Friday mystery object #410 answer

Last week I gave you this rather fishy skull to have a go at identifying:

There was a lot of discussion about what it could be, which is unsurprising, since there are a LOT of fish species – with over 34,000 possibilities. This one proved additionally confusing, since it seems to have no teeth, as mentioned in the comments by Adam Yates. Fortunately, Wouter van Gestel flagged that some species with several rows of teeth tend to lose those teeth during preparation if it’s not done with sufficient care, which is useful to know.

I picked this object because I get a lot of requests for identifications of fish skull bones and this specimen is helpful, as it has the various bones of the skull labelled individually:

This specimen also happens to be a fish from a family that often comes up for identification. The neurocranium (or braincase) has a fairly characteristic shape in these fish, which is best seen from above on the skull (although unfortunately it’s not labelled in the image below):

The neurocranium tends to be a bone that gets found on beaches quite commonly. In fact, I have had a similar neurocranium as a mystery object in the past, so you may have seen one here before:

This shape is what I expect to see from members of the True Cod family, the Gadidae. Clearly a lot of other people recognised this as well, since Chris kicked off the comments with references to Gadus, Cod and Pollock and there were lots of suggestions of Cod and Pollock (AKA Saithe) on Twitter:

Unfortunatley, this is where it gets more complicated. Differentiating between different Gadids isn’t always easy. The size suggests it will be one of the larger members of the family – Cod, Haddock or Pollock being the main focus. Haddock is easy enough to dismiss, since they have small mouths, with lower jaws (composed mainly of the dentary and articular bones) that don’t project as far as we see in this specimen.

After that it gets really quite tricky – to the point where I am now doubting the original identification we have for the mystery object. This specimen was labelled as an Atlantic Cod Gadus morhua, although the original identification when acquired from Rowland Ward was Pacific Cod Gadus macrocephalus. But after a lot of searching of images from some pretty reliable online resources, I’m increasingly convinced that the specimen is a Pollack, Pollock or Saithe Pollachius pollachius (Linnaeus, 1758) – N.B. I’m ruling out P. virens since the lower jaw proportions are wrong.

The reason I’m thinking Pollack is based around a few small features of a couple of the bones of the skull. In particular, I’m interested in the shape of the hyomandibular and the opercular (Osteobase has these elements for Cod, but unfortunatley not Pollack). To give you an idea of the differences, here are the Cod elements (superimposed in blue) alongside the same bones of the mystery object (tinted red):

These differences are consistent across the skull specimens of Cod and Pollack that I’ve managed to find. The Cod has notch in the upper leading edge of the hyomandibular, unlike the Pollack, which has a more obtuse smooth line along the leading edge. The Cod also has a notch in the trailing lower edge of the opercular, that is just seen as a slight concavity in the Pollack.

I’d be interested to hear what you think about these suggested features!

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