Repairing Parvinatator (basal ichthyosaur) using DGS

The basal ichthyosaur,
Parvinatator (Fig. 1), has been presented or interpreted, as a very odd sort of ichthyosaur, especially for a basal form. All figures of it show an anteriorly leaning orbit, a giant postorbital, a dorsal naris and an ornate and extremely elevated retroarticular process. These traits are not found in other ichthyosaurs.

Recent access to the original paper and photos therein clarified several problems using DGS (digital graphic segregation).

Parvinatator wapitiensis
(Nicholls and Brinkman, 1995; Early Triassic), was once considered the basalmost ichthyosaur. Here, in the large reptile tree, it nests between Chaohusaurus and Qianichthyosaurus. The skull is almost complete and the fore flippers were quite large. Unfortunately very little else of this ichthyosaur is known.

Figure 1. Parvinatator in situ (upper left) with DGS colors applied for bone identification. As originally interpreted (upper right). Reconstructed, repairing the jugal break (lower left). Flipper (lower right).

Figure 1. Parvinatator in situ (upper left) with DGS colors applied for bone identification. As originally interpreted (upper right). Reconstructed, repairing the jugal break (lower left). Flipper (lower right). Now Parvinatator looks more like its sisters, although it still has very deep ‘jowls.’

The reconstruction above
indicates that the posterior skull had been taphonomically rotated forward, breaking the gracile jugal below the orbit. The posterior jugal never expands in sister taxa. Closer inspection indicates the posterior part of the expanded postorbital process of the jugal is the anterior quadratojugal, which is separated from its posterior portion by a matrix break.  These and other bones are corrected and realigned here alongside the original illustration (upper right). Now Parvinatator looks more like it’s sister taxa, except for that really deep posterior mandible.

The funny looking retroarticular process
is probably a cervical and its rib. Nothing like it can be found in sister taxa.

The manus (flipper)
demonstrates that certain side-by-side phalanges in digits 4 and 5 fused together. Perhaps fusion, rather than loss of digits, is how some ichthyosaurs had fewer digits.

Reconstructions are important
Parvinatator shows us that you have to score the animal as it was in vivo, not in situ.

References
Nicholls EL and Brinkman DB 1995. A new ichthyosaur from the Triassic Sulphur Mountain formation of British Columbia. – In: Sarjeant WAS (ed.): Vertebrate fossils and the evolution of scientific concepts: 521–535 London (Gordon & Breach).

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