How Did Birds Lose Their Teeth?

Modern birds are completely toothless, yet the vast majority of their ancestors possessed teeth. As a result, the evolutionary transition from toothed jaws to edentulous beaks is often regarded as one of the most representative examples of trait loss in evolution.

However, tooth evolution in Mesozoic birds was far more diverse than is often appreciated. Different groups varied considerably in the number of teeth on the premaxilla, maxilla, and dentary. Some species were completely toothless, whereas others retained numerous teeth, and tooth crown size also differed substantially among taxa.

Phylogeny of Mesozoic Birds. Green indicates basal birds, blue indicates enantiornithines, and purple indicates ornithuromorphs. Yellow stars denote completely edentulous genera.（Image source：Zhou, Y et al. (2026).， CC BY-NC-ND 4.0 。）

Previous studies generally divided the evolution of avian dentition into three stages: fully toothed, partially toothed, and completely toothless. It is now recognized that complete tooth loss evolved independently in multiple Mesozoic bird lineages, including confuciusornithids, enantiornithines, and ornithuromorphs. However, exactly how tooth number and tooth size evolved in different jaw bones has remained poorly understood.

As research has progressed, two long-standing hypotheses have largely been rejected: first, that teeth disappeared because they made flight too heavy; and second, that a persistent evolutionary trend continuously drove tooth reduction throughout avian evolution. Nevertheless, detailed analyses of how tooth number and crown size evolved across different jaw bones have remained limited.

To address this question, researchers compiled a large dataset of well-preserved Mesozoic bird fossils and quantified tooth number on the premaxilla, maxilla, and dentary, as well as average tooth crown size. They then reconstructed the evolutionary history of tooth reduction and tooth loss using phylogenetic comparative analyses together with multiple evolutionary models.

Archaeopteryx（Image source：H. Raab， CC BY-SA 3.0 。）

The study examined 31 well-preserved fossil specimens representing 21 Mesozoic bird genera, including basal birds, enantiornithines, and ornithuromorphs, and encompassing both toothed and toothless taxa.

Because dental information remains incomplete for many Mesozoic bird species, the analyses were performed primarily at the genus level. For Archaeopteryx, whose internal taxonomy remains controversial, different specimens were treated as separate operational species in order to reconstruct patterns of tooth evolution within the phylogeny.

The analyses showed that the common ancestors of the major Mesozoic bird lineages still possessed teeth, with no evidence supporting the existence of an edentulous common ancestor. In other words, toothless birds did not inherit their beaks from a single toothless ancestor; instead, edentulous beaks evolved independently in multiple evolutionary lineages.

Confuciusornis（Image source：paleobear， CC BY 2.0 。）

Based on the reconstructed evolution of tooth crown size, the researchers identified at least four independent events of complete tooth loss, represented by Confuciusornis, Gobipteryx, Archaeorhynchus, and Schizooura. Although Archaeorhynchus and Schizooura are closely related ornithuromorphs, the analyses indicate that they each evolved toothlessness independently.

Archaeorhynchus（Image source：Jonathan Chen， CC BY-SA 4.0 。）

The reduction of teeth also did not occur simultaneously across different parts of the jaw. Teeth on the premaxilla, maxilla, and dentary were independently reduced or lost in different lineages. For example, several clades represented by Confuciusornis and Gobipteryx independently lost their maxillary teeth during evolution.

In contrast, Mengciusornis is currently the only known bird that lost only its dentary teeth while retaining teeth elsewhere in the jaws. This suggests that the reduction of dentition in birds followed a highly diverse evolutionary pattern rather than a single, fixed sequence.

The researchers then examined the evolutionary models describing changes in tooth number and tooth crown size. Their results indicate that tooth number in Mesozoic birds did not evolve under a consistent trend toward tooth loss. Instead, variation in tooth number is best explained by the Brownian motion model, indicating that it was primarily driven by evolutionary time and phylogenetic history rather than by continuous natural selection favoring tooth loss. Consequently, the long-standing hypothesis that tooth reduction evolved to reduce the weight of the skull during flight was not supported.

The evolution of tooth crown size, however, followed a somewhat different pattern. Although tooth number did not show a directional trend, tooth crown size exhibited a weak but detectable signal of directional selection toward smaller teeth. In other words, teeth did not become progressively fewer through time, but they did tend to become gradually smaller.

Because tooth crown size showed only a very weak relationship with body size, this trend is unlikely to have resulted simply from changes in overall body size. Instead, the researchers suggest that smaller tooth crowns may have been associated with shifts in feeding behavior or diet. For example, some Mesozoic birds may have gradually specialized on smaller prey.

Jeholornis（Image source：Tiouraren， CC BY-SA 4.0 。）

The analyses indicate that complete tooth loss occurred independently multiple times during the evolution of Mesozoic birds. The toothless forms found among basal birds, enantiornithines, and ornithuromorphs are not directly related to one another, but instead represent separate evolutionary events.

In contrast, partial tooth reduction in different parts of the jaw appears to have been more complex. For example, the researchers suggest that Mengciusornis and Schizooura may have inherited certain patterns of partial tooth reduction from a common ancestor.

Longipteryx（Image source：Jonathan Chen， CC BY-SA 4.0 。）

As new fossils continue to be discovered and described, an increasing number of toothless enantiornithines have been recognized. The researchers therefore suggest that additional completely toothless species may yet be found among lineages with highly reduced dentition, such as Jeholornis, Longipterygidae, and Mengciusornis.

Finally, the study suggests that tooth reduction in Mesozoic birds was likely influenced by multiple factors acting together. These may have included the evolution of the digestive system, such as the development of a keratinous beak and gizzard, shifts in diet and ecological niche, and developmental mechanisms shared with other theropod dinosaurs. Because different evolutionary lineages experienced different selective pressures, tooth reduction occurred independently in multiple groups, resulting in an overall pattern dominated by random evolution rather than a single directional trend.

（Author: Bai Leng）

Reference：

Zhou, Y., Zhao, Y., Guo, Y., Wei, X., Zhang, F. (2026). Evolution of tooth number and size reveals random evolution and weak directional selection of dentition toward independent tooth loss in Mesozoic birds. Avian Research