Which ichthyosaur is the most primitive? Part 2

Updated April 13, 2015 with a flatter cranium reconstruction.

Among the Triassic ichthyosaurs I have tested, this one, Mikadocephalus, now nests as the most primitive. Note the plesiomorphic skull bones: a parietal without a crest or trough,  and an overall similarity to the outgroup taxa described below.

Figure 1. Mikadocephalus (Middle Triassic) now nests as the basalmost ichthyosaur, next to Eohupehsuchus and Wumengosaurus on one side and Utatsusaurus and Grippia on the other. Here the former premaxilla is the dentary and a few other bones are reidentified. And it's big, which means there are more primitive ichthyosaurs yet to be discovered in the Early Triassic.

Figure 1. Mikadocephalus (Middle Triassic) now nests as the basalmost ichthyosaur, next to Eohupehsuchus and Wumengosaurus on one side and Utatsusaurus and Grippia on the other. Here the former premaxilla is the dentary and a few other bones are reidentified. And it’s big, which means there are more primitive ichthyosaurs yet to be discovered in the Early Triassic.

Mikadocephalus gracilirostric (Maisch and Matzke 1997) was described from “an almost complete skull” from the Anisian-Ladinian of Switzerland that “does not fit into any of the currently recognized families of Triassic ichthyosaurs.” 

It is a large ichthyosaur with a skull around 50 cm long. That size is probably due to its late appearance in the Middle Triassic. Undiscovered Early Triassic sisters were probably much smaller.

Oddly,
the largest bones of the skull, the dentaries, were originally thought to be absent. Here the former premaxillae are identified as paired dentaries (yellow) and the premaxillae are smaller bones found elsewhere, splintered and separated into dorsal and tooth-bearing portions. Colorizing and reconstructing the bones using DGS made these identifications possible.

Figure 2. Wumengosaurus, a proximal outgroup taxon to hupehsuchids + ichthyosaurs.

Figure 2. Wumengosaurus, a proximal outgroup taxon to hupehsuchids + ichthyosaurs has a skull similar to that of Mikadocephalus.

Yesterday we looked at a number of basal ichthyosaur skull temple regions and it was quite apparent that this area underwent great changes that are not apparent in Mikadocephalus. Rather Mikadocephalus has a more primitive skull, like that of Eohupehsuchus (Fig. 3) and Wumengosaurus (Fig. 2).

Figure 2. Eohupehsuchus has a skull similar to that of the basalmost ichthyosaur, Mikadocephalus.

Figure 3. Toothless Eohupehsuchus is a hupehsuchid that has a skull similar to that of the basalmost ichthyosaur, Mikadocephalus.

It is unfortunate
the post-crania of Mikadocephalus is unknown. In 1997 Eohupehsuchus and Wumengosaurus were unknown. Even so, had a reconstruction of Mikadocephalus been created in 1997 I think we would have known about its special status a long time ago. Or at least the status of the dentaries would have been realized. The authors nested Mikadocephalus between Cymbospondylus and Temnodontosaurus out of an list of eight ichthyosaurs.

References
Maisch M and Matzke AT 1997. Mikadocephalus gracilirostris n. gen., n. sp., a new ichthyosaur from the Grenzbitumenzone (Anisian-Ladinian) of Monte San Giorgio (Switzerland). Paläontologische Zeitschrift 71(3/4):267-289.

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