Xinminosaurus, yet another basal ichthyopterygian

I had no idea
so many basal ichthyopterygians were out there. Oddly, their original authors suspected the same, but did not put forth cladograms to support their hunches. Plus, some were Middle Triassic in age, while more derived taxa are found in Early Triassic strata. Finally, the proximal outgroups for ichthyosaurs (Fig. 2) were not recognized.

Figure 1. Xinminosaurus in situ and with DGS reconstructed.

Figure 1. Xinminosaurus in situ and with DGS reconstructed.

Xinminosaurus catactes (Jiang et al. 2008, Middle Triassic, GMPKU-P-1071, 1.6m). is another basalmost ichthyopterygian known for over 7 years now. Distinct from its closest kin, the teeth of Xinminosaurus were large squarish blocks. The paddles were short and broad with just a few extra phalanges (3-5-5-5-2) on the manus.

Figure 2. Cladogram of ichthyosaurs and kin with five putative Shastasaurus specimens in pink.

Figure 2. Cladogram of ichthyosaurs and kin with Xinminosaurus nesting close to the base of the Ichthyopterygia or as a transitional taxon proximal to that clade. 

Xinminosaurus had smaller cervicals than in Thaisaurus. The humerus was shorter. The scapula was not as tall. The hind limbs were shorter, more paddle-like. All these traits are more ichthyosaurian. So these taxa (Fig. 2), together with Wumengosaurus, provide a gradual accumulation of ichthyosaurian traits.

The origin of ichthyosaurs
is not such a mystery when you employ 530 taxa, but this topology was recovered when only half the current number of taxa were known, when Stereosternum was the sister to the Ichthyopterygia. The rest have been added over the last four years.

Whenever basal ichthyosaurs are mentioned,
Cartorhynchus and Omphalosaurus are considered. The large reptile tree found Cartorhynchus nested close to the pachypleurosaur, Qianxisaurus. Omphalosaurus is known by too few bones to be included in the large reptile tree, but earlier, it was considered close to Sinosaurosphargis.

Jiang D, Motani R, Hao W, Schmitz L, Rieppel O, Sun, Sun Z 2008. New primitive ichthyosaurian (Reptilia, Diapsida) from the Middle Triassic of Panxian, Guizhou, southwestern China and its position in the Triassic biotic recovery. Progress in Natural Science 18 (10): 1315.


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