Chicken teeth!

[David Marjanovic <david.marjanovic@gmx.at>]

Matthew P. Harris, Sean M. Hasso, Mark W. J. Ferguson & John F. Fallon: The 
Development of Archosaurian First-Generation Teeth in a Chicken Mutant, 
Current Biology 16(4), 371 -- 377 (21 February 2006)

Summary (refs removed):
"Modern birds do not have teeth. Rather, they develop a specialized 
keratinized structure, called the rhamphotheca, that covers the mandible, 
maxillae, and premaxillae. Although recombination studies have shown that 
the avian epidermis can respond to tooth-inductive cues from mouse or lizard 
oral mesenchyme and participate in tooth formation, attempts to initiate 
tooth development de novo in birds have failed. Here, we describe the 
formation of teeth in the talpid^2 chicken mutant, including the 
developmental processes and early molecular changes associated with the 
formation of teeth. Additionally, we show recapitulation of the early events 
seen in talpid^2 after in vivo activation of [beta]-catenin in wild-type 
embryos. We compare the formation of teeth in the talpid^2 mutant with that 
in the alligator and show the formation of decidedly archosaurian 
(crocodilian) first-generation teeth in an avian embryo. The formation of 
teeth in the mutant is coupled with alterations in the specification of the 
oral/aboral boundary of the jaw. We propose an epigenetic model of the 
developmental modification of dentition in avian evolution; in this model, 
changes in the relative position of a lateral signaling center over 
competent odontogenic mesenchyme led to loss of teeth in avians while 
maintaining tooth developmental potential."

The teeth were hidden under the beak.

Quote from inside the article:
"We show the initiation of tooth developmental programs as well as the 
formation of conical, saber-like structures on the lower jaw of the ta^2 
chicken. The structures formed are similar to those seen in the 
first-generation teeth of the alligator in position, histological 
differentiation, and morphogenesis. This finding is consistent with the idea 
that developmental programs are hierarchical and that atavisms will 
reinitiate early steps before later processes of more complex teeth. 
Previous reports interpreted tooth formation in light of knowledge of 
mammalian tooth development and thus searched for the elusive chick molar. 
Our work demonstrates a phylogenetic framework in which to interpret the 
latent ability of avian embryos to form teeth apart from mammalian tooth 
development."