Ancient sea saw arrives at the Museum

27 January 2015

Fossil Shizorhiza stromeri

The fossil nose with the saw teeth highlighted   

An exceptional example of the saw-like nose of an extinct fish has made its way into the Museum’s collection.

The 1.5-metre-long fossilised nose, or rostrum, belonged to a relative of modern rays called Shizorhiza stromeri. It ranged widely across an ancient ocean between 72 and 66 million years ago.

‘When we saw that a specimen of Schizorhiza with what appeared to be a complete rostrum, we were very excited,’ said Dr Zerina Johanson, the principal investigator of a NERC-funded research project into sharks and rays at the Museum.

The new specimen is the largest fossil of its kind. Fossil finds of S. stromeri are most often isolated teeth or fragments of the nose. The new specimen will provide researchers with information on the size of the fish, as well as insights into the evolution of its saw-like teeth.

‘In comparison to other rostrum-bearing sharks and rays, six metres would be likely. So it may well have been over a tonne in weight,’ said Dr Charlie Underwood from Birkbeck University, London, and a Museum collaborator.  

Sawfish

Reconstruction of Schizorhiza stromeri © C. Underwood

 

Sea saw

Along the edges of the rostrum in the new specimen are hundreds of small, sharp, ‘saw teeth’ that would have given the fish’s head the general semblance of a jagged chainsaw. Based on previous specimens, those teeth that make up the serrated blade of the saw are underlain by a tightly packed series of replacements, waiting to take their place at the surface.

Because most of the rostrum is preserved, Dr Johanson and her colleagues hope to find out more about where on the skeleton the saw teeth would have been produced during the fish’s lifetime.

‘We think the tip of the rostrum is a special region where tooth-like structures are generated, and we should be able to determine this from the new specimen,’ she said. Such research may also shed light on the evolution of teeth in general – both inside and outside the mouth.

These ‘teeth’ are not true teeth, but are actually thought to be highly modified versions of tooth-like structures (known as denticles) that cover the bodies of sharks and some rays. But unlike these denticles that normally aid in streamlining, the saw teeth of S. stromeri were used for feeding.

‘It is likely it slashed into shoals of small, soft-bodied fish and squid and sucked up the dead and injured,’ said Dr Underwood. ‘The teeth from the mouth are minute, under two millimetres long, and would have had little function other than gripping.’

Dangerous seas

Miners working on the phosphate plains of northern Morocco discovered the 1.5-metre long specimen as they excavated rock for the production of fertiliser. 

The rocks in which the fossil was discovered indicate that the fish swam in shallow coastal waters of the Tethys Sea, an ancient ocean that that existed around this time, over 66 million years ago. Sawfish alive today occupy a similar habitat.

The phosphate deposits also contain a diverse assemblage of marine life, including many large predators.

No conclusive fossils of S. stromeri have been found after this time, strongly indicating that – along with the famous demise of non-avian dinosaurs on land – this species didn’t make it past the mass extinction at the end of the Cretaceous Period. 

  • by Alex Riley