Despite being an important tool for understanding ancient species, fossilised hair is five times rarer than feathers, new research has found.

Fossils of body coverings contain unique data on the ecology and lifestyle of extinct animals, including what colour they might have been.

They also might affect our understanding of when kinds of body coverings, such as feathers and hair, evolved.

“This pattern of where and when we do find fossilised feathers and hairs can be used to inform where we look for future fossil discoveries,” said first author Chad Eliason, a researcher at the Field Museum of Natural History in Chicago who conducted the research while a postdoctoral fellow at the University of Texas at Austin in the US.

In this study, published in the journal Proceedings of the Royal Society B: Biological Sciences, the researchers used data on fossil type and age to determine that hair probably evolved much earlier than current fossil samples indicate.

Fossil beds that preserve soft tissues like hair and feathers are called lagerstatte and are rare on their own.

The researchers were interested in understanding how frequently different types of body coverings were found preserved in these exceptional sites, which include the Yixian Formation in China and the Green River Formation in the western US.

Eliason and his collaborators assembled the largest known database of fossilised body coverings, or integument, from land-dwelling vertebrates, a group known as tetrapods, collected from lagerstatte. They found that unlike feathers, hairs are extremely rare finds.

“Mammal hair has been around for more than 160 million years yet over that time we have very few records,” Eliason said.

The rarity might be explained by feathers and hair containing different types of the protein keratin, which may impact the likelihood of fossilisation.

However, the study noted that the lack of hair samples could have nothing to do with fossilisation, and be explained by the collecting behaviour of paleontologists, with a single feather usually being much easier to identify than a single hair.