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An ancient, insect-like sea creature with a fan-shaped tail and a shell wrapped around its body, it swam upside down and looked like a taco. But this taco could bite back.
Newly discovered fossils of the extinct arthropod Odaraia alata recently gave scientists their first glimpse of Odaraia’s jaw-like structures, called mandibles. These small, paired appendages near the mouth bite, hold and tear food, and arthropods with these mouthparts are called mandibulates.
The first mandibulates evolved in oceans during the Cambrian Period (541 million to 485.4 million years ago) and include modern crustaceans, insects and millipedes, such as centipedes and millipedes. Whether they cut, tear or grasp, mandibulates help arthropods do their jobs, and mandibulates diversified so successfully that they now make up more than half of all animal species, according to the Royal Ontario Museum.
Identifying Odaraia’s mandibles solves a long-standing mystery about how the animal caught its meals and suggests that Odaraia is among the earliest mandibles in the arthropod family tree, researchers reported July 24 in the journal Proceedings of the Royal Society B: Biological Sciences.
The species was described in 1912 from fossils found in the Burgess Shale in British Columbia, Canada, in rocks dating to about 505 million years ago. However, the heads of these fossils were incomplete. This left scientists uncertain whether Odaraia belonged to the mandibulates, since head appendages are key to classifying extinct arthropods, said lead author of the study Alejandro Izquierdo-López, who conducted the research at the Royal Ontario Museum while earning a doctorate in the University of Toronto’s department of ecology and evolutionary biology.
For the new study, researchers studied about 150 fossils collected by the Royal Ontario Museum during expeditions between 1975 and 2000. Most of the specimens were new material that had not previously appeared in scientific publications, Izquierdo-López said.
“There were only a few published before,” he said in an email. “We had clear mandibles in just over 10, which shows how hard it is to find them preserved!”
The preserved mandibles were previously only hinted at by muscle scars in other Odaraia specimens, the study authors reported. Odaraia’s newly discovered mouthparts “are stout, short appendages with a row of teeth,” Izquierdo-López added. “This is exactly what we would expect a mandible to look like.”
Their discovery underscores that even in known species, new fossils can still be full of surprises, said Dr. Joanna Wolfe, a research associate in the department of organismal and evolutionary biology at Harvard University.
“It’s important to revisit species that we already know about. In this case, they (the study authors) had a lot of new material,” Wolfe said. “Sometimes features are only visible on one specimen, so you always have to look.”
Prey-catching leg hairs
Odaraia was about 15 centimeters long and looked out at its ocean home through large eyes on stalks. Its body was divided into dozens of segments, with more than 30 pairs of thin legs.
It was encased in what’s known as the taco shell, a tubular shield that folded around Odaraia’s body, leaving its head sticking out in front and its tail sticking out in back. Many arthropods have this taco-like feature, known as a bivalve shell, “including living arthropods such as ostracods (seed shrimp) and fan shrimp,” Wolfe said.
The shell folded over Odaraia’s limbs, so it may not have been able to walk on the seafloor, according to the Royal Ontario Museum. Instead, the sea-bug taco likely moved about like modern horseshoe crabs do: by swimming upside down.
While the feet may not have been used for walking, they were likely important for catching food, such as smaller Cambrian marine animals, the researchers reported. When they examined the fossils, they found stiff, hair-like structures called setae covering the animals’ feet. These tiny spines may have captured food, much like rows of baleen in the mouths of whales filter seawater and trap plankton.
“We think the spines between the legs interlock, creating a net to capture passing prey,” Izquierdo-López said.
This type of feeding is common among modern crustaceans, which have different types and lengths of bristles that they use to capture food, Wolfe added.
More mandibulate mysteries
One feature that surprised and intrigued the scientists had never before been seen in Cambrian animals: a single tooth-like structure between the jaws of Odaraia.
“We still don’t know exactly what it is, even if we compare it to today’s mandibles,” Izquierdo-López said. “However, we think it was probably used together with the mandibles to further chew the food. This structure may have evolved into other similar structures in millipedes or in crabs, but we can’t say more yet.”
Finding additional fossils could shed light on the function of this structure and help to unravel other unusual details about Odaraia, such as the presence of three small eyes between two larger ones. Previous studies briefly described these light-sensing organs, though the researchers were unable to detect the rudimentary eyes in their scans.
“We couldn’t see those three eyes very well in this study, but we can’t completely rule out their presence,” Izquierdo-López said. “Future specimens may reveal an even more complex head than we have now.”
Mindy Weisberger is a science journalist and media producer whose work has appeared in Live Science, Scientific American, and How It Works magazine.
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