The Slow and Steady Growth of Trilobites

An animal’s size shapes the way that it lives and interacts with the world, impacting its lifespan, its locomotion, and its adaptability to its surroundings.

Even though studying the growth of ancient species is tricky, since biologists cannot observe the process directly, biologists believe that studying the size of fossilized creatures could clarify important questions about the lives and the evolution of early animals. Here’s what trilobites were, where they lived, and what scientists say about their unique growth rate.

Introduction to Trilobita: What Are Trilobites?

Trilobita, or ancient marine creatures often referred to as trilobites, swam through the seas and skittered across the sea floors millions of years ago and seem to testify to the foreignness of the past. Research in Paleobiology reveals that these strange animals actually lived their lives at a familiar pace, growing at a super-slow rate like modern marine crustaceans.

Where Did Trilobites Live?

Trilobites lived in various oceanic environments. They thrived in seas and oceans, inhabiting shallow shorelines to deeper waters. Some trilobites were benthic, living on the seafloor, while others were pelagic, swimming in open waters. Their fossils, found worldwide, indicate their adaptability to diverse marine habitats.

Are Trilobites Invertebrates?

Yes, trilobites were invertebrates. As arthropods, they were part of a group that includes today’s insects, arachnids, and crustaceans. Trilobites had exoskeletons, segmented bodies, and multiple appendages, typical of invertebrates, but lacked a backbone.

Read More: What Are the Oldest Fossils in the World?

When Did Trilobites First Appear?

Trilobites first appeared during the Cambrian Period, which began around 521 million years ago. This era marked a significant event in Earth’s history known as the Cambrian Explosion, a period characterized by a rapid diversification of life forms in the ocean. Trilobites emerged as one of the dominant marine organisms during this time, showcasing a remarkable variety in size, shape, and structure.

How Old Are the Trilobite Fossils?

Despite the difficulty of studying ancient species, a pair of researchers has recently found that the 450-million-year-old trilobite species, Triarthrus eatoni, took 10 years to grow to a length of approximately four centimeters, a growth rate and age that resembles several species of modern marine crustacean.

Read More: A Kaleidoscope of Evolution: The Colorful Cambrian Explosion

When Did Trilobites Go Extinct?

Trilobites had an impressively long existence of about 270 million years. Trilobites went extinct about 252 million years ago during the Great Permian Extinction, where nearly 90 percent of all life on the planet was wiped out. 

Why Did Trilobites Go Extinct?

The extinction of trilobites can be attributed to a combination of dramatic environmental changes including climate change, ocean acidification, reduced oxygen levels, and habitat disruption.

Climate Change

Significant global warming, partly due to massive volcanic eruptions, like those in the Siberian Traps, impacted trilobites. These eruptions released large amounts of greenhouse gasses, leading to a warmer climate.

Ocean Acidification

The increased CO2 levels caused the oceans to become more acidic. This change in marine chemistry was detrimental to many marine organisms, particularly those with calcareous shells or exoskeletons, like trilobites.

Reduced Oxygen Levels

The warmer and more acidic oceans likely experienced reduced oxygen levels (anoxia), making it difficult for many marine species to survive.

Habitat Disruption

The climatic and geochemical changes would have drastically altered marine habitats, impacting the food chains and ecological niches that trilobites and other marine organisms depended on.

Read More: The Late Permian Mass Extinction Explained

How Did Trilobites Grow?

“T. eatoni lived in low-oxygen environments and, similarly to extant crustaceans exposed to hypoxic [low oxygen] conditions, exhibited low growth rates,” says Daniel Pauly, a study author and a biologist at the University of British Columbia, in a press release.

“Low-oxygen environments make [it] more difficult for water-breathers to grow and add to the difficulties of breathing through gills, which, as 2D surfaces, cannot keep up with the growth of their 3D bodies. Thus, under hypoxic conditions, they must remain small if they are to maintain the rest of their body functions.”

Comparing Trilobites to Modern Marine Creatures

These findings suggest that the ancient world was not so different than the world today, at least in terms of the slow maturation of small marine creatures. The findings also represent a significant step forward for the investigation of the lives of trilobites and other fossil species.

“These findings provide the first reasonable estimates of absolute growth in early animals using methods known to accurately characterize growth in comparable living species,” says James Holmes, another study author and a paleobiologist at Uppsala University, in a press release. “They show us that nearly half-a-billion years ago, growth in marine arthropods like trilobites was similar to modern examples like crustaceans living in today’s oceans.”

Trilobite Fossils Reveal Growth Rate

Researchers worked withinformation from 295 trilobite fossils collected in New York state. Using this information, they completed a length-frequency analysis, an approach initially created to approximate the growth rates of living sea creatures that lack visible age indicators.

The researchers then compared their estimates of the growth of T. eatoni with corresponding information on the growth of modern crustaceans. The comparisons revealed relatively similar rates of growth.

“These consistencies,” the researchers conclude in their paper, “are encouraging and suggest that the methods proposed here could be profitably applied to other species.”

Read More: This Evidence Upended Theories on the First Animal to Reach Land

This article was originally published on July 25, 2022 and has since been updated by the Discover staff.

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