Tag Archives: cartilaginous 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.

Perch (Ray-Finned Bony Fish)

Fish diversity took off once dinosaurs went extinct

Today, ray-finned fish make up 99% of all fish species, but it wasn’t always like this. In an attempt to find out what triggered this spectacular multi-niche dominance, paleontologists traveled back in time sort of speak and analyzed ancient fossils to see what the fish diversity makeup looked like millions of years ago. Intriguing enough, the ray fish practically exploded in their diversity right after the last great mass extinction which occurred 65 million years ago. An asteroid impact wiped out thousands of species, including all dinosaurs. But there was now enough room for other creatures to take their place. On land, mammals started filling in the large-scale niches eventually reaching a dominant position. In the water, it was the ray-finned fish that seized the opportunity.

Perch (Ray-Finned Bony Fish)

Perch (Ray-Finned Bony Fish)

In contrast to cartilaginous fish (sharks, rays, skates), ray-finned fish (Actinopterygii), as the name suggest, have their fins supported by a rigid skeleton. as the name indicates. These fin rays are sometimes known as lepidotrichia. About half of all vertebrate species are ray-finned fish and know about 24,000 species, with new ones added regularly. Ray-finned fish vary in size from very small, like the Paedocypris progenetica (the smallest vertebrate in the world measuring only a third of inch long), to very large like the ocean sunfish (Mola mola), which can be up to 12 feet long and weigh over a ton.

It’s quite difficult to find and interpret Actinopterygii fossils due to preservation considerations. So, researchers at the Scripps Institution of Oceanography, University of California had to think outside the box for a moment. Instead of hunting for fossils one at a time, they drilled marine sediments and took samples from six sites around the world, including the Atlantic and Pacific oceans. They then determined the ratio of fossilized teeth from ray-finned fishes to the fossilized scales from another major group of fish: sharks (ichthyoliths). 

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At the end of the Cretaceous, some 66 million years ago, sharks heavily outnumbered ray-fined fish. Then, after the asteroid crashed into the planet,  the ratio of these ray-finned fish remains shot up dramatically, quickly outnumbering those of sharks. But shark species didn’t go extinct – the numbers stayed more or less flat. Most likely, the extinction event killed off a lot of marine species, including the thriving  ammonites which competed with fish for food. Left enough room to expand, ray-finned fish quickly climbed the evolutionary ladder, as reported in the Proceedings of the National Academy of Sciences.

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