For nearly 270 million years, the ancient trilobite creatures roamed the Earth and survived two mass extinctions. How then did the resilient species suddenly become extinct?
Trilobite fossils have revealed that the creatures were invertebrate marine anthropods made up of thousands of species around the world. They have an armor shell that most likely served as their defense by curling up amid predators. It is also believed that they lived off plankton and had a symbiotic relationship with certain bacterias in the ocean.
Volcanic activity in Serbia for nearly two million years brought about the world's third mass extinction (Permian-Triassic extinction) due to massive amounts of carbon dioxide release. Atmospheric carbon gas triggered ocean acidification and killed off masses of marine species. Calcium isotope evidence from a 2010 study revealed that nearly 95% of marine species vanished at the time, including trilobites.
Surviving the First Mass Extinction
According to Melanie Hopkins from the American Musem of Natural History, the decline of trilobites occurred even before the Permian-Triassic extinction. During of time of the extinction, she explained, there weren't many trilobites left.
The evolution of trilobites, such as changes in shape and body size, affected their survival. The earliest trilobites during the Cambrian period were highly diversified, most likely due to the lack of competitive species. As they adapted to the changing global environment, physical aspects such as limbs and segments evolved as well.
A few million years later during the Ordovician Period, there were more competitive species for food and predators as well. Developing different eye positioning or the ability to roll into a ball as a defense mechanism was necessary to compete against other species on the ocean floor. Eventually, such factors may have hindered the trilobites from recovering after the first two mass extinctions they survived.
The first mass extinction they experienced was about 444 million years ago during the Ordovician-Silurian boundary. According to Christopher Muscato from the University of Northern Colorado, "global temperatures cooled and the sea levels fell."
At the time, many creatures had adapted to shallow and warm waters, making it impossible for them to survive cooler temperatures and deeper waters. Afterward, trilobite species went from thousands to hundreds and 85% of the Earth's species went extinct.
Gradual Disappearance of Trilobites
Even as ecosystems recovered and other species thrived, trilobites could not diversify as they previously had, said Hopkins. Moreover, competition in marine food webs was persistent and also hindered their recovery.
375 million years ago, the second mass extinction during the Late Devonian period occurred over a long interval, Hopkins explained. Several factors such as a supernova event, plant evolution causing reduced levels of carbon dioxide in the atmosphere, and continental drifts all contributed to the extinction. Hopkins said that after the Devonian period, only the Proetidae family of trilobites were left.
Researchers have yet to determine how the Proetidae family persisted until the Permian period or what factors hindered their diversification. By the time that volcanic activity caused global warming during the Permian-Triassic boundary, trilobites were completely wiped out.
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