10.1. The Origin of Dental Tissue

The evolution of new types of hard tissues like cartilage, bone, dentin and enameloid was key to the diversification and success of chordates. This chapter reviews the origin of teeth, their diversification in the major groups of vertebrates and their organization in mammals. Human teeth are studied in detail in the next chapter.

Origins of hard tissues and teeth

As studied in the chapter on skull, the evolution of progressively harder tissues in chordates promoted the origin of vertebrae, skulls, jaws and teeth. Lampreys and hagfishes lack bones or a mandible but they have a cartilaginous skeleton. Lampreys use sharp “teeth” to attach to their prey, but these teeth are hardened by keratin and they lack any dentin or enameloid. They are analogous (similar function) but not homologous (originated by common descent) to our teeth.

Teeth with dentin and enamel evolved in fishes, and modern paleontological evidence is gradually revealing when and how. Several hypotheses have been proposed as to how teeth evolved, and they can be grouped into Outside-In and Inside-Out propositions.

The outside-in propositions are based on the finding that most cartilaginous fishes like sharks and rays have a tough skin covered with tiny tooth-like scales (placoid scales or dermal denticles). In most species, all dermal denticles are oriented in one direction, making the skin feel very smooth if rubbed in one direction and very rough if rubbed in the other. Each placoid scale has a pulp, covered by a softer and a harder mineralized layer of tissue that resemble dentin and enamel, respectively. These scales are proposed to have originated the teeth of tetrapods. The scales would have covered the entire body as a protective structure. At the edge of the mouth, however, these structures could become useful at improving the grip of a bite. They would then be selected to become more internalized, larger and assume specialized shapes for capturing prey and processing food.

The inside-out hypotheses suggest that teeth could have evolved in the mouth first, and then covered the body as dermal denticles. Several types of fishes have pharyngeal teeth, which are saliences on the walls of the pharyngeal arches (which also hold the gills) that help to grip and retain food items. Dentin and enamel could have evolved in the pharyngeal teeth of fishes, then spread through the mouth and the outer skin. The teleost bony fish Denticeps clupeoides has most of its head covered by dermal teeth, which were not present in the common ancestors of bony fish. This shows that dermal teeth could originate from oral teeth.

As an alternative to both directions of spread of teeth, dermal and oral teeth could have evolved independently. The evidence for oral teeth, however, seems to indicate a single origin.

Dermal and oral teeth are common and well developed in cartilaginous fishes. When uncovering older evidence, researchers have to carefully establish the evolutionary history of a fossil and the homology of the structures being compared across species. The current fossil record has many gaps and some evidence is tentative, but it seems most likely that dermal scales evolved much before teeth with dentin. Dermal scales have been found in various groups of jawless ostracoderm and placoderm fishes, whereas enameloid teeth are not found at all in most placoderms. Teeth with dentin and a pulp cavity are first seen in arthrodirans which form the most recent subgroup of placoderms. Pharyngeal teeth have been described from older jawless fishes (ostracoderms), but studies failed to find evidence of common structure between them and modern teeth and developmental studies did not reveal any major separation between the origin of oral teeth and dermal denticles. The most recent evidence is therefore favoring the outside-in hypothesis, in which dermal scales would have covered the body and started to specialize as teeth in arthrodiran placoderm fishes, which originated the cartilaginous and bony fishes, and all tetrapods.

Summary

Teeth with pulp, dentin and enameloid have evolved in jawed fishes. Other fishes and many invertebrates have tooth-like structures formed by keratin. Teeth may have evolved from dermal denticles or may have originated in the mouth and the earliest evidence of complete teeth in the mouth was found within the latest group of placoderms, which is thought to have originated cartilaginous fishes, bony fishes and tetrapods.

Key terms

Vertebrate, Cyclostomata, placoid scales, dermal denticles, tooth evolution, pharyngeal teeth, placoderm, ostracoderm, jawless fish, Arthrodira, pharyngeal arch

Credits

Figure 1 by Richard from Canton, United States - Knoxville zoo - chimpanzee teeth / File:Knoxville zoo - chimpanzee teeth.jpg, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=6012995

Figure 2 by Petter Bøckman - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=12990830

Figure 3 by Pascal Deynat/Odontobase - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=15564795

Figure 4 by The original uploader was BiotopAG at German Wikipedia - Transferred from de.wikipedia to Commons., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=13632385

Figure 5 by Epipelagic - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=24543340