Giant Bony Fish Swam in Prehistoric Seas

Darwin described the fossil record as being like individual words in a book when the rest of the sentence, the rest of the paragraph, the rest of the chapter was missing.  One of the strongest arguments put forward against Darwin’s theory of natural selection was why did the fossil record that show many different types of intermediate forms as natural selection led to the evolution of a new species from an older species?

Darwin argued that the fossil record was so poor and fragmentary that the lack of evidence to support his theory was only to be expected, after all, only parts of the geology of the world had been explored at the time of Darwin’s ground-breaking (no pun intended) “On the Origin of Species”.

Even today the paucity of the fossil record is widely accepted, despite the fact that very much more of the world’s geology has been explored than in Darwin’s day.  However, there are still huge gaps, such as the fact that during the mid Jurassic around 170 million years ago there were giant plankton eating fish and then nothing until the ancestors of whale sharks, manta rays and such like plus the first filter feeding whales appeared during the end of the Mesozoic/beginning of the Cenozoic and began to fill this ecological niche.

A team of scientists from the UK and America may have helped to “plug” this gap in the fossil record as they have re-interpreted known fossils and examined new specimens determining that filter feeders were around in prehistoric oceans after all.  The team of researchers, writing in the journal “Science” have identified fossil evidence that shows such fish existed between the time of Leedsichthys and the end of the Cretaceous 65 million years ago.

Leedsichthys (Leedsichthys problematicus) was a member of the Pachycormidae, a group of ray-finned fishes known only from Mesozoic fossils.  Named after the discoverer and one of the first researchers into these huge fish, Alfred Leeds, this fish is only known form a few isolated remains.  One of the problems of cartilaginous fish is that they tend not to leave many fossil remains, their soft cartilage does not preserve anywhere near as well as the bony skeletons of Teleosts for example.

Their are no close living relatives of the Pachycormidae today, the closest relatives are believed to be the North American Bowfin fish.  Leedsichthys was a large animal with a powerful tail. broad fins, an elongated body and a huge, gaping mouth.  When feeding these enormous fish would open their mouths very wide and swim forward causing large amounts of sea water to pass over their gill-rakers filtering out any plankton and small fish fry which was this animal’s prey.

An Illustration of Leedsichthys

Picture Credit: Robert Nicholls

These animals are reminiscent of the ray-finned filter feeders of today, creatures such as the Whale shark and the Basking shark.  Like these extant species of fish, Leedsichthys probably travelled great distances searching for food.  This life style is indicated by the geographical spread of Leedsichthys fossils – from East Anglia in England, to France, Germany and South America (Chile).

The international team of scientists that carried out the review of Pachycormidae fossil material included academics from Glasgow and Oxford Universities, plus researchers from Fort Hays University in Kansas, DePaul University in Chicago and the University of Kansas.  The team believes that they have uncovered a “missing piece in the evolutionary study of fish, mammals and ocean ecosystems.”

The project began in Glasgow, with a review of the known remains of the giant Jurassic fish Leedsichthys, in conjunction with the excavation of a new specimen of this creature in Peterborough (Cambridgeshire, England).  At first thought to have been the largest fish that ever existed, with an estimated length in excess of 25 metres, scientists now believe that Leedsichthys was not quite so huge, with some specimens perhaps as much as 17 metres long.  Leedsichthys was viewed as an isolated example of a large filter feeder in the oceans during the Mesozoic.  There was still a gap in the fossil record between the last fossils of this Jurassic leviathan and the first appearance of modern filter-feeders towards the end of the Cretaceous.  The first whales were toothed predators, filter feeding whales such as the Blue or the Grey evolved later, but mammals did not attempt to fill this ecological niche for many millions of  years after the extinction of the dinosaurs and marine reptiles.

Dr. Jeff Liston, from Glasgow University, was responsible for the excavation in Peterborough and found the new specimen to be an anomaly.

He commented:

“The breakthrough came when we discovered additional fossils, similar to Leedsichthys, but from much younger rocks”.

Dr. Liston went onto add:

“These specimens indicated that there were giant filter-feeding fishes for much longer than we thought.  We then started to go back to museum collections, and we began finding suspension-feeding fish fossils from all round the world, often unstudied or misidentified.”

Several of the most important new fossils – all from the same extinct bony fish family as Leedsichthys – came from sites in Kansas (United States), but other specimens from as far afield as Dorset, Kent and Japan were examined by the team as they attempted to piece together the story of large filter feeders in ancient oceans.

Dr Liston added:

“The fact that creatures of this kind were missing from the fossil record for over 100 million years seemed peculiar.  What we have demonstrated here is that a long dynasty of giant bony fish filled this space in time for more than 100 million years.  It was only after these fish vanished from the ecosystem that mammals and cartilaginous fish such as Manta rays, Basking sharks and Whale sharks began to adapt to that ecological role.”

The teams findings have “implications for our understanding of biological productivity in modern oceans, and how that productivity has changed over time”.

One of the best preserved Kansas specimens had previously been interpreted as being a fanged, predatory swordfish.  However, when the team began to prepare the specimen in more detail they discovered a toothless gaping mouth, with an extensive network of gill-rakers to extract huge quantities of microscopic plankton.

The team have named and described this new type of Pachycormidae – Bonnerichthys in honour of the Kansas family who discovered the fossil.

An Illustration of the Kansas Pachycormidae – Bonnerichthys

“Big Mouth” Bonnerichthys

Picture Credit: Robert Nicholls

Although, not quite the size of Leedsichthys the Kansas specimen indicates that these fish could grow up to 5 metres in length.

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