This is not to say that these animals are known only from teeth. Morganucodon for example is known from many bones. Unfortunately, in the area of Wales where they are commonly found, the skeletons are all broken up and the bones are often broken, and always separated from each other (disarticulated), making it difficult to do more studies with them. However, once in a while, a very well preserved Mesozoic mammal pops up, which is where Spinolestes xenarthrosus comes in. Described this week in Nature by Thomas Martin and colleagues, this new mammal is very interesting.
|Fossil of Spinolestes xenarthrosus from Martin et al. (2015).|
Spinolestes, amazingly, has a number of organs preserved including the outer ear, and possible lung, and liver tissue. The authors have even identified the presence of a muscular diaphragm. Spinolestes is characterized by having a mane containing long “guard hairs” along the neck and shoulder region, and longer hairs along the middle of the back and tail, making a hairy crest along the midline of the animal, while the rest of the body is covered in more typical shorter, soft underfur. In addition to these hairs, it also had “protospines” along the back of the hip-region, which are larger than hairs and formed by several smaller hair-like filaments merging together, similar to how spines are formed in modern mammals like hedgehogs. These features in combination show that mammals evolved this covering of a softer undercoat, denser and thicker guard hairs, and stiffer spines already in the Early Cretaceous, relatively early on in mammal evolution, a feature that is still seen in mammals today.
|Lifelike reconstruction of Spinolestes xenarthrosus by Oscar Sanisidro|