Chapter 1: The Fascinating World of Mammalian Headgear

Cranial appendages, commonly referred to as "headgear," exhibit a remarkable variety among ruminant mammals. From the diminutive ossicones of giraffes to the massive antlers of moose, these structures have evolved from a shared ancestor, yet they display significant differences in size and form due to varying gene expression patterns.

A stunning collection of these adaptations can be observed in the Richard Gilder Center for Science, Education, and Innovation, part of the Louis V. Gerstner, Jr. Collections Core at the Museum. Scientists have long pondered the evolutionary roots of these unique structures. According to lead researcher Zachary Calamari, a paleontologist and assistant professor at Baruch College, the diversity of horns and antlers has sparked considerable debate among experts regarding their origins.

While collectively termed "headgear," scientists classify these structures differently based on their distinct tissue compositions and growth mechanisms as seen in various taxonomic families.

Section 1.1: Antlers vs. Horns

Antlers, characteristic of deer and their relatives, originate from a small bony structure known as a pedicle. Each year, antlers grow out from these pedicles and are initially enveloped in a vascular layer of skin called "velvet," which nourishes the developing bone. Once the growth period concludes, the velvet dries up and is shed.

In contrast, the headgear of cattle, bison, and similar species remains permanent and is encased in a tough keratin layer, akin to human nails. Unlike antlers, which are typically grown by males (with the notable exception of reindeer), both genders of horned animals develop these structures, though females often have smaller or less pronounced horns.

Subsection 1.1.1: Ossicones and Pronghorns

Ossicones, found in giraffes and okapis, represent another form of headgear. These bony projections start as cartilage that transforms into bone and fully fuses with the skull as the animal matures. Interestingly, while giraffe ossicones are generally covered in skin and fur, the tips of okapi ossicones remain exposed bone in adulthood.

Pronghorns showcase a hybrid of horn and antler traits. Their headgear consists of a bony core sheathed in keratin, similar to horns, but with branches that are shed seasonally, much like antlers. While typically present in males, around 30% of female pronghorns also develop these structures, which are unique to the pronghorn antelope species.

Section 1.2: The Evolutionary Timeline of Headgear

Approximately 170 modern ruminant species exhibit their distinct headgear, with many more identified in the fossil record. This diversity first appeared around 15 million years ago during the Miocene epoch, serving various purposes such as species recognition, mate attraction, and male combat for mating rights.

The largest known headgear belongs to the extinct Megaloceros giganteus, or "Irish elk," which boasted antlers spanning up to 13 feet. Often discovered in Ireland's bogs, these impressive creatures vanished around 7700 years ago.

Chapter 2: Investigating the Ancestral Links

Motivated by the intriguing questions surrounding mammalian headgear, Professor Calamari initiated his research into its evolutionary history while pursuing his doctorate at the Museum's Richard Gilder Graduate School. Utilizing computer-based 3D shape analysis and transcriptome sequencing, he and his team compared the genetic sequences of cattle horns, deer antlers, and pig skin.

Their findings indicate that all horns and antlers share a common ancestry, arising as paired bony projections from the frontal area of the skull. As Professor Calamari elaborated, "Our results reveal that the gene expression patterns supporting a single origin for horns and antlers may differ significantly from those of other bones."

Co-author John Flynn emphasized the role of cranial neural crest cells in this process. These cells, which give rise to various craniofacial structures, exhibit remarkable flexibility, resulting in the diverse shapes and functions seen across vertebrates.

"It's fascinating that these same cells are responsible for antler formation," Dr. Flynn noted. The distinct gene expression patterns observed in cattle horns and deer antlers provide robust evidence of their shared ancestry, suggesting that variations in other ruminant headgear, such as ossicones and pronghorns, may also stem from a common ancestral trait.

"This genomic research not only enhances our understanding of evolutionary history but also sheds light on the mechanisms of bone formation in mammals," Professor Calamari remarked.

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

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Originally published at Forbes.com on 27 May 2024.