Cartorhynchus: rebuilding the small ichthyosaur mimic

Mistakes were made here
earlier while reconstructing Cartorhynchus, the basal sauropterygian ichthyosaur-mimic. Those mistakes are corrected here (Figs. 1, 2) and already updated in earlier posts. All of these repairs further cement the relationship of Cartorhynchus to its sister, Sclerocormus  (Fig. 3) and its ancestral sister, Qianxisaurus (Fig. 4), taxa nesting near the base of the Eosauropterygia, not the Ichthyopterygia in the large reptile tree (LRT, 1401 taxa).

Figure 1. New tracing and reconstruction of the basal sauropterygian with flippers, Cartorhynchus.

Figure 1. New tracing and reconstruction of the basal sauropterygian with weak flippers, Cartorhynchus. Note the flipped maxilla, now convex ventrally. The pectoral girdle is rebuilt based on that of Qianxisaurus. See text for details. Compare pectoral elements to Qianxisaurus in figure 5.

Cartorhynchus lenticarpus (Motani et al. 2014; Early Triassic) was originally considered a strange basal ichthyosauriform and a suction feeder. Here it nests with Sclerocormus and Qianxisaurus as a basal eosauropterygian representing a new clade of ichthyosaur-mimics with a very early appearance of flipper-like limbs. Neotony played a part in the appearance of a short rostrum, large eyes, short neck, poorly ossified phalanges and small size. The supratemporal was large here, and the splenial can be seen in lateral view, though just barely. These are also results of neotony as most sauropterygians lack them. The outgroup taxon, Pachypleurosaurus, fuses the large supratemporal and squamosal

Ichthyosaurs have the following traits by convergence.
Ichthyosaurs have robust scleral rings (eyeball bones) while most eosauropterygians do not. Distinct from most eosauropterygians and like ichthyosaurs, Qianxisaurus has small supratemporals and gracile scleral rings. The splenials are not visible in the present exposure. Like Cartorhynchus, the digits of Qianxisaurus are not well developed. 

Figure 2. Cartorhynchus reconstruction in lateral and dorsal views with new lateral view skull and pectoral girdle.

Figure 2. Cartorhynchus reconstruction in lateral and dorsal views with new lateral view skull and old invalid dorsal view skull. The new pectoral girdle is in place here. The flippers seem to be relatively immobile. The tail was the main propulsive organ. Neotony created this sauropterygian with large eyes, short snout, short neck and digit-less limbs.

The premaxilla
of Cartorhynchus was tiny, ideal for nipping small food items. Small teeth were present, contra the original interpretation. The naris and orbit were quite large, distinct from all candidate sister taxa. The in situ maxilla was taphonomically flipped, so Cartorhynchus actually had a ventrally convex maxilla. The heavy ribs and flat bottom make Cartorhynchus look like a bottom feeder. The elevated dorsal neural spines suggest a dorsal fin (Fig. 2). The small scapula and coracoid suggest a weak, passive pectoral flipper. A long tail was probably present, as in its larger sister, Sclerocormus (Fig. 3). This would have been the primary propulsive organ. Note the lack of large flippers in Sclerocormus. Qianxisaurus is the last known common ancestor.

Figure 1. Large Sclerocormus and its much smaller sister, Cartorhynchus. These nest with basal sauropterygians, not ichthyosauriforms.

Figure 3. Large Sclerocormus and its much smaller sister, Cartorhynchus. These nest with basal sauropterygians, not ichthyosauriforms.

Qianxisaurus chajiangensis (Cheng et al. 2012; Fig. 4) is a Middle Triassic basal eosauropterygian based on a virtually complete articulated skeleton with all digits poorly ossified. Cheng et al. 2012 nested Qianxisaurus as derived from a sister to Wumengosaurus, a taxon that nested closer to thalattosaurs, ichthyosaurs and mesosaurs in the LRT. The Cheng et al. (2012) study had some odd nestings including Kuehneosauridae (rib gliders) a little too close to turtles and thalattosaurs. The LRT widely separated these taxa, as befitting their utterly distinct morphologies.

Figure 4. Slight changes to the temple region of Qianxisaurus shows the reappearance of the suptratemporal, which had been lost in more primitive taxa only to be reacquired here and further elaborated in Cartorhynchus.

Figure 4. Slight changes to the temple region of Qianxisaurus shows the reappearance of the suptratemporal, which had been lost in more primitive taxa only to be reacquired here and further elaborated in Cartorhynchus.

The LRT
nests Qianxisaurus between Pachypleurosaurus and LariosaurusPistosaurus nests as an outgroup in the Cheng et al. (2012) tree, but closer to Simosaurus in the LRT.

The upper temporal fenestrae of Qianxisaurus
were smaller than in Pachypleurosaurus. The supratemporal formed the posterior rim without fusion to the squamosal. In Qianxisaurus the retroarticular process of the mandible was smaller than in related taxa. The teeth of Qixiansaurus were unusual with a slightly constricted cylinder and short conical crown.

Despite the small size of the ilium
at least four sacrals were present.

The Cartorhynchus-like pectoral girdle of Qianxisaurus.
The scapula (green Fig. 5) had a slim strap-like morphology. The clavicles were broader laterally, meeting medially in an arch shape, as in Cartorhynchus (Fig. 1).

Figure 5. The Qianxisaurus pectoral girdle is ancestral to the Cartorhynchus pectoral girdle with similarly shaped elements. Compare to figure 1.

Figure 5. The Qianxisaurus pectoral girdle is ancestral to the Cartorhynchus pectoral girdle with similarly shaped elements. Compare to figure 1. Interclavicle is hidden in situ and hypothetical here based on phylogenetic bracketing. 

Mimic taxa
appear occasionally in the LRT, which, so far, has been able to lump and split mimics by testing them against all available candidate sisters. Motari et al. 2014, for all his experience and expertise in ichthyosaurs, failed to add basal eosauropterygians, like Qianxisaurus, to their taxon lists and so was not able to consider this possibility. Better not to assume things, but to let the software perform an unbiased analysis starting with a wide gamut of taxa like the LRT.

Correcting mistakes
is part of the scientific process, whether they be internal or external. 

References
Cheng YN, Wu XC, Sato T and Shan HY 2012. A new eosauropterygian (Diapsida, Sauropterygia) from the Triassic of China. Journal of Vertebrate Paleontology. 32 (6): 1335. doi:10.1080/02724634.2012.695983
Jiang D-Y, Motani R, Huang J-D, Tintori A, Hu Y-C, Rieppel O, Fraser NC, Ji C, Kelley NP, Fu W-L and Zhang R 2016. A large aberrant stem ichthyosauriform indicating early rise and demise of ichthyosauromorphs in the wake of the end-Permian extinction. Nature Scientific Reports online here.
Motani R et al. 2014. A basal ichthyosauriform with a short snout from the Lower Triassic of China. Nature doi:10.1038/nature13866

wiki/Qianxisaurus
wiki/Cartorhynchus
wiki/Sclerocormus

 

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