Photograph by Bernard Radvaner, Corbis
Published April 16, 2014
Paleontologists have long thought that sharks hit on the right combination of body shape and internal anatomy early on, and that evolutionary forces didn't tinker much with the design over the following hundreds of millions of years. But a handful of bones in a 325-million-year-old shark-like fossil could upend this idea.
Structures that supported the fossil shark's gills more closely resemble those of modern-day bony fishes—like tuna or goldfish—than those of living sharks, reports a team of researchers led by the American Museum of Natural History (AMNH) in New York City. The arrangement of these bony or cartilaginous arches is also similar to the gill supports found in ancient, extinct fishes called placoderms and acanthodians.
Rather than being "living fossils," "modern sharks are in this respect evolutionarily quite advanced," said study co-author John G. Maisey, a vertebrate paleontologist at the AMNH.
If scientists want greater insight into these gill supports—which are believed to have given rise to the vertebrate jaw and human ear bones—as well as early fish evolution, they should look to modern fishes rather than living sharks, Maisey and colleagues argue in their study, published today in the journal Nature.
Peering Into the Past
Sharks have been held up as good examples of a primitive jawed vertebrate for hundreds of years, said Maisey. "And yet, there's never been an attempt to look at early fossil sharks." That's partly because there's a limited number of well-preserved gill arches from ancient sharks, and partly because of limitations in technology.
Maisey and colleagues were lucky enough to find a well-preserved fossil in the Ozark Mountains of Arkansas. The fossil contained the three-dimensional arrangement of the gill arches in a newfound species of ancient shark, Ozarcus mapesae. They were able to scan the fossil and create a high-resolution computer model of the bones.
They compared O. mapesae's gill arch arrangement with that found in ancient and modern fish. "The basic arrangement we found in Ozarcus would characterize the common ancestor of all these forms," Maisey said.
Modern sharks, though, have a different arrangement of gill supports that likely evolved after O. mapesae. So if scientists want to study the evolution of these arches and the jaws that followed, they shouldn't be looking at modern-day sharks, Maisey and colleagues concluded. They should be studying bony fishes instead.
"We're throwing down the gauntlet," said Maisey. "This is the real condition in an early shark, and it changes how we have to think about the evolution of jawed vertebrates."
A Place on the Family Tree
Data from sharks and fishes during this time period, around 325 million years ago, is so rare that any new information is welcome, said Martin Brazeau, a paleontologist at Imperial College London. "The ultimate conclusions [to this study] could very well change a bit, but I think the general picture's solid."
However, Brazeau, who was not involved in the new study, cautioned that the paper's authors did not include something called an "outgroup" in their analysis. "You can think of it like a control group in an experiment," he said. An outgroup can tell you whether the characteristic you're looking at, gill arch arrangements in this case, is a novel or ancestral feature within a group of organisms. (See "Fish Fossil Has Oldest Known Face, May Influence Evolution.")
But part of the reason the researchers didn't perform a more comprehensive analysis, Maisey said, is because there are so few fossils with which to compare gill arch arrangements.
Matt Friedman, a paleontologist at the University of Oxford in the U.K., agreed that the correct placement of Ozarcus within the fish family tree will need some more study. But he's excited because the field is in a kind of renaissance—thanks to advances in technology that allow studies like this—in how researchers look at the evolution of these animals.
Shark enthusiasts can't expect the idea that sharks are living fossils to disappear all that quickly, though. "It's an entrenched idea," said Friedman. "You see it in textbooks." Changing those will take time.
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This only proves the point that most of the creatures have evolved into in their modern relatives with little similarities to the original generation. There have been quiet a few reasons for that evolution, like inter breeding, environmental changes, and even evolution of earth (with introduction of more elements, because of meteor showers, to the earth) has created big difference in the evolution of all the creatures.
What I want to know is why did this very successful group (sharks and rays) never develop an out and out kelp munching herbivorous member. I don't mean the accidental consumption of phytoplankton by whale and basking sharks. Rays have crushing teeth which could deal with kelp or seagrass but in all their long LONG history no shark or ray specie has exploited these resources.
Oxford! This article is truly amazing, it shows how humans overlook true happenings when they are set on an idea they think is correct. Wanting to be an archaeologist I found this quite fascinating how little evidence there is for fish fossils. Of course this makes sense due to decaying and no real natural mummification happening in the sea. I wonder what else will be discovered shortly.
Interesting article. I would have loved to have seen a picture of the well-preserved fossil and the three dimensional view spoken about in this article.
the fossil record is so incomplete that scientists are constantly having to revise what they think actually happened. and the disturbing part is that they call the theory of evolution real science. based on such a spotty fossil record makes the theory of evolution based more on wild eyed assumptions than true scientific inquiry.
@Brian Edwards when you're an alpha, why settle for them greens when you can munch on flesh, like nature dictates?
@james george You seem to be a "god of gaps" adherent - whenever there's a gap in a theory, you shove god into the picture.
I'd love you on my jury if I was ever on trial and guilty.
@james george the theory of evolution is real science, you need to look up the terms of what a theory is, and then get off your big box of hate and discontent. The best part about the science mentioned here it that they admit to learning new things, and they strive to correct the scientific record.
@james george Not at all. Additional research and scientific inquiry advances and strengthens the theory; it does not make it "wild-eyed assumption."
The scientists are discovering new evidence. They are thoroughly studying and vetting the evidence. They are doing the research. They are publishing their findings so that any scientist anywhere in the world who so wishes can review and critique it. They are adding to the vast bulk of human scientific knowledge. I don’t know what world you live in, but here in the REAL world that IS true scientific inquiry. The “alternative” has been a centuries old abysmal failure at producing anything similar. Also, the fossil record for sharks may be spotty, but the fossil record is abundant for other classes of organisms. Further, the Theory of Evolution is supported and confirmed by MULTIPLE lines of evidence, not just the fossil record.
@KENNETH LANE Or maybe you could stop fishing and let our oceans be. Much better, eh?
@D Ram I appreciate that but in all that time they never once experimented?
@Hellmuth Schreefel Agreed. But it is interesting how spotty the record is for all elasmobrachii due to the cartilage skeletons. I would say that this group is a poor example to hinge an "Anti-evolution" debate on; and yet the teeth alone can say so much about they development (or lack thereof). Thanks for putting it back to "the man" though.
@Neil Dewar @Nada Ahmed @KENNETH LANE Agreed on most points. There is a salt water fish we call a Coral Trout, but that is just the colloquial name. Otherwise, I figured I would take the piss for fun. There is the possibility of over-fishing the Ozarks ocean supply, but we just call that over-fossicing ;oP
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