Cymbospondylus – primitive or derived?

Cymbospondylus petrinus is a 20-30 foot (up to 8.33 m) long Middle Triassic ichthyosaur with a long, low toothy skull, short broad paddles and a long low, tail (Fig. 1).

Figure 1. Cymbospondylus skull and overall in lateral view.

Figure 1. Cymbospondylus skull and overall in lateral view.

The question is: 
Is Cymbospondylus primitive and derived from Chaohusaurus and Grippia (as per Motani 1999)? Or is Cymbodpondylus derived and derived from Mixosaurus (as per Maisch and Matzke (2000, 2003) and the large reptile tree)?

Cymbospondylus appears to be primitive.
It has the long snaky body of basal ichthyosaurs, like Utatsusaurus and Thaisaurus.

However, if Cymbospondylus nests between Grippia and Mixosaurus
it is a giant nesting between two relatively small to tiny ichthyosaurs.

Figure 2. Cymbospondlyus compared to sister taxa according to the large reptile tree to length (above) and to scale (below). Shown in gray is Shonisaurus popularis, which is compared to to Shonisaurus sikanniensis.

Figure 2. Cymbospondlyus compared to sister taxa according to the large reptile tree to length (above) and to scale (below). Shown in gray is Shonisaurus popularis, which is compared to to Shonisaurus sikanniensis.

Motani (1999)
nested Cymbospondylus at the base of the Ichthyosauria between Chaohusaurus + Grippia and Mixosauria (Mixosaurus and all higher ichthyosaurs (Merriamosauria).

Maisch and Matzke (2000, 2003)
nested Cymbospondylus a little higher, between Mixosauria and Merriamosauria.

Figure 2. Ichthyosaur subset of the large reptile tree.

Figure 2. Ichthyosaur subset of the large reptile tree.

The large reptile tree (Fig. 2) nested Cymbspondylus petrinus between Mixosaurus and the toothless Guanlingsaurus liangae YGMR SPC V03017 + the possibly toothless giant Shonisaurus sikanniensis (apart from ‘Cymbospondylus’ buchseri, which here (Fig 2) nests with Shonisaurus popularis in a distinct clade). So we should expect several taxa transitional between Mixosaurus and these giants and near giants.

Despite their long, snaky look, Cymbospondylus and kin are not primitive, but may have reverted to that morphology as they grew to larger and larger size.

References
Leidy J 1868. Notice of some reptilian remains from Nevada: Proceedings of the American Philosophical Society 20:177-178.
Merriam JC 1908. Triassic ichthyosauria with special references to the American forms. Memoirs of the University of California 1: 1-196.
Yin G-Z, Zhou X, Cao Y, Yu Y and Luo Y 2000. A preliminary study on the earlyLate Triassic marine reptiles from Guanling, Guizhou, China. Geology-Geochemisty 28(3):1–23 (Chinese with English abstract).

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