Teeth

Differences in Dental Growth Between Mammals and Other Vertebrates

  • Mammal Tooth Replacement

    • Mammals grow and replace their teeth differently than other vertebrates.

    • Notable lack of continuous replacement:

    • Mammals have one set of baby teeth.

    • Adult teeth replace baby teeth during youth.

    • Once an adult tooth is lost, it does not regrow.

  • Comparison with Reptiles

    • Dinosaurs share similarities in tooth replacement with reptiles like crocodiles and sharks.

    • Reptiles continuously grow new teeth throughout their lives:

    • When a tooth falls out, a new tooth is ready to replace it immediately.

  • Tyrannosaurus rex Tooth Replacement

    • Dinosaurs such as Tyrannosaurus rex replaced each tooth approximately every 1.5 to 2 years.

    • Tooth replacement occurred irrespective of wear or damage.

    • New teeth develop beneath older teeth.

    • Roots of older teeth dissolve and are absorbed back into the jaw (process referred to as resorption).

  • Tyrannosaurus Jaw Anatomy

    • Example of a lower jaw of a tyrannosaur, missing teeth with new teeth growing in.

    • Once the roots are almost resorbed, the old tooth becomes loose.

    • The loose teeth remain until they naturally fall out during activities such as feeding.

    • Feeding sites often reveal shed teeth among the skeletons of prey, providing fossil evidence.

    • Shed teeth may be broken or fully resorbed.

Hadrosaur Dental Structures

  • Hadrosaur Jaw Examination

    • Display of one half of the lower jaw of a hadrosaur, showcasing features:

    • Outward appearance reveals a wide, flat surface for grinding plant material.

    • Inner side displays multiple layers of teeth—at least three visible rows.

  • Teeth Count in Hadrosaurs

    • Large hadrosaurs typically had around 1,000 teeth at any one time, including:

    • Active teeth used for eating.

    • Replacement teeth beneath the active teeth.

  • Hadrosaur Dietary Adaptations

    • Hadrosaurs were herbivorous, specifically adapted to grind tough plant materials.

    • Their teeth experience accelerated wear compared to carnivorous species.

    • Consequently, hadrosaurs frequently replaced their teeth with a higher count of replacement teeth growing beneath each working tooth.

  • Examples of Hadrosaur Teeth

    • Complete hadrosaur tooth used for grinding before significant wear occurs.

    • Worn down teeth shown, illustrating the uneven wear due to the enamel (outer layer) being slower to wear than dentin (inner layer).

  • Dental Battery Concept

    • The configuration of the teeth creates a dental battery.

    • Teeth are inset from the outer surface to prevent chewed plant material from falling out of the mouth.

Comparison Between Mammals and Dinosaurs

  • Contrast Between Mammalian and Dinosaur Dentition

    • After losing baby teeth, mammalian herbivores (e.g., antelopes) develop one set of tall adult teeth;

    • Hadrosaurs have multiple rows of tightly packed smaller teeth as a configuration for grinding, similar adaptations found in some mammals.

  • Convergent Evolution

    • Although dental batteries exist in both hadrosaurs and ceratopsians, they evolved independently rather than from common ancestors.

    • Primitive members of these lineages lacked dental batteries, indicating this feature is a result of convergent evolution, a concept to be explored later in the course.

Functional and Structural Adaptations of Teeth

  • Grinding Mechanics in Hadrosaur vs. Ceratopsian Jaws

    • Hadrosaur jaw movements occur side to side and forward/backward;

    • The grinding surfaces are predominantly horizontal.

    • Ceratopsian jaws present a different functional morphology;

    • Grinding surfaces are nearly vertical, featuring sharp edges formed by enamel.

    • These edges cut plants before the smaller pieces are ground by the flat grinding surfaces of the dental battery.

Review of Key Concepts

  • Identification of Animals with Dental Batteries

    • Check which animals possess dental batteries:

    • A, Diplodocus

    • B, Triceratops

    • C, Tyrannosaurus

    • D, Corythosaurus

    • Correct answers include B (Triceratops) and D (Corythosaurus); both possess dental batteries, while having evolved these features independently.

Examination of Various Dinosaur Teeth

  • Different Types of Dinosaur Teeth

    • Pachycephalosaurs and Ankylosaurs:

    • Characterized by small leaf-shaped teeth.

    • Ceratopsians:

    • Exhibit a sharp cutting surface with a flat grinding surface at the back of the tooth.

    • Hadrosaur Teeth:

    • Part of the dental battery, closely packed and designed for grinding.

    • Sauropod Teeth (e.g., Diplodocus):

    • Peg-like structure suitable for their feeding habits.

    • Carnivorous Theropod Teeth:

    • Example from a tyrannosaur showcases serrated edges ideal for cutting flesh.

    • Giganotosaurus teeth display long roots attaching them into the jaw.

    • A large tyrannosaur tooth illustrates a prominent serrated cutting edge, enhancing carnivorous feeding efficiency.