Extinction and the fossil record
Biology · Key Stage 3 · The School
Why the fossil record is incomplete
FOSSILS need rare, exact conditions to form. A body must be buried fast, with little oxygen to slow decay. Then it needs time and pressure to turn into rock. Most living things decay away completely and leave nothing at all. Soft bodies with no shell or bone fossilise even more rarely. So the fossil record leans heavily toward things with hard parts.
What extinction means
EXTINCTION means a species has completely died out. No members of it remain anywhere on Earth. No future generation of it can ever be born. This is not the same as a population simply shrinking. It is permanent. A species cannot come back once every member has died with none left to breed.
Causes of extinction
Extinction can happen slowly. A species can lose out to a rival that competes better for the same resources. Or its habitat can change faster than it can adapt. Extinction can also happen fast, through a sudden disaster — a huge asteroid strike, a massive volcano, or a new disease — that leaves a species no time to adapt at all.
Question: had coelacanths died out? Follow one real case, step by step. Scientists first described a coelacanth (say see-luh-kanth) in 1836, from a fossil fish tail. Step 1: look at the span of the fossils. The Natural History Museum says fossil coelacanths range in age from 409 to 66 million years old. That is a span of 409 minus 66 = 343 million years. Step 2: look at the end of the record. The youngest known fossil is 66 million years old. No younger fossil was known. They left fossils across 343 million years, then none for 66 million years, so scientists assumed that coelacanths had died out. Step 3: test the assumption. In December 1938 Marjorie Courtenay-Latimer, curator of the East London Museum in South Africa, saw an unusual fish. It had been caught in a fishing net and was about 1.5 metres long. A fish expert, J. L. B. Smith, examined it and had no doubt that it was a true coelacanth. The living fish is a different species from the fossil ones, but it belongs to the same group. Answer: coelacanths had not died out, even though the record showed none for about 66 million years. The Natural History Museum says animals that seem to be extinct and then reappear tend to share two traits. They live in a small area, and in a habitat where fossils rarely form. So a gap can hide a group of animals that is still alive. In general, a gap may mean that no fossil formed, that erosion wore the rock away, or that the fossils have not been found yet. A gap alone does not prove that a species was gone. Even so, the fossil record still gives a good picture of the broad pattern of life over time.