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2 Odontology : A History of Teeth

       Peter P. Emily

       Lakewood, CO, USA

      The history of teeth mirrors the evolution of the world. As environmental changes occurred, teeth needed to adapt for survival. Except for fossilized contents, teeth were the best clue to the diets that changed dentition over time.

Primate and human dental evolution were nearly as varied as the evolution of carnivores and herbivores. Following, is some of the many evolutionary concepts of herbivore and carnivore evolution. The various theories on dental evolution hold that teeth evolved from scales of dermal denticles. Oskar Hertwig showed that skin denticles and teeth are homologous – skin denticles being very similar to tricuspid secodont teeth. Evidence of the ascending Paleozoic fish and reptiles show dermal plates throughout the palate that led to a primitive dental function. The mouth of Pelycosaur Edaphosaurus is an example of palatal nodules. The area of the mouth covered in teeth was greatest among these ancient reptiles. They were an adaptation for eating hard‐shelled prey, as seen in the Placodonts from the later Triassic period (see Figure 2.1).

      As teeth became more specialized, the fixation to the jaws by bony attachment evolved. In particular, Pleurodont, Acrodont, Thecodont, and Prothecodont dentition evolved. These forms of attachment still remain to the present day.

      In the long Carboniferous period, adaptation to land evolved among the oldest reptiles, the synapsid Pleosaurs. The Transition from gill‐breathing to lung‐breathing tetrapods took place toward the end of the Devonian period, 200 plus million years BCE. Therapsids bridged the chasm between the reptiles and primitive mammals. Therapsidia had several families that were partially herbivorous and partially carnivorous. Dicynodon had only one single tooth on each side of the maxilla. Some were toothless herbivores. A second subgroup of therapsids was composed of theridonts and cynodonts. They were possibly the first to show mammal‐like heterodontous dentition.

Figure 2.1 Pelycosaur Edaphosaurus.

      Source: A.S. Romer [1].

      The Pelycosaurs showed the beginnings of a segregation of the dentition into pre‐canine and post‐canine tooth series. Later, advanced synapsids possessed differentiation of tooth form in different regions of their dentition. In the late Triassic period, some forms of reptiles such as Ictidosauria, had mammal‐like teeth, yet belonged to a class of reptiles.

Pelycosaurs show clear evidence of heterodonty in some species with numerous teeth of equal size but with two larger anterior maxillary teeth, which points to the possible beginning of canine teeth. Dimetrodon and Edaphosaurus both possessed similar body structure but their mode of life differed appreciably since according to their dentition, Edaphosaurus was an herbivore, and Dimetrodon a predacious carnivore (see Figures 2.2– 2.4 ).

Early Evolution

From their reptilian ancestors, the earliest mammals inherited dentition subdivided into incisiform, caniniform as well as premolar and molariform teeth. Anterior post canines were lost and not replaced. Molariform teeth seemed to have been added to the distal arcade, a dental characteristic typical to mammals, though this characteristic was not seen in early mammals. Unlike tribosphenic teeth, their occlusal surfaces require extensive abrasion to come into occlusion. Marsupials and placentals differentiated from a common ancestral stock of mammals possessing tribosphenic dentition. This differentiation took place before the end of the early Cretaceous, approximately 100 million years BCE.

Figure 2.2 Dimetrodon.

      Source: A.S. Romer [2].

      Figure 2.3 Edaphosaurus (From Romer [1968] [2]).

      Source: Teeth and Dentition in the Different Groups of Vertebrates page 172, 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. Previously published in A.S. Romer an L.I. Price's article “Review of the Pelycosauria” in Geological Society of America Special publications, Volume 28. GS of A Special Publications allows “use up to three items (…figures…) from GSL published material without permission or charge with acknowledgement of source.”

      The 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.

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