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An ancient beetle-like sea creature with a fan-shaped tail and a carapace that wrapped around its body swam upside down and looked like a taco—but this taco could bite.
Newly found fossils of the extinct arthropod Odaraia alata recently gave scientists their first look at Odaraia’s jaw-like structures, called mandibles. These small, paired appendages near the mouth bite, hold and tear food, and arthropods with these mouths are called mandibles.
The first mandibulates evolved in the oceans during the Cambrian period (541 million to 485.4 million years ago) and include modern crustaceans, insects, and myriapods such as centipedes and millipedes. Whether it’s cutting, tearing or grasping, mandibles help arthropods get the job done, and mandibles have diversified so successfully that today they make up more than half of all animal species, according to the Royal Ontario Museum.
The identification of the mandible in Odaraia solves a long-standing mystery about how the creature captured food and suggests that Odaraia is among the oldest 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 found in the Burgess Shale, British Columbia, Canada, in rocks dating back to about 505 million years ago. However, the heads of these fossils were incomplete. That left scientists unsure whether Odaraia was a mandibulate, since head appendages are crucial for classifying extinct arthropods, said the study’s lead author Alejandro Izquierdo-López. He conducted research at the Royal Ontario Museum during his doctoral studies at the University of Toronto’s Department of Ecology and Evolutionary Biology.
For the new investigation, the researchers examined around 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.
“Only a few have been published before,” he said in an email. “We had clean mandibles for just over 10, which shows how hard it is to find them preserved!”
Jean-Bernard Caron/Royal Ontario Museum
Scientists first discovered fossils of Odaraia in the Burgess Shale in 1912.
Preserved mandibles were previously only indicated by muscle scars in other Odaraia specimens, the study authors said. Odaraie’s newly found mouthparts “are strong, short appendages with a row of teeth,” Izquierdo-López added. “This is exactly what we’d 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, Research Fellow in the Department of Organismic and Evolutionary Biology at Harvard University.
“Revisiting species we know from before is important. In this case, (the study authors) had a lot of new material,” Wolfe said. “Sometimes the features are only visible on one specimen, so you should always look.”
Odaraia was about 15 centimeters long and gazed at her ocean home with large stalked eyes. Its body was divided into dozens of segments with more than 30 pairs of spindly legs.
The casing was a so-called taco shell – a tubular shield that folded around Odarai’s body so that her head stuck out the front and her tail stuck out the 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 carapace folded over Odaraia’s limbs, so she may not have been able to walk on the sea floor, according to the Royal Ontario Museum. Instead, the sea beetle taco likely emerged like modern horseshoe crabs: by swimming upside down.
Jean-Bernard Caron/Royal Ontario Museum
Odaraia probably belonged to the earliest mandibulates, a group of arthropods with chewing mouths called mandibles.
While its legs may not have been used for walking, they were probably important for catching food, like smaller Cambrian sea creatures, the researchers said. When they examined the fossils, they found stiff, hairy structures called setae lining the animals’ legs. These tiny spines could have trapped food, just as rows of baleen in whales’ mouths filter seawater and trap plankton.
“We think the spines could connect between the legs to form a web to catch passing prey,” Izquierdo-López said.
This type of feeding is common in many modern crustaceans, which have different types and lengths of bristles they use to capture food, Wolfe added.
Other mysteries of the mandible
One feature that puzzled and intrigued scientists had never been seen before in Cambrian animals: a single toothed structure between Odaraia’s mandibles.
“We still don’t know exactly what it is, even if we compare it to today’s jaws,” Izquierdo-López said. “However, we think it was probably used together with the mandibles to further chew food. This structure may have evolved into other similar structures in centipedes or crabs, but we cannot say more to date.”
Finding more fossils could shed light on the function of this structure and may help shed light on other unusual details about Odaraia, such as the existence of three small eyes between two larger ones. Previous studies have briefly described these light-sensitive organs, although the researchers did not detect rudimentary eyes in their scans.
“We didn’t see these three eyes very well in this study, but we can’t completely rule out their presence,” Izquierdo-López said. “Future samples may reveal an even 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.