Tag Archives: bony fish

Illustration of giant Jurassic fish Leedsichthys next to human diver for scale. Credit: Wikimedia Commons.

Ancient bony fish that was larger than a whale shark and a fast swimmer

Illustration of giant Jurassic fish Leedsichthys next to human diver for scale. Credit: Wikimedia Commons.

Illustration of giant Jurassic fish Leedsichthys next to human diver for scale. Credit: Wikimedia Commons.

Bony fish make up roughly 95% of all fish species. Intriguingly, despite the sheer amount of species, their sizes are not particularly varied, and bony fish are generally quite small. The biggest bony fish in the world is the ocean sunfish, which weighs around 2.3 metric tons. Meanwhile, cartilaginous fish grow to be far bigger, with species such as whale sharks weighing up to 34 metric tons. Scientists have attempted to explain this huge discrepancy by pinning it on metabolic constraints. However, such an assertion misses the bigger picture, a new study seems to suggest.

The international team of researchers from Spain, Brazil, and the UK studied an ancient bony fish called Leedsichthys problematicus, which swam in the oceans of the Jurassic era more than 160 million years ago. This was one of the largest bony fish ever to have lived. It was 15 meters long and weighed up to 45 metric tons, which means it was even larger than even today’s whale sharks.

Pieces of Leedsichthys fossils were first found by the British collector Alfred Leeds in 1889. Similar remains were subsequently found at other sites, from northern Germany to Normandy, Mexico and the Atacama desert in Chile. The giant fish was a vertebrate suspension-feeder that fed on large quantities of plankton.

Leedsichthys may be key in settling a debate among biologists that has been going on for some time: why are bony fish so small compared to their cartilaginous relatives? One argument says that large animals generally have to function with less oxygen per tissue mass, and since bony fish appear to have higher metabolic requirements than cartilaginous ones, it would simply be physically impossible for them to grow past a certain threshold. Such an assertion, however, is strongly disproved by ancient species such as Leedsichthys.

The new study suggests that the metabolic constraint argument doesn’t hold water in this discussion, and we need to look for something else. By the author’s calculations, which used data from living bony fish, Leedsichthys would not only have survived, but thrived too. According to results published in the journal Paleontology, the giant fish could have swum at speeds of up to 17.8 kilometers per hour, while still keeping its tissue properly oxygenated.

Scientists still don’t know why there aren’t any big bony fish alive today, but at least we now know that their metabolism isn’t to blame.

The 415-million-year-old fish Janusiscus provides evidence of a common bony and cartilaginous fish. Credit: SAM GILES, MATT FRIEDMAN, AND MARTIN BRAZEAU

Ancient 420-million-year-old fossil hints of bony fish and cartilaginous fish common ancestor

As far as charting the tree of life goes, a basic indicator used to distinguished between classes of animals is the skeleton. Fish, for instance, can have a cartilaginous skeleton and here were remind sharks, rays and skates, or a bony skeleton like the sturgeon or ocean sunfish. In fact, bony fish or Osteichthyes as they’re also known represent the largest class of vertebrates in existence today. Based on fossil evidence and genome analysis, scientists know that the two groups diverged from a common ancestor around 420 million years ago, but we’ve yet to find actual fossil of it. Things are shaping up though after paleontologists have identified  an Early Devonian fish from Siberia, approximately 415 million years old, which bears features of both classes.

In search of the common fish ancestor

The 415-million-year-old fish Janusiscus provides evidence of a common bony and cartilaginous fish. Credit: SAM GILES, MATT FRIEDMAN, AND MARTIN BRAZEAU

The 415-million-year-old fish Janusiscus provides evidence of a common bony and cartilaginous fish. Credit: SAM GILES, MATT FRIEDMAN, AND MARTIN BRAZEAU

Initially, the specimen was classified in a 1992 paper as a bony fish belonging to the genus Dialipina, based on the scales and the head bones’ similarity to those of bony fish called Dialipina from the New Siberian Islands. Martin Brazeau at Imperial College London found it odd, however, that a bony fish was this old, so he requested more details. Eventually, him and his team were convinced that this was worth a thorough investigation.

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The scientists performed micro CT scans to peek inside the delicate structure of the bones that encase the fragile head of the fish, whose fossil was only one centimeter long.  Because of the shape of the skull roof and the enamel on the scales, the fish was naturally classified as a bony one. Inside, however, things are a bit different. The CT scans showed how the skull is traversed by  nerves and blood vessels around the brain more closely resembled those of cartilaginous fish. As such, the fish has features of both classes. The fossil was eventually named Janusiscus schultzei, in honor to  the two-faced Roman god Janus.

The feeds previous speculations that suggested that both classes of jawed fish  had features of bony fish, but the cartilaginous ones eventually lost these. It also supports a 2014 study that showed that a 325-million-year-old fossil shark had a surprising number of bony fish features, suggesting that the ancestor also had these features and that sharks may be more specialized than originally believed.

“[…] Both groups evolved different adaptations, and they’ve also retained different primitive features from their ancestor,” Giles explains. “Each group has found a different way of approaching the problem of living in the sea.”

Janusiscus is a fascinating discovery,” says John Long, a paleontologist at Flinders University in Adelaide, Australia. It’s also one that couldn’t have been made without the use of a detailed CT scan, he notes. “Such use of modern technology is transforming the way we do paleontology by revealing new layers of information in these critical transitional fossils.”

Findings appeared in Nature // via Science