Scientists have determined from fossil evidence that horses have existed for over 55 million years. The earliest horses from the eocene and miocene periods contained oral cavities of short crowned brachydont teeth better equipped for browsing soft, leafy vegetation, fruit and groundcover of the ancient woody forests. The horse evolved over time, and the current form and type of dentition in the horse is believed to have evolved about 15 to 20 million years ago. This evolution allowed adaptation that was better suited to grazing. The changes included a longer cheek teeth arcade and a deeper skull and jaw to better accommodate the hypsodont (high-crowned) tooth. The hypsodont tooth (a tooth that continues to erupt from the jaw over a very prolonged number of years) is better equipped to handle the increased wear that occurs when grinding the more abrasive grass containing silica and trace amounts of grit from the surrounding soil surface and plant roots.
Horses are among other things;
- Heterodont-meaning that they contain incisors, canines, premolars and molars
- Hypsodont-meaning that their teeth continually erupt
- Anelodont-meaning that their teeth will stop growing
- Diphysodonts-meaning that they contain both deciduous (baby teeth) and permanent teeth
There are quite a few anatomical disparities that allude to the differentiation between human teeth and those of the modern equidae. Horse’s teeth are hysodontic by nature which means their teeth continually erupt throughout most of their lifetime. We say most because horse’s teeth are also anelodontic by nature which means their teeth do stop growing. In the young horse the majority of the tooth is enclosed within the alveolar tooth socket deeply beneath the gum line to a very short root; this in the horse is called, the reserve crown. The rest of the tooth that is visible above the gum line is called, the crown of the tooth. Up until the age of about 7 years the reserve crown continually fills with secondary dentine due to stimuli and attrition. At this stage the cheek teeth along the arcades of a mature adult horse is usually in the vicinity of approximately 4 1/2 to 5 inches in length, which finishes the anelodontic stage that is characteristic of equine hypsodontic teeth. Through the grinding action required to chew, horse's teeth are abraded away at approximately 3-4 mm every year, however the hysodontic nature of a horses teeth allows for continual eruption of the reserve crown at the same rate that the teeth are abraded.
Horses contain 12 incisors, 6 in the upper maxilla, and 6 in the lower mandible, all of which are initially deciduous precursors, or baby teeth. These are used for prehension (picking food up so it can be chewed) along with the lips of the horse. The horse also contains up to 4 canine teeth, (not common in mares) two in the upper maxilla on either side of the jaw, and also two in the lower mandible. These canine teeth erupt and are situated in the inter-dental space between the incisors and the premolars of the horse. Horses also can contain up to 16 pre-molars, though this is rare indeed. It is much more common to see just 12-14 pre-molars in the majority of horses. This numerical discrepancy in premolars between individual horses is wholly dependent on how many 1st premolars an individual horse erupts. The 1st premolars are called wolf teeth, and a lot of horses do not have them, however many horses do have them. They'll often be a combination of either one wolf tooth (unilateral) or two wolf teeth (bilateral) and these are usually situated in the upper maxilla and labial to the 2nd premolar or 1st cheek tooth. We say usually, because on rare occasions the 1st premolars or wolf teeth can erupt in the lower mandibular arcade. On rare occasions an equine dentist will come across a horse that has a full complement of 16 premolars, which coincides with a full complement of 1st premolars or wolf teeth, with two (bilateral) each in the maxillary, and two (bilateral) each in the mandibular arcades.