Underwhelming Fossil Fish of the Month: July

By Mark Carnall, on 30 July 2013

It’s very trendy to point out that with the combination of Google, Wikipedia and smart phones, we now have more information at our fingertips than any of the great thinkers, including Charles Darwin, ever had access to. Although this may be technically true, a lot of that information we can access is videos of cats, this tome on Luke Skywalker’s wife and several terabytes of saucy Harry Potter fan fiction (we don’t dare link to). In fact it’s probably a good job that Darwin didn’t have such distractions in the palm of his hand as we may have never ended up with On the Origin of Species because he filled his days watching videos of Japanese men synchronised walking.

However, for the fossil fish that feature in this series, Underwhelming Fossil Fish of the Month, it’s safe to say that Darwin had access to almost as much information about these uninspiring, unimportant and all around underwhelming fossils as we have today and July’s fossil fish is no exception.

Psammodus tooth plate Grant Museum of Zoology

LDUCZ-V1474 Lovely

Here we have a rather lovely specimen which has been identified as the tooth of Psammodus sp. Psammodus is a genus of cartilaginous fish (one of the many ways of being a fish, today’s cartilaginous fish include sharks, rays, skates and rat, elephant and rabbit-fishes). Quite where and how Psammodus fits into this group is very poorly understood, some authorities placing them in the Chimaera wastebasket group that includes the aforementioned rat, elephant and rabbit-fish (interesting(?) aside, there is more than one way of being an elephantfish or rabbit-fish, explaining quite how is a blog post for another time). Before we get tied up in knots, it’s sufficient to say that Psammodus is probably a sharky, chimaera-ey kind of fish with these large flat teeth that are similar to the teeth we find today in rays and sharks that specialise in crushing up the shells of molluscs and crustaceans. These impressive teeth, like the one we see here, are often described as pavement-like, which is one of those accessible science terms you see come up again and again even though it doesn’t really mean anything (measurement of lengths and volumes in football fields being another one, American football? Association football? Gaelic Football?). Perhaps it just means they are big and flat, which to my mind is pavement slab-like. Either way, they’re big and flat like this one here.

If the label can be trusted, this specimens was collected from Carboniferous limestone in Carrickmacross County Monaghan Ireland. That probably allows us to speciate this specimen as Psammodus rugosus. Psammodus and their relatives appear in the Carboniferous period and appear to go extinct in the Permian.

Preservation Without spending too much time looking into it, it appears that all we currently know about this genus comes from specimens of teeth like this one. In fact the 8 species of Psammodus are descriptively named, indicating what you might expect the teeth to look like- porosus (porous), trapeziformis (shaped like a trapezoid), rugosus (corrugated as with this specimen) and cornutus (horned). The specimen we have here is very typical of what is found as a fossil and as with many vertebrates, particularly cartilaginous fish who don’t have a bony skeleton, the hard teeth stand a much better chance of fossilising to the extent that it’s the only evidence we have of this group of animals at all.

Research A quick look at where this genus is mentioned in the literature and Psammodus is more often than not listed as being present (essentially Psammodus was ‘ere) or merely referenced when comparing the teeth to the teeth of other fish as well as appearing on a number of dry 19th century lists of palaeozoic fishes. The majority of references I could find were from the mid 1800’s many of which Darwin would have had access to.

In Society Any decent, accessible illustrated guide book to fossils will no doubt mention Psammodus, possibly with an image of a tooth but that’s about it. Oh and inspiringly they resemble pavements, where would we be without pavements? In fields I guess.