Free Planet of the Bugs: Evolution and the Rise of Insects by Scott Richard Shaw

brachiopods resemble the shape of an ancient Roman lamp. They also resemble clams, but the resemblance is only superficial, as the two shells of a clam are similar to each other in size, while brachiopods have a smaller top shell and a larger bottom one. Lamp shells peaked in diversity during the Ordovician but retained high species richness over Silurian times. Some of the brachiopods cemented their shells to surfaces to keep them in place, so they were important in building the structure of Silurian reefs. So abundant werethe lamp shells in Paleozoic seas that they now are probably the most common fossils in the middle-eastern United States. The very first fossil that I discovered as a child was a brachiopod lamp shell, found protruding from a rock along the banks of the Mississippi River. The state of Kentucky has declared any brachiopod as its state fossil, not bothering to name any particular genus or species; there are just too many of them.
    One Giant Leap for Arthropod-Kind
     
    The coral reef ecosystems may have been the biological pinnacle of Silurian times, but since insects are fundamentally terrestrial animals, the story of land colonization must still be told, with a slightly more arthropodan bias. It may have taken tens of millions of years, but eventually species richness on land did outpace that of the oceans; the complexity of our tropical forest ecosystems has vastly outstripped the complexity of our ocean reefs ever since. The pitter-patter of those little arthropod feet echoes loudly across the ages and had profound implications in shaping life’s subsequent diversity.
    Many biologists have long assumed that plants needed to colonize the land first and to establish ecosystems for animals to occupy. That may not be the case, as some good evidence suggests. Namely, there are trace fossils of arthropod footprints, fossilized tracks, dating to sediments from the Late Ordovician. Even if terrestrial plants were present then, it’s clear from the footprints that arthropods were walking out on the open wet soils, quite separate from plants, at the earliest of times on land.
    If arthropods were strolling on the beaches more than 443 million years ago, what they were doing there? They may have been avoiding deepwater predators. We must assume that the very first animals to walk on land were arthropods that lived in the shallowest waters, in the intertidal zones. Our longtime companion the moon played a significant role in the evolution of life by creating these pools and the tides that shape them. When the tides ebbed and flowed, any arthropods that could survive on the moist shorelines at low tide would have benefited greatly, simply by avoiding the big predators. As the Silurian progressed, the coral reefs presented an increasingly hostile environment. While the tide moved out, predators that breathed with gills,such as sea scorpions, cephalopods, fish, and even large trilobites, swam into the deeper waters. The little arthropods that survived along the shorelines enjoyed a peaceful safe haven, perhaps.

     
    FIGURE 3.1. A coiled millipede is a quintessential example of a myriapod: a long, multisegmented arthropod with lots of legs. Creatures somewhat like these were among the first animals to colonize land. (Photo by Kenji Nishida.)
     
    Two groups of arthropods appear to have colonized the shorelines at about the same time: the arachnids and the myriapods. The arachnids were the scorpions and the group from which spiders, mites, and their relatives are descended; the myriapods were long, multisegmented, multilegged creatures, the group from which millipedes, centipedes, and insects evolved. Let’s look at each of these animals in turn and consider how and why they might have migrated to the beaches.
    Sting Time on the Beach
     
    Among the oldest fossils of terrestrial animals are the first scorpions, dating from the Late Silurian. We may call the Silurian scorpions “terrestrial” because they clearly