Stable morphological types and mosaicism in the macroevolution of birds (Neornithes)

The apparent morphological lacunae dividing the macrotaxa of modern birds (orders of Neornithes) present a serious impediment to the construction of macroevolution models. However, the discovery and detailed investigation of multiple groups of fossil birds in recent decades contributed to the elucidation of the process of the formation of modern higher taxa and allowed for analysis of the macroevolution process that occurred in this group of vertebrates during the Cenozoic period. The present article is a concise review of the “transitional” taxa with a focus on representatives of the Galloanseres clade. Almost all of the transitional bird forms exhibit a mosaic morphology, although certain stable morphological traits or combinations of traits occur in different groups and may represent either a primitive or an advanced feature for a specific group. The stability of these morphological features is due to the correlation between ontogenetic processes, whereas the independent evolution of individual morphological characteristics is related to the modular organization of ontogeny. The problem of the emergence of the morphological type of modern Passeriformes is discussed with modular organization of the birds’ feet as an example. Mosaicism of the features in the transitional forms is regarded as the only macroevolutionary pathway possible, given the restraint on parallel optimization of multiple systems of the body. Consequently, stem taxa with mosaic morphology are regarded as ancestors of crown taxa. Complete filling of the lacunae between taxa does not occur during macroevolution, since the “transitional” taxa are separated from the respective descendant and ancestor taxa by lacunae as well. Notably, the evolutionary formation of some higher taxa of birds started with the formation of advanced morphology of the jaw apparatus and the entire skull, while the postcranial skeleton initially remained plesiomorphic.