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This fossilized structure was found close to several coelacanth bones, from the Late Jurassic – Early Cretaceous of Uruguay (South America). The accompanying fauna include theropod dinosaurs, crocodyliforms, turtles, ginglymodian and dipnoan fishes.
It is bean-shaped, compact, measuring about 1,5 cm long, and it has what appear to be growth layers. These layers are continued through the interior of the piece, as can be seen from high resolution CT scan (see figure). Some kind of periodicity between thin dense layers, and thick less dense layers can be perceived, with all of them becoming narrower towards the flat face of the piece.
It is tempting to assume it is an otolith. But after consulting several colleagues around the world who work with fossil and Recent otoliths, they tend to disagree with this. Those who work with teleostean otolihts rejected this possibility, mainly based on general morphology (which is rather simple) and kind of growth. Those who are more familiarized with fossil otoliths from other groups than Teleostei were more cautious (even more considering the fact that otoliths are almost unknown in fossil coelacanths and other sarcopterygians). Nevertheless, all of them agreed with the idea this is a biogenic structure, with some kind of seasonal growth.
Mineral composition was approached through EDS but the results are not conclusive (for now). Main elements are Ca, P, O and C, but I’m unable to discriminate between calcium carbonate/phosphate. Si and O seem to be more abundant in the fractures and interstices (maybe it’s diagenetic silicon oxide).
SEM images are not conclusive either. Each layer seems to be constituted mainly of a matrix of micrometric granular bodies, or filiform bodies, agglomerated (see figures).
After several months studying this piece without reaching a taxonomic identification, I’m turning to fellow paleontologists and neontologists, to ask for your help.
Thanks in advance!
Hello Pablo; They look like otoliths (inner ear bones) to me. Fish have one or more of them in each ear capsule. As the fish grows the "stones" grow by concentric accretion...the bands on the specimens suggest that. Cheers, Jim Des Lauriers
Dear James:
Thank you very much for your answer.
Indeed, that’s the question. I wrote to some world experts in fossil and Recent otholiths. Those who work with teleostean otolihts rejected the possibility, based on general morphology (it is quite simple, and the insertion point for the macula –sensitive tissue– apparently cannot be seen), and the kind of growth (as far as I understand, during otolith growth the outer layers cover the inner ones, so the growth layers should not be visible from the surface, contrary to this case).
But those researchers who are more familiarized with other groups than Teleostei were more cautious, and weren’t able to reject the possibility that this could be an unknown morphotype of otolith.
Hello Pablo; Here is a little test of the growth pattern. In known, modern, otoliths, the age of the fish can be determined by sectioning the otolith and then counting the annual growth rings. Are you brave enough to section one of these? Best regards, Jim Des Lauriers
YYes! In fact, I made the high resolution CT scan as a previous step, prior to an invasive technique. In the CT slices you can see an alternation between thick and thin layers, and the general pattern of layers suddenly becomes more compressed towards the flat and wide face of the piece. This is congruent with a seasonality growth, while the narrowing of the pattern could indicate sexual maturity (thus minor growth rate).
After seeing this, I sectioned the piece to see the ultrastructure in SEM. Even more, some of the SEM images are from the interior of the piece.
Pablo; Well, those results should make a strong argument for otoliths. Lovely! Jim Des Lauriers
It looked like a fossilized otolith to me, but I'm not sure. On this topic, consult:
Otoliths in Fossil Coelacanths
by Clack (1996)
Dear José:
Thank you for your comment. Indeed, I know the work of Clack (1996). In one particular specimen of Diplurus illustrated in this work, the otoliths seem to be preserved in 3D. Perhaps their morphology could be similar to this piece, but I need better images of that specimen (Iwrote to AMNH staff to ask for potos, adn I waiting for their response).
