Evolution of ruminant headgear: a review

Abstract

The horns, ossicones and antlers of ruminants are familiar and diverse examples of cranial appendages. We collectively term ruminant cranial appendages 'headgear'; this includes four extant forms: antlers (in cervids), horns (in bovids), pronghorns (in pronghorn antelope) and ossicones (in giraffids). Headgear evolution remains an open and intriguing question because phylogenies (molecular and morphological), adult headgear structure and headgear development (where data are available) all suggest different pictures of ruminant evolution. We discuss what is known about the evolution of headgear, including the evidence motivating previous hypotheses of single versus multiple origins, and the implications of recent phylogenetic revisions for these hypotheses. Inclusion of developmental data is critical for progress on the question of headgear evolution, and we synthesize the scattered literature on this front. The areas most in need of attention are early development in general; pronghorn and ossicone development in particular; and histological study of fossil forms of headgear. An integrative study of headgear development and evolution may have ramifications beyond the fields of systematics and evolution. Researchers in organismal biology, as well as those in biomedical fields investigating skin, bone and regenerative medicine, may all benefit from insights produced by this line of research.

Healed antler fracture from a giant deer (Megaloceros giganteus) from the Pleistocene in Poland

“…They should thus not be confused with horns, which are found only in bovids and are composed of a scabbard-like keratinous sheath covering a permanent bony horn core (Davis et al, 2011). The specimen from Barycz came from an adult individual with incomplete fusion of cranial sutures.…”

Understanding the effects of the bovine POLLED variants

“…Horn and scur morphology Horns of bovids are permanent, paired and symmetrical appendages that vary vastly in morphology between species and even breeds (Davis et al 2011). Horns have two main parts: a 'dead' keratin outer sheath and a bony inner core of 'living' tissue (Zhu et al 2016).…”

“…Horns have two main parts: a 'dead' keratin outer sheath and a bony inner core of 'living' tissue (Zhu et al 2016). Between the keratin sheath and bony core are several layers of tissue: the periosteum (tissue that lines the bones), subcutaneous connective tissue, dermis and epidermis (Davis et al 2011). True horns have a bony core that is attached to the frontal bones and a frontal sinus that extends into the horn spike.…”

Decoupling Tooth Loss from the Evolution of Baleen in Whales

“…In some vertebrate clades, integrative approaches have been successful in illuminating the evolutionary origin of novel integumentary and tissue systems (e.g., feathers in archosaurs, headgear in ruminants; Prum, 2005;Davis et al, 2011). For the origin of baleen in mysticetes, this problem remains one of the more difficult questions to answer in cetacean macroevolution for reasons relating to the challenges of studying living mysticetes.…”