tagonistic to the needs of a respiratory membrane. So—to build an external coating that resists desiccation and then suffocate? Or build a surface respiratory structure that allows the diffusion of oxygen into the body but risk desiccation through this same structure? This dilemma had to be overcome by any land conqueror, and it was apparently so difficult that only a very small number of animal phyla ever accomplished the move from water to land. Some of the largest and most important of current marine phyla certainly never made it: there are no terrestrial sponges, cnidarians, brachiopods, bryozoans, or echinoderms, among many others, for instance.
Identification of the first land animals has relied on a fossil record that is notoriously inaccurate when it comes to small terrestrial arthropods. The oldest fossils of land animals all appear to be small spiders, scorpions, or very primitive insects dating back to between 420 million and 410 million years ago—right about the transition from the Silurian to the Devonian. All of these animals, however, have very weakly calcified exoskeletons and are rarely preserved. By the late Silurian or early Devonian, however, the rise of land plants also brought ashore the vanguards of the animal invasion, and it is clear that multiple lines of arthropods independently evolved respiratory systems capable of dealing with air. The respiratory systems in today’s scorpions and spiders provide a key to understanding the transition of arthropods from marine animals to successful terrestrial animals. Of all body structures required to make this crucial jump, none was more important than respiratory structures. It also seems apparent that the earliest lungs used by the pioneering arthropods would have been transitional structures nowhere near as efficient as the respiratory structures in later species. But in a very high-oxygen atmosphere, air can diffuse across the body wall of very small land animals—and the first land animals all seemed to be small—and can enter the body through even the primitive lung structures.
Of the phyla that made it onto land—the arthropods, mollusks, annelids, and chordates (along with some very small animals such as nematodes)—the arthropods were preevolved to succeed, for their all-encompassing skeletal box was already fashioned to provide protection from desiccation. But they still had to overcome the problem of