Evolutionary and Morphological Diversity of Cranial Appendages in Artiodactyls: A Review

Abstract

Cranial appendages in herbivores are vital for their survival and have evolved from a common ancestor around 15-20 million years ago and are of four types: antlers, ossicones, horns, and pronghorns. The interest may actually beneficial, as the study of ruminant headgear may enable important applications in the fields of veterinary and human medicine. Antler development, is seen as a key to understanding a suite of skin–bone interactions, including healing, regeneration and neural control of bone development. Horns consist of conical keratinous sheaths which enclose the horn core, arising from the frontal bone. They vary greatly in size, forms and curvatures. Unlike antlers, both the bony core and keratin covering are never shed naturally. Horns act as a shield which protects the animal during fight (Bighorn Sheep). Female bovids that are larger in size and live in an open habitat are more likely to have horns (African buffalo). Horns also function as shock absorber. Antlers undergo periodic regeneration, with deer antlers being a model for studying bone growth and regeneration. Antler growth is energetically expensive and induced by testosterone. Ossicones, found in giraffes and okapis, are permanent and differ from horns and antlers. Pronghorns have a keratinous sheath that sheds annually and a forked appearance due to branching. Each type of cranial appendage has unique characteristics and functions, highlighting the diversity of these structures in herbivores and their evolutionary significance.