Evolution of tooth structure in the Equoidea.

The study observes changes in the tooth structure of the Equoidea – a group that includes horses and their extinct relatives – over their evolutionary history, noting a more complex dental structure in later species.

Evolved Histological Structures

• The research focuses on changes in the histological structures (microscopic anatomy) of teeth in the Equoidea group. Over their evolutionary history, these structures have become increasingly complex, especially in the molars which have become hypsodont or elongated.
• The teeth of Hiracotherium, an early member of Equoidea, featured straight bands known as Hunter-Schreger bands. In later species, including the modern horse (Equus), this feature evolved into a more complex pattern.

Changes in Enamel Prisms

• The study also discusses changes in enamel prisms, the primary structural unit of tooth enamel. In Hiracotherium, the prisms had an arched form with an alternating pattern, while in Equus, they evolved to an oval form arranged in straight rows.
• The arrangement of prisms in rows separated by interprismatic sheets in Equus may have augmented the strength of the tooth enamel. This structural change aligns with the evolution of hypsodont molars.

Ameloblasts and Enamel Development

• The paper suggests that over the course of Equoidea evolution, the number of ameloblasts – cells responsible for enamel formation – increased, while their size became smaller.

Dentine Evolution

• Concerning dentine, the hard, dense, bony tissue forming the bulk of a tooth, the research shows that peritubular dentine, a highly mineralized type of dentine, was scarcely observed in Hiracotherium but became apparent in later species such as Mesohippus and Equus.
• A transition between these structural changes is noticeable in the molars of Mesohippus, which are not hypsodont.

Implication of the Study

• The findings suggest that these transformations in the histology of the Equoidea’s tooth structure possibly predated the changes in the overall shape of molar teeth during their evolutionary history.