Exoskeletons in the Fossil Record Because the hard arthropod exoskeleton fossilizes more readily than the remains of many other animals, the evolutionary history of the phylum Arthropoda is abundantly represented in the fossil record.
Much more is known about this phylum than other invertebrate phyla because of this phenomenon.
Entire classes of arthropods that have left no modern descendants are known today because their substantial body armor appears in various marine strata.
Examples of this are well documented in the remains of the extinct marine groups of the trilobites (similar to modern horseshoe crabs) and the eurypterids (giant "water scorpions").
In the case of the trilobites, many fossils are actually the result of cast-off exoskeletal moltings rather than the carcasses of the dead animals.
This illustrates the fact that, for hundreds of millions of years, arthropods have maintained a lifestyle and evolutionary approach to physiological problems that are similar to those of modern forms.
X-ray photography has been employed successfully to penetrate the hard, mineralized fossils of extinct and ancestral forms of modern arthropods, showing in good detail the internal structures of exoskeletons and other tissues.
Radiographic images produced by this technique demonstrate the continuity of structure and life shared by members of this extremely successful phylum for a period extending beyond the early Cambrian period (500 million years before the present).
The exoskeleton is an evolutionary advantage shared by all arthropods.
This advantage, along with their body and limb segmentation, has allowed them to move into myriad ecological niches, first in the sea and later on land, in freshwater, and finally in the air.
As a phylum, arthropods are arguably the most successful of all metazoan animal phyla; they exceed all others combined in terms of the number of species, diversity, and the number of individual organisms.
This ubiquity in all biomes and climates and in virtually every conceivable niche in every ecosystem makes them a force that has a constant influence on human life.