By Ella Davies Reporter, BBC Nature
An ancient spiral-toothed fish has been reconstructed from fossil evidence by scientists.
US researchers used CT scans to build a computer model of what Helicoprion looked like and how it ate.
They were also able to resolve a continuing puzzle over whether the unique saw-like spirals were located inside or outside the mouth.
The findings show the animals were more closely related to modern chimaeras, or ratfish, than sharks.
The study is published by researchers from Idaho State University in the Royal Society journal Biology Letters.
The university’s Museum of Natural History has the largest public collection of fossilised Helicoprion in the world.
The fish lived 270 million years ago but because they were largely formed from cartilage, which does not preserve well, their fossil record comprises unusual spiral structures.
Referred to as “whorls”, these features have been compared to spiralling saw blades and have puzzled the scientific community for over a century.
Early theories suggested that they were actually used for defence and were located on the fish’s upper or lower jaws, or even the dorsal fin.
In order to solve the mystery, Dr Leif Tapanila and colleagues investigated the most complete fossil in the collection.
The fossil, discovered in Idaho, has a whorl measuring 23cm with 117 individual teeth. Unlike other specimens, the fossil also includes impressions of the cartilage structures.
The team used a high-powered CT scan, which uses X-rays to create a detailed computer image, in order to fully analyse what was inside the rock.
“When we got the images back, we could easily see that we had the upper and lower jaw of the animals, as well as the spiral of teeth,” said Dr Tapanila.
“For the first time we were able to very clearly image how that spiral of teeth relates to the jaw.”
The scientists found that the spiral was connected to the fish’s lower jaw, in the back of the mouth.
“Imagine that… instead of having a tongue, you have this large spiral of teeth,” Dr Tapanila explained.
“Only maybe a dozen teeth are poking up out of your lower jaw so you can bite.”
“The rest of those teeth are stored inside and are not being used, those are your baby teeth – the teeth you had when you were younger.”
Dr Tapanila said this discovery supports the argument that unlike sharks, which constantly replace their teeth, Helicoprion retained its teeth permanently.
Rest of story – http://www.bbc.co.uk/nature/21589719