Meeting Abstract
Mosaic evolution describes the presence of a mixture of derived and pleisomorphic characteristics. This condition can be generated as a result of interrelated clusters of traits, or modules, evolving with different tempo and mode. Mosaicism is a major factor shaping avian evolution: for example, the dissociation of the hind limb and forelimb allowed for the diversification of locomotor behaviours and the radiation of crown birds. We evaluate cranial modularity in birds using high-dimensional geometric morphometrics in a broad taxonomic sample (352 species) and find that the skull, like the post-cranium, is highly modular. We show that evolutionary dynamics are heterogeneous among modules and that evolutionary rate is linked to developmental origin. The face and cranial vault evolve faster than the basicranium and jaw joint. Fast-evolving regions are derived from anterior mandibular-stream cranial neural crest (CNC), or from multiple embryonic cell populations, compared to slow-evolving regions that develop from mesoderm or posterior mandibular CNC only. Evolutionary rates are also variable across the avian phylogeny, with each module showing unique patterns. For example, the rostrum is characterized by punctuated evolution at the origin of major clades, whereas cranial vault shows high rates in lineages with ornamental structures. Taken together, these findings illustrate that the modular organization of the avian cranium allowed for semi-independent tempo and mode across skull regions, facilitating the diversification of birds.
