145-million year-old mini mammals discovered at English beach
The earliest known placental species renews debate over the emergence of mammals. Andrew Masterson reports.
The long and tortuous evolutionary journey of mammals that led to the emergence of Homo sapiens and other great apes began more than 145 million years ago with the appearance of some small placental species that no doubt spent a lot of time dodging the attentions (or just the feet) of late-Jurassic dinosaurs.
And the earliest fossilised examples of these eutherian mammals – that is, species either with placentas or more closely related to placental mammals than marsupials – have been discovered not in some distant exotic wilderness but in the cliffs edging the genteel English county of Dorset.
Reporting the journal Acta Palaeontologica Polonica, a team led by Steve Sweetman from the University of Portsmouth, UK, unveils teeth from two new species unearthed from the cliffs of Durlston Bay on the east coast of Dorset.
Both species were small, and one was possibly a burrower that fed on insects.
The significance of the teeth was initially noticed by co-author Grant Smith, an undergraduate student, who thought they did not look right for the age of the rock deposit he was investigating and passed them to Sweetman for comment.
“I was asked to look at them and give an opinion and even at first glance my jaw dropped!" he recalls.
"The teeth are of a type so highly evolved that I realised straight away I was looking at remains of Early Cretaceous mammals that more closely resembled those that lived during the latest Cretaceous -- some 60 million years later in geological history.”
The question of the identity of the earliest eutherian mammal on the fossil record is a surprisingly contentious one. Other contenders include three specimens from China – one from the late Jurassic and two from the early Cretaceous – but all three have been thrown into doubt after renewed molecular and morphological analysis.
In the absence of other claimants, thus, Sweetman’s Dorset finds may at least temporarily hold the crown.
Although they both date to the beginning of the Cretaceous, their existence, the scientists write, supports the contention that eutherian mammals emerged earlier, during the Jurassic.
One of the new species, by the way, has been formally named Durlstotherium newmani. The first part of the name references the geographic location in which it was found. The second is a tribute to a man called Charlie Newman, who is not a palaeontologist but is, in fact, the landlord of a pub called the Square and Compass – the nearest place to the cliffs in which fossil hunters can get a beer.