Friday, 15 January 2016

Underwhelming fish fossil of the year

The title is a bold claim, particularly for so early in the year. This fossil is obviously not a big flashy dinosaur, it is two tiny bits of tooth plates from a Cretaceous fish. However, it has special meaning for me for two reasons. Firstly, I am on a quest to publish on all groups of vertebrates (not actively pursuing but it would be nice to do) and this will cross of teleosts (the bony rayed fishes - most of the things you count as fish today) along with dinosaurs, marine reptiles, and mammals (these groupings may also get more specific as I get further along too). Secondly, and more exciting for me, is these fossils are the first I've found myself that have been published: (if you want a .pdf email me)

A bit of background, the fossil is from the Kallamedu formation of SE India. The area has been published on a fair bit as it is one of the largest Cretaceous exposures of fossil bearing strata in India. That being said it is still remarkably poorly represented in publishing when compared to the North and Central regions known for the intertrappean layers where small pockets of material lay between the massive Deccan trap eruptions layers. These areas are interesting for the sheer size of the erupted material as it covers an area the size of Texas twice over, and to a depth of up to 2km in places. The eruption has also been implicated by some for climactic events that led to the demise of the dinosaurs (either with or because of the meteor that hit Mexico).

I digress again, back to the site in the Kallamedu. The fossils found in the formation are typical of Gondwanan locales of similar late Cretaceous age, with teeth of crocodilians, abelisaurid and troodontids dinosaurs, bones of titanosaur sauropods, bothremydid turtles, and fish scales. The sediment is a mix of clays and sandstones and has been interpreted as a fluvial/deltaic region with occasional marine influence. Thus it may not come as a surprise you find fish teeth in it, however the type of teeth is what makes them interesting.

After much ado:

From Halliday et al., 2016 - Photographs in lateral, occlusal, and apical views of specimens DGUF/145 (left) and DGUF/146 (right). Scale bar = 1cm.
So there you have it, two tiny pieces of tooth plates, both barely more than a centimetre in size. They aren't just any standard tooth plate, they are what are called pharyngeal toothplates. In fish that feed on shells and hard objects, they develop what is functionally a second set of jaws in their throats designed to crush. It is these teeth that help determine which group they belong too. The size and apparent replacement pattern meant we took it a step further and CT scanned these specimens.

From Halliday et al., 2016 - CT scans of DGUF/145 (A) and DGUF/146 (B) in lateral (1), occlusal (2), and apical (3) views. The bony matrix has been digitally removed to illustrate the stacking of the teeth. Convex teeth are organized in vertical stacks; each stack is arranged in a row offset from adjacent rows. The apical surface of the teeth shows no foramen housing a pulp cavity, as is also true for NHM-UK 38814.
From the CT scans we can see the teeth are small, rounded on the occlusal (top) surface, covex on the apical (bottom) side, and arranged in nice linear columns. All of these are important for not just figuring out the group of fish that they belong to (Phyllodontidae), but also narrow it down to probably Egertonia sp.. It's tough to be more specific as we lack any more material from the fish, although there are living relatives (bonefish and ladyfish) which give us an insight into probably biology (shell and crustacean crushing, shallow salt water fish), and they have similar teeth although they are stacked alternatively.

The most interesting thing to derive from two otherwise minute underwhelming fossils is that the fish is otherwise unknown from India in the Cretaceous. The taxa has only ever been found once before in the Cretaceous, although indeterminate fossils belonging to the family have been found across Eurasia from this time. The only other definitive occurrence is from Madagascar showing the close links these two regions maintained in terms of what animals were there despite the fact that Madagascar and India were joined until about 85 million years ago (about 20 million years earlier than these fossils). Keep an eye out for another paper by Halliday et al., that is following hot on the heels of this one (mainly as it was reliant on this publishing before it could be) addressing the faunal similarities between these areas.

Anyway, that's all from me on my underwhelming fish fossil. I thought I could give you a lovingly crafted drawing of what part the fossils come from on a fish (see the entertaining blog from which this one post derives inspiration, but could not do it justice. So instead here is a picture of a black drum showing off its crazy throat teeth that do look superficially similarly (but I can tell you they are very different indeed):

A black drum (Pogonias cromis) showing off its smiles, inside and out. From