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An ancient bug-like sea creature with a fan-shaped tail and a carapace 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 have recently given scientists their first glimpse of Odaraia’s jaw structures, known as mandibles. These small paired appendages near the mouth are called mandibulates, they hold and grind food, and arthropods with these mouth parts are called mandibulates.
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 myriopods, such as centipedes and millipedes. Whether they’re crawling, clawing or biting, mandibles help arthropods get the job done, and mandibulates have diversified so that they make up more than half of all animal species today, according to the Royal Ontario Museum.
The identification of mandibles in Odaraia solves a long-standing mystery about how the creature took its meals, and suggests that Odaraia is among the earliest mandibulates 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 at 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. This left scientists unsure whether Odaraia was among mandibulates, as head appendages are crucial for classifying extinct arthropods, said lead study author Alejandro Izquierdo-López. He conducted the research at the Royal Ontario Museum while pursuing a doctorate in the University of Toronto’s department of ecology and evolutionary biology.
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, said Izquierdo- López.
“Only a few have been published before,” he said in an email. “We had clear mandibles in just over 10, which shows how hard it is to get 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 strong, short appendages with a row of teeth,” Izquierdo-López added. “This is exactly what we would expect a manible to look like.”
Their discovery highlights that new fossils can still be full of surprises even for known species, said Dr. Joanna Wolfe, research associate in the Department of Organic and Evolutionary Biology at Harvard University.
“It’s important to look again at species that we know before. 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 should always look.”
Leg grubs that attract prey
Odaraia measured about 6 inches (15 centimeters) long and watched its ocean home through large eyes on the stalks. His body was divided into many parts, with more than 30 pairs of spindly legs.
It consisted of the so-called taco shell – a tubular shield that wrapped around Odaraia’s body, leaving its head sticking out of the front and its tail poking out from the back. Many arthropods have this taco-like feature, known as a bivalve carapace, “including living arthropods like ostracods (seed reedrons) and fan shrimp,” Wolfe said.
The carapace folded over Odaraia’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 got around like modern horseshoe crabs do: swimming upside down.
Although its legs may not have been used for walking, they were probably important for grinding food like the small Chabracha creatures, the researchers reported. When they examined the fossils, they found stiff hair-like structures called setae lining the animals’ legs. These tiny pins may trap food, as layers of baleen in whales’ mouths filter seawater and pass plankton.
“We think the spines could interlock between the legs, creating a net that would 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, Wolfe added.
More mandibulate mysteries
One feature that puzzled and intrigued scientists had never been seen before in Cambrian animals: a single tooth-like structure between Odaraia’s mandibles.
“We still don’t know what it is, exactly, even when comparing it to mandibulates today,” said Izquierdo-López. “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 centipedes or trees, but we can’t say more, so far.”
The function of this structure could be clarified if additional fossils are found, and it may help to improve other unusual details about Odaraia, such as the presence of small eyes between the two large ones. Previous studies briefly described these light-sensitive organs, although the researchers did not detect the underlying eyes in their scans.
“We were not able to see those three eyes very well in this study, but we cannot completely deny their presence,” said Izquierdo-López. “Maybe future specimens will reveal one even more complex than what 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.
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