Did the frontal migrate to the nasal position in pliosaurs – or not? – part 1

A recent online paper
by Benson et al. (2013) had me scratching my head. What looked like nasals were identified as frontals (Fig. 1, green circle). And the prefrontals contacted the parietals, which were extended anteriorly beyond mid orbit. The nasals were absent, according to Benson et al. (2013) and they’re not the only ones. Benson et al. were following tradition. Knowing that this could really screw up – or clarify – phylogenetic analysis, I dived into pliosaurs, a subject I barely knew. Here are the results of my studies. (And yes, I sent these to Dr. Benson, but he stood by his interpretation).

Figure 1. Are the frontals mislabeled or overlooked? Here are several views and interpretations of Pliosaurus kevani together with two other plesiosaur skulls, both of which have fairly standard nasals and frontals.

Figure 1. Are the frontals mislabeled or overlooked? Here are several views and interpretations of Pliosaurus kevani together with two other plesiosaur skulls, both of which have fairly standard nasals (in magenta) and frontals (in white). The bright blue bones are a tooth and an unidentified crescentic fragment. Note that Benson et al. identify the bone over the naris as a frontal (fr). There might be a suture between the frontal and parietal but there’s also a small crack there, but that’s not important.  The frontals may be fused to the parietals but not to each other. They have not migrated. In the revised figures (in color) the frontal contacts the orbit, as in the compared taxa.

Benson et al. (2013) identified the bone over the naris in Pliosaurus kevani as a frontal, rather than a nasal, contra the identification in related forms like Simosaurus and Plesiosaurus (Fig. 1). Such identifications are not novel to Benson et al. In fact, they go back about a century (Williston 1903, Fig. 2) who didn’t realize the frontals had fused to the parietals but not to each other. It’s really hard to get rid of the frontals. But it’s easy to get rid of frontals if they are fused to the parietals and the resulting bones are called the parietals.

Figure 2. Brachauchenius showing the fusion of the parietal and frontal and the resulting confusion over the nasal.

Figure 2. Brachauchenius from Williston 1903, showing the fusion of the parietal and frontal and the resulting confusion over the nasal. Fusion and abrasion have both contributed to this confusion.

Evidently each parietal fused to each frontal
without fusing medially, and that was not realized. So Williston didn’t know whether the nasal was the nasal or the frontal. A century later, the problem has not gone away.

This goes back to a very old problem in palaeontology.
Often when two bones fuse, the resulting bone is given one name. So the other bone becomes absent, when in reality it is present, but incorporated or fused (Fig. 2).

For the nasals to become the frontals
there has to be a transitional taxon that shows this. But their are no such fossil taxa. If the nasals and frontals fuse, the nasals are still there. If the parietals and frontals fuse, the frontals are still there. We don’t have an inborn bias against nasals, so why did they get sent to the list of absent bones?

Is it possible that the frontals fused to the parietals?
Or was the frontal/parietal suture overlooked?
Either way the nasal can remain over the naris, as it is in related taxa. Otherwise we have to accept that the frontals migrated to take the place and shape of the nasals, without creating vestigial nasals in the process of moving out.

In all other aspects, the Benson et al. paper is well done and well presented.

Final note:
Whether you agree or disagree with the above assessment, this is Science at its best, repeating the observation or experiment and confirming or refuting the original observation or experiment. If there’s a disagreement, third and fourth parties are encouraged to repeat the observation and report. That’s what they call consensus, which is currently in the Benson et al. camp among pliosaur aficionados.

Tomorrow, a look at more evidence from more pliosaur frontals.

References
Benson RBJ, Evans M, Smith AS, Sassoon J, Moore-Faye S, Ketchum HF and Forrest RF 2013. A Giant Pliosaurid Skull from the Late Jurassic of England. PLOSOne online here.
Williston SW 1903. North American Plesiosaurs (Part 1). Field Columbian Museum, Publ. 73, Geological Series 2(1):1-79.

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