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An ancient bug-like sea creature with a fan-shaped tail and a shell wrapped around 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 recently provided scientists with their first look at Odaraia’s jaw 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 million to 485.4 million years ago) and included modern crustaceans, insects, and millipedes such as centipedes and centipedes. Whether cutting, tearing or grasping, mandibles help arthropods get their work 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 mandibles in Odaraia resolves a long-standing mystery about how the creature caught its food 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 back to about 505 million years ago. However, the heads of these fossils were incomplete. That left scientists unsure whether Odaraia belonged to the mandibles, since head appendages are critical for classifying extinct arthropods, said lead study author Alejandro Izquierdo-López. He conducted the research at the Royal Ontario Museum while pursuing a PhD in the Department of Ecology and Evolutionary Biology at the University of Toronto.
For the new investigation, 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 previously appeared in scientific publications, Izquierdo-López said.
“Only a few have been published before,” he said in an email. “We had clear mandibles in just over 10, which shows how difficult it is to find them preserved!”
Jean-Bernard Caron/Royal Ontario Museum
Scientists first discovered Odaraya fossils in the Burgess Shale in 1912.
Preserved mandibles had previously only been hinted at by muscle scars in other Odaraia specimens, the study authors report. Odaraya’s newly discovered mouthparts “are stout, short appendages with a row of teeth,” Izquierdo-López added. “This is exactly what we would expect the lower jaw to look like.”
Their discovery underscores that even for known species, new fossils can still be full of surprises, said Dr. Joanna Wolff, a research associate in the Department of Organismic and Evolutionary Biology at Harvard University.
“Revisiting the species we’ve known before is important. In this case, they (the study authors) had a lot of new material,” Wolff said. “Sometimes the characteristics are only seen in one specimen, so you always have to look.”
Odaraia was about 6 inches (15 centimeters) long and peered into its ocean home through large eyes on stalks. Its body was divided into dozens of segments, with more than 30 pairs of spindle-shaped legs.
Surrounding him was what was called a taco shell, a tubular shield that folded around Odaraya’s body, leaving his head protruding from the front and his tail protruding from the back. Many arthropods have this taco-like feature, known as a bivalve carapace, “including living arthropods such as ostracods (seed shrimp) and fan shrimp,” Wolff said.
The carapace folded over Odaraya’s limbs, so it may not have been able to walk on the sea floor, according to the Royal Ontario Museum. Instead, the sea bug taco probably spread like modern horseshoe crabs: swimming upside down.
Jean-Bernard Caron/Royal Ontario Museum
Odaraya was probably among the earliest mandibles, a group of arthropods with chewing mouthparts called mandibles.
Although its legs may not have been used for walking, they were likely important for catching food as smaller Cambrian sea creatures, the researchers report. When they examined the fossils, they found hard, hair-like structures called setae covering the animals’ legs. These tiny spines may have trapped food, much like the rows of baleen in whales’ mouths that filter seawater and trap plankton.
“We think the spines can intertwine between the legs, creating a web to catch passing prey,” Izquierdo-López said.
This type of feeding is common among many modern crustaceans, which have different types and lengths of setae they use to capture food, Wolff added.
More mandibular mysteries
One feature that puzzled and intrigued scientists had never been seen before in Cambrian animals: a tooth-like structure between Odaraya’s mandibles.
“We still don’t know exactly what it is, even when we compare it to mandibles today,” Izquierdo-López said. “However, we think it was probably used together with the mandibles for further chewing of food. This structure may have evolved into other similar ones in centipedes or crustaceans, but we cannot say more at this time.
Finding additional fossils may clarify the function of this structure and may help clarify other unusual details about Odaraya, such as the existence of three small eyes between the two larger ones. Previous studies 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 rule out their presence completely,” Izquierdo-López said. “Future specimens may reveal an even more complex head than we have today.”
Mindy Weisberger is a science writer and media producer whose work has been published in Live Science, Scientific American, and How It Works magazine.
