It is important to get a head
Lauren Sallan, post-doctoral student at the University of Chicago has equalled her prestigious colleagues with recent achievements in evolutionary understanding. The idea that the development of features in the head precedes that in other areas such as body shape is a hard one to prove. For the first time her team has looked at "cephalisation" and all it entails in fossils rather than extant species. The whole of adaptive radiation will be affected by this attitude placing feeding before other evolutionary pressures that drive diversification. Previously Cichlid fish and Darwin's finches from the Galapagos Islands have been used to study this kind of adaptation.
Lauren and co-author Matt Friedman, PhD, lecturer in palaeobiology at the University of Oxford and a former colleague at Chicago, looked at two different adaptive radiations in the fossil record. These occurred when "Ray-finned fish" radiated 360 million years ago, after a mass extinction, and when some of their descendants, the Acanthomorph fish took advantage of the post- dinosaur period, when they were made extinct in the Cretaceous period (ending only 65 million years ago.) Quantifying the differences between fin position, jaw shape and body depth enabled the research to compartmentalise the timing of any diversification. The ancient sturgeon here is the nearest living species to the early ray-finned fish, while a gar fish shows an interesting jaw modification from early acanthomorph fish. (it's also called the alligator fish!)
Ancient sturgeon via Shutterstock
and - not-so-ancient gar fish via Shutterstock
Results were just as was hoped. Cranial features always preceded change in body features, just as some of Darwin's finches show beak shape changing before body shape. The fish could develop spiny or very blunt "teeth" in the jaws long before newer species illustrated body change. "We have these two entirely separate radiations, and in both of them the pattern is heads first. So feeding might be more important to diversification than habitat use," said our intrepid researchers.
A new food source is thought to drive diversification before the species adapt to their new environmental influence. "It's not something within the animals themselves; it's more opportunity that matters" says Lauren. However the pair do have reservations about the full import of their discoveries.
Lauren Sallan with one of her fossils; Credit: University of Chicago
"Evolution is really complex, and it's not really clear that there should be only one model," Lauren Sallan said. "It might be that this model might apply to fishes in certain time periods, or might apply to vertebrates, but a lot more investigation is needed to see whether that is actually true." The research however is truly outstanding (alongside some interesting research recently on "walking" lung fish) as epic in its influence on our understanding of evolution.
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