“Saber-toothed squirrel from age of dinosaurs discovered,” announced the headline in the Christian Science Monitor. The discovery of skull and jaw fragments has reinvigorated the debate about the origins of mammals and which of these tiny creatures represent evolutionary dead ends.
It is hard enough to get taxonomists to agree on the classification of modern organisms which are presented to them; imagine the verbal fisticuffs when they are asked to classify extinct animals which existed in the Jurassic Period and which are known only from fossilized mandibles and teeth. Just such a group of organisms exists, and the scientific and paleontological world has its collective knickers in a twist.
Imagine a time when the climate is shifting dramatically and there is a mass extinction of the species which dominated the known world. This is what happened at the end of the Mesozoic Era. Fortunately for life on Earth, living under the feet of the giant reptiles who ruled the land during happier times were some primitive warm-blooded creatures. These tiny creatures survived the dramatic shift which eventually gave rise to the Age of the Mammals and the subsequent domination by mammals on the continental land masses.
So how did mammals come from these humble beginnings to arrive in full glory in the 21st century? Does a modern Homo sapiens have any common ancestry with a saber-toothed squirrel?
In order to determine ancestry, or phylogeny, taxonomists use a classification scheme which is described at http://www.helium.com/items/2477190-how-scientists-classify-organisms. Paleontologists try to fit fossil remains into modern classification schemes in order to understand the genealogy of modern organisms.
To understand the argument in the scientific world, it is necessary to understand that mammals are divided into two subclasses: Protheria and Theria. Theria are creatures in which modern taxonomic characteristics can easily be recognized and used for classification.
The subclass Prototheria is further broken into two infraclasses: Allotheria and Eutheria. Eutheria includes the placental mammals.
The infraclass Allotheria is where some of the controversy begins. These “mammals” actually lay eggs and have some other anatomical characteristics not typically associated with higher mammals. It contains only two orders: Monotremata and Multituberculata. Living Monotremes are the platypus and the echidna, or spiny anteaters, of Australia and Papua New Guinea.
The Multituberculata are an extinct order which flourished during the Jurassic Period. Often classified within the Multituberculata are the Haramiyidae. The Multituberculata are tiny animals known only from fragments of jawbones and teeth. Unlike the rodents who supplanted them, they have teeth which show a puncture-crushing closure pattern, rather than a grinding-crushing pattern. This suggests a carnivorous diet rather than the more typical rodent vegetarian diet.
New fossil discoveries from this same evolutionary time period have served to illuminate the potential path for the origin of mammals. The Haramiyids were a very diverse group and were occupying ecological niches which would eventually be invaded by other mammals. Although the rodents eventually supplanted terrestrial Haramiyids, there were arboreal niches which were eventually occupied by primates.
A new discovery in China has the potential to either explain or further confuse the origin of mammals and the previous placement of the Haramiyids both within the Multituberculata and on the mammalian family tree. This fossil has a mandibular middle ear, shows a gradual transition of shape of vertebrae from the upper spine to the lower and has a very primitive “ankle.” However, the molars have very high crowns and the multiple roots are only partially fused. Unlike other known Haramiyids, the upper and lower tooth cusps alternate, which is a specialization associated with the grinding needed for vegetarians rather than puncturing, most often seen in carnivores.
Rather than settle the debate, this discovery is bound to provide full employment for taxonomists and paleontologists as they debate where this new creature fits in the evolutionary scheme. Does it fit in the current scheme alongside the known mammalian precursors, or have the Haramiyids once again been ousted from their niche as a precursor to true mammals and relegated to total oblivion by another species?