Character transformations and their functional significance as a key to the evolution of hystricognath Rodentia

Abstract

Hystricognathi represent a monophyletic taxon within Rodentia. Since phylogenetically analyzed morphological systems are essential for revealing evolutionary processes, this study identifies evolutionary character transformations on the stem lineage of Hystricognathi as derived from the author's own work and the literature. Data so far indicate that evolutionary transformati ons in the rostral head region, the loss of tactile ability in the outer nasal skin and the mobile arrangement of the associated cartilage, were allied with a switch from om ni vorous to her bivorous and fiber-rich nutrition. Additional character trans formations in the skull assist in digesting such food. Structures associated with reproduction and placentation show a remark able proportion of derived character conditions: the chorioallantoic placenta has a ring-shaped organization and growth structure which optimizes the capacity for passive diffusion, a subplacenta occurred as a speci a lized region responsible for pla cental invasion and the in verted yolk sac facilitates substance exchange with the main placenta. Finally, precocial new borns evolved as a derived condition within Rodentia. All things considered, a mode of reproduction is indicated, which does not demand excessive additional energy intake by the mother and is in accor dance with her low energetic diet. Hystricognathi possess major character trans for mations that represent prerequisites for their successful radiation at the time when more open ecosystems and grasslands evolved during Earth history. The analysis resulted in the reconstruction of a life-near picture of the hystricognath stem species pattern with high explanatory power in terms of changes in space and time and their interdependence with biodiversity.