Plants and Animals

Utterly Bizarre Ancient Fish Had A Ginormous, Plankton-Scooping Mouth

February 9, 2016 | by Robin Andrews

Photo credit: Om nom nom: An artist's impression of the Rhinconichthys eating up plankton. Robert Nicholls

The Cretaceous Period, extending from 145 to 66 million years ago, featured some of the most visually striking dinosaurs we know of, including the terrifying Dakotaraptor and the exceedingly horny Kosmoceratops. Its oceans were also populated by some truly bizarre aquatic creatures, including Rhinconichthys. As a new study published in Cretaceous Research details, this ancient bony fish had a gaping jaw that opened like a parachute canopy unfurling in the wind.

Although this genus of sea-dwelling animal was already known to paleontologists, fossil finds are incredibly rare. In fact, before now only one distinct species was known, initially identified from a site in England in 2010. This study brings the total number of species up to three, which reveals that these highly-specialized fish were globally distributed.

The two new family members were found in modern-day Colorado and Hokkaido, Japan. The latter was previously, erroneously identified as the same species as the original fossil specimen, but this study concludes that it’s a distinct species. The Colorado specimen is particularly noteworthy, as it extends the range of the genus to the Cretaceous-age Western Hemisphere.

“This tells just how little we still know about the biodiversity of organisms through the Earth's history,” Kenshu Shimada, a paleobiologist at DePaul University and one of the authors of the study, said in a statement. “It's really mindboggling.”

Each had their own unique physical characteristics, but all three Rhinconichthys species used the same feeding method that whale sharks, basking sharks, and manta rays use today: suspension feeding. This involves using a mouth-based apparatus to trap and consume nutritious matter or prey floating in the water, which in many cases includes plankton.

A modern whale shark feeds in the same way as the ancient Rhinconichthys did. Rich Carey/Shutterstock

Rhinconichthys was incredibly well-adapted to feed this way: It had a cavernous mouth that could swing wide open, thanks to a pair of bones called hyomandibulae that formed an oar-shaped lever. This adaptation meant that their upper and lower jaw could extend from being essentially parallel to almost at right angles to each other when the mouth was fully opened, allowing the fish to feed on vast amounts of plankton in one go.

In fact, it’s likely that these bizarre-looking, 92-million-year-old beasts spent most of their days swimming around with their mouth fully open. Curiously, unlike many modern suspension feeders, they had fairly large eyes; these were more likely used to look out for larger predators than for spotting plankton in the water.

Unfortunately, although the skulls of these fish have been fairly well preserved, the rest of their bodies haven’t been. This means that much of the anatomy of Rhinconichthys can only be inferred from related species, including its size. For example, the English specimen was smaller than the other two, suggesting that it was perhaps a juvenile, whereas the Japanese and American fossils were adults. With this in mind, the authors estimate that a fully-grown adult Rhinconichthys could have grown up to 2.7 meters (8.9 feet) in length.

Rhinconichthys belonged to a group of fish named pachycormids, which became extinct along with the non-avian dinosaurs at the end of the Cretaceous.

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