Overgrown Bone — A ROM Paleontologist identifies the fish with the swollen head

Originally published in the Summer 2010 edition of ROM Magazine.

Bluecheek Silver grunt

Q. I found this object on a beach in Oman. I think it might be part of a fish skull. If it is, can you tell me what kind of fish it is from?
Mike Silver, Toronto

A. You are indeed correct: it is part of a fish skull, but not just any ordinary fish skull. I knew the moment I saw the swollen and thickened bones that this skull has hyperostosis, which means “excessive thickening of the bone.” Several skulls in our comparative fish-bone collection display this same phenomenon.

Originally scientists interpreted these bone overgrowths as tumours, but we now know that they are a regular part of the growth process in some species.  Exactly why some fish do this and others don’t is still a mystery.

Within a species the same bones are always hyperostotic.  But across species, the patterns are different—so the overgrowth is species-specific. In many fish, part of the skull is affected, such as the supraoccipital of the spadefish. In other, such as the crevalle jack, it’s the ribs, vertebrae, or other skull bones that are involved.

Even though I knew this was a hyperostotic skull, I still had trouble identifying your particular fish species. ROM ichthyologists (fish specialists) pointed me to various books on fishes of Oman and the Persian Gulf, but none had skeletal illustrations. I was pretty sure no similar skull would be in our comparative collections since we have few from that part of the world, but I checked them all just to be sure—and found no match. The thousands of whole preserved fish in the ichthyology collection were not likely to be any help either, because I needed to see their skeletons.

The solution turned out to be the Internet.  I started searching for illustrations of hyperostotic fish bones using the names of the fish that looked most similar to your fish skull, such as several species from the drum family. This led me to an archaeological site report published in Germany in 2002, which discussed a huge collection of bones excavated from a medieval mosque in Siraf, Iran.  Lo and behold, illustrated in one of the report’s figures was a fragment of a fish skull that matched yours exactly.

The illustration matched so well because the non-hyperostotic parts of the skull, which are thinner and more delicate, had broken away in both the recent skull you found washed up on the beach, and in the archaeological fish bone, which represents the remains of someone’s meal back in medieval times.  Whether the cause was natural or human-induced, the breakage pattern was the same.

Thanks to this illustration, we now know that your skull is from a bluecheek silver grunt (Pomadasys argureus).  These tasty fish still live in the Middle East today.

Thank you for bringing in such an intriguing specimen and donating it to our collection.

Images of Fish Skulls

1. Photograph of Mike Silver’s incomplete fish skull from Oman. Top view.

2. A complete bluecheek silver grunt skull with hyperostosis. Top view.

3. A complete bluecheek silver grunt skull with out the hyperostosis. Top view.

Photos: Bluecheek silver grunt, courtesy of T. Gloerfeit-Tarp and P.J.Kailcla, 1984. Skulls 2 and 3 courtesy of A. von den Dresch and A. Dockner, 2002.