How Did Invertebrates Fare During Earth's Mass Extinctions?

The evolutionary history of life on Earth has been punctuated by several significant mass extinction events, collectively known as the Big Five, which have had profound effects on the diversity and distribution of species. Among these events, invertebrates have shown both remarkable resilience and vulnerability, with varying degrees of impact on different groups.

The End-Ordovician Extinction (445 Ma)

One of the earliest known mass extinctions, the end-Ordovician extinction, primarily affected marine taxa. Vertebrates, being less abundant at this time, were not heavily impacted. Specifically, reef-building organisms like brachiopods, echinoderms, ostracods, and trilobites were among the hardest hit. This event serves as a stark reminder of how interconnected and sensitive marine ecosystems can be, even in the absence of abundant vertebrates.

The Late Devonian Extinction (380-360 Ma)

This period marks a series of smaller extinctions that were collectively larger than they appear individually. The vertebrates that faced significant losses were the placoderms. Invertebrates fared worse, with cephalopods experiencing the most significant decline followed by corals, brachiopods, crinoids, stromatoporoids, trilobites, and ostracods. These extinctions highlight the complex interdependencies within marine ecosystems and the vulnerable nature of sessile and planktonic invertebrates.

The Permian-Triassic Extinction (251 Ma)

Often referred to as the Great Dying, the Permian-Triassic extinction is the most severe of the Big Five. It led to the extinction of 50% of all animal families and 80% of marine species, impacting both marine and terrestrial life. Among vertebrates, the only significant loss was the therapsids. However, invertebrates bore the brunt of this event. Trilobites went extinct completely, and brachiopods and crinoids also suffered heavy losses, followed by bryozoans. This event underscores the global nature of these mass extinctions and their remarkable breadth in affecting multiple groups of organisms.

The End-Triassic Extinction (200 Ma)

The end-Triassic extinction marked another significant event in the extinction timeline. Among vertebrates, three major groups were severely affected: marine reptiles, conodonts, and Crurotarsi (a group that includes crocodiles). Invertebrates faced significant losses, with ammonoids being the most affected, followed by bivalves, gastropods, and bryozoans. This extinction event highlights the regional and global impacts of mass extinctions on marine life.

The Cretaceous-Tertiary Extinction (65 Ma)

The Cretaceous-Tertiary (KT) extinction, famously marked by the extinction of non-avian dinosaurs, also caused significant losses among invertebrates. Ammonites, belemnites, and rudists went extinct, alongside corals, bivalves, and echinoderms. Insects and plants were only locally and temporarily affected with no species-level extinctions. This event demonstrates the varying degrees of impact during mass extinctions, with some taxa being more resilient than others.

In conclusion, invertebrates have consistently faced the largest losses during Earth's mass extinctions. This can be attributed to the nature of the fossil record, highlighting the importance of hard-shelled organisms and the vast diversity and ecological roles that invertebrates fulfill. The resilience and vulnerability of these organisms provide valuable insights into the dynamics of mass extinctions and the resilience of Earth's life forms.

Keywords: invertebrate extinction, mass extinction events, fossil record