Diverse Headgear Of Hoofed Mammals Evolved From A Common Ancestor

From the small ossicones of a giraffe to the gigantic antlers of a moose, they all evolved from the same ancestor but are dramatically different in size and shape due to differences in gene expression

Cranial appendages, or “headgear,” come in many different shapes and sizes in ruminant mammals. In the fossil record, antlers, horns, and similar growths were first seen on ancestral ruminants in the Cervidae family — deer, bison and their relatives. Where did this fancy headgear come from? Was there just one common ancestor, or many, that gave rise to all the ruminant mammals that grow such a vast array of bony headgear?

“Horns and antlers are incredibly diverse structures, and scientists have long debated their evolutionary origins,” said the lead author of a new study, palaeontologist Zachary Calamari, an assistant professor at Baruch College and the CUNY Graduate Center and a research associate at the American Museum of Natural History.

Although these bony growths are collectively known as ‘headgear,’ biologists use different names for them because their tissue composition and growth patterns are distinct to each taxonomic family (Figure 1).

Antlers, for example, are found in deer and their relatives. At the root of each antler is a small bony growth known as a pedicle, and every year, antlers grow out of these pedicles. Antlers are made of bone but as they grow, they are covered with ‘velvet,’ a thin layer of skin and blood vessels that nourishes the growing bone underneath. Later, when growth has ceased for the season, velvet dries and is scraped off.

In deer, antlers are only grown by males — with the exception of reindeer, where both sexes grow them. For reasons that remain mysterious, antlers are shed seasonally, and the energetically-demanding process of growing new ones begins again.

In contrast to antlers, the bony headgear of cattle, bison and their relatives is permanent. Also unlike antlers, horns are covered with a tough coating of keratin, which is the same material that human fingernails are made of. Additionally, both sexes grow horns, although females’ horns may be smaller, shorter or thinner than males’.

Ossicones are another sort of headgear in ruminant mammals. These short bony growths, which are found in both male and female giraffe and okapi, begin as cartilage growths, and harden into bone and completely fuse to the skull by puberty. Interestingly, although ossicones in giraffe are usually covered in skin and fur, the tips of okapi ossicones are usually unprotected bone in adults.

Pronghorns’ headgear is a blend of the characteristics of both horns and antlers. The core of a pronghorn is bone that is covered by a keratin sheath, like horns. But as with antlers, the keratin sheath forms branches and is shed seasonally. Pronghorns are usually found in males but approximately 30% of females also grow them. Pronghorns are found only in one living species, the pronghorn antelope.

There are about 170 modern ruminant species with their own distinctive headgear, and many more can be found in the fossil record. This headgear first appeared in the fossil record around 15 million years ago in the Miocene era, and is used to recognize other members of the same species, to attract mates, and often, to fight other males for access to them.

The largest known example of headgear are probably found in the now-extinct giant deer or “Irish elk,” Megaloceros giganteus, which was one of the largest deer to ever live, whose enormous antlers could reach a 13-foot spread. The remains of these animals are often found in bogs in Ireland, hence their common name. They went extinct around 7700 years ago.

Inspired by the many mysteries surrounding mammalian headgear, Professor Calamari began investigating its evolutionary origins when he was a doctoral student at the Museum’s Richard Gilder Graduate School. To approach this question, he used computer-based 3D shape analysis and he sequenced transcriptomes — genes that are expressed in a specific tissue at a specific time.

In this study, Professor Calamari and his collaborator compared cattle horn transcriptome sequences to deer antler and to pig skin transcriptomes. They found that all horns and antlers evolved from a common ancestor as paired bony outgrowths from the animals’ “forehead,” the area near the frontal bones of the skull.

“In addition to gene expression patterns that support a single origin of horns and antlers, our results also show the regulation of gene expression patterns in these structures may differ from other bones,” Professor Calamari explained.

“Our results provide more evidence that horns form from the cranial neural crest, an embryonic cell layer that forms the face, rather than from the cells that form the bones on the sides and back of the head,” said the study’s co-author, palaeontologist John Flynn, who is with the AMNH.

Cranial neural crest cells give rise to a diverse lineage of cells in vertebrates, including bones, cartilage, nerves and connective tissues of the craniofacial skeleton. The remarkable developmental flexibility of cranial neural crest cells supports the development of the remarkable variety of craniofacial shapes and functions, from filter feeding sieves to powerful, toothy jaws in vertebrates.

“It is striking that these are the same cells that form antlers,” Dr Flynn commented. “And the distinctive patterns of gene expression in cattle horns and deer antlers, relative to other bone and skin tissue ‘controls,’ provide compelling evidence of shared origin of fundamental aspects of these spectacular bony structures in an ancient ancestor.”

“This genomic research not only gets us closer to solving an evolutionary mystery, but also helps us better understand how bone forms in all mammals,” Professor Calamari stated.

“These results help us understand the evolutionary history of horns and antlers and could suggest that differences in other ruminant cranial appendages, like ossicones and pronghorns, are also elaborations on a shared ancestral cranial appendage.”

Source:

Zachary T. Calamari & John J. Flynn (2024). Gene expression supports a single origin of horns and antlers in hoofed mammals, Communications Biology 7, 509 | doi:10.1038/s42003-024-06134-4