In 1824, Étienne Geoffroy Saint-Hilaire, dissecting embryos of rose-ringed parrots, testified “to the existence of a dental apparatus in birds,” according to the subtitle of his paper. Although he used the term “tooth”, he described simpler rudiments than those of mammalian teeth. That’s why they were quickly called “pseudodents”. But apart from terms, what is their true nature? Are these rudimentary teeth or something else entirely? Recent research by Stefan Lurian and his team at the Free University of Brussels on parrots, the bird family that includes parrots and parrots, provides decisive answers.
Paleontologists have described several bird fossils with teeth, such as the mythical Archeopteryx – and a consensus emerged that the loss of the latter occurred at least six times in the Cretaceous period between 100 and 80 million years ago. In an article published about ten years ago, Antoine Louchard and Laurent Viriot of the Ecole Normaleum in Lyon outlined all the hypotheses that explain this particular phenomenon.
Teeth, so diverse today in mammals, have been replaced in birds by the beak/stomach pair. The beak, or rhamphoteque, is used to capture food, which is then stored in the crop before the gizzard, which contains the pebbles (or gastroliths), mechanically grinds it up. Three embryological mechanisms appear to be central to the formation of this beak: delayed tooth development—odontogenesis—by inactivating certain key genes, such as those responsible for the enamel layer; loss of contact between the cell wall of the oral cavity – the oral epithelium – and the underlying supporting tissue, the dental mesenchyme, from which teeth are formed in mammals; a change in the function of genes that pass from odontogenesis to the formation of a ramphoteque.
The loss of teeth brings undeniable advantages to flying animals with a high metabolism. This reduces body weight and, given the lightening of the head, leads to a shift in the center of gravity down and towards the back of the body. No chewing is involved in the mouth, the quick capture of food allows for immediate flight, which protects the bird from possible predators. Efficient digestion of food by the stomach shortens the duration of digestion, which seems to help meet the frequent nutritional needs associated with a high metabolism. Finally, the plasticity of the ramfoteka allows rapid evolutionary acquisition of specialization. Under the same name “beak” what is the difference between the beak of an eagle, a flamingo, a pelican, a duck or a heron! Changing a beak is much easier than changing teeth.
But why pseudo teeth? In some fossil birds, they are much more impressive than in parrots. This is the case for the odontopterygiformes, very large pelagic birds that lived from the beginning of the Tertiary to the end of the Pliocene, between 60 million and 2.5 million years ago. Pseudo-teeth, which bristle tomi (cutting parts of the beak) of one of them, sea pelagornis, attracted the attention of Antoine Louchard and Laurent Viriot, who wondered if anatomical and histological examination could explain their origin. However, each false tooth is built on a bone outgrowth, and there is a periodicity of a certain pattern – between two large false teeth, three small ones, one medium, three small ones – repeated at least six times on each half-jaw. Such an organization necessarily requires precise embryonic control.
Thus, Antoine Louchart and Laurent Viriot suggested that in birds with pseudo-teeth, oral epithelial cells remain able to interact with underlying cells and that the first stages of tooth development are not inactivated. Specifically, gene expression BMP4, which plays a role in vertebrate skeletal and dental development and is also known to be a major contributor to bird beak morphology, as has been demonstrated for Darwin’s finches. However, with one difference: in the absence of dental mesenchymal cells – odontoblasts – there can be no formation of teeth. Then the epithelium will directly interact with bone cells, which will lead to the development of bone tubercles. In other words, instead of interacting with odontoblasts, epithelial cells will interact with bone mesenchymal cells, osteoblasts. Thus, the teeth would disappear due to the lack of contact between the two tissues, but the genetic signals would still be present and directed towards structuring the pseudo teeth. Obviously, it is impossible to get evidence of gene activation from a fossil.
It was then that the study of pseudo-teeth in parrots by Stefan Lurian’s group attracted worldwide attention. From a histological point of view, there is a similarity between P. mauretanicus and those of these birds which also correspond to extensions of the deep bones. Embryologically, we do not see the folding of the epithelium, which is characteristic of the formation of a tooth germ. And from a molecular point of view, it is surprising to find the expression of certain classical genes for the development of mammalian odontogenesis, such as Shh, BMP4, Pitks2 and Pax9.
All this indeed testifies to the high conservatism of the same gene expression program. In mammals, it builds teeth. In birds, it deviates and forms ramfoteka and false teeth. Thus, the latter are extensions of the mandibular bone formed by genes that produce teeth. We then understand why things could have evolved so quickly, given that all the necessary genetic substrate was already present. With the same molecular toolkit, one can build teeth or a beak, with or without pseudoteeth.