Post by harrypristis on Aug 5, 2007 13:52:39 GMT -5
Jaws, Teeth of Earliest Bony Fish Discovered
John Roach
for National Geographic News
August 1, 2007
Fossils of sardine-size fish that swam in ancient oceans are the earliest
examples of vertebrates with teeth that grow from their jawbones, according
to new a new study.
The fish, which lived 420 million years ago, are a "very modest" beginning
for the jaw-and-tooth pattern widespread in nature today, said study
co-author Philippe Janvier, a paleontologist at the National Museum of
Natural History in Paris, France.
"It's really the first evidence that we have of the earliest bony fishes-the
earliest ancestors of all the fishes that have the[ir] bones and teeth
implanted in the bones of the jaw," Janvier said.
Modern bony fish such as cod, herring, and coelacanths have this tooth
arrangement. So do tetrapods-four-limbed creatures such as frogs,
crocodiles, and humans, which are all descendants of bony fishes. (Related:
"Fossil Fish With 'Limbs' Is Missing Link, Study Says" [April 5, 2006].)
When a bony fish or a tetrapod loses a tooth, a new one grows from the bone
below the void, whereas other jawed vertebrates, such as sharks, have teeth
that grow from inside their gums. Sharks have skeletons of cartilage instead
of bone.
Shark teeth are lined up in "families." New teeth grow at the inner end of
their respective tooth family, and old teeth fall off at the end of an
inside-out progression-similar to a conveyor belt.
Though fossil representatives of the earliest members of each of these
living groups are well known, the earliest stages of jawed vertebrate
evolution presents a fuzzier picture.
The new fossils help clarify these questions, Janvier said.
Transition Fossils
The researchers discovered the telltale bony fish fossils among fragments
collected on the Swedish island of Gotland in the Baltic Sea. Other
fragments came from boulders carried to Germany by glaciers a few million
years ago.
Some of the fossils belong to the species Andreolepis hedei and others to
Lophosteus superbus, fish previously identified by scale and head bone
specimens.
Whether they were truly bony fish or more like sharks was an open question,
however.
Two of the new fossils suggest a direct link to bony fish: tooth-bearing
jawbones.
What's more, the bones show a tooth pattern that is in between the tooth
rows of sharks and bony fishes.
Though these ancient bony fish teeth grew from a bone, old teeth remained
attached to the bone. New, larger teeth grew at the inner end of each tooth
file.
"It shows a sort of transition between the shark condition and the bony fish
condition," Janvier said.
Within 20 million years after Andreolepis hedei and Lophosteus superbus
lived, the first bony fish with much larger teeth characteristic of modern
bony fish and tetrapods appear in the fossil record. This was during the
Devonian period, 416 to 359 million years ago. (Related: "Ancient Fish
Fossil May Rewrite Story of Animal Evolution" [October 18, 2006].)
"That's very important because it allowed the bony fishes to become
predators," Janvier said.
Sharks also existed in the Devonian, but they were "humble compared to the
bony fishes," he said.
The first bony fishes probably ruled the seas, rather than sharks, because
the bony fishes' teeth lasted a longer time in the jaw.
"Then, later on, the sharks ... became much larger and big predators,"
Janvier added.
The study appears in tomorrow's issue of the journal Nature.
Sorting Characteristics
Michael Coates, a biologist at the University of Chicago in Illinois who
studies early vertebrate evolution, was not part of the research team.
He said the discovery of rare fossils like the ancient bony fish allows
scientists to sort general, primitive characteristics of all jawed
vertebrates from the more specialized features that distinguish sharks from
bony fishes.
The new study, he noted, clearly shows that Andreolepis and Lophosteus are
bony fishes, but their tooth pattern raises a question about what makes a
shark a shark.
"Growing teeth in this serial manner around the jaw margin-which once upon a
time looked like it was unique to sharks-now looks like it is a general
system."
news.nationalgeographic.com/news/2007/08/070801-jawed-fish.html
John Roach
for National Geographic News
August 1, 2007
Fossils of sardine-size fish that swam in ancient oceans are the earliest
examples of vertebrates with teeth that grow from their jawbones, according
to new a new study.
The fish, which lived 420 million years ago, are a "very modest" beginning
for the jaw-and-tooth pattern widespread in nature today, said study
co-author Philippe Janvier, a paleontologist at the National Museum of
Natural History in Paris, France.
"It's really the first evidence that we have of the earliest bony fishes-the
earliest ancestors of all the fishes that have the[ir] bones and teeth
implanted in the bones of the jaw," Janvier said.
Modern bony fish such as cod, herring, and coelacanths have this tooth
arrangement. So do tetrapods-four-limbed creatures such as frogs,
crocodiles, and humans, which are all descendants of bony fishes. (Related:
"Fossil Fish With 'Limbs' Is Missing Link, Study Says" [April 5, 2006].)
When a bony fish or a tetrapod loses a tooth, a new one grows from the bone
below the void, whereas other jawed vertebrates, such as sharks, have teeth
that grow from inside their gums. Sharks have skeletons of cartilage instead
of bone.
Shark teeth are lined up in "families." New teeth grow at the inner end of
their respective tooth family, and old teeth fall off at the end of an
inside-out progression-similar to a conveyor belt.
Though fossil representatives of the earliest members of each of these
living groups are well known, the earliest stages of jawed vertebrate
evolution presents a fuzzier picture.
The new fossils help clarify these questions, Janvier said.
Transition Fossils
The researchers discovered the telltale bony fish fossils among fragments
collected on the Swedish island of Gotland in the Baltic Sea. Other
fragments came from boulders carried to Germany by glaciers a few million
years ago.
Some of the fossils belong to the species Andreolepis hedei and others to
Lophosteus superbus, fish previously identified by scale and head bone
specimens.
Whether they were truly bony fish or more like sharks was an open question,
however.
Two of the new fossils suggest a direct link to bony fish: tooth-bearing
jawbones.
What's more, the bones show a tooth pattern that is in between the tooth
rows of sharks and bony fishes.
Though these ancient bony fish teeth grew from a bone, old teeth remained
attached to the bone. New, larger teeth grew at the inner end of each tooth
file.
"It shows a sort of transition between the shark condition and the bony fish
condition," Janvier said.
Within 20 million years after Andreolepis hedei and Lophosteus superbus
lived, the first bony fish with much larger teeth characteristic of modern
bony fish and tetrapods appear in the fossil record. This was during the
Devonian period, 416 to 359 million years ago. (Related: "Ancient Fish
Fossil May Rewrite Story of Animal Evolution" [October 18, 2006].)
"That's very important because it allowed the bony fishes to become
predators," Janvier said.
Sharks also existed in the Devonian, but they were "humble compared to the
bony fishes," he said.
The first bony fishes probably ruled the seas, rather than sharks, because
the bony fishes' teeth lasted a longer time in the jaw.
"Then, later on, the sharks ... became much larger and big predators,"
Janvier added.
The study appears in tomorrow's issue of the journal Nature.
Sorting Characteristics
Michael Coates, a biologist at the University of Chicago in Illinois who
studies early vertebrate evolution, was not part of the research team.
He said the discovery of rare fossils like the ancient bony fish allows
scientists to sort general, primitive characteristics of all jawed
vertebrates from the more specialized features that distinguish sharks from
bony fishes.
The new study, he noted, clearly shows that Andreolepis and Lophosteus are
bony fishes, but their tooth pattern raises a question about what makes a
shark a shark.
"Growing teeth in this serial manner around the jaw margin-which once upon a
time looked like it was unique to sharks-now looks like it is a general
system."
news.nationalgeographic.com/news/2007/08/070801-jawed-fish.html