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

trilobites declined to about thirty-five families, but there were still only five families of fishes. At the same time there were about fifty families of cephalopods, large predatory squids with coiled shells. We might well have combined the Ordovician time with the Cambrian and called it all the “age of trilobites.” Or if you want to reflect the changes in the biological communities, then we should call it the “age of the cephalopods.” But calling attention to the appearance of the first fishes is just a manifestation of our egos. If you looked at Early Ordovician communities, you would still see lots and lots of trilobites but not many fishes.
    What really happened then is that some descendents of little
Pikaia
sprouted gills and fins and started swimming. Several million years spanned between the early
Pikaia
fossils and fishes’ appearance, so we must suppose that however vulnerable that modest little creature may have been, she still had established a way of life that allowed her to survive for millions of years among the Late Cambrian arthropod communities. Then, at the start of the Ordovician, and with the appearance of fishes, there is no longer any trace of little
Pikaia
. Never again do we see soft little worms with notochords or vertebrae. Maybe the lineage of
Pikaia
was transformed entirely into fishes. Or, perhaps, those descendents that became fishes turned right around and gobbled up the last
Pikaia
.
    Since the start of the decline of the trilobites coincides with the appearance of fishes, it’s tempting to think that fishes played an important role in their disappearance. Probably they were a contributing factor, but it’s obvious that nothing changed very quickly. Trilobite diversity started to decline after the Cambrian, but they didn’t disappear entirely until the end of the Permian, about 250 million years later. Fish diversity increased only slowly and at first the fish were jawless, no doubt nibbling in the sediments. It took tens of millions of years before fish developed bone-crushing teeth to crack shells. The trilobites retained their advantage of hard external skeletons, and they were vulnerable to predators only during their small planktonic stages, and during their soft-shelled molting phase. Also, from the Ordovician to the Permian times, while trilobites were in gradual decline, the familydiversity of cephalopod squids was always much greater than the diversity of fishes, and marine crustaceans were on the rise. Another predatory arthropod group made its first appearance in the Late Cambrian, the eurypterid sea scorpions. These nasty creatures had large armor-crushing claws, so they probably could consume large trilobites far more efficiently than either fish or cephalopods, which may have consumed more small planktonic forms of the trilobites. Finally, even the trilobites may have contributed to their own demise by evolving predatory species. During the Ordovician time there appeared the giant predatory trilobite
Isotelus
, which reached sizes of 16 to 28 inches. This ferocious monster has been dubbed the “
Tyrannosaurus
of the Ordovician” and has been declared the state fossil of Ohio. So although increasing predation pressure was a perhaps factor in the decline of the trilobites, it is clear that the fishes were not the main reason for their decline, and probably fishes were not the dominant trilobite predators.
    To truly understand the disappearance of the trilobites, we need to look beyond the predators and examine some aspects of trilobite biology. Danita Brandt, a trilobite biologist at Michigan State University, has been studying the molting processes of trilobites, and she may have uncovered an important clue regarding their decline. Ironically, it seems that the key to trilobite’s initial success, their external skeleton, may also hold the secret of their demise. It seems that although trilobites were among the first to successfully develop an arthropod skeleton,