The Dance of Extinction

The figures pass by, ghosts of beings from another time.

Most of the species that went extinct belonged to various clades all across the tree of life as each lineage took its own losses. However, the most obvious losses are sizeable groups which left no descendants.

Some simply had too little diversity. Monotremes, the famous egg-laying mammals, were trapped on their island continent as it froze. The sea levels eventually fell enough to allow passage to New Guinea and beyond, but this same passage allowed waves of invaders to make their way into the continent and compete with remaining natives. They simply had all their leathery eggs in one basket, and too few of those eggs, too. Even lower in diversity, the ginkgo was just a single species representing an entire ancient division of plants, and thus had no possibility of survival.

Large felids, or cats, disappeared as most flightless large predators did, but their smaller kin did not last much longer. Their diets were too restrictive, unable to subsist on plant matter and largely dependent on small mammals. When small mammal populations crashed in the cold, cats had little to eat; even birds were no option, for they were quick to flee to warmer areas. Unlike cats, the similarly carnivorous toothed whales made it through the extinction, thanks to the ocean's relative stability as compared to land. Baleen whales, however, did not survive. Their food sources underwent severe boom-and-bust cycles in their changing environments, not yielding enough to feed their enormous bulk.

Crocodilians and cycads, both famously old lineages, had the same issue: Their reliance on warm temperatures. Cycads, those recognizable background plants of many dinosaur paintings, had hugged the tropics for millions of years, chasing the warm, wet conditions they once thrived in many ages ago, but which they could chase no more. Crocodilians' fate was similar, with most crocodilians living in tropical or subtropical areas. Some had adaptations to low temperatures, such as the American alligator and its ability to brumate in frozen lakes. However, this only sealed their fate, for they did not flee as their environment froze. Such dependence on the tropics also took out an entire order of fish: Characiformes, including piranhas and tetras.

However, there is one worldwide loss both unusual and prominent: The eusocial wasps. Wasps in general are incredibly important to their ecosystems, serving as pollinators, predators, prey, and much more. This was true of the eusocial wasps as well – the subfamilies Polistinae, Vespinae, and Stenogastrinae – which inhabited most of the world in their organized hives. Wiping out wasps in their entirety would be nigh impossible, but not so for the eusocial ones. Their fatal flaw was in their life cycle: wasp hives had no winter protection, so every winter the entire population was culled save for hibernating queens. When the extinction came, eusocial wasp populations crashed worldwide. Those living in areas that were once temperate died in an unending hibernation, while those in what were once tropical areas faced huge die-offs in the unforeseen low temperatures. What few survivors remained couldn’t maintain healthy hive sizes in the short warm periods of the year, and what's worse, there was nowhere on the globe safe from random freezes. Within a thousand years after the mass extinction, eusocial wasps were wiped out, leaving ripples felt worldwide.

Significant as these losses were, these two were the true symbols of the extinction's severity: Crinoids and Triops.

Crinoids were a group of echinoderms – the same phylum as starfish and sea urchins – and were war-torn survivors. Evolving in the Ordovician period roughly 460 million years ago, these ancient creatures faced severe diversity losses several times in Earth's long history, though they always managed to squeeze by. Their vulnerability was in their anatomy, for their bodies were filled with interlocking calcium carbonate structures called ossicles, making up most of their weight. While this made them distasteful to most predators, it also made them very sensitive to increased acidity. This extinction proved too much: Not only was the increase in acidity fast and severe, but it was permanent, never again backing down to normal levels. The crinoids are finally extinct.

Finally, the humble Triops, the famous genus of crustaceans that has existed since at least the Jurassic period. Living in warm seasonal pools, they can tolerate very poor water quality and attain breeding age in less then a week after hatching, producing eggs that can survive in parched earth for years. This extinction could never have eliminated them all. However, what is remarkable is that even these survivors lost several species. The fact that even Triops suffered is perhaps the most poignant indicator of how much things have changed.