One of the most interesting areas in the evolution of animals is the crossover from reptiles to mammals. Numerous hybrids exist which continually switch back and forth between these categories of animal or have to be included in both at the same time. One of the disputed families is that of the Tritylodontidae. Following the initial discovery of their fossilized teeth in 1847 and 1866 in Württemberg, Germany, this was considered the oldest family of mammals.
Their skulls are actually very mammal-like and their dentition resembles that of present-day rodents. However, the teeth themselves look completely different. Due to the similarity between their diets, the teeth of the Tritylodontidae and rodents developed in parallel, but today's rodents are not descendent from this family of fossil animals. Various other characteristics – in the jaw joint, the small bones in the ear or on the shoulder girdle, for example – indicate that Tritylodontidae are actually reptiles.
To find out what food this family of animals preferred, the teeth of the Kayentatherium species were examined at the Steinmann Institute at the University of Bonn using a zoom microscope for large object fields from Carl Zeiss. The new system enables scientists to create very high-resolution images with outstanding depth of field. Accordingly, they can photograph key areas on the tooth's surface. These "facets" are formed during the process of chewing, by the contact of the teeth in the upper and lower jaws These areas are thus polished smooth and they reflect light.
Scratches also appear on the facets, which reflect the movements of the lower jaw during chewing. Through the analysis of these scratches, statements can be made about the chewing cycle, and conclusions drawn about the animal's preferred food. For the Kayentatherium, this was exclusively plants. In this respect, they are similar to modern-day rodents, who also eat primarily plants. While rodents move their lower jaw from back to front, the Kayentatherium always chewed from front to back. The molars feature steeply-angled sharp edges arranged in pairs, one behind the other; which are extremely effective in the breakdown of food. However, it is impossible to say which types of plant the Kayentatherium ate. The habitat of the time gives some clues in this respect. It largely consisted of Equisetum, a horsetail which blanketed the ground like grass.
January 16, 2013