The horns, antlers, and other bony structures (cranial appendages) growing from the skulls of even-toed, hoofed mammals like deer, antelopes, and giraffes (the group of hoofed mammals called Pecora) are an evolutionary mystery. Cranial appendages are often used to determine how species are related to the different families of Pecora, because each family is distinguished by the presence of a specific type of appendage. When fossil species lack cranial appendages, it is difficult to assign them reliably to any particular family. These appendage-less fossil species could provide necessary information about the ancestral origins of cranial appendages in each family, but the inability to confidently place them in the context of pecoran evolutionary relationships precludes their use in studies of cranial appendage evolution. Prior studies have shown that variation in the way structures change as an animal grows can result in the origination of totally new structures. A better understanding of cranial appendage development will therefore provide much-needed context for hoofed mammal cranial appendage evolution and help resolve the question of their origins. It will also contribute to research on how skin and connective tissues contribute to bone growth and have implications for understanding pathways and patterns of skull development, with methods and results that are broadly applicable to skull and bone growth research in all vertebrates, including humans. This research will form the basis of a curriculum on genomic analyses and genetics for middle school students in the American Museum of Natural History's Lang Science Program, piloted with diverse groups of New York schoolchildren and available to teachers and schools across the country.
This project will test two competing hypotheses: 1) the broad range of distinctive cranial appendages observed in fossil and extant pecoran artiodactyls do not share a common evolutionary origin, 2) portions of cranial appendages, such as the permanent bone pedicles of antlers and the bone cores of horns, may be homologous across some or all of the appendage types in this clade. Samples will be collected from the cranial appendage forming tissues of at least three individuals each from the four extant cranial-appendage-bearing families of Pecora (Bovidae, Cervidae, Giraffidae, and Antilocapridae) and an appendage-less outgroup taxon from Tragulidae, at 3 developmental stages: birth, one month, and two months old. RNA will be isolated from these tissues and sequenced using next-generation sequencing technologies. Gene expression profiles sequenced from tissues at a second location on the body of each individual will provide a somatic control to estimate gene expression specifically related to cranial appendage growth (the cranial appendage transcriptome). These transcriptomes will be optimized onto a phylogeny to test for homology of the genes responsible for cranial appendage growth across and within age groups.
