- The earliest mineralized tooth-like structures are known as placoid scales, which appeared in primitive jawless fishes (ostracoderms) around 520 million years ago. Placoid scales were formed by odontogenic cells and showed rudimentary pulp cavities.

Early vertebrates and primitive teeth

- In some early vertebrates, like acanthodians and cartilaginous fish, denticles emerged. Denticles were similar to placoid scales but more complex, featuring dentine, pulp, and an enamel-like tissue called vitrodentine.

Rise of true enamel

- The first evidence of true enamel was found in early sarcopterygian fish in the Devonian period (419-359 million years ago). This enamel was formed by ameloblasts and composed of hydroxyapatite crystals arranged in a unique matrix.

Enameloid in amphibians

- Amphibians like the extinct labyrinthodonts developed complex teeth covered by enameloid, composed of dentine-like materials and apatite crystals.

Thecodont teeth and mammals

- Thecodont teeth are characterized by the attachment of the tooth to the surrounding bone through a socket. This tooth type originated in early reptiles and diversified throughout the reptile lineages, including the ancestors of mammals.

Heterodont and specialized teeth

- Mammalian ancestors began to show varying tooth shapes, leading to the development of heterodont dentition in Cynodonts, a key group leading to mammals. Different tooth types like molars, canines, and incisors emerged.

Complex adaptations in mammalian enamel

- As mammals continued to evolve, there were progressive advances in enamel's structure and chemical composition. Primates (including humans) further refined enamel complexity, resulting in dense mineralized enamel to withstand dietary abrasives.

Modern enamel's refinement

- Throughout mammalian evolution, enamel features underwent optimizations as part of dietary specializations, adaptations to the environment, and increased complexity of chewing structures.

In summary, tooth enamel evolution started with rudimentary mineralized structures in early fish, transformed into true enamel in sarcopterygians, refined with enameloid in amphibians, became more complex in reptiles, varied with heterodont dentition in mammal ancestors, and reached great complexity and diversity in modern mammalian lineages.