Evolution of the Mammalian Tooth

   

The primary origin of teeth was once believed to be derived from the dermal denticles. However, there has been much speculation about this hypothesis in the last 15 years (Witten et al., 2014). Now there are two main but opposing hypotheses regarding the origin of the teeth - the outside-in hypothesis and the inside-out. The outside-in hypothesis claims that, as previously stated, the teeth evolved from the denticles. Contrarily, the inside-out hypothesis states that they derived from an endodermal origin (from the neural crest mesenchyme) (Witten et al., 2014). The latter hypothesis believes that these endodermal teeth evolved independently among the different classes of vertebrates. This post will focus more on the evolution of mammalian teeth from our synapsid ancestors.

Early mammals evolved from synapsid reptiles - mammal-like reptiles evolved around 300 million years ago and the basal mammals about 210 million years ago (Lucas, 2006). The mouth of the early reptiles was extremely similar to extant reptiles and includes large numbers of simple, conical teeth. 

Differences Between Reptilian and Mammalian
Dentition
 https://jurassiccoast.org/fossilfinder/focus-on-fossils/mammals/

 Their teeth had no roots and were kept in position by simple fusion to bone (Lucas, 2006). They lost teeth periodically throughout their life, and these teeth could be continuously replaced.

As they began to evolve from their synapsid ancestors, the early mammals developed fewer teeth, housed in bony sockets along the jaw bone and that only replaced once (had two generations) (Lucas, 2006). There was also the development of a clear differentiation of tooth types including incisors, canines and post-canines.

Modern humans, and many other extant mammals, have these three classes of teeth. The incisors, found at the front of the jaws, have a simple shape. In most mammals they have come to a point at the bottom, but in higher primates (humans included) they are flattened to create a working surface (Lucas, 2006). These help us grip and tear our food.

Types of Teeth
http://biology-igcse.weebly.com/human
-teeth-and-dental-decay.html

The canines are used for ripping and tearing tough foods,
such as meats. Therefore they are not found in animals that are
strictly herbivorous - cows, sheep etc.

The postcanines - referred to as the molars and premolars - are the teeth at the back of the mouth that help us grind our food into smaller components to aid in swallowing and digestion.