatal views of the in situ specimen.

Figure 1. Diandongosaurus skull. The DGS method shows the dorsal and palatal views of the in situ specimen.

Using DGS,
the skull of Diandongosaurus (Fig. 1) is only slightly different than originally described. The prefrontal does not meet the postfrontal in this or any other basal sauropterygian. The premaxilla/maxilla suture is shifted slightly forward so that the premaxilla has only 4 teeth.

Figure 2. Diandongosaurus exposed in ventral view, skull in dorsal view. Note the small size. At 72 dpi this image is 6/10 the original size.The last common ancestor of Diandongosaurus and Pachypleurosaurus was a sister to Anarosaurus at the base of the Sauropterygia.

Figure 2. Diandongosaurus exposed in ventral view, skull in dorsal view. Note the small size. At 72 dpi this image is 6/10 the original size.The last common ancestor of Diandongosaurus and Pachypleurosaurus was a sister to Anarosaurus at the base of the Sauropterygia.

While we’re discussing the base of the Sauropterygia…
I realized that all sister taxa of Cartorhynchus (Fig. 4) have about 19 cervicals and 19 dorsals. Originally I had reconstructed Cartorhynchus withe its pectoral girdle close to the skull, where it was found in situ. But the pectoral girdle was much wider than the ribs in that area. Of course crushing is involved, but if you move the pectoral girdle closer to the 19th cervical, then everything appears to fit a little better (Fig. 3). Those posterior cervical ribs were dorsalized, indistinct from the dorsal ribs based on available data. Perhaps a closer look would show the line of demarcation.

Figure 1. Although the pectoral girdle was preserved just behind the skull, in all sister taxa there are about 19 cervicals and 19 dorsals. Plus the pectoral girdle itself is very wide, better suited to the widest ribs. Perhaps Cartorhynchus had a longer neck than commonly assumed.

Figure 3. Although the pectoral girdle was preserved just behind the skull, in all sister taxa there are about 19 cervicals and 19 dorsals. Plus the pectoral girdle itself is very wide, better suited to the widest ribs. Perhaps Cartorhynchus had a longer neck than commonly assumed.

You might remember 
Cartorhynchus (Motani et al. 2014) was originally considered a type of basal ichthyosaur. Having a short neck was part of that decision. Lacking the correct generic sister taxa (Fig. 4) was also part of that decision. A few score revisions nested Cartorhynchus as a sister to Qianxisaurus, which also has poorly ossified manual digits.

Figure 4, Subset of the large reptile tree: the marine younginiformes, including the Enaliosauria (Sauropterygia + Mesosauria + Thalattosauria + Ichthyosauria)

Figure 4, Subset of the large reptile tree: the marine younginiformes, including the Enaliosauria (Sauropterygia + Mesosauria + Thalattosauria + Ichthyosauria)

A second paper on Diandongosaurus 
(Sato et al. 2013) just came to my attention, restudied on the basis of a new specimen, also in ventral view.  It is preserved straight as an arrow. (Fig. 5).

Figure 5. Sato et al. specimen of Diandongosaurus.

Figure 5. Sato et al. specimen of Diandongosaurus.

The Sato et al team
nested their specimen at the base of the nothosauroids (Nothosaurus, Corosaurus, Lariosaurus), but they did not include Palatodonta, Pappochelys and other basal placodonts. Instead they used Placodus and Cyamodus to represent all placodonts. The Sato et al. team also used many suprageneric taxa, except among the pachypleurosaurs.

By duplicating the deletion
of all but two placodonts, the large reptile tree recovered Diandongosaurus at the base of the the Sauropterygia, basal to Pachypleurosaurus. So no change there. However, Placodus and Cyamodus now nested between Wangosaurus and Simosaurus among the basal plesiosaurs, one node away from the nothosaurs.

By duplicating the taxon list 
of Sato et all. (as best as I could using 25 similar or the same taxa) Diandongosaurus did not change its nesting between Anarosaurus and Pachypleurosaurus. Likewise, the sauropterygians did not change their topology. However, other taxa were all over the place. The soft shell turtle, Odontochelys, nested at the base with the thalattosaur, Askeptosaurus. The rhynchosaur, Hyperodapedon and the choristodere, Champsosaurus nested with the placodonts, Placodus and Cyamodus. Claudiosaurus nested with Prolacerta, Trilophosaurus, Iguana (representing Squamata) and Proterosuchus (representing Archosauriformes). These odd nestings demonstrate the importance of having a broad gamut study to enable a verifiable narrowing of focus on a subset of that broad gamut study. Otherwise, its just scattershot, as shown above.

References
Motani R et al. 2014. A basal ichthyosauriform with a short snout from the Lower Triassic of China. Nature doi:10.1038/nature13866
Sato T, Cheng Y-N, Wu X-C and Shan H-Y 2013. Diandongosaurus acutidentatus Shang, Wu & Li, 2011 (Diapsida: Sauropterygia) and the relationships of Chinese eosauropterygians. Geological Magazine 151:121-133.
Shang Q-H, Wu X-C and Li C 2011. A new eosauropterygian from Middle Triassic of Eastern Yunnan Province, Southwestern China. Vertebrata PalAsiatica 49(2):155-171.

 

wiki/Cartorhynchus

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