Fossil Brain Case Reveals New Insights About Ancient Shark

by AMNH on Mar 19, 2018 2:37 pm

A CT scan showing the unique serrated edges of a Carcharopsis tooth, measuring approximately 5 millimeters. The blue lines are canals within the tooth.
Allison Bronson

A new study led by Museum scientists provides new insights into a 320 million-year-old shark that was previously only known by its unusual teeth and fragmented jaws. A newly uncovered fossilized brain case of Carcharopsis wortheni, found in the Fayetteville Shale of Arkansas, is helping researchers better place the ancient shark in the tree of life.

Carcharopsis lived during a critical point in evolutionary history, following the end-Devonian extinction event, when nearly 95 percent of vertebrate species went extinct. The late Paleozoic shark was originally described in 1843 based on its distinctive serrated teeth, a feature that is common in modern sharks but rarely found in early shark specimens.

Great white shark swims through a school of fish.

Great white sharks, Carcharodon carcharias, are named for their serrated (carcharos) teeth (odon).
Courtesy of Terry Goss/Wikimedia Commons

“They look a little like what you’d see in a great white shark, but 320 million years old and with different enamel,” said lead author Allison Bronson, a Ph.D. student in the Museum’s Richard Gilder Graduate School. “This is really early to see serrated teeth.”

CT scan of fragmented Carcharopsis skull.

The first known cranium fossil belonging to Carcharopsis was discovered in 2007 by Royal Mapes, a research associate at the American Museum of Natural History.
Allison Bronson

The first known cranium belonging to the extinct shark was discovered in 2007 by Royal Mapes, a retired Ohio University professor and Museum research associate, who donated the specimen along with some 540,000 other fossils to the Museum. Mapes is a coauthor on the new Carcharopsis study, published in the journal Papers in Palaeontology, along with Division of Paleontology Curator John Maisey.

[Watch the video below for more about the Mapes collection at the Museum, including fossil shark specimens.]

[MUSIC PLAYING]

NEIL LANDMAN (Curator, Division of Paleontology): This was a tremendous collection of around 550,000 specimens of marine invertebrates and vertebrates.

Royal Mapes and his wife Gene Mapes, they were professors at Ohio University. And over the last 45 years, they've been collecting fossils.

Most of the fossils they collect are marine fossils in Oklahoma, Arkansas, Texas. We don't have a good representation of that in our collections already.

Ammonoids belong to the group cephalopods, part of the mollusks. And the cephalopods are distinguished from the clams and the snails in that cephalopods learned how to swim in the water. So when you think of cephalopods, there's octopus, there's squids, there's the pearly nautilus. And the ammonoids are a big part of it. But that group of cephalopods is now extinct. What we see is their beautiful outer shell. But there is a complex internal anatomy, including the jaws.

And in the Mapes collection, there are many specimens that have preserved their jaws inside the shells. It's a very rare occurrence. It takes very unusual circumstances. But it's just perfect for our research. And it lets me understand the behavior and anatomy of ammonoids a lot better.

JOHN MAISEY (Curator, Division of Paleontology): Royal and his wife have an innate ability to collect and find, and collect fossil sharks and other fossil fish in remarkably good preservation in places where nobody else has been able to find them.

These are almost three dimensional. They're not flat and crushed and broken. They're beautifully-preserved fossils that we can then scan, and in the computer, we can extract the skeletal structures in a great deal of detail.

This is a complete head of a little tiny shark. Inside this rock, there's every little piece of all these gill arches, and all the rest of the skeleton. The jaws and other things. Everything's preserved in exquisite detail, but you can only see it by scanning it and processing the scan using computer technology. And that's what we've been doing.

This is one of these fossils from Arkansas, which I think should be renamed Sharkansas, that is so spectacular. Even though it doesn't look like, in scientific terms, it's really significant. It's a major discovery.

It's not just getting a whole bunch of new fossils, but it's what they are, and what they represent that's really important to us.

LANDMAN: You always look to future generations.

I see the value of this collection, and the treasures in this collection, that graduate students in the future, curators in the future, are going to be able to mine this collection for generations to come.

[MUSIC PLAYING]

The researchers used high-resolution computed tomography (CT) imaging to examine the cranium, a tooth, and an isolated tooth base. Using the scans, they were able to reconstruct the internal canals of the teeth for the first time and found that these are similar to those found in modern sharks.

The arrangement of the shark’s blood vessels—also revealed through CT scans—suggests that Carcharopsis was probably closely related to the group of ancient cartilaginous fish from which today’s sharks and rays evolved. However, more complete fossils are needed to firmly position it in the tree of life.