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Trilobites are strange creatures – they look like giant swimming potato bugs wearing helmets and have lived on Earth for 270 million years. These armored invertebrates, whose species once numbered in the thousands, thrived in the oceans as they scavenged and burrowed for food, and even managed to survive two mass extinctions.
But about 252 million years ago, trilobites disappeared from the fossil record. What ultimately wiped out this resilient backyard class?
The disappearance of the trilobite coincided with the Late Permian extinction (also known as the Permian-Triassic extinction), the third and most devastating of the mass extinction events. Volcanic eruptions in Siberia spat out huge amounts of lava for about 2 million years, according to Melanie Hopkins, associate curator of paleontology at the American Museum of Natural History in New York City. These flaming eruptions sent billions of tons of carbon dioxide into the atmosphere, triggering Ocean acidification, which in turn made it very difficult for marine animals to survive, according to a 2010 article published in the journal Proceedings of the National Academy of Sciences. Up to 95% of marine species succumbed to the Late Permian extinction, also known as the Great Dying, including trilobites.
Related: How long do most species last before they become extinct?
The trilobites, however, had already started a downward spiral towards extinction by this point. “By the time you get to this mass extinction, there aren’t a lot of trilobites around,” Hopkins told Live Science. This is because environmental and evolutionary changes have reduced this class of creatures.
Hopkins is studying the changes in the shape and size of the body of trilobites over time, and how these factors affected their survival. When the trilobites emerged at the start of the Cambrian period (541 million to 485 million years ago), they were extremely diverse, potentially because there weren’t many competitors, Hopkins said. Early Cambrian adaptations of trilobites were primarily related to growth and development, such as changes in the number of segments or limbs they had.
But during the Ordovician period, which began around 485 million years ago, competition and predation came into play more than ever before. At the moment “a lot of [trilobite] Adaptation is clearly tied to ecology, ”Hopkins said. Some trilobites have developed different eye positioning, harder exoskeletons, or the ability to roll into a ball. These adaptations, paleontologists suspect, have made trilobites more efficient on the increasingly competitive ocean floor. And over the long run, these pressures may have limited the recovery of trilobites after the mass extinctions to come.
Then came the world’s first mass extinction: the Ordovician-Silurian extinction around 444 million years ago, brought on by global cooling and falling sea levels, according to the Department of Environmental Sciences. Land of the University of Southern California. The number of species of trilobites, once in the thousands, has dropped into the hundreds, according to the American Museum of Natural History. Although food webs and ecosystems have remained intact, trilobites “never quite diversify or reach the numbers they had previously achieved,” said Hopkins. Growing competition in their ocean habitats may be what has kept them from bouncing back completely.
The second mass extinction, the Late Devonian, struck trilobites from around 375 million years ago. The late Devonian extinction was slower and the cause less specific than before and after. It’s harder to study because it has happened over a long period of time, Hopkins said, but it has likely led to slower evolution and diversification. Although the direct cause is less clear, the effect of the second extinction on the trilobites was profound. Entire orders – in biology, animals are classified into orders, families, groups and, finally, species – have disappeared. After the second extinction, there was only one family left in the Trilobita class: the Proteus’.
“That’s all that was left,” Hopkins said.
We don’t know what made Proteus’ so resilient. They were relatively simple creatures compared to some of the most massive and monstrous trilobites that have existed. By the third extinction, the end of the Permian, competition, predators and environmental changes had turned the odds against ancient Proetida. They couldn’t resist the global warming events triggered by volcanic eruptions.
The details of what made trilobites so resilient and vulnerable are still under study. One way to learn more about why they died out, Hopkins said, “is to understand why they never diversified to the same extent again. But that question remains unanswered.”
Originally posted on Live Science.
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