Cartorhynchus compared to ichthyosaurs and sauropterygians

While phylogenetic analysis nests the new ichthyosaur-mimic, Cartorhynchus, with pachypleurosaurs, sometimes it helps to put the contenders side-by-side (Fig. 1). I’ve also updated the odd pectoral girdle and traced the visible palatal elements since Cartorhynchus was first presented here (which has been updated).

Figure 1. Sauropterygian sisters to Cartorhynchus (green) compared to ichthyosaurian sister candidates. No ichthyosaurs have a short snout and flat belly. Cartorhynchus and sauropterygians swim with their flippers. All ichthyosaurs swim with their tails. Cartorhynchus nests between Pachypleurosaurus and Qianxisaurus in the large reptile tree.

Figure 1. Sauropterygian sisters to Cartorhynchus (green) compared to ichthyosaurian sister candidates. No ichthyosaurs have a short snout and flat belly. Cartorhynchus and sauropterygians swim with their flippers. All ichthyosaurs swim with their tails. Cartorhynchus nests between Pachypleurosaurus and Qianxisaurus in the large reptile tree. That’s where you find a small premaxilla and large clavicles.

Cartorhynchus certainly has a distinct morphology, even for a pachypleurosaur.  But then pachypleurosaurs are basal to a wide range of marine reptiles including placodonts, plesiosaurs, thalattosaurs (including Helveticosaurus and Vancleavea), ichthyosaurs and mesosaurs.

The large head, short neck and flippers instead of limbs set Cartorhynchus apart from other basal sauropterygians. Placodonts also have a short neck and short rostrum, so it happens.

Like all pachypleurosaurs,
Cartorhynchus has both an anterior and posterior coracoid (Fig. 2) forming a chest shield like a plesiosaur. That makes it a flipper swimmer, not a tail swimmer, like ichthyosaurs, which evolved from long, mesosaur-like sauropterygians, like Wumengosaurus. No ichthyosaur has a flat robust gastralia basket, wide rib cage, and short rostrum like Cartorhynchus and the pachypleurosaurs have. Note the long premaxillary ascending process makes contact withe the frontals, as in Pachypleurosaurus. The palate is more pachypleurosaur-like than ichthyosaur-like.

The scapula-coracoid
In many pachypleurosaurs and their descendants, the anterior coracoid and scapula are fused together. Many illustrations of pachypleurosaurs don’t note this, but call the unit a scapula. You can discover this for yourself by looking at a wide variety of clade members. In Cartorhynchus, the scapula is not fused to the coracoid (Figs. 2, 3).

Figure 2. Cartorhynchus lenticarpus in situ showing palatal and pectoral elements

Figure 2. Cartorhynchus lenticarpus in situ showing palatal and pectoral elements. What Motani et al. considered a displaced angular is here interpreted as the anterior coracoid. Yes, the scapulae are preserved upside down. That helps make more sense of the flipped up clavicles. In vivo they were appressed to the elongate interclavicle oriented laterally. The palate morphology is closer to pachypleurosaurs than to ichthyosaurs.

Cartorhynchus and basal ichthyosaurs share many traits.
But Cartorhynchus and basal sauropterygians share a few more. That tips the scales in favor on sauropterygians, based on the hypothesis of maximum parsimony. Ichthyosaurs can also trace their ancestry through basal pachypleurosaurs. So they’re not that far removed.

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. Cartorhynchus reconstruction with palate and pectoral elements in color. This is a wide, flat specimen, like other basal sauropterygians, like Qianxisaurus. The palatal elements can be seen through the orbit and broken rostrum with line extensions completing their outlines. Even the twin tiny internal nares can be seen here. In ichthyosaurs the internal nares are set much further back in the skull. 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.

The new palatal data confirms the pachypleurosaur affinities of Cartorhynchus. Note the presence of internal nares essentially below the external nares. In Wumengosaurus and ichthyosaurs the internal nares are set further back in the skull.

The new interpretation of the ultra-wide interclavicle makes the overall shape more fusiform, despite the presence of a small neck, and it locates the scapula as far laterally as the widest ribs, which makes more sense in the reconstruction based on the squared off proximal humerus giving it room to move dorso-ventrally, more like an underwater wing.

References
Motani R et al. 2014. A basal ichthyosauriform with a short snout from the Lower Triassic of China. Nature doi:10.1038/nature13866

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