When you think of mammals you probably immediately think of something furry. In fact hair is ubiquitous amongst the mammal groups (except of course those that have secondarily lost it like the cetaceans) and so palaeontologists have been very keen to find traces of it in the fossil record. Discovering when fur first evolved in the mammals would give us a clearer understanding of why and how our ancestors developed this unique trait. And it really is unique. Although there are many other animals, from bees to pterasaurs, which have (or had) a covering of ‘fur’ true ‘hair’ is limited to the mammals. There are two main parts to hair, the ‘follicle’ which is embedded in the skin and from which the hair grows, and the ‘shaft’ which is the main body of the hair. The shaft is made of a protein called keratin which is also the main constituent of your nails. In modern mammals hair has been adapted for a variety of function from water proofing to camouflage so it is very important stuff and has been an integral part of our family’s ecological success. Until now though the antiquity of this trait has been a little uncertain. Now enter a brand new fossil discovered earlier this month (October 2015).
This new mammal, officially named Spinolestes xenarthrosus, is 125 million years old, putting it back to the Cretaceous at a time when dinosaurs still dominated the planet. While the reptiles experimented with feathers this tiny creature scurried around at ground level feasting on the insects of its wetland home. This particular specimen was around the size of a juvenile rat and weighed in at somewhere between 50-70 grams but although it was small it was perfectly formed. The fossil is exquisitely preserved by virtue of an unusual process called phosphatic fossilisation, which has allowed individuals hairs and internal organs to still be visible under a scanning electron microscope.
Everything about this fossil is extraordinary. It is the earliest mammal ever discovered to have hair, and the preservation is good enough that the follicles are visible too. From this the researchers were able to discover that Spinolestes grew its hair in the way seen in modern mammals meaning that the origin of our own hair probably dates back to a similar time. That’s not all. This incredible little creature also has preserved traces of internal organs, including the liver and curved muscle of the diaphragm. This is the earliest ever example of mammalian organs found in the fossil record. It even has the earliest ever external ears seen in our lineage.
So here we have one of the earliest known animals ever to have hair, the earliest record of internal organs, and the earliest external ears, but there is one final first for our list. It is also the earliest example of a particular type of skin infection called dermatophytosis. Never heard of it? Well chances are you have really because in humans its sometimes called ‘athlete’s foot’. The medical name dermatophytosis refers to the skin condition caused by a group of closely related fungi, while the colloquial name is usually reserved for the same infection when its found on the feet. Its sometimes called ‘ringworm’, which is deceptive because its not caused by a worm at all, but this name does neatly encapsulate the symptoms. Usually these are a circular rash or lesions which are very itchy and often enflamed. Its very common in humans and many of our fellow mammals, but thankfully it is also relatively easy to treat. Most people catch it from exposure to other people’s bare skin, or from surfaces touched by other peoples’ bare skin, hence its association with contact sports. The fungi which cause it all feed on keratin meaning they usually colonise areas of the body with hair or close to the nails but they can feed directly off the keratin in your skin too. They like warm, moist conditions and apparently they have been feeding on mammals since at least the early Cretaceous. In the case of Spinolestes the evidence comes from those unique hair samples, some of which are oddly truncated. This is a clear clinical sign in modern mammals of this particular class of skin disease and so it seems that Spinolestes was a sufferer.
This infection probably wasn’t too much of a problem for Spinolestes and was unlikely to have been the cause of death, but it does show how ancient many of our modern-day parasites really are. Since the days of the very first animals there have undoubtedly been parasites evolving to exploit them but its always nice to find actual physical proof.
Reference: Martin, T., et al. 2015. A Cretaceous eutriconodont and integument evolution in early mammals. Nature. 526(7573). DOI: 10.1038/nature14905