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.
|Lifelike reconstruction of Spinolestes xenarthrosus by Oscar Sanisidro|