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An ancient, insect-like sea creature with a fan-shaped tail and a shield wrapped around its body that swam upside down and looked like a taco – but this taco was capable of biting.
new Excavations A recent study of the extinct arthropod Odaraya alata has given scientists their first look at jaw-like structures called mandibles. These small, paired appendages near the mouth bite, grip and tear food, and arthropods with these mouthparts are called mandibles.
The first jaws evolved in the oceans during the Cambrian period (541 million to 485.4 million years ago) and include modern crustaceans, insects, and many legs, such as millipedes and millipedes. Whether jaws help arthropods cut, tear, or grip, jaws have diversified so successfully that they now make up more than half of all animal species, according to Royal Ontario Museum.
The identification of the lower jaw in Odaraya solves a long-standing mystery about how the creature caught its meals, and suggests that Odaraya is among the oldest lower jaws in the arthropod family tree, researchers reported July 24 in the journal Nature Community. Proceedings of the Royal Society B: Biological sciences.
The species was described in 1912 from fossils found in the Burgess Shale of British Columbia, Canada, in rocks dating back about 505 million years. However, the heads of these fossils were incomplete. That left scientists uncertain whether Odaraia belonged to the jawed mandibles, since head appendages are important for the classification of extinct arthropods, said lead study author Alejandro Izquierdo Lopez. He conducted the research at the Royal Ontario Museum while earning his doctorate in the Department of Ecology and Evolutionary Biology at the University of Toronto.
As part of the new investigation, 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-Lopez said.
“Only two of these bones have been published before,” he said in an email. “We found apparent jaws in just over 10 of them, 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.
The preserved lower jaw was previously known only from muscle scars in other specimens of Odaraya, the study authors reported. The newly discovered mouthparts of Odaraya “are short, robust appendages with a row of teeth,” Izquierdo-Lopez added. “This is exactly what we would expect the lower jaw to look like.”
Their discovery confirms that even for known species, new fossils can be full of surprises, they say. Dr. Joanna Wolfa research associate in the Department of Organismic and Evolutionary Biology at Harvard University.
“It’s important to revisit species we already know,” Wolf said. “In this case,[the study authors]had a lot of new material. Sometimes features are only visible in one specimen, so you always have to look.”
Odaraya was about 6 inches (15 centimeters) long and looked out over its ocean habitat through large eyes mounted on stalks. Its body was divided into dozens of segments, with more than 30 pairs of slender legs.
What encased it was a so-called taco shell — a tubular shield that wrapped around Odaraya’s body, leaving its head sticking out the front and its tail sticking out the back. Many arthropods have this taco-like feature, known as a bivalve shield, “including living arthropods like seed shrimp and fan shrimp,” Wolf said.
The armor folded over Odaraya’s limbs, so it may not have been able to walk on the sea floor, according to Royal Ontario MuseumInstead, the sea bug taco probably spread the same way modern horseshoe crabs do: by swimming upside down.
Jean Bernard Caron/Royal Ontario Museum
Odaraya is likely among the oldest of the jawed arthropods, a group of arthropods with chewing mouthparts called jaws.
Although their legs probably weren’t used for walking, they were likely important for catching food like smaller sea creatures of the Cambrian, the researchers said. When they examined the fossils, they found hard, hair-like structures called setae lining the animals’ legs. These tiny spines may have trapped food, much like the rows of fins in whales’ mouths filter seawater and trap plankton.
“We think the spines could intertwine between the legs, creating a web that would capture passing prey,” Izquierdo Lopez said.
This type of feeding is common among many modern crustaceans, which have different types and lengths of bristles that they use to capture food, Wolf added.
More lower jaw mysteries
One feature that puzzled and intrigued scientists was that it had never been seen before in Cambrian animals: a tooth-like structure between the jaws of Odaraya.
“We still don’t know exactly what this structure was, even when compared to today’s lower jaw,” says Izquierdo López. “But we think it was probably used with the lower jaw to chew food more thoroughly. This structure may have evolved into other similar structures in millipedes or crabs, but we can’t say much more, yet.”
Finding additional fossils could clarify the function of this structure, and could help explain other unusual details about Odaraya, such as the presence of three small eyes between the larger ones. Previous studies have briefly described these light-sensing organs, although the researchers did not detect primitive eyes in their surveys.
“We couldn’t see these three eyes well in this study, but we can’t completely rule out their existence,” 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.