Next Generation Science Standards:
- MS-LS4-1. A
Key Vocabulary: Index fossil, segmentation, extinction, habitat, paleontologist, adaptation, species, environment, geologic era, geologic period
Trilobites were one of the most successful animals ever to exist on Earth, living for nearly 270 million years and occupying every marine biome. From warm and shallow seas to deep oceans, to the frigid waters around our poles, they thrived for 50 million years longer the dinosaurs. But what made trilobites so successful? And what can they tell us about life millions of years ago? Dr. Brenda Hunda, a paleontologist with the Cincinnati Museum Center, explains why trilobites were so resilient and how scientists are using them to learn about the Earth’s past.
What were the trilobites?
The term “trilobite” refers to a class of over 20,000 different species of extinct arthropods, a group of invertebrate animals with an exoskeleton, such as spiders, insects, and crustaceans. First appearing on Earth about 520 million years ago, they looked similar to modern horseshoe crabs: a 3-lobed body with a broad head and segmented thorax. While no modern trilobites exist, they thrived in the oceans of early Earth. “Not only were trilobites on Earth for a very long time, but they had a very wide range of habitats,” says Dr. Hunda. “Trilobite niches extended from inshore, intertidal habitats to deep marine habitats.”
This wide range of habitats led to trilobites coming in all shapes and sizes. The smallest adult trilobite was just under a millimeter long, with the largest over 70 cm in length. Some species had a bullet-like body good for fast swimming, while others had enlarged eyes, good for scanning for food at different levels in the water. The bottom-feeding trilobites had a large head which would rest on the seafloor and filter the water for plankton while the rest of their body hidden under the sand or mud. There are even fossils that show trilobites who evolved to lose their eyes, indicating that they lived in the low light environment of the deep ocean.
Perhaps the most innovative adaptation was a species of trilobites that scientists hypothesize lived near deep ocean sulfur vents. In this extreme environment, symbiotic sulfur-eating bacteria grew in the gills of the trilobites, providing them with energy. This high diversity in their body types and the ability to adapt to their environment is what made trilobites successful for so long.
So why are there no trilobites today? “The class as a whole suffered many extinctions in its 270 million year history, but the final decline started in the Late Devonian Period (around 350 million years ago),” Dr. Hunda explains. A period of worldwide glaciation resulted in the oceans becoming toxic, which greatly reduced trilobite diversity. This was followed by major tectonic activity resulting in the formation of the supercontinent Pangaea; reducing many near-shore habitats and causing a lot of global environmental changes. With their habitats disappearing, trilobites were already on the decline. By the time of the end-Permian mass extinction 250 million years ago, more than 90% of all marine species on the planet and the trilobite’s fate was sealed.
An index fossil worth noting
The vast geographic range and long period in which trilobites lived make their fossils incredibly useful to researchers. Trilobites are known as index fossils, fossils used by scientists to make inferences on the ages of rock layers. “Trilobites allow geologists to date the rocks they are found in and correlate them with other rocks of similar ages around the world,” Dr. Hunda says. “This helps us understand the timing of events in the fossil and geologic record, which is critical to reconstructing the history of life and our planet.”
One example of this reconstruction uses the trilobite species Paradoxides, the fossils of which are found in eastern Canada, Sweden, Wales, the Czech Republic, and Morocco. While these locations are separated from each other by thousands of miles today, 500 million years ago they were joined in a supercontinent called Gondwana. It’s fossils such as Paradoxides that provided evidence for plate tectonics.
Examining trilobites also provides a unique window into the mysterious undersea world from half a billion years ago. Due to their hard exoskeleton, a rare feature for marine animals at the time, the entire bodies of trilobites are more easily preserved. This gives researchers a detailed look at their adaptations and vital clues on the environment at the time in which they lived. Dr. Hunda’s own work involves how trilobite populations in the Late Ordovician Period (around 450 million years ago) adjust their distribution when water depth shifts through time. “Their morphology, geographic distributions, and preservation all provide information on past environments, biological interactions (food webs, symbiosis, commensalism, etc.), and ecosystem evolution,” says Dr. Hunda.
Paleontologists are discovering new trilobite species nearly every day, with much of modern research focusing on how the group’s diversity evolved over their 270 million year history. In 2017, exceptionally well-preserved trilobite fossils dating from over 500 million years ago were found in China. These fossils had a complex digestive system, putting the evolution of a such an adaptation a full 20 million years before scientists had thought to have occurred. “Even though we know a lot about trilobites, there is still so much more to learn!” says Dr. Hunda.
But how are the fossils of a species that died out hundreds of millions of years ago relevant in today’s world? “Without trilobites, we would be missing a huge piece our planet’s history,” Dr. Hunda explains. “Trilobites have been critical to our current understanding of the evolution of life on Earth and events in Earth’s history, which informs how our modern Earth and life on it came to be, and even what we can expect in the future.” In knowing our history, scientists are able to better address modern issues relating to climate change, biodiversity, and extinction.
Perhaps the biggest lesson that we can learn from Trilobites is how they were able to adapt and flourish in so many different environments. Their adaptability and evolutionary innovation through all of these drastic Earth and life events are astonishing.” Dr. Hunda says. The ability to adapt their bodies to what the environment demanded allowed trilobites to flourish or an impressive 270 million years. Though humans may like to think of themselves as a more resourceful and adaptive species than other living things, we have yet to stand a similar test of time.