Paleontologists studying rocks from Morocco have unearthed the most exquisitely preserved trilobite fossils yet discovered. The new lifelike fossils update our understanding of the evolution and biology of these extinct ocean-dwelling arthropods.
The details are so great that soft tissue parts of the trilobites, including the mouth and digestive tract, are clearly visible, researchers report June 27 in Science. Such parts are typically lost as the animals turn into fossils.
“These trilobite fossils represent the most complete specimens found to date, not only preserving the hard exoskeleton but also the soft parts in 3-D, such as the antennae, walking legs and the digestive system,” says paleontologist John Paterson of the University of New England in Armidale, Australia.
After dinosaurs, trilobites are perhaps the most recognizable fossil animals (SN: 9/27/23). They proliferated in the ocean for about 270 million years before going extinct at the end of the Paleozoic era, some 252 million years ago.
Trilobite fossils are extremely common because their hard exoskeletons make it relatively easy for the animals to become fossils. But just as it’s rare to discover any trace of soft-tissue preservation in dinosaurs, so it is with trilobites.
To uncover how these trilobites and their tissues became so well preserved, Paterson and his team enlisted Robert Gaines, a geologist at Pomona College in Claremont, Calif., and an expert in how the soft parts of animals become fossils.
It happened like this: First a volcano exploded, and superheated ash flowed from the eruption into nearby coastal waters. The ash dissolved and then remineralized out of the water, covering the exposed trilobites and entombing them in a matter of hours to days.
The key step in this process, Gaines says, is that the ash hit water before hardening around the trilobites; without the cooling effects of ocean water, the hot ash would have burned the trilobites away.
Gaines studies similar fossil preservation in other, older fossils, such as an arthropod called Aegirocassis, an alien-like animal with what appears to be a strange baleen-style feeding apparatus (SN: 3/11/15). “I recognized the similarities immediately,” Gaines says. “They pointed to the same process operating more than 20 million years earlier.”
Besides being ready for a museum showcase, the fossils open new windows onto trilobite biology and evolutionary history.
“The clarity of the preservation is astonishing and is of fundamental importance,” says Nigel Hughes, a paleontologist at the University of California, Riverside who was not involved in the new work. “It provides a level of preservation detail that unequivocally confirms a number of conjectures made based on less well-preserved material, which demonstrates the power and importance of exceptional preservation.”
The fossils confirm, for instance, that trilobites ate using the many pairs of legs stretching from their head to their torso. They chewed food along a central groove while passing food particles toward a tiny mouth.
“Food processing took place along the entire length of the animal,” Hughes says.
This differs from other arthropods, such as crustaceans, which have more specialized limbs along their body lengths, used for tasks from self-defense to swimming.
“We don’t yet know for sure, but it seems likely that this basic limb style endured throughout the history of trilobites, and the lack of limb specialization may be part of the reason for their ultimate demise,” Hughes says.
Discovering more well-preserved trilobites could only help clarify the evolutionary story of these fossil icons.
Volcanos, including ones near coasts, erupted relatively often over the vast stretches of geologic time, Paterson says. That means this kind of pristine preservation may be more common than scientists think.
“Geology and paleontology students at universities are often told that fossils are found only in sedimentary rocks,” Paterson says. “But our new study completely contradicts that notion. I hope that our work will encourage others to reprogram their search image in the hunt for amazing fossils.”
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