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Tooth loss in birds

From: Ben Creisler

A new online paper. The paper is not free, but I have included the outline
and figure captions, which are posted.

Antoine Louchar and Laurent Viriot (2011)
>From snout to beak: the loss of teeth in birds.
Trends in Ecology & Evolution (advance online publication)

All living birds are toothless, constituting by far the most diverse
toothless vertebrate clade, and are striking examples of evolutionary
success following tooth loss. In recent years, an unprecedented number of
Mesozoic birds have been described, illustrating the evolution of dentition
reductions. Simultaneously, major advances in experimental embryology have
yielded new results concerning avian edentulism. Reviewing these lines of
evidence, we propose hypotheses for its causes, with a prominent role for
the horny beak during development. A horny beak and a muscular gizzard
functionally ?replaced? dentition for food acquisition and processing,
respectively. Together with edentulism itself, these features and others
contributed to the later success of birds, as a result of their high
performance or additional functionality working in concert in these complex

Article Outline
Tracking the roots of bird beak success
Evo-devo insights
Inactivation of odontogenetic genes
Epithelium?mesenchyme shift
Diversion of gene function
Further changes following tooth loss
Tooth loss in avian evolution
Patterns of tooth loss
Timing of tooth loss
Crucial innovations for edentulism
Tooth loss, rhamphothecae and gizzards in other tetrapods
Towards a model
Concluding remarks
Appendix A. Supplementary data


Figure 1. 

Occurrences of tooth loss and dentition reduction in Aves in a phylogenetic
and temporally constrained framework. The white bars shows taxa with
complete dentition; the orange bars, taxa with partially reduced dentition,
and the red bars, toothless taxa. Two superimposed bars indicate that the
two conditions are currently possible alternative hypotheses. The
phylogenetic framework, and the dentition and temporal data follow the
references in the supplementary material online. Isolated avian teeth:
lower Barremian of Spain [ca. 128 million years ago (Ma)]; lower Aptian of
China (125?120 Ma); Campanian of Alberta, Canada (between 83 Ma and 71 Ma);
late Maastrichtian of Belgium (65.8 Ma; Ornithuromorpha). # indicates the
minimal number of independent cases of edentulism; and the orange
parentheses indicate the minimal number of independent groups, or
continuums, of partial dentition reductions. The independent cases of
partial reduction are identified using phylogeny combined with the
recognition of different patterns that cannot be earlier stages of others
(Figure 2). § In this lineage, the partial reduction is only incipient
(teeth remain in premaxillaries). Parentheses can overlap, because the
position of some taxa is not sufficiently well resolved. In the
Enantiornithes, the inclusion of taxa in parentheses is speculative for the
same reasons. Partial dentition reductions are either independent or
branched at the base of lineages later evolving edentulism. The four cases
of edentulism probably originated within three of the independent
continuums of partial reductions, including two cases of edentulism from
the single group of partial reduction in the Ornithuromorpha, most
parsimoniously. The star indicates the approximate shift from a lower to
higher degree of metabolism (Box 1). In alternative phylogenies, the
Scansoriopterygidae, Troodontidae, Dromaeosauridae and Oviraptorosauria
would be placed within and at the base of Aves after the divergence of
Archaeopterygidae [0110] , [0135] , [0140] , [0145] and [0150] .
Incidentally, a recent analysis places Archaeopteryx more distant from
basal birds such as Jeholornis, than the Scansoriopterygidae, but with
?only tentative statistical support? [31]. In this hypothesis, the Aves as
defined in the main text [22] would contain at least part of the
Scansoriopterygidae, Troodontidae, Dromaeosauridae and Oviraptorosauria,
rather than exclude Archaeopteryx from Aves, as the authors suggest [31].
However, we follow the most widely accepted hypothesis for Aves here (see
main text). The legend to Figure 2 contains details of (a?j).

Figure 2. 

Examples of patterns of partial dentition reduction in birds, illustrated
by different Cretaceous species (showing right side only). Toothed parts of
jaws are highlighted in orange and the premaxillary in grey. Lettering
refers to Figure 1. (a)Jeholornis prima, (b)Sapeornis chaoyangensis,
(c)Cuspirostrisornis houi, (d)Boluochia zhengi, (e)Longipteryx
chaoyangensis, (f)Longipteryx sp. [3], (g)Rapaxavis pani, (h)Yanornis
martini, (i)Hesperornis regalis, (j)Ichthyornis dispar. Abbreviations: d,
dentary; m, maxillary; pm, premaxillary. Other patterns are known (see the
supplementary material online). Based on data and references in the
supplementary material online.

Figure 3. 

Occurrences of tooth loss, partial dentition reduction and potential
correlates in tetrapods, in a phylogenetic framework. Attributes are
indicated for taxa in which they apply to at least one lineage. Larger
ellipses in the Aves indicate a higher frequency of independent events
(Figure 1). A cross (?) indicates an extinct group. A ?gizzard? icon
between parentheses indicates that the characteristic is unconfirmed for
this taxon. A ?metabolism? icon between parentheses means that homeothermy
is incomplete, and a question mark is added to indicate hypothetical cases.
A ?flight? icon between parentheses indicates that flight was probably not
as sustained and active as in the Neornithes. The following are features
thought to help overcome tooth loss: §, internal tracheal bony spines,
which help to crush eggs; *, specialized tongue; **, elongated protractile
sticky tongue; ***, baleen; ¶, all but one species, with total edentulism
in females; Ø, callous pad of hardened gum on premaxillaries; Ω,
keratinous grinding plates; #, rhamphotheca associated with propalineal jaw
movements. Other rhamphothecae are associated a priori with arcilineal jaw
movements; μ, keratinized beak and palatal or tongue spines (with
horny grinding plates in the platypus and elongated tongue in echidnas; the
platypus has teeth when juvenile, and a beak when adult [1]). Incidentally,
rhamphotheca-like elements in mammals are not homologous with the
rhamphotheca in archosaurs. They are composed of α-keratin in mammals
as opposed to β-keratin in birds. The consensus phylogenetic framework
follows the following sources: for mammals [89]; for theropods, the
framework adopted in the main text (Figure 1) and, for other tetrapods,
that used in [0005] and [0190] , as well as references below. Dentition
reductions, rhamphothecae sensu lato and other feeding-apparatus features
are taken from [0005] and [0190] , and additional sources for theropods
[0230] , [0450] and [0455] , pterosaurs [0175] , [0460] , [0465] and [0470]
, crocodylomorphs [95], snakes [96] and mammals [0485] , [0490] and [0495]
; gizzards from [0055] , [0205] , [0210] , [0230] and [0450] ; homeothermy
from Box 1 and [75]; and sustained active flight, see main text.
Representatives of edentulous taxa are illustrated.

Figure 4. 

Proposed evolutionary interactions related to the loss of teeth in birds.
Several major morphological, physiological and behavioral innovations
favored or made possible (arrows) the evolution of other innovations in a
complex way: some facilitated edentulism in birds, whereas others led to
avian evolutionary success following, and despite, tooth loss, as the Aves
are the most speciose class of extant tetrapods. Dashed arrows represent
less obvious influences. The horizontal distribution of events reflects
approximately their relative temporal occurrences, when known, although
some cannot be assigned to a well-defined relative placement; for example,
tooth loss occurred several different times during the Mesozoic (see main
text for details and references). The extinction of toothed birds close to
the Cretaceous?Paleogene (K?P) crisis might derive from contingent effects
of the sudden asteroid collision [100]. It could also derive in part from
physiological characteristics, such as incomplete homeothermy and
endothermy, which might have made non-Neornithes less resilient to the deep
and rapid climatic and food-web changes that characterized the crisis
[100]. The model might, in part, not only apply to tooth loss on the line
to Neornithes, as a few of the innovations depicted here are observed in
association with tooth loss in other lineages (e.g. that of Confuciusornis
or Gobipteryx[3]).

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