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An ancient, insect-like sea creature with a fan-shaped tail and a shell surrounding its body swam upside down and looked like a taco — but this taco could bite back.
Newly discovered fossils of the extinct arthropod Odaraia alata have recently given 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 mandibles.
The first mandibles evolved in the oceans during the Cambrian Period (541 to 485.4 million years ago) and include crustaceans, insects, and modern myriapods, such as millipedes and centipedes. Whether used for cutting, tearing, or grasping, mandibles help arthropods get the job done, and mandibles have 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 creature captured its meals, and suggests that Odaraia is among the earliest mandibles in the arthropod family tree, the researchers reported July 24 in the journal Proceedings of the Royal Society B: Biological Sciences.
The species was described in 1912 from fossils discovered in the Burgess Shale in British Columbia, Canada, in rocks that were about 505 million years old. However, the heads in those fossils were incomplete, so scientists weren’t sure if Odaraia belonged to the mandibulates, because head appendages are essential for classifying extinct arthropods, said Alejandro Izquierdo-López, lead author of the study. He conducted his research at the Royal Ontario Museum while pursuing a PhD in the University of Toronto’s department of ecology and evolutionary biology.
For the new study, the researchers examined 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 yet appeared in scientific publications, Izquierdo-López said.
“Only a few specimens had been published before,” he said in an email. “We had clear mandibles on just over 10 specimens, which shows how difficult it is to find them preserved!”
Jean-Bernard Caron/Royal Ontario Museum
Scientists first discovered Odaraia fossils in the Burgess Shale in 1912.
The preserved mandibles were previously hinted at only by muscle scars in other Odaraia specimens, the study authors reported. The newly discovered Odaraia mouthparts “are short, robust 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 for known species, new fossils can still be full of surprises, said Dr. Joanna Wolfe, a research associate in Harvard University’s Department of Organismal and Evolutionary Biology.
“It’s important to revisit species that we already know about. In this case, they (the study authors) had a lot of new information,” Wolfe said. “Sometimes features are only visible in one specimen, so you always have to look at them.”
Odaraia was about 15 centimeters long and observed its ocean through large eyes perched on stalks. Its body was divided into dozens of segments, with more than 30 pairs of slender legs.
The shell was what’s called a taco-shaped carapace, a tubular shield that folded around Odaraia’s body, leaving its head sticking out in front and its tail in back. Many arthropods have this taco-like feature, known as a bivalve carapace, “including living arthropods like ostracods (seed shrimp) and fan shrimp,” Wolfe said.
The shell folded over Odaraia’s limbs, which may have prevented it from walking on the seafloor, according to the Royal Ontario Museum. Instead, the marine insect taco likely moved the way modern horseshoe crabs do: by swimming upside down.
Jean-Bernard Caron/Royal Ontario Museum
Odaraia was probably one of the first mandibles, a group of arthropods with chewing mouthparts called mandibles.
The animal’s legs may not have been used for walking, but they were probably important for capturing food, like those of small Cambrian sea creatures, the researchers said. When they examined the fossils, they found stiff, hair-like structures called setae that lined the animal’s legs. These tiny spines could have trapped food, much like the rows of baleen in whales’ mouths filter seawater and trap plankton.
“We think the spines could intertwine between the legs, creating a net that would capture passing prey,” Izquierdo-López said.
This type of feeding is common among many modern crustaceans, which have different types and lengths of bristles that they use to capture their food, Wolfe added.
Other Mandible Mysteries
One feature that has intrigued and puzzled scientists has never been seen before in Cambrian animals: a single tooth-like structure between Odaraia’s mandibles.
“We still don’t know exactly what it is, even comparing it to today’s mandibles,” Izquierdo-López said. “However, we think it was probably used with the mandibles to chew food more. This structure may have evolved into other similar structures in millipedes or crabs, but we can’t say more at this point.”
The discovery of additional fossils could clarify the function of this structure and help clarify other unusual details about Odaraia, such as the existence of three small eyes between the two larger ones. Previous studies have briefly described these light-sensitive organs, although the researchers did not detect the rudimentary eyes in their scans.
“We couldn’t see these three eyes very well in this study, but we can’t completely deny their presence,” Izquierdo-López said. “Future specimens could reveal an even more complex head than the one 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.