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

Abstract

Modern orders of Neornithine birds are separated by distinct morphological gaps, hampering the development of particular models. of macroevolutionary transformations. However, recent decades have witnessed the discovery and extensive study of many fossil groups of birds, which shed light on the origin of modern higher taxa. These fossils further allow analyzing the whole process of the macroevolutionary change in this group of vertebrates during the Cenozoic. Here, a brief review is presented of the "transitional" taxa with special attention to representative of the clade Galloanseres. Almost all transitional groups of fossil birds do display mosaic morphology. It is noted that many stable morphologies (characters and character complexes) persist in different groups, occurring either as primitive states, or as advanced ones. The stable recurrence of these characters is caused by the persistence of stable ontogenetic pathways. On the other hand, independent evolution of various morphologies can be explained by the modular organization of the development. The modularity in the structure of the foot is used here as an example to show a possible way of the origin of the foot in extant perching birds (Passeriformes). The mosaic morphology of the transitional taxa is seen as the only possible way to overcome the evolutionary constraint of the parallel optimization of numerous organismal systems. It is noted that in course of macroevolutionary change, no complete filling of morphological gaps occur. Transitional forms are separated by additional gaps from their ancestors and descendants. Hence, stem groups are seen as the most probable ancestors of crown taxa. It is further noted that the evolutionary formation of some higher taxa of the living birds was initiated by the evolution of advanced morphology of the feeding apparatus and skull, while change in their. postcranial morphology was delayed.