Research on giant sea scorpions (eurypterids) – the largest bugs that ever lived – has shed new light on why eurypterids became so large and eventually died out.
In the 1930s, Romer argued that eurypterids became larger and larger over time because they were in a kind of arms race with giant fish called placoderms. Presumably the larger would be the predator of the smaller, so the smaller grows. Others have suggested that a greater abundance of oxygen in an ancient atmosphere (and thus sea) just made things big. New research seems to indicate that both ideas are still valid.
There are two main evolutionary lineages of eurypterids. One of them, the predatory eurypterids, grew to up to 2.5 meters in length and may have done so in competition with placoderms. The second group did not grow as long, and were sea floor scavengers, may have grown long because of environmental conditions.
According to James Lamsdell, the lead author of the paper describing this finding,
… the evolution of the two main eurypterid lineages was quite different. The giant predatory eurypterids increased in size but decreased in diversity as placoderms become more common, while the other form of eurypterids that were initially small scavengers, only reached their massive size later on when many other invertebrates also increased in size.
Eventually, the giant predatory eurypterids seem to have lost the arms race, about 400 millin years ago. The placoderms won. The scavenging eurypterids lasted another 140 million years or so, and their size increase happened later as well owing to the timing of optimal ecological conditions. They were killed off by the Permian Extinction, which just goes to show that natural selection does not design organisms to cope with events that have not happened yet!
Carsten F. Dormann, Bernd Gruber, Marten Winter, & Dirk Herrmann (2009). Evolution of climate niches in European mammals? Biology Letters