Getting to the Root of Fur

First thing in the morning, my mind is on autopilot. I’m mostly relying on muscle memory to get me through the routine of feeding the cats, washing up, and getting dressed as my brain boots up. But the other day an unexpected question smacked me out of my usual stupor. On that particular morning I needed to shave, and my cat Margarita jumped up onto the sink to see what strange thing I was doing. When I looked into the sink and saw the scraped-off stubble, with my furry assistant trying to swat the stream of water from the faucet, I was struck by the unique body covering that signifies that the cat and I are part of the same evolutionary family – hair.

But where did hair come from? I’ve written plenty about the integument of dinosaurs and the ever-growing record of enfluffled saurians, but I couldn’t recall when hair became a defining feature of my ancestry. So with a few pointers from paleontologist Adam Huttenlocker, I started to sift through the literature to find out why I have to rake blades across my face every few days to remove itchy hairs instead of grooming feathers or shedding scaly skin.

We know that fur has to be ancient. The evolutionary tree of modern mammals is a very furry one, and the last time the three major lineages of today’s beasts – the monotremes, marsupials, and placentals – shared a common ancestor was in the Triassic, over 220 million years ago. There’s no indication that hair evolved more than once, so fur has to be at least that old.

Some spectacular fossils back up the notion that Mesozoic mammals were furry. A skeleton of Eomaia, a 125 million year old mammal found in China’s Liaoning Province, was preserved with a halo of fossil fur. Even older, the 165 million year old mammal Megaconus and the almost-mammal Castorocauda were fossilized with parts of their pelts, as have other exceptionally-preserved Mesozoic beasts found in China.

But fur didn’t necessarily start with mammals. If we look to feathers, for example, plenty of non-avian dinosaurs had them. Evolution just allowed birds to co-opt the insulating, flashy structures to take to the air. The same was probably true of fur, with sparse protohairs preceding full pelage. To get down to the root of hair, we need to go back beyond the first mammals.

Mammals belong to a much larger group of vertebrates called synapsids. For a long time the non-mammalian parts of this family tree – like the sail-backed Dimetrodon and weasel-like cynodonts – were called “mammal-like reptiles”, but this was off the mark. Better to call them protomammals, as they were more closely related to mammals than any reptile. Fur probably evolved within their protomammal ranks. The question is where.

The problem is that the fossil record isn’t always obliging when it comes to direct evidence. For some of the earliest synapsids, bones make up the majority of their fossil record. But there are a few scattered clues about what they looked like on the outside.

In 1975, paleontologist Robert Reisz described the anatomy of Archaeothyris – a small, lizard-like synapsid that lived about 306 million years ago. Among the preserved material were belly scales. These weren’t the same as the beta-keratin-coated scales of today’s lizards and snakes, but analogous structures. Basically, pseudoscales. And additional finds, including body impressions, suggest that the bellies of early synapsids were dotted with these same structures. What the rest of their bodies looked like, we don’t know.

During their evolution, protomammals eventually lost their pseudoscales. In 1968 the Russian paleontologist P. Chudinov described skin impressions for a wildly-ornamented protomammal named Estemmenosuchus. This synapsid didn’t appear to have any scales, but instead smooth “glandular” skin similar to that of a frog or a hairless mammal.

Unfortunately, though, the original material Chudinov studied hasn’t been reevaluated in some time, and similar specimens from other animals have been lacking. Multiple fossil finds are needed to clear up the ancestral protomammal state and when hair started to become a synapsid feature. Yet, even as they await additional direct evidence, paleontologists can look to other hints to see when protomammals started to get fluffy.

Hair isn’t just for insulation or display. Hairs are also an essential part of the mammalian sense of touch, and having so many little sensors over the body certainly changed the brains of the first synapsids to bear fur. By combining direct evidence with the shape of prehistoric brains, University of Texas paleontologist Tim Rowe and colleagues were able to make the case that hair was already an essential mammal trait by the time the group scurried out onto the evolutionary stage.

Drawing from brain endocasts created from CT scans of the 200-195 million year old mammals Morganucodon and Hadrocodium, Rowe and colleagues were able to track how mammal brains evolved. Among other alterations, the researchers saw that the early mammals had an expanded neocortex – part of the brain dominated by the sense of touch. Paired with the fact that other Mesozoic mammals and their almost-mammal relatives, such as Castorocauda, definitely had thick fur, the enlargement of the neocortex in Morganucodon and Hadrocodium hints that fur had originated sometime earlier so that it could play such an important sensory role by the time the first mammals evolved.

Whether or not prehistoric bone bears other hallmarks of hair requires further research to discern. Associating external integument to internal scaffold is no simple task. But the fossil record is full of surprises and no doubt contains additional evidence to explain how we and our mammalian kin came to be, including the long backstory behind why I have a fluffy companion who is fascinated by the ritualized removal of the hair evolution gave me.

References:

Blackburn, D., Hayssen, V., Murphy, C. 1989. The origins of lactation and the evolution of milk: a review with new hypotheses. Mammal Review. 19, 1: 1-26.

Carroll, R. 1969. Problems of the origin of reptiles. Biological Reviews. 44, 3: 393-431.

Kielan-Jaworowska, Z., Hurum, J. 2006. Limb posture in early mammals: sprawling or parasagittal. Acta Palaeontologica Polonica. 51, 3: 393-406

Ji, Q., Luo, Z-X., Yuan, C-X.,Wible, J., Zhang, J-P., Georgi, J. 2002. The earliest known eutherian mammal. Nature 416: 816–822. doi:10.1038/416816a

Niedźwiedzki, G., Bojanowski, M. 2012. A supposed eupelycosaur body impression from the Early Permian of the Intra-Sudetic Basin, Poland. Ichnos. 19, 3: 150-155

Reisz, R. 1975. Pennsylvanian pelycosaurs from Linton, Ohio and Nýřany, Czechoslovakia. Journal of Paleontology. 49, 3: 522-527.

Rowe, T., Macrini, T., Luo, Z. 2011. Fossil evidence on the origin of the mammalian brain. Science. 332: 955-957.

Zhou, C., Wu, S., Martin, T., Luo, Z. 2013. A Jurassic mammaliaform and the earliest mammalian evolutionary adaptations. Nature. 500: 163-167. doi: 10.1038/nature12429