19 february 2010

Fossils net plankton-eating giant

Science

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Giant plankton-eating fishes roamed the prehistoric seas for over 100 million years before they were wiped out in the same event that killed off the dinosaurs, new fossil evidence has shown.

An international team describe how new fossils from Asia, Europe and the US reveal a previously unknown dynasty of giant plankton-eating bony fishes that filled the seas of the Jurassic and Cretaceous periods, between 66-172 million years ago.

The team report their findings in this week’s Science.

‘Today’s giant plankton-feeders – such as baleen whales, basking sharks and manta rays – include the largest living vertebrate animals, so the fact that creatures of this kind were missing from the fossil record for hundreds of millions of years was always a mystery,’ said Dr Matt Friedman of Oxford University’s Department of Earth Sciences, an author of the report.

‘We used to think that the seas were free of big filter feeders during the age of dinosaurs, but our discoveries reveal that a dynasty of giant fishes filled this ecological role in the ancient oceans for more than 100 million years.’

Diagram showing reconstruction of Bonnericthys and position of bones Several of the most important new fossils came from deposits in Kansas in the USA, with other remains from as far afield as Dorset and Kent in the UK, and Japan. Some members of this filter-feeding fish group are estimated to have been up to 9 metres long, a similar size to modern plankton-eating giants such as the basking shark.

‘One of the reasons these big fishes were overlooked or misidentified lies in their anatomy,’ said Dr Friedman. ‘Over their evolutionary history, these fishes reduced the amount of bone in their skeletons, probably to save weight, with the consequence that most of their hard parts were easily scattered after death. As it turns out, the only parts you routinely find in the fossil record are their well-developed forefins.’

With few clues to go on, palaeontologists had argued that the owner of these isolated fins looked something like the modern-day swordfish. This changed when some of Dr Friedman’s colleagues began cleaning a fossil that preserved skull bones along with the fins.

Dr Friedman said: ‘Instead of finding a head with a long sword-like snout and jaws lined with predatory fangs, they found something completely different: long, toothless jaws supporting a gaping mouth, and long, rod-like bones that contributed to the huge gill arches needed to filter out enormous quantities of tiny plankton.’ The team named this fish Bonnerichthys, honouring the Kansas family who discovered the fossil.

Remains of similar giant plankton-eating fishes had been known from much older rocks, but they were thought to be a short-lived and unsuccessful evolutionary experiment. ‘As soon as we recognised that these animals had a longer history than anyone thought, I started examining museum collections and found more examples that had been overlooked or misidentified,’ explained Dr Friedman. Revisiting previously collected fossils netted the team evidence that these fishes thrived for millions of years and colonised many parts of the globe.

Intriguingly the ancestors of large modern filter-feeders such as baleen whales and whale sharks only appeared after the extinction of Bonnerichthys and its relatives, suggesting that today’s filter-feeders evolved to fill the ecological niche left behind by these plankton-eating contemporaries of the dinosaurs.

Bonnericthys illustration by Robert Nicholls
Illustration of Bonnericthys by Robert Nicholls
The research team consisted of scientists from Oxford University (UK), DePaul University, Chicago (US), Fort Hays State University, Kansas (US), University of Kansas (US), University of Glasgow (UK), and Triebold Paleontology Inc & Rocky Mountain Dinosaur Resource Centre, Colorado (US).

A report of the research, entitled ‘100-Million-Year Dynasty of Giant Planktivorous Bony Fishes in the Mesozoic Seas’, is published in this week's Science.