Cranial appendages, or “headgear,” come in many different shapes and sizes. But while antlers and horns are similar, they have key differences—as do more obscure appendages like ossicones and pronghorns.
“Cranial appendages have evolved a bunch of times, and there are a lot of different types of them,” says Zachary Calamari, a student at the Museum’s Richard Gilder Graduate School who studies the evolution and development of horns and antlers.
In the fossil record, antlers, horns, and similar appendages are first seen on the ruminants that have them today—cows, deer, and other animals related to them—in the Miocene era, around 15 million years ago. And they’re not just for looks. Many species use their headgear in clashes over potential mates.
As for what’s different, here’s how to tell cranial appendages apart.
Antlers
Antlers are found in the Cervidae family—deer and their relatives. At the root of each antler is a small, bony growth called a pedicle, and every year, antlers grow out of these pedicles. Antlers are made of bone, and covered with "velvet"—a thin, soft layer of skin and blood vessels that gets scraped off the antler over time.
Later in the year, those antlers are shed, making room for a new set to grow in. That's why hikers will occasionally find antlers laying on the ground, discarded by their owners. It also means that the huge antlers of a male elk, for instance, are the incredible product of just a single season of growth. Antlers are also an exclusively male accessory, with one exception—in reindeer, antlers can be found on both sexes.
Horns
While you’ll occasionally find a shed antler, that’s not true for horns—they’re attached permanently to species in the family Bovinae, or cows and their relatives. Where antlers grow out of a bony stub, horns have a full core of bone.
Instead of skin, horns are covered in a tough coating of keratin, the same substance that makes up human fingernails. Horns are also more likely to be a unisex accessory, with males and females of a species both sporting the appendages.
Ossicones
Horns and antlers are the most common kinds of headgear, but they’re not the only one present in modern mammals. Giraffes and okapi sport short, bony growths called ossicones. Ossicones begin as cartilage growths, and harden over the course of an animal's life. By puberty, these growths have hardened into bone and fused to the skull completely.
While ossicones are typically covered in skin and fur, the tips of the growths in some okapi are simply unprotected bone.
Pronghorn
Pronghorns are a singular cranial appendage, blending some traits of horns and antlers. The core of a pronghorn is bone, with a keratin sheath. But that horn-like sheath not only branches like an antler—it also gets shed every year. This hard-to-classify piece of headgear is found in just one animal today—the pronghorn
While today’s animals display a wide array of cranial appendages, it’s nothing compared to the diversity of horns and antlers that was once seen in mammals. To get a look at some of the strange and spectacular horns and antlers found in the fossil record, take a guided tour of the Hall of Advanced Mammals with Zachary Calamari in the video below.