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An ancient, worm-like sea creature with a fan-shaped tail and a shell that wraps around its body swam upside down and looked like an taco, but it could bite back.
New Discoveries fossil Fossils of the extinct arthropod Odaraia alata recently provided scientists with their first glimpse of Odaraia’s jaw-like structures, the mandibles. These paired tiny appendages near the mouth are used to chew, grasp, and tear food, and arthropods with these mouthparts are called mandibles.
The first mandibles evolved in the oceans during the Cambrian Period (541 to 485.4 million years ago) and include modern crustaceans, insects and myriapods such as centipedes and millipedes. Their mandibles help them perform arthropod tasks such as cutting, tearing and grasping, and mandibles have diversified very successfully, accounting for more than half of animal species today. Royal Ontario Museum.
Identifying Odaraia’s mandibles solves a long-standing mystery of how the creature captured prey and suggests that Odaraia belongs to the oldest mandibularia in the arthropod family tree, the researchers reported July 24 in the journal Nature. 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. The fossil rocks date back to about 505 million years ago. However, the heads of these fossils were incomplete, which meant scientists weren’t sure if Odaraia belonged to the mandible family, because head appendages are crucial for classifying extinct arthropods, said lead author Alejandro Izquierdo Lopez, who conducted the research at the Royal Ontario Museum while pursuing his doctorate in the University of Toronto’s Department of Ecology and Evolutionary Biology.
For the new study, the researchers looked at about 150 fossils collected by the Royal Ontario Museum during expeditions between 1975 and 2000. Most of the specimens were new material that had never been published in a scientific publication before, Izquierdo-Lopez said.
“Only a few have been published to date,” he said in an email. “Just over 10 mandibles remain in clear condition, highlighting how difficult they are to find in such a preserved state!”
Jean-Bernard Caron/Royal Ontario Museum
Scientists first discovered Odaraia fossils in the Burgess Shale in 1912.
The preserved mandible, previously only hinted at by muscle scars in other Odaraia specimens, according to the study authors. Odaraia’s newly discovered mouthparts “are a stout, short appendage with a row of teeth,” Izquierdo Lopez added. “This is exactly how we expected the mandible to look.”
Their discovery highlights that new fossils, even for known species, are full of surprises, they say. Dr. Joanna WolfResearch Associate in the Department of Biological Evolution at Harvard University.
“It’s important to re-examine previously known species, and in this case they (the study authors) had a lot of new material,” Wolf said. “Sometimes features are only visible in one specimen, so you should always look closely.”
Odaraia was about six inches (15 centimeters) long, had large, stalked eyes that peered out at its marine habitat, and had dozens of segments on its body with more than 30 pairs of slender legs.
Encased in what’s known as an octopus shell, a tubular shield that encases Odaraia’s body, with its head jutting out the front and its tail protruding out the back. Many arthropods have bivalve shells with this octopus-like feature, “including modern arthropods such as giant shrimps and sea shrimps,” Wolf said.
Odaraia’s shell was folded over its limbs, which may have prevented it from walking on the ocean floor, he said. Royal Ontario MuseumInstead, the turtle octopus likely moved around by swimming upside down, like modern horseshoe crabs.
Jean-Bernard Caron/Royal Ontario Museum
Odaraia was probably one of the earliest members of the Mandibles, a group of arthropods with chewing mouthparts called mandibles.
The researchers report that while the legs may not have been used for walking, they were likely important for catching prey, such as small Cambrian marine creatures. Examination of the fossils revealed that the undersides of the animal’s legs were lined with stiff, hair-like structures called setae. These tiny spines could have captured prey in the same way that the baleen lining a whale’s mouth filters seawater and captures plankton.
“We think that the spines intertwine between the legs to form a web that captures passing prey,” Izquierdo Lopez says.
This type of feeding method is common to many modern crustaceans, who use setae of different types and lengths to capture food, Wolf added.
More Mandibra Mysteries
One feature that has puzzled and intrigued scientists is one never before seen in a Cambrian animal: a single tooth-like structure between Odaraia’s lower jaws.
“We still don’t know what it is exactly, even when we compare it to modern mandibles,” Izquierdo Lopez says, “but we think it was probably used together with the mandible to further chew food. This structure could have evolved into similar structures in centipedes and crabs, but we don’t know any more than that right now.”
The discovery of further fossils may shed light on the function of this organ and may also help reveal other unusual details about Odaraia, such as the presence of three small eyes between two large ones. Previous studies have briefly described these light-sensitive organs, but the researchers did not detect any primitive eyes in their scans.
“Although the three eyes were not clearly visible in this study, we cannot completely rule out their presence,” Izquierdo Lopez said. “Future specimens may reveal a more complex head than we have today.”
Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American, and How It Works magazine.
