Survivors: The Animals and Plants that Time has Left Behind. Richard Fortey

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      3. Gustav Vigeland’s sculpture of a survivor, the scorpion, Frogner Park, Oslo, Norway.

      Let me describe these early days in more detail. I have already remarked that the horseshoe crabs had set out upon their distinctive path before the first land plants had advanced upon harsh and barren shores; although recent discoveries suggest that a few simple plants may have already ventured onto mud flats. These had probably not yet been followed by insects or spiders. There were fishes already, of a primitive cast, but they had no ambitions then to invade the land, although a few species may have nudged into waters that were not fully marine. The seas abounded with trilobites, which occupied every ecological zone from shallow shores to ocean deeps. These prolific arthropods must have been many times more abundant than the early relatives of the horseshoe crabs. They evidently had an advantage at the time. This may have been the evolution of their robust dorsal ‘shell’ of calcite, which allowed them to develop spines, armour and a tough anchor for muscles, as well as an ability to roll up into tight, impregnable balls when threatened. Trilobites soon learned an array of different feeding habits; some were predators, some ate soft mud, others swam in the open seas. They died out some 255 million years ago. By contrast, the relatives of Limulus may have stayed conservatively on the sea floor as scavengers and predators. The horseshoe crabs on Delaware Bay have an exoskeleton of chitin, which is a natural polymer that is quite tough and flexible, although no substitute for stony calcite. But the horseshoe crab has turned what might have been a weakness to advantage by developing an exceptional immune system. Survival in the long term may depend on more subtle features than armour alone.

      It is possible to trace the horseshoe crab story still further back, into the Cambrian Period more than 500 million years ago, to a time when many of the major types of animals converge towards their common ancestors. The Cambrian was an interval of unprecedented evolutionary activity and I shall describe its special features in more detail in the next chapter. Early relatives of Limulus have been identified in Cambrian strata, but they include some species that look a little different from their hardy survivors. Some of them also look more like early trilobites, such as Olenellus, a form with a big head-shield surrounded by a narrow rim; one might expect a family resemblance if they are indeed closer to a common origin. There are important differences, too. Where the limbs of trilobites are known, they are similar all along the length of the animal: the paired limbs are each split into a walking leg carrying a comb-like branch near its base that in all likelihood functioned as a gill. They are not subdivided into different ‘packages’ in different parts of the body separating walking and feeding appendages in front from gills behind, as they are in Limulus. To add to this, all trilobites had typical ‘feeler’ antennae near the front of the head, and none had the strange chelicerae. This may not be so important, since having antennae seems to have been a general property of primitive arthropods. It might just be that the trilobites still retain this one characteristic more primitive than Limulus and its allies, but they could yet have descended from a common ancestor. Trilobites are abundant fossils on account of their easily preserved calcite hard parts. Fossils of unmineralised animals are altogether more unusual. Spectacular recent discoveries of fossils of soft-bodied animals preserved within Cambrian rocks have been made in China and Greenland. These have revealed an almost embarrassing variety of undoubted arthropods early in the Cambrian. Some of them might seem to bridge the differences between Limulus and its allies and the trilobites, but for every feature that points one way, there seems to be another that suggests something else. For more than a decade now palaeontologists have argued about how these fossils should be classified, and about the only thing they all agree upon is that the Cambrian threw up many animals with curious combinations of characteristics that were probably winnowed out by subsequent evolution. It is not, perhaps, so surprising that ‘mixed up’ animals lived at this Cambrian time, because all the arthropods were not genetically far apart then – they would have had the subsequent 500 million years СКАЧАТЬ