plant‐eating ancestral reptilian dinosaurs had leaf‐shaped homodont dentition. Since they were browsers, the need for broad occlusal surfaces was not seen until the evolution of grazers.
Early mammals of the Triassic period wore the occlusal surfaces into occlusion as did their reptilian contemporaries. They had dentition unlike teeth of the tribosphenic pattern. Their occlusion required a significant amount of abrasion before they matched. The earliest mammals investigated by Moss and Pool lacked the prismatic structure of enamel.
The earliest mammal molariform teeth, prior to development of a true tribosphenic molar, produced a different action. This was more of a shearing action between the crests of the trigon and stylar shelf and those of the talonid and trigon. Food could still be crushed between these segments. By the early Cretaceous period,100 million years BCE. Mammals with fully developed tribosphenic dentition were in existence.
Figure 2.4 Snout fragment of an ichthyosaur (After Quenstedt from Peyer [3] [1937]).
Source: Teeth and Dentition in the Different Groups of Vertebrates page 144, as published in Comparative Odontology, by Bernhard Peyer, translated and edited by Rainer Zangerl, with a forward by Alfred S. Romer with permission. University of Chicago Press 1968. Library of Congress Catalog Card number 66‐20578. Originally appeared in Handbuch der vergleichenden Anatomie der Wirbeltiere Gesamt‐Inhaltsubersucht published originally by Urban & Schwarzenberg and acquired by Elsevier.
Even though mammals existed as far back as 200 million years BCE, most of the dentition had been extensively altered during the previous 100 million years BCE, or more. The extinction of the dinosaurs around 65.5 million years BCE precipitated a rapid evolution in mammalian dentition.
Enamel
One of the dental features distinguishing the dentition of reptiles from mammals is their enamel formation. Reptiles have non‐prismatic enamel while mammals have prismatic enamel. Reptile enamel is characterized by greater structural variety and lesser complexity than mammals. The evolution of non‐prismatic enamel to prismatic enamel was probably the evolution of Tomes' processes in modern man. This process is the change from flat secretory surfaces to a complex assortment of enamel corrugation in mammals, which secretes prismatic enamel.
How did mammalian dentition evolve from these prehistoric reptiles? Many theories have been proposed. Unfortunately, an adequate picture of the diversity of organization and dental evolution that existed in this class of animals during the Mesozoic era is not clear.
Dental Evolution
The two major theories of evolution of prehistoric reptilian homodont dentition to multi‐cusped teeth were the Concrescence Theory and the Differentiation Theory as discussed by Owen (Owen 1845). The Concrescence Theory states that multi‐cusped mammalian molars are formed by fusion of a number of simple conical teeth. Support for this theory came from multi‐tuberculates in which molars had distinct cusps. It was assumed that each cusp represented a reptilian tooth, as later multi‐tuberculates had more cusps than older species. The Cope–Osborn Differentiation Theory stated that the most complex mammalian molar originated from a single‐cusped reptile tooth with formation of cusps in front and behind the original. This led to the additional cusps changing position relative to the main cusp in order to form a triangle. A heel is added by evolutionary differentiation to the triangle in the lower molar, such as the lower carnassial first molar.
One significant characteristic of the ancient carnivores is the size of the carnassial teeth in relation to their diets. In general, most carnivore dentition has changed little when compared with herbivores. Charles Darwin (1809–1882) initiated much controversy concerning evolution, including that of teeth. Darwin's early writing was a text entitled “Zoonomia,” in which was presented the theory of inheritance by acquired characteristics. Though Zoonomia was based more on speculation than science. His fossil records of the horse are one of the classic fossil records showing evolution of teeth. Darwin's “The Origin of Species” was a monumental look at the evolution of species through natural selection. The documentation of the evolution of teeth grew immeasurably in the nineteenth century. It was profoundly influenced by the Cell Theory, with J. Muller and Oskar Hertwig provoking thought.
The most extensive research and publication of the morphology and evolution of teeth was produced by Richard Owen in 1856. He classified over 13,000 specimens at the British Museum. His publication dealt a great deal with teeth, evolution and hereditary changes. Owen was most remembered for his disputed belief in Darwin's theory of evolution and his coining of the word “dinosaur.” Owen's two volumes, entitled “Odontology,” published in 1840–1845 became an invaluable research tool for more than 150 years. Frank Colyer's, “Variation and Diseases of the Teeth of Animals” in 1936 is one of the most complete treatises of Odontology.
Molar Evolution
The evolution to the Tribosphenic molar was the signifying molar form particular to mammals. With the exception of the Platypus and the Echidna, all living mammals are descendants of a common stock characterized by tribosphenic dentition. In 1936, Simpson coined the word “tribosphenic” as a replacement for the Cope‐Osbornian “tritubercular” and “tuberculo‐sectorial” descriptions used respectively for essentially euthemorphic upper and lower molars.
In contrast, tribosphenic dentition is not found in any of the mammals that appear in the late Triassic, 190 M.Y.B.C.E. The fossil record of the Jurassic period, 180–35 million years BCE shows instances of convergence. Only in the early Cretaceous, 135–25 million years BCE was there a definite record of mammalian tribosphenic dentition. In the middle to late Cretaceous period, the fossil record shows a major adaptive radiation to this kind of dentition.
By the early Cretaceous period, mammals with fully tribosphenic molars were in existence and had begun to diversify. These early mammals were small carnivores and omnivores. None showed modification of the dentition characteristic of mammalian herbivores, nor did the first mammals with tribosphenic dentition show modifications of mammalian herbivores.
The tribosphenic dentition of early cretaceous mammals was not adapted to the demands of an herbivorous diet. The origin of tribosphenic dentition was one of mammalian adaptation's major changes in the terrestrial flora during this period. The herbivores of the Cretaceous period were rodent‐like creatures that branched off the early mammalian stock before the origin of the tribosphenic dentition. The Cretaceous period was a long 71‐million‐year period, 135–66 million years BCE. This is one‐third the total history of the class of Mammalians. A more rapid dental evolution began during the Cenozoic, 64 million years BCE and evolved without dinosaurian competition.
In 1907, Osborn‐Cope presented four principle tenets of molar evolution that were later revised to a degree by Hershovitz:
First Principle: The primitive tritubercular type. The tritubercular was ancestral to many if not all of the molar teeth.
Second Principle: The origin of the tritubercular type from a single reptilian cone by the addition of lateral denticles.
Third Principle: Cusp addition or differentiation. New denticle cusps or smaller cones on the sides of original reptilian cone added by budding or outgrowth.
Fourth Principle: Reversed upper and lower triangles. In the lower molars, the reptile cone is external and
