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What fossils tell us
Every fossil tells a story
Every fossil has a story to tell because every fossil was once a living member of an ecosystem in which it interacted with other animals and plants and with its physical environment. We can use fossils as tools to help us to:
Of course the fossil record is not complete. Not every animal or plant that lived has become a fossil – which is just as well or there would be no room for us on the planet! But despite the gaps, the fossil record gives us a rich view of life through time since it first began more than 3,500 million years ago.
Palaeobiology
Palaeobiology involves reconstructing fossils as living animals or plants. What did they look like? How did they move? What did they eat? Where did they live? How did they originate?
A big fish story
In one of the Big Monster Dig programmes, the team visits a quarry near Peterborough where the giant fish Leedsichthys has been discovered. So far, the only part of it to have been excavated is its head, which is six meters long and contains hundreds of gill rakers – used to sieve out plankton from the water.
The present is the key to the past
But what did the rest of Leedsichthys look like? The answers involved a fishy shopping trip for Sarah and Joe to Leicester's famous fish market. Why? Because when trying to investigate how fossil animals lived 'the present is the key to the past'.
We know a great deal about many living fish because we can watch them swimming, eating and mating. Biologists have noticed that in living fishes the shape of the body and tail fin is related to their lifestyle.
Different shapes for different lifestyles
Consider, for example, the tuna. Everything about the shape of the tuna is designed for cruising efficiently through the water: in particular, the tail is tall and thin. A pike, on the other hand, is specialised for rapid acceleration. Fast-accelerating fish have a tail that is broad and deep, so that a tail-flick displaces a large volume of water and generates large forward thrust.
The other major category of fish is those that are specialised for manoeuvring, such as the butterfly fish. These fish have deep bodies when looked at side on, but from the front or back appear thin. They don't have the cruising ability of a tuna, nor the acceleration of a pike, but when it comes to manoeuvring in and out of tight spaces they are unrivalled.
Similar purpose, similar design
So, if you find a fossil fish you may be able to interpret some of its palaeobiology by looking at its general shape and that of the tail fin. Leedsichthys was a plankton feeder, and like all plankton feeders it needed to cruise very efficiently through the water. We would expect, therefore, that it had a tall thin tail like that of a tuna, and indeed the Leedsichthys tails that have been found are tall and thin.
Plankton is not very nutritional and is hard to digest, so we can deduce that Leedsichthys probably had a very long gut to help it process the plankton and a long body to house this long gut. And, finally, we know that modern-day plankton feeders, like basking and whale sharks, have large heads and mouths to scoop up as much plankton as possible; Leedsichthys was the same. A similar purpose produces a similar design.
Reconstructing ancient environments
Environmental reconstruction uses fossils and evidence from the sediments in which they are found to interpret ancient environments. This is possible because different animals require different environmental conditions in order to survive. If we can work out what those requirements were for a particular fossil (this can be quite tricky), we can say something about the environment in which they lived. In this way, it is possible to use fossils to interpret past climates, water salinities, levels of oxygenation and many other factors.
Marvellous molluscs
Molluscs are a large group of animals with more than 120,00 living species. They include familiar animals such as slugs, snails, squids, octopuses and many shellfish such as mussels, oysters and clams.
The Big Monster Dig team visited Latton Quarry in Wiltshire to investigate a Pleistocene (see Timeline) mammoth graveyard. In the Pleistocene gravels many mollusc remains, particularly clams (bivalves) and snails (gastropods), were collected by teams of fossil-finding enthusiasts.
Interpreting the environment
One of the advantages for palaeobiologists in working with relatively young fossils (in this case a mere 200,000 years old) is that many of the snails and clams have undergone virtually no evolution since that time and we still find their descendants today. This means, for example, that if a particular snail inhabits only marshy, grassland today, then it almost certainly lived in the same environment 200,000 years ago. So, by finding freshwater and land-dwelling molluscs on the site, the Big Monster Dig team was able to build up a very detailed and accurate picture of what the environment would have looked like through the eyes of a mammoth (and it was pretty good, with plenty of water to drink and grass to eat).
Pleistocene mollusc finds can be used to interpret the types of vegetation present, water depth, speed of water flow and many other factors. For example, a snail called Bithynia tentaculata was found close to the mammoth remains that indicates considerable weed growth in fresh water. Land-dwelling molluscs found close to the mammoth remains tell us that the ground was marshy in places, that the dominant vegetation was grass and that there were no trees or tall shrubs.
Older fossils
The real detective work comes in where the fossil finds are much older and don't look exactly the same as their living relatives. It's even more of a problem when the fossil animals are now extinct and have no living relatives. Here it may still be possible to use these fossils in environmental reconstruction, but it is necessary to be very careful and use as many lines of evidence as possible from different fossils and enclosing rocks.
Stratigraphy
Fossils can be used as indicators of specific time periods. This is possible because animals and plants have evolved over time, so that particular fossils are restricted to certain intervals of time. Using this, geologists can match rock layers of the same age across vast areas.
Ammonites
Ammonites are spectacular and distinctive fossils. They became extinct at roughly the same time as the (non-avian) dinosaurs at the end of the Cretaceous period. Ammonites are important finds, because not only are they beautiful but they also help geologists to date and correlate rocks.
This is because ammonites evolved rapidly and individual species were only around for a short time before dying out and being replaced by another ammonite species. In the programme about Leedsichthys, the Big Monster Dig team was able to use the ammonites Kosmoceras and Erymnoceras, found by the Boys Brigade, to date the sediments in which the Leedsichthys was found to 155 million years ago.
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