Callorhinchus - Chimaeroid

Fossil photo | Wed 14 Dec 2011  

Anterior portion of a right palatine tooth plate of the chimaeroid Callorhinchus, collected from the lower part of the Gearle Siltstone. Chimaeroids are the sister group to the elasmobranchs (sharks and rays). Like sharks, chimaeroids have a skeleton of cartilage. Whereas sharks and rays have a highly mobile upper jaw (palatoquadrate), connected to the cartilaginous cranium via ligaments it is fused to the cranium in chimaeroids. The latter have a single opening for the gills on each side behind the skull whereas sharks and rays have separate gill slits for each gill arch. Sharks and rays replace their teeth with slightly larger ones throughout their lives whereas chimaeroids have three pairs of teeth (mandibular teeth in the lower jaw and vomerine and palatine teeth in the upper jaw) that grow continuously (like the front teeth of a rat) and are never replaced. The mid-Cretaceous rocks in the Giralia Anticline contain one of the most diverse chimaeroid assemblages known from anywhere in the world of this age.

The image below depicts the modern species Callorhinchus milli, the elephant fish, which occurs in moderately deep water along the southern coast of Australia, between Esperance and Sydney. The modern species of Callorhinchus are ‘living fossils’ having changed very little over the last 105 million years.

Elephant Fish (Australian Ghost Shark), Melbourne Aquarium
Elephant Fish (Australian Ghost Shark), Melbourne Aquarium, Photo by Fir0002, Copyright under GNU Free Documentation License.