Catching a prehistoric whopper | University of Oxford
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Catching a prehistoric whopper

Pete Wilton

In the world of fossils it's usually befanged predators or their feathered bird-like cousins that steal the limelight.

So finding out about Bonnericthys, one of the unsung heroes of the Jurassic & Cretaceous - as part of working on this news story - has been an unexpected treat.

These giants were about as far from the flesh-eating inhabitants of Jurassic Park as you can get: 9m-long fish gliding around the prehistoric seas hoovering up plankton much like the benign basking sharks, whale sharks and blue & grey whales we know today.

Big & small fry
Making the video turned up a lot of fascinating detail and context that didn't make the final cut: like the fact that these large filter-feeders shared the ancient oceans with the ancestors of smaller fry living off the same resources, such as early herrings and anchovies.

'These familiar fishes survive to the present day, but the most striking difference between these suspension feeders and the extinct ones we've studied is size,' Matt Friedman, of Oxford University's Department of Earth Sciences, told me.

'The extinct fishes reaching lengths of 9m or more are giants on any scale, but they are particularly massive in comparison to living bony fishes that thrive on plankton.'

The awkward size and shape of some of the museum specimens is one reason why they were misidentified or ignored, but another factor is the peculiar anatomy of the giant plankton-eater:

'First is the enormous mouth, with long, slender jaws that bear no teeth whatsoever - a feature common to filter feeders,' Matt explained.

'Another important feature is the enormous gill skeleton: Fishes use gills to breathe, but suspension feeders have co-opted their gill arches to extract plankton from the water. They pull off this trick helped by structures called gill rakers, finger-like projections that extend off the front of the gill arches, which can assume elaborate shapes in suspension feeders.'

'As for the body of these animals, it would have been very streamlined, with a well-developed pair of fins just behind the head, and a massive, crescent-moon shaped fin at the back of the tail.'

Things fall apart
These specialised lightweight bodies, fine-tuned by evolution, contain skeletons with very little bone - so that they tend to fall apart after death, leaving only fragments of skull and flipper to sink to the ocean floor and be preserved.

It was a series of new finds from excavations around the world that helped unlock the story of Bonnericthys:

'The most important fossil specimens for our study came from rocks laid down in western Kansas near the end of the age of dinosaurs, about 80 million years ago. We've also got other pieces of this same fish from other parts of the US including New Jersey, South Dakota, Wyoming, and Alabama,' Matt told me.

'Other examples of this group of fishes that we report for the first time came from Kent and Dorset here in the UK, as well as from as far away as Japan.'

These giants proved to be quite the globe-trotters, but it is the longevity of their 100m-year-dynasty that marks them out as an evolutionary success story rather than just an interesting experiment, Matt comments:

'They cruised the oceans for at least 100 million years. To put this in context, that's longer than the giant baleen whales have been around, and longer than any of the groups of  massive filter-feeding sharks.'

'Another way to think about these fishes is by comparing them to mammals: mammals have been dominant on land for something like 65 million years, the time from the end of the age of dinosaurs up to today. I think we'd all agree that mammals are a pretty successful group.'

'Of course these giant fishes never achieved the diversity of mammals, but they were in the oceans for a longer period of time than mammals have been the dominant land vertebrates.'