The fossil record can tell us about the structure and way of life of past animals and the sequence in which they appeared: the facts about the time dimension may enable us to root evolutionary trees.
Obviously the fossil record is very patchy and very incomplete: animals lacking hard skeletons are less likely to leave fossil evidence. Even where a group has as many fossils as the arthropods, some animals once very common (such as the trilobites) occur in large numbers while rarer forms may not be represented at all. Extinct animals may present us with a baffling mixture of characters.
Nevertheless, not only can the fossil record tell us about the morphology of past organisms, it can also with careful interpretation reveal facts about animal mechanics, and also about ecological interactions and the nature of past ecosystems.
The succession of life forms that can be traced is sufficiently reliable for geologists to date rocks by the fossils which they contain. As the sequence in which the main groups of animals occurred is determined, new finds continually push the earliest appearance of each group back in time. Direct ancestry, however, is not revealed. While every fossil must have a nearest living relative, we can only very rarely identify it, and in any case the chance of finding a direct ancestor is vanishingly small. Among fossils as among living animals, a supposed ‘missing link’ between phyla can seldom be authenticated, although it may be disproved. Yet where fossils are plentiful a group of intermediate and possibly transitional forms can sometimes be identified – information that molecules can never provide.
Barnacles on the seashore give us a nice example of information about evolutionary change. From the earliest fossils to modern forms there is a trend of reduction of the number of lateral plates round the body. The rich fossil record enables us to trace this reduction in seven of the eight lineages of barnacles in this family, a remarkable example of parallel evolution. Why should it have occurred? Fewer plates means fewer junctions between plates, and observations
of living animals show that common predatory gastropods attack barnacles at these junctions. The plate reduction coincided with the rise of this family of predacious gastropods in the Cretaceous. Further, the one lineage with no plate reduction is Chenobia, barnacles living on turtles and free from gastropod predation.