Friday, 16 October 2015

Exceptionally preserved Early Cretaceous mammal

There's an ongoing theme/belief in vertebrate palaeontology that if you want to work on Mesozoic mammals, you have to like teeth. This stems from the fact that a large number of early mammal or mammaliaform fossils are actually teeth, and the different species, genera, or even families are primarily distinguished from each other due to the features of their different teeth, and in particular their molars. This can be related to their exact dental formula (i.e. how many incisors, canines, premolars and molars they have), the number of cusps found on specific teeth, etc. In fact, many names of species or families come from the teeth: Morganucodon, one of the best known early mammals means "Glamorgan tooth", from the Vale of Glamorgan in Wales where it was first found, identified first by a tooth; and eutriconodonts have three ("tri") cones or cusps ("con") on their teeth, while many more end with "dont" or "don" or "dens", all different ways to say teeth.

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 xenarthrosus is a newly described eutriconodont mammal from the Early Cretaceous of Spain. The skeleton reveals a variety of functional features, suggesting Spinolestes was a proficient mover on land, and may have dug into the ground when necessary, but not necessarily adapted for this kind of lifestyle as it lacks the dental and skeletal characters typically associated with animals that habitually live underground. It had vertebrae similar to those found in xenarthrans (anteaters, armadillos and sloths), a condition that it evolved convergently (meaning the existence of this feature in both groups is not due to shared ancestry, but rather it evolved twice). While these are all interesting features, what is the most remarkable about this specimen is the soft tissue and integumentary structure preserved.

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
Martin T, Marugán-Lobón J, Vullo R, Martín-Abad H, Luo Z, Buscalioni AD. 2015. A Cretaceous eutriconodont and integument evolution in early mammals. Nature 526: 380-384.