Edwardchristhoper.blogspot.com - After a century of colourful guesses, CT scans have revealed what’s
really going on inside the nightmarish jaw of Helicoprion, a large, 270
million-year-old cartilaginous fish with an elaborate whorl of teeth set
in the middle of its mouth.
In 1899, Russian geologist, Alexander Petrovich Karpinsky, gave this
six-metre-long fish the name Helicoprion, meaning “spiral saw”, based on
a fragmentary fossil found in Kazakhstan. Because the saw he was
describing had been separated from the rest of the body, Karpinsky
couldn’t be sure where it would have fit, so initially he suggested that
it started in the fish’s mouth, and curled upwards along the snout as
an external coiled mass of fused-together teeth. Think a sawfish’s saw,
only curled upwards. Further guesses were made during the early 1900s by
a number of researchers from around the world, including American
palaeontologist Charles Rochester Eastman. Eastman had issues with the
idea that such an unwieldy apparatus could have possibly sat inside this
poor creature’s face. Publishing in a 1900 edition of
The American palaeontologist,
Eastman favoured the idea that the whorl protruded from somewhere along
the length of the fish’s back, acting as some sort of defensive
display, perhaps.
The many faces of Helicoprion.
Reconstructions of Helicoprion since 1899. Earliest models (a – d)
posited the whorl as an external defensive structure, but feeding
reconstructions dominate more recent hypotheses. Artwork © Ray Troll
2013.
A few years later, Karpkinsky followed Eastman’s train of though, and
suggested that the Helicoprion’s whorl could have formed part of the
animal’s tail, or perhaps extended from its dorsal fin, or sat lower
down on its back. In 1907, American ichthyologist, Oliver Perry Hay,
found a fossilised specimen that was still sitting in its natural
position, and judging from this, favoured the jaw theory. But did it sit
in the upper or lower jaw? And did it sit in both? Such questions were
impossible to answer with the few and fragmentary specimens these
researchers had to work with.
Regardless, the general consensus in the earliest hypothetical
reconstructions of Helicoprion was that this terrible, toothy whorl
surely served a defensive purpose. Later this century, this perception
has changed, and researchers moved towards the idea that the whorl was
used mainly for feeding, and therefore was associated with the
creature’s jaw.
A recent reconstruction of
Helicroprion with the tooth whorl sitting inside the mouth like a
tongue. Credit: Mary Parrish, Robert Purdy, Victor Springer and Matt
Carrano from the Smithsonian National Museum of Natural History
In 1950, a crucial Helicoprion whorl specimen was discovered by
Danish palaeontologist Svend Erik Bendix-Almgreen in the Waterloo Mine
near Montpelier, Idaho. Named IMNH 37899 and housed in the Idaho Museum
of Natural History, it was first described by Bendix-Almgreen in 1966.
It might have been seriously crushed and disarticulated, but along with
the 117 discernible serrated tooth crowns sitting on a spiral with a
diameter of 23 cm was some very telling cranial cartilage. This proved
for the first time that at least some of the whorl was contained inside
Helicoprion’s mouth.
But that didn’t limit the possibilities. Over the past fifty years,
researchers have suggested that the whorl extended awkwardly from the
lower lip, curling underneath the chin; sat inside the mouth where the
tongue should be; or perhaps sat further down towards the throat.
Leif Tapanila with two of the largest Helicoprion whorls in the world. Credit: Ray Troll
Now a team led by Leif Tapanila from the Department of Geosciences at
Idaho State University, and curator of the Idaho Museum of Natural
History, have gained unprecedented insight into the structure of
Helicoprion’s skull. IMNH 37899 was scanned using an ACTIS scanner at
the University of Texas High-Resolution X-ray CT Facility, and from
this, a scaled, 3-D computer-generated model of the animal’s skull was
generated.
“Our reconstruction posits that the tooth whorl is a singular,
symphyseal [fused] structure of the lower jaw that occupied the full
length of the mandibular arch,” the team reported in
Biology Letters
yesterday. This means that instead of extending past the lower jaw and
coiling underneath the chin, as had been previously suggested, the whorl
grew
inside the lower jaw. This way, just as sharks have
multiple rows of teeth that are continuously replaced, Helicoprion had a
partly concealed tooth factory that began near the area where the upper
and lower jaws meet, ran over the mouth wear the tongue would be if it
had one, and then into the cartilage supported by the lower jaw (see
first image).
“Continual growth of the whorl pushes the tooth–root complex in a
curved direction towards the front of the jaw, where it eventually
spirals to form the base of the newest root material, and this process
continues to form successive revolutions,” the researchers say. “At some
time, prior to a complete 360 degree evolution of spiral growth, tooth
crowns are concealed within tessellated cartilage on the upper jaw.”
As Helicoprion didn’t have any teeth on his upper jaw, the team
suggests that the predatory fish would have broken down its soft-bodied
prey, such as cephalopods and small fish, by repeatedly slicing them
with a single row of serrated teeth. When it closed its closed its lower
jaw, the whorl of teeth were pushed backwards, “providing an effective
slicing mechanism for the blade-like serrated teeth and forcing food to
the back of the oral cavity”.
Tapanila and colleagues suggest that the Helicoprion’s jaw could have
extended past 50 cm long, and some tooth whorls would have boasted some
150 teeth. The team also says that the creature is not a shark, as
others have assumed, but a chimaera (Holocephalan), which is a group of
cartilaginous fish also known as ratfish or ghost sharks that branched
off from the sharks 400 million years ago. “It was always assumed that
the Helicoprion was a shark, but it is more closely related to ratfish, a
Holocephalan,” says Tapanila. “The main thing it has in common with
sharks is the structure of its teeth, everything else is Holocephalan.”
Source : http://blogs.scientificamerican.com
