Beaks and teeth: An evolutionary example of negative correlation between traits?

AGUILAR-PEDRAYES, Isaura; Montana State University, Bozeman, MT.


Across the evolutionary history of tetrapods, diverse jaw structures have evolved for obtaining food. About two-thirds of current species use teeth and the rest (birds and turtles) use beaks. Most studies involving embryonic developmental and paleontological research of birds and some extinct relatives in the fossil record, seem to indicate that beak acquisition is accompanied by irreversible tooth loss. Currently there are many hypotheses that state an adaptive advantage for this trade-off, but haven’t been tested in a macroevolutionary scale and most studies are biased towards this taxon. These include lightening the avian body as an adaptation of flight, tactile sensitivity for both food grasping and chick rearing, and (or) having a beak as a “less metabolic expensive” alternative to teeth. Non-adaptive alternatives have yet to be explored thoroughly—for example, trait evolution correlation and phylogenetic history. The objective of this study is to test if tooth count reduction coevolved with the appearance of the beak, that is a keratinized epidermis that covers the jaws. This research will focus on dinosaur taxa. I collected the following data from Aves, a group that includes taxa from Archaeopteryx to extant Neornithes: beak presence/absence and tooth-count. I will also add non-avian saurischians and ornithischians known to have developed beaks. I will also leverage Bayesian phylogenetic comparative methods to test for negative selection (reduction) on tooth count by beak presence, accounting for phylogeny. Lastly, I will assess if phylogenetic error affects our results by using two alternative phylogenies for Dinosauria. Literature research mentions birds and non-avian theropods independently evolved beaks three times, and tooth loss evolved at least four times in Mesozoic birds. Preliminary results using BayesTraits V3 favor a linear regression model where tooth count reduction in the premaxilla is correlated with beak presence, accounting for phylogenetic signal (λ≈1); in other words, the appearance of a keratinous surface in the premaxilla (beak) seems to contribute to tooth count reduction. Adding non-avian theropods and ornithischian might help clear this inquiry. Also, in a broader aspect this research will provide insights into the evolution and diversification of animals, and help determine if current hypotheses apply for all beaked animals that ever lived.