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Leedsichthys: the big fish
 Leedsichthys: the big fish
Leedsichthys
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Monster fish
This story was slightly different from the usual Big Monster Dig investigation because the star of the show – the giant Leedsichthys – had already been found. The first bits of this monster fish were spotted by one of Dave Martill's students two years previously. It was identified as another example of a monster fish first discovered in a nearby brick pit by Alfred Leeds (after whom the creature was named) more than 100 years ago.
Since the discovery, Dave, along with fellow palaeontologist Jeff Liston and a motley crew of helpers, had spent months living in the disused brick pit and digging up fish bones. As the main fossil in the show had already been found, the questions put to the team by Alfred Leeds's great great grandchildren were: exactly how big was this fish and how did it manage to grow so large?
Marine environment
To answer these questions the team had to explore the world in which the fish lived. To do this they needed to look for the fossilised remains of other animals that lived at the same time and to take a close look at the sediments that contained the fossils. They enlisted the help of the local boys brigade, who did a great job of fossil hunting, and soon had a huge pile of ammonites, belemnites and other fossils.
These confirmed that the fish was living in the sea (and not a lake or river) and also helped to date the rocks as being around 155 million years old. Meanwhile, at the main dig site, Dave was turning up a few interesting finds, including the teeth of a Liopleurodon – a mean predator that undoubtedly would have preyed on our gentle giant.
Fishermen's tale
Fishermen are notorious for exaggerating the size of their catch and Dave and Jeff were no exception. Wild claims as to its possible dimensions were being bandied about, so why didn't they just measure it?
Of course, that would have been the obvious solution, except for one small problem: the main body of the fish was missing. In Leedsichthys, as with many fish, the head and tail were made of bone, while parts of the body were made of cartilage. While the bone has a fairly good chance of being fossilised, the softer cartilage decays very rapidly. Consequently, they could measure the head and tail, but they had no idea how big the missing bit was.
Safe in the knowledge that since the middle bit had got away no one could disprove him, Dave proposed that the creature could have been up to 30 metres in length – four times the size of any living fish! Sarah Gabbott planned a trip to the Leicester fish market to test his theory.
Filter feeder
The team knew from previous research that Leedsichthys was a filter feeder, similar to modern basking sharks and blue whales. It used thousands of tiny bones called gill rakers, capturing the tiny microscopic organisms on a grid as it passed water through its gills. Once they knew what it ate and how it got its food, it was time to work out how it had got so big.
Two issues were important here. First, what was the palaeogeography – the geography of this area during the Jurassic period – like? The geography of any area changes dramatically over geological time with the movements of the plates on the Earth's surface, changes in sea level and long term changes in climate. What did Peterborough look like in the Jurassic?
Jurassic Peterborough
The sediments in the brick pit were mud that was deposited on a sea floor. The water must have been fairly deep because the mud showed no signs of having been influenced by waves. The team also knew that the UK was much further south during this period, closer to the Tropic of Cancer. The configuration of the continents meant that Peterborough sat in a narrow seaway that connected two large oceans to the east and west.
The next step was a bit of rough science to work out how much food was available in this seaway. Sea bed sediments are made up of a combination of silt and clay that has washed or blown in from the land and organic material that comes from the bodies of dead things – mainly microscopic algae, plankton and other small creatures. A crude estimate of how much material was in the water can be obtained by looking at how much is in the sediment.
How much did Leedsichthys eat?
To do this, John Howell took Lucy, the Big Monster Dig presenter, off to the nearby London Brick Company, where they conducted an experiment. The Oxford Clay produces very fine bricks. Part of the brick-making process involves heating the green (uncooked) bricks until all of the water and organic material in the clay has been removed and only the inert clay is left, baked and hard.
Once the water content of a brick has been calculated, the remaining weight difference before and after baking tells us how much organic material was contained in the sediment. In this case, the sediment comprised a staggering 8-10% organic material, indicating a sea that was very rich in food. Many modern filter feeders, such as basking sharks, keep growing as long as there is food available. Leedsichthys was probably the same and the abundant food supply helped it to become very big. But how big is very big?
Measuring fish heads and tails
Sarah had returned from her shopping trip with a bag full of statistics to show the team. She had measured the lengths of the heads and tails from a number of fish in the market in order to work out the relationship between the length of their heads and total length. Broadly speaking (and there was much heated debate over this), the fish at the market had a total length of around five times the length of their heads. So a fish with a 10-centimetre head would be 50 centimetres in length, and a Leedsichthys with a six-metre head would be ... 30 metres. Sometimes even fishermen's tales don't exaggerate.
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