The origin of the bird's beak: new insights from dinosaur incubation periods

Abstract

The toothless beak of modern birds was considered as an adaption for feeding ecology; however, several recent studies suggested that developmental factors are also responsible for the toothless beak. Neontological and palaeontological studies have progressively uncovered how birds evolved toothless beaks and suggested that the multiple occurrences of complete edentulism in non-avian dinosaurs were the result of selection for specialized diets. Although developmental biology and ecological factors are not mutually exclusive, the conventional hypothesis that ecological factors account for the toothless beak appears insufficient. A recent study on dinosaur incubation period using embryonic teeth posited that tooth formation rate limits developmental speed, constraining toothed dinosaur incubation to slow reptilian rates. We suggest that selection for tooth loss was a side effect of selection for fast embryo growth and thus shorter incubation. This observation would also explain the multiple occurrences of tooth loss and beaks in non-avian dinosaur taxa crownward of Whereas our hypothesis is an observation without any experimental supports, more studies of gene regulation of tooth formation in embryos would allow testing for the trade-off between incubation period and tooth development.

Ultramicrostructural reductions in teeth: implications for dietary transition from non-avian dinosaurs to birds

“…The lack of complexed enamel in most Cretaceous birds suggests their substantially weakened role in resisting wear during food processing. The simplified and thin enamel layer may also be related to a fast incubation time and a lack of adequate time for mineral deposition [30,31]. In comparison to the commonly reported paralleled enamel found in dromaeosaurids [32], even enantiornithines with relatively thick enamel show a simplified crystal structure without a clearly layered structure.…”

“…In the face of intensified competition from carnivorous theropods and pterosaurs, changes of feeding ecology to a more herbivorous diet could be one of the major selective factors ruling on the evolution of Cretaceous stem birds, as exemplified by tooth changes in both exterior morphology and internal microstructures. In addition to feeding adaptation, a recently hypothesized developmental process (i.e., incubation duration) may also have constrained tooth growth [30]. Fast embryonic growth and a shortened incubation time may have restricted tooth development compared with non-avian theropods with a longer period of incubation [31].…”

The distribution of dental features in non-avian theropod dinosaurs: Taxonomic potential, degree of homoplasy, and major evolutionary trends

“…Functional morphology. Tooth loss and edentulism in theropods have been recently investigated by Zhou and Li (2009), Davit-Béal et al (2009), Louchart and Viriot (2011), Zanno and Makovicky (2011), Lautenschlager et al (2013), Meredith et al (2014), Bhullar et al (2016), Wang et al (2017b), Erickson et al (2017) and Yang and Sander (2018). The classical reasons for the origin of edentulism are related to developmental economy and for food acquisition.…”

“…It is noteworthy that this approach could be tested on other dinosaurs closer to the bird lineage (e.g., Torvosaurus; Araújo et al, 2013). Yang and Sander (2018) suggested that the incubation period is a selection factor for edentulism or, in other words, the selection for tooth loss was a side effect of the selection for fast embryo growth and thus shorter incubation. This hypothesis, however, lacks the support of experimental evidence.…”

Trophic shift and the origin of birds

“…Regarding the reduction or loss of the teeth of birds, it is traditionally attributed to lightening the body for flight (Zhou et al 2010;Zheng et al 2011). This, however, cannot explain the occurrence of numerous toothed Mesozoic birds (e.g., Enantiornithes and Ichthyornis) (Zhou et al 2010;Yang and Sander 2018; O'Connor 2019), and hence teeth were probably not a limiting factor for flight (O'Connor and Zhou 2015; Chiappe and Qingjin 2016;Mayr 2017;Zhou et al 2019). Alternatively, teeth reduction or loss is considered to be partly due to the functional replacement by the muscular gizzard (Louchart and Viriot 2011;Zheng et al 2014;O'Connor 2019).…”