The partial skull of an armored fish that swam in the oceans over 400 million years ago could turn the evolutionary history of sharks on its head, researchers have said.
Bony fish, such as salmon and tuna, as well as almost all terrestrial vertebrates, from birds to humans, have skeletons that end up made of bone. However, the skeletons of sharks are made from a softer material called cartilage — even in adults.
Researchers have long explained the difference by suggesting the last common ancestor of all jawed vertebrates had an internal skeleton of cartilage, with bony skeletons emerging after sharks had already evolved. The development was thought so important, living vertebrates are divided into “bony vertebrates” and “cartilaginous vertebrates” as a result.
Among other evidence for the theory, the remains of early fish called placoderms — creatures with bony armor plates that also formed part of the jaws — shows they had internal skeletons made of cartilage.
But now a startling discovery has upended the theory: researchers have found the partial skull-roof and brain case of a placoderm composed of bone.
The fossil, about 410 million years old and reported in the journal Nature Ecology & Evolution, was unearthed in western Mongolia in 2012, and belongs to a placoderm that has been dubbed Minjinia turgenensis and would have been about 20-40cm in length.
“This fossil is probably the most surprising thing I have ever worked on in my career. I never expected to find this,” said Martin Brazeau of Imperial College London and first author of the research.
“We know a lot about [placoderm] anatomy and we have hundreds of different species of these things — and none of them has ever shown this kind of bone.”
The new discovery, he said, casts doubt on the idea that sharks branched off the evolutionary tree of jawed vertebrates before a bony internal skeleton evolved.
“This kind of flips it on its head, because we never expected really for there to be a bony internal skeleton this far down in the evolutionary history of jawed vertebrates,” said Brazeu. “This is the type of thing [which suggests] maybe we need to rethink a lot about how we think all of these different groups evolved.”
While the team say that one possibility is that bony skeletons could have evolved twice — once giving rise to the newly discovered placoderm species and once to the ancestor of all living bony vertebrates — they say a more likely possibility is that an ancestor of sharks and bony vertebrates actually had a bony skeleton, but that at some point in their evolutionary history the ability to make bone was lost in sharks.
Brazeau said the new findings adds weight to the idea that the last common ancestor of all modern jawed vertebrates did not resemble “some kind of weirdo shark” as is often depicted in text books. Instead, he said, such an ancestor more likely resembled a placoderm or primitive bony fish.
Daniel Field, a vertebrate palaeontologist at the University of Cambridge who was not involved in the work, welcomed the findings.
“Evolutionary biologists were long guided by the assumption that the simplest explanation — the one that minimized the number of inferred evolutionary changes — was most likely to be correct. With more information from the fossil record, we are frequently discovering that evolutionary change has proceeded in more complex directions than we had previously assumed,” he said.
“The new work by Brazeau and colleagues suggests that the evolution of the cartilaginous skeleton of sharks and their relatives surprisingly arose from a bony ancestor — adding an extra evolutionary step and illustrating that earlier hypotheses were overly simplistic.”
The town of Baolai (寶來) is located along the Southern Cross-Island Highway in the upper reaches of Kaohsiung City. After suffering a devastating setback at the hands of Typhoon Morakot, the town’s tourism industry is finally showing signs of recovery. While the town itself has many commercial hot spring offerings for tourists, the adjacent Baolai River also has at least five different wild hot springs available to those with a more adventurous spirit. SHIDONG AND WUKENG Just before entering the town of Baolai, make two right turns to reach the bridge across the Baolai River. Immediately after crossing this bridge, there is
In October of 2002 the James Ossuary exploded into the public consciousness. The artifact, a burial box in which bones were interred, was announced at a press conference in Washington prior to undergoing any form of scholarly authentication. It had an inscription that read in Aramaic: Ya’akov bar Yosef akhui di Yeshua (“James, son of Joseph, brother of Jesus”). Its promoters presented the thing as the first real concrete link to the historical Jesus. It was an obvious fake, and at that time I was administrating two enormous discussion groups devoted to early Christian history, which hosted numerous scholars in
Jan. 25 to Jan. 30 It was the beginning of the end when German sergeant Hans Jurgen Radis walked out of the Dutch-controlled Fort Zeelandia and surrendered to the besieging army of Cheng Cheng-kung (鄭成功, also known as Koxinga). The Dutch had already been trapped in the fort for nine months, and they were sick, hungry and in despair. After one defection during the early days of the siege, Dutch commander Frederick Coyett set up checkpoints around the fort’s perimeter, in what is today’s Tainan. Radis told his bunkmate he was going hunting, but by the time they
“Well, if it cannot happen this year because of the pandemic,” Tourism Bureau Director General Chang Shi-chung (張錫聰) says at the end of his interview with Cycling Shorts last week, “at least we’ll be ready to promote it next year.” Chang is discussing the Year of Cycling Tourism (自行車旅遊年) that has long been planned for this year. He has spent the previous 30 minutes introducing the various infrastructure projects undertaken over recent years and those proposed for the next few. Essentially, the Bureau, under the Ministry of Transportation and Communications (MOTC), has been pulling together resources from a wide range of