Originally proposed in the 1930s, ‘Romer’s theory’ argues that eurypterids evolved in an ‘arms race’, alongside early vertebrates – giant armour-plated fish called placoderms – which is why they became so large. Subsequently, scientists have thought that eurypterids evolved to their huge size due to higher levels of oxygen in the atmosphere in the past, and other environmental factors.
The new research shows that both views are correct as the two main eurypterid lineages faced different pressures.
The first group – the giant predatory eurypterids that were up to 2.5 metres long and had swimming paddles – became large due to competition with placoderms in line with Romer’s theory. The second group – initially smaller that walked and scavenged on the sea floor – grew to a huge size due to environmental factors.
Previous research had not differentiated between the two lineages, nor tested either theory statistically and thus overlooked the fact that different pressures affected the two groups independently.
The new study, by James Lamsdell and Dr Simon Braddy from the University of Bristol, which is published today in Biology Letters, compared patterns of the size and numbers of different types, of eurypterids and early fishes.
James Lamsdell, lead author on the paper explains: “We found that 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.”
The demise of the giant predatory eurypterids coincided with the appearance of large placoderms around 400 million years ago. Lamsdell and Braddy show there is a peak in their diversity before placoderms appear in the fossil record, then they rapidly decrease in numbers, increasing in size as they do so before they eventually die out 370 million years ago. This suggests that they attained their huge size by competing with early vertebrates such as placoderms, a battle which they eventually lost.
The scavenging eurypterids on the other hand avoided competing with vertebrates and outlasted the placoderms. They also attained massive sizes, reaching almost 2 metres long, but not until 300 million years ago when they had to cope with life in less salty water, where being bigger is better for helping to regulate such things as the chemistry of blood fluids. They died out due to massive changes in the environment, along with 95% of life, during the Permian extinction 260 million years ago.
Dr Simon Braddy added: “This research indicates that ecology and competition with other animals is as important as environmental change in explaining why some bugs were so big in the past.”