Mother of all teeth
The discovery of two rare fossils in the UK has added a new branch to the mammalian tree, says Debkumar Mitra
Palaeontologists have unearthed the fossilised teeth of two rat-like creatures which lived 145 million years ago on the Dorset coast of southern England. What makes this discovery so exciting is that the teeth belonged to animals from a branch of the mammalian tree that led to modern mammals, including Homo sapiens. According to researchers from the University of Portsmouth, mammals as different in appearance as the huge blue whale and the tiny pygmy shrew are descended from the "Dorset rats". The research was recently published in the journal Acta Palaeontologica Polonica.
A paper published earlier in the journal Science claimed that the ancestor of all placental mammals evolved less than 4,00,000 years after the mass extinction of dinosaurs 65 million years ago. The research used DNA studies to draw a hypothetical creature - yet to appear in fossil records - that was a tree-climbing, insect-eating mammal weighing less than 250 grams. In shape and size it was between a small shrew and a mid-sized rat.
Steven C. Sweetman, lead author of the Acta Palaeontologica Polonica paper, said in an email interview, "Mesozoic mammals were, with one notable exception, generally shrew or rat-sized and with a similar body plan as can be seen from the fossils discovered in China and from skeletons discovered elsewhere."
Like many discoveries in pala-eontology, this one too happened by chance. As part of his undergraduate dissertation project, Ports-mouth's Grant Smith was tasked with taking fossil samples from the cliffs around Durlston Bay, near Swanage, Dorset. "Grant returned with 50 kilograms of rock samples. They were processed and sieved, the purpose being to isolate micro-vertebrate remains," says Sweetman.
When Smith examined the residue under a binocular microscope, he found a most unusual tooth that was obviously mammalian but which his supervisor, David Martill, could not recognise. Martill called Sweetman - who primarily studies small vertebrates that lived during the time of the dinosaurs - and asked if he could identify it.
"I was shocked," says Sweetman. "The tooth resembled those of mammals living some 60 million years later. The following day Grant called me to say that he had found another one. Sure enough, this was the tooth of an advanced mammal but an obviously different and larger one." According to the researchers, these mammals were small, furry creatures that were most likely nocturnal. From the teeth, researchers deduced that one belonged to a burrower that probably ate insects, while the other belonged to a larger mammal that may have eaten plants as well. Both these animals lived in the early Cretaceous period.
What makes the researchers so sure that the teeth belonged to the ancestor of all mammals, including humans?
"The teeth are of a very advanced form and clearly belong to eutherian mammals. Eutheria includes the ancestor of placentals, which includes creatures as diverse as shrews, whales and humans," says Sweetman. "These teeth are so far the earliest record of eutherian mammals and therefore the oldest known ancestors of humans. They do not represent the ancestors of all mammals; there are a great many others in now extinct groups."
In 2011, Chinese researchers discovered fossilised remains of a mouse-like mammal, which they said lived under shadow of the dinosaurs 160 million years ago. The researchers called it Juramaia sinensis and included it in the eutheria group. How then can a 145-million-year-old eutherian mammal be called the oldest identified ancestor of human beings?
"Cladistic analyses [methods of reconstructing the evolutionary tree] and morphological studies [of the form and structure of the organism] undertaken since publication of the paper in which Juramaia was described have shown that it is a stem therian and not a eutherian. The current opinion of many workers in the field is that the Juramaia's tooth morphology is sufficient evidence to exclude it from eutheria," says Sweetman. Hence, the teeth are "undisputed fossils" of mammals belonging to the line that led to human beings.
Drawing the mammalian family tree from fragmented, worn out fossils is an exciting but treacherous process. A scientific battle is currently raging among dinosaur experts. A new controversial theory has virtually thrown the accepted dinosaur family tree out of the window. As of now, the mammalian tree appears to be safe.
Till someone comes up with a new fossil, let us welcome Durlstotherium newmani - one of the new species has been named - as our ancestor.
• In mammals, teeth have reached their highest peak of evolution; they are more complicated and more efficient than in other vertebrates.
• Living or dead, teeth have much to contribute to the study of palaeonto-logy, morphology and ecology. They are the hardest part of any mammal and, therefore, the part that is most often fossilised and recovered.
• The present day human teeth morphology is a result of mammalian evolution that began about 225 million years ago. From a simple cone to a complex and diverse pattern of cones and ridges, teeth evolved in response to changes in the Earth’s environment.