Anshunsaurus (thalattosaur) juvenile notes

Anshunsaurus (Liu 1999) was a late middle Triassic (Landinian) thalattosaur close to Askeptosaurus. It is known from an adult and a juvenile.

From the abstract: “A marine reptile from the Ladinian deposits near Xingyi is described and identified as a juvenile of Anshunsaurus wushaensis on the basis of similar skull proportions and many postcranial characters. Based on this specimen and observations of the holotype of A. wushaensis, there is no distinct ontogenetic differentiation in the length of the jugal. The absence of an astragalus in the holotype, and the greater length of metacarpal V relative to metacarpal IV, could be due to intraspecific variation. Ossification is not synchronous for corresponding elements on both sides of the body.”

This asymmetry is the key point to this post, and something to consider when thinking about adult/juvenile matching.

Figure 1. The two Anshunsaurus skull, adult and juvenile.

Figure 1. The two Anshunsaurus skull, adult (from Liu and Rieppel 2005) and a  juvenile (from Liu 2007). Scale bar is 10cm. So on a 72dpi screen the skulls are shown at 0.8 scale or 20 percent smaller.

What are thalattosaurs?
Liu and Rieppel 2004 wrote, “The phylogenetic position of thalattosaurs within amniotes is controversial: They have been suggested to be diapsids with possible affinities to the Lepidosauromorpha (Romer, 1956; Rieppel, 1998), Archosauromorpha (Evans, 1988), to be Neodiapsida inc. sed. (Benton, 1985), or else to be the sister taxon of Sauria (Mu¨ller, 2004) or of Ichthyopterygia (Müller, 2003).”

According to the large reptile tree, Müller is correct.

The Triassic marine reptile Anshunsaurus huangguoshuensis was originally described by Liu (1999) as a sauropterygian (likely a plesiosaur or nothosaur) on the basis of a dorsal view of a skull. The specimen was recognized as thalattosaurian by Rieppel et al. (2000). So errors happen.

Figure 2. The manus and pes of the large and small Anshunsaurus specimens.

Figure 2. The manus and pes of the large and small Anshunsaurus specimens. In the manus note the difference in metacarpal/digit proportions along with carpal ossification. In the pes note similar differences, and also a lengthening of mt4 and a narrowing of its proximal articulation. Individual variation or speciation?

Genus differences
Anshunsaurus differed from the previously known but similar Askeptosaurus by the maxilla forming part of the  anteroventral orbital margin; fusion of the postorbital and postfrontal; the posterolateral process of the frontal extending posteriorly far beyond the anterior margin of lower temporal fossa, narrowly approaching but not contacting the supratemporal; the long and slender ventral process of the squamosal extending to the lower margin of the cheek; jugal with an elongate posterior process; lateral exposure of the angular equal to that of the surangular; deltopectoral crest on the humerus developed; fibula expanded.

Species differences
A. wushaensis
 was slightly smaller and differed from the holotype A. huangguoshuensis in the following characters (Rieppel et al., 2006):

  1. relatively smaller skull relative to the glenoid-acetabulum length;
  2. short posterior process of the jugal that does not extend backward beyond the midpoint of the lower temporal fossa;
  3. neural spines in the posterior dorsal region that are not taller than their anteroposterior width and with a distinct ornamentation of vertical grooves and ridges near their dorsal margin;
  4. cruciform interclavicle with a broad-based anterior process;
  5. ectepicodylar groove and notch on humerus distinct;
  6. entepicondyle well developed, with ridge on ventral side of medial margin but no foramen;
  7. metacarpal V slightly longer than metacarpal IV;
  8. loss of one phalanx in fourth digit;
  9. iliac blade posterodorsally expanded; 
  10. seven ossified tarsals.

In the skull, the major difference between A. huangguoshuensis and A. wushaensis is the degree of extension of the posterior process of the jugal.

Rieppel et al. (2006) report on the new species, “The specimen here described is remarkable with respect to the asymmetry of the maxillae, scapulae, and ilia. The scapular and iliac asymmetry is due to different degrees of ossification. It indicates that the corresponding elements on two sides are not synchronous in the process of ossification.”

Worth remembering.

References
Liu J 1999. New discovery of sauropterygian from Triassic of Guizhou, China. Chinese Science Bulletin 44: 1312–1315.
Liu J 2007. A Juvenile Specimen of Anshunsaurus (Reptilia: Thalattosauria). American Museum Novitates 3582, 9 pp.
Liu J and Rieppel O 2005. Restudy of Anshunsaurus huangguoshuensis (Reptilia: Thalattosauria) from the Middle Triassic of Guizhou, China. American Museum Novitates 3488, 34 pp.
Rieppel O, Liu J and Li C 2006. A new species of the thalattosaur genus Anshunsaurus (Reptilia: Thalattosauria) from the Middle Triassic of Guizhou Province, southwestern China. Vertebrata PalAsiatica 44: 285–296.

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