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Dianmeisaurus: perhaps not a new pachypleurosaur
A new paper
by Shang and Li (2015) presents an exquisitely preserved small (< 50cm) Middle Triassic pachpleurosaur (basal sauropterygian, IVPP V 18630) they name Dianmeisaurus (Fig. 1-3, pdf online here). Shang and Li noted resemblance to contemporary specimens attributed to Diandongosaurus Shang et al., 2011 and Dianopachysaurus Liu et al., 2011a, both similarly without a constricted snout and with a large orbit and small upper temporal fenestra. They reported the new genus, Dianmeisaurus, is characterized by extremely narrow frontals and a mandibular articulation close to the occipital condyle, a large and stout anterolateral process of the clavicle, the proximal ulna is much wider than the distal end and 41 presacral vertebrae. Other traits are distinct from one sister or the other.
Phylogenetic analysis
Shang and Li added this new taxon to a flawed cladogram by x and nested Dianmeisaurus between Diandongosaurus and Majiashanosaurus with 49 MPTs. Keichousaurus and Dianopachysaurus are also clade members close to the base of the Nothsauroidea.
By contrast,
I added Dianmeisaurus to the large reptile tree and found it nested with Keichousaurus and Hanosaurus at the base of the Nothosauroidea. Since the post-crania is completely visible (except for the ilium) the differences are all in the skull, which is exposed ventrally and only slightly disarticulated. Note the rather vague tracing by Shang and Li. By contrast, I traced more bones. They had first hand access to the fossil itself. I worked from published images using DGS (digital graphic segregation) and created a reconstruction in several views that closely match sister taxa.
Figure 1. Dianmeisaurus with tracing by Shang and Li 2015 and with tracings using DGS bottom from right to left: mandible and occiput, ventral view of dorsal elements, palatal elements, and at left all combined. Above are reconstructions in dorsal, lateral and palatal views. Note the differences with the original tracing. Click to enlarge. The depth of the premaxilla and length of the pmx teeth are guesses as dimensions are largely hidden by the overlying dentaries.
Distinct from Shang and Li
I found the preorbital and postorbital regions sub equal. The pterygoids and and palatines are narrower. The maxillary processes are wider. Circumorbital bones are defined. A posterior parietal and occiput are separated and identified.
In the large reptile tree
Dianmeisaurus is similar to Hanosaurus, but differs from Keichousaurus in the following traits:
- Orbit not > lateral temporal fenestra
- Pineal foramen ≥ 0.20 minimum parietal length
- Posterior parietal > 40º in dorsal view
- Last maxillary tooth extends to posterior orbit
- Phalanx number on pedal digit 4: five
- Pedal 4.1 length/width less than 3/1.
Dianmeisaurus is similar to Keichousaurus, but differs from Hanosaurus in the following traits:
- Orbit ≥ rostrum
Keichousaurus is similar to Hanosaurus, but differs from Dianmeisaurus in the following traits:
- Frontal and parietal fusion
Other traits,
such as humerus vs. femur length, are not preserved or exposed in all three taxa. Keichousaurus traits are based on a published drawing. The current loss of resolution could be due to bad data, or could resolve itself with more pertinent taxa. Based on the above traits, Dianmeisaurus appears to be closer to Hanosaurus, which closely resemble one another. If congeneric, they are not conspecific.
Personal note
This is the 1500th blog post for this WordPress site.
References
Shang Q-H and Li C 2015. A new small-sized eosauropterygian (Diapsida:Sauropterygia) from the Middle Triassic of Luoping, Yunnan, southwestern China. Vertebrata PalAsiatica 10:265-280. online pdf here
Source: https://pterosaurheresies.wordpress.com/2015/12/09/dianmeisaurus-perhaps-not-a-new-pachypleurosaur/
