About 70 percent of animals on Earth, including millipedes, bees, shrimp and crabs, are arthropods with mandibles, or pincer-like jaws. To understand how organisms with this anatomical feature became so diverse and successful over the past 500 million years, scientists looked inside the taco-shaped shell that protected an enigmatic creature that once swam in prehistoric seas.
This creature, the arthropod Odaraia alata, lived in the shallow seas of the Middle Cambrian period about 508 million years ago. Sometimes compared by scientists to modern-day shrimp, Odaraia was about 8 inches long, a large size for its time. Its unique hard shell, which may remind you of Taco Tuesday, likely helped it propel itself through the water, sometimes upside down.
An analysis of fossils, published Wednesday in the journal Proceedings of the Royal Society B, presents the first solid evidence that Odaraia had mandibles and likely collected food in the open ocean, not just near the seafloor. The study fills critical gaps in the evolutionary record of arthropods and the establishment of marine food webs.
Paleontologists have been wondering about Odaraia’s existence for more than a century. They hypothesized in 1912 and again in 1981 that the marine bug might filter feed using mandibles, as shrimp and barnacles do today. But the fossil evidence was inconclusive.
“A lot of things about Odaraia remained secret to us,” even after the full analysis in 1981, said Alejandro Izquierdo López, a paleontologist who led the new study while at the University of Toronto.
Dr. Izquierdo-López measured and documented 150 previously unanalysed Odaraia alata Dr. Izquierdo López’s specimens come from the collections of the Royal Ontario Museum. He selected 24 exceptionally well-preserved fossils for closer study, which yielded exquisite detail on the animal’s head and legs. Such features rarely survive geological burial, but the fossils were originally discovered in British Columbia, Canada, in the Burgess Shale, a rock formation with unusually high rates of high-quality preservation of soft body parts. Dr. Izquierdo López’s specimens are no exception.
“I’m amazed,” said Russell Bicknell, a paleontologist at the American Museum of Natural History who was not involved in the study. “It’s crazy how beautiful these animals are.”
The fossils had transparent mandibles, lined with tiny teeth. Between the jaws was a strange single tooth, probably used for grinding food.
“It looks like a trident,” said Dr. Izquierdo López. “It’s something that has never been observed in any other Cambrian animal.”
Dr. Izquierdo López’s analysis also revealed that Odaraia’s “very bizarre” legs were covered with dozens of large spines and hundreds of small spikes. The legs interlocked to create a mesh, similar to that of a fishing net. The animal may have used its cascade of legs to funnel water through its tube-like shell, capturing plankton and other prey in the net. It then sent the food up to its mouth, where mandibles and teeth crushed it.
The fossil finds support previous assumptions that Odaraia was a filter feeder. But relatively wide spaces between the spines on its legs suggest that the animal could have caught prey up to a centimeter long. Odaraia also has some of the largest eyes relative to its body size of any animal from that time, a common feature among predators.
Dr. Izquierdo López then put forward the hypothesis that Odaraia could have been both a filter feeder and a predator. “It’s an idea that deserves to be explored,” he said.
“This is an animal that has discovered the mechanism of suspension feeding, which is quite innovative for such an early period,” Dr Bicknell said. The fossils offer “an amazing insight into how we think this type of animal functioned.”
This flexibility may have helped Odaraia thrive in the water column, paving the way for the evolutionary success of other mandible-equipped arthropods. While its taco-shaped shell no longer exists, its way of feeding has survived for half a billion years.
Scientists aren’t sure what Odaraia’s closest living relative is; the taco-shaped shell hasn’t been seen since the animal roamed the Cambrian seas. Some forms of shrimp are potential relatives of Odaraia, but there’s still much to learn, Dr. Izquierdo López said.
The two paleontologists, if given the opportunity, would subject Odaraia to a taste test.
“They were probably pretty meaty,” Dr. Izquierdo López said. “I would probably do something with parsley and garlic.”
He added: “You would just have to take a lot of legs off.”
As for Dr Bicknell, who has dual New Zealand and Australian citizenship: “Honestly? Prawns on a barbecue.”
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