Spineless Giants Track Oceanic Revolutions

We’re fascinated by superlative size. That’s why humungous dinosaurs regularly make headlines, and Carboniferous arthropods – dragonflies and millipedes that reached B-movie sizes by dint of higher atmospheric oxygen – are paleo-documentary regulars. And beyond their size, we’re transfixed by why and how such giants could evolve. What’s strange, then, is that we haven’t paid more attention to the giant marine invertebrates of the prehistoric past.

Today’s giant and colossal squids were hardly the first invertebrate giants to inhabit the seas. Squishy and shelly critters with sizes over a foot and a half long have evolved multiple times during the last 500 million years and are well-known among paleontologists who specialize in spineless species. Endoceras giganteum, a 451 million year old cephalopod that lived inside an elonged cone of a shell, could get to be about 15 feet long, and there are rumors of lost specimens 30 feet in length. The 404 million year old sea scorpion Jaekelopterus rhenaniae has been estimated to be over eight feet long, and the 465 million year old trilobite Hungioides stretched nearly three feet long. And that’s just a few of prehistory’s immense invertebrates.

 

The shell of an immense Endoceras. From Klug et al., 2014.
The shell of an immense Endoceras. From Klug et al., 2014.

But is there any pattern to the origin of these tentacled and joint-legged giants? That’s the question behind a newly-published study by Universität Zürich paleontologist Christian Klug and colleagues in the journal Lethaia. Within a window of 500 to 300 million years ago, Klug and coauthors looked to see if the occurrence of giant cephalopods and arthropods in space and time corresponded to changes in oxygen levels, temperature, and sea level.

There was no simple connection between the environmental factors – all implicated as possible triggers for gigantism – and large body size among the marine invertebrates. Instead, the superlative species seemed to cluster around two times when marine life flourished.

Within their 200 million year window, Klug and colleagues found, the largest shell-covered cephalopods evolved about 475 million years ago. The largest trilobites weren’t very far behind, at 468-460 million years old, and another group of archaic arthropods – the weird anomalocaridids – counted their largest members in the 488-472 million year range. All three groups independently evolved huge size within a 28 million year window that coincides with the Great Ordovician Biodiversification Event – an evolutionary pulse that not only spun off many new species, but also allowed new, complex marine ecosystems to evolve.

Many of the other species in the study – including large cephalopods, trilobites, and sea scorpions – cluster around another, narrower window. All of these different groups again generated giant species between 400 and 383 million years ago during an event called the Devonian Marine Nekton Revolution. In brief, seafloor and deep water habitats were so saturated with species that competition drove the evolution of more animals capable of living suspended within the water column.

Huge, cone-shelled cephalopods and massive trilobites didn’t evolve because of simple causes such as an abundance of oceanic oxygen or dips in ocean temperature. Giants evolved in cool and warm seas at high and low sea levels and at various oxygen concentrations. Instead, remarkable body size among the marine invertebrates appears to be tied to major diversification events. Whatever allowed the group as a whole to flourish, Klug and colleagues point out, is what gave rise to giants.

Maps showing the distribution of marine invertebrate giants through time, with the oldest at the bottom. The pink swaths are the limits of tropical seas. From Klug et al., 2014.
Maps showing the distribution of marine invertebrate giants through time, with the oldest at the bottom. The pink swaths are the limits of tropical seas. From Klug et al., 2014.

But environmental shifts may explain a different part of the evolutionary picture.

Between 500 and 370 million years ago, most of the giants lived at high latitudes, closer to the poles. But starting at 370 million years ago, their occurrences creep down the latitudes towards the equator. And it was during this time that glaciers were forming in the southern hemisphere, the global sea level sank, temperatures dropped, and oxygen levels rose more than 20%, creating cooler, nutrient-rich seas. Perhaps these cooler, more productive seas created new constraints for where the biggest invertebrates could live.

This could be a coincidence. Klug and coauthors point out that sampling bias – where fossils are and can be collected – might influence this pattern, and note that perhaps continental shifts created rich, productive shelf environments that altered where giant species were likely to evolve. But the fact that all the groups investigated in the study – belonging to distinct lineages different in anatomy and physiology – followed this pattern suggests there’s some as-yet-unknown cause for the global shift. Paleontologists can pick out the biggest of the big, but why marine giants emerged where they did requires the continued imagination and investigation of long-lost seas.

Reference:

Klug, C., de Baets, K., Kröger, B., Bell, M., Korn, D., Payne, J. 2014. Normal giants? Temporal and latitudinal shifts of Palaeozoic marine invertebrate gigantism and global change. Lethaia. doi: 10.1111/let.12104.

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