From snout to beak: the loss of teeth in birds

Evidence for a single loss of mineralized teeth in the common avian ancestor

“…Edentulism also occurs in turtles and mammalian lineages (anteaters, baleen whales, and pangolins) (2, 3), with some mammals lacking tooth enamel (aardvarks and sloths) (4), but these clades are less diverse than birds and together comprise only~385 species. Modern birds share a common ancestry with toothed, maniraptoran theropod dinosaurs (5). Instead of teeth, modern birds use two highly specialized structures, a rhamphotheca (horny beak) for food acquisition and, as part of the digestive tract, a muscular gizzard for food processing (5).…”

Cranial anatomy ofErlikosaurus andrewsi(Dinosauria, Therizinosauria): new insights based on digital reconstruction

“…Keratinous beaks (rhamphothecae) evolved several times within non-avian theropods (Louchart and Viriot, 2011) and have been suggested to be present in (amongst others) Ornithomimosauria, Oviraptorosauria, and Therizinosauria. Among these groups, only ornithomimosaurs provide direct evidence of a preserved keratinous sheath (Norell et al, 2001;Barrett, 2005).…”

“…Although the presence of a rhamphotheca in E. andrewsi is well supported, the extent of its keratinous sheath is less clear. Extant birds exhibit a plethora of sheath morphologies, which cover various portions of the premaxilla, maxilla, and dentary (Hieronymus and Witmer, 2010), whereas a rhamphotheca most likely covered the edentulous parts of the jaws in most Mesozoic birds (Louchart and Viriot, 2011). For E. andrewsi, the presence of a rhamphotheca on the premaxilla can be deduced by the presence of several neurovascular foramina pitting the rostral and lateral surfaces (Morhardt et al, 2009;Hieronymus and Witmer, 2010;Louchart and Viriot, 2011).…”

“…Extant birds exhibit a plethora of sheath morphologies, which cover various portions of the premaxilla, maxilla, and dentary (Hieronymus and Witmer, 2010), whereas a rhamphotheca most likely covered the edentulous parts of the jaws in most Mesozoic birds (Louchart and Viriot, 2011). For E. andrewsi, the presence of a rhamphotheca on the premaxilla can be deduced by the presence of several neurovascular foramina pitting the rostral and lateral surfaces (Morhardt et al, 2009;Hieronymus and Witmer, 2010;Louchart and Viriot, 2011). Similarly, the rostrolateral surface of the maxilla bears neurovascular foramina, indicating that it might have been covered by the rhamphotheca, with its caudal-most extent marked by the large foramen for the nasopalatine nerve (Hieronymus and Witmer, 2010).…”

“…Although the lack of preserved cranial elements, particularly premaxillae and maxillae, makes it difficult to track the presence/absence of rhamphothecae in Therizinosauria, Beipiaosaurus inexpectus (Xu et al, 1999(Xu et al, , 2009a and Jianchangosaurus yixianensis (Pu et al, 2013) both have an edentulous premaxilla and a ventrally deflected dentary bearing a characteristic lateral shelf, indicating that a keratinous rhamphotheca evolved early in the clade. Similarly, a rostral rhamphotheca is assumed to have covered the edentulous elements in Ornithomimosauria, Oviraptorosauria, and Avialae (Louchart and Viriot, 2011). However, the development of a complete upper and lower rhamphotheca paralleled by partial or complete edentulism is a derived character that appears within different maniraptoriform lineages (Ornithomimosauria, Oviraptorosauria).…”