Palaeontologists from the University of Portsmouth have discovered two new fossil teeth from the Cretaceous rocks of Dorset. They are from the branch of the mammal tree that led to modern mammals – including humans. Their findings suggest the origin of modern mammals lies even earlier in geological history, as supported by previous fossil finds from China.

The new fossils were found by an undergraduate student, Grant Smith. “Grant was sifting through small samples of earliest Cretaceous rocks collected on the coast of Dorset … in the hope of finding some interesting remains,” said Dr. Steven Sweetman, a research fellow at the university who led the research. Grant knew he had a mammal, but didn’t realise he’d discovered something so special. “Unexpectedly he found not one, but two quite remarkable teeth.”

Grant and his supervisor David Martill, the paper’s co-authors, called on Sweetman to take a look and give his opinion. Sweetman recalls, “my jaw dropped! I realised straight away I was looking at remains of Early Cretaceous mammals.”

Two fossil teeth of the Purbeck Mesozoic mammals, Durlstotherium (A1-4) and Durlstodon (B1-4), named after Durlston Bay in Dorset. These Jurassic mammals are ancestors to placental mammals, and are oldest of their kind found in Europe. Photograph: SCS/Sweetman et al. 2017

These teeth belong to two different animals, both members of the group known as Eutheria: the group that includes all modern mammals. They have been named Durlstodon and Durlstotherium, after Durlston Bay in Dorset, where the fossils were collected.

Mammals are animals that possess fur, feed their young on milk, and have complex specialised teeth. They have their origins in the Triassic at the same time as the dinosaurs. They split into many different groups during the Jurassic and Cretaceous, including branches that died out long before the impact that killed the non-avian dinosaurs. The groups of mammals that survived the extinction event include the monotremes (platypus and echidna), the marsupials (such as kangaroos and koalas), the multituberculates (which then died off around 35m years ago), and the eutherians, or placental mammals. This last group is now the most successful mammal lineage on earth, having evolved creatures as diverse as the blue whale, the grasshopper mouse, and homo sapiens.

Fossils from the first placental mammals are relatively rare in the UK. While these Dorset fossils may seem underwhelming, comprising only two molar teeth with no roots, the unique specialisations of mammal teeth for processing food result in complex tooth shapes. These shapes evolve in patterns that allow palaeontologists to pinpoint what group a mammal belongs to, meaning that even a handful of teeth allow scientists to identify new species.

A mandrill monkey yawns in his enclosure at the zoo in Dresden, eastern Germany, showing off his complex teeth: insicors, canines, premolars and molars. Photograph: Matthias Hiekel/AFP/Getty Images

The evolution of mammals has been well fleshed out since the first mammal from the Mesozoic (the time period that includes the Triassic, Jurassic and Cretaceous) was found in English Jurassic rocks in the early 1800s. Recently new fossil discoveries – particularly from China – have transformed our understanding of early mammals. Whereas researchers used to believe Mesozoic mammals were simple, rat-like animals cowering in the shadows of dinosaurs, we now know that by the end of the Jurassic, various mammal lineages were experimenting with different ecologies, including otter-like semi-aquatic forms, and mole-like diggers. Even 160m years ago, mammals were more diverse and successful than anyone had previously suspected.

However, there are still some gaps in our knowledge. The exact origins of eutherian mammals is among them. The earliest eutherian mammal currently known, Juramaia, was described in 2011. It comes from the Jurassic rocks of Tiaojishan Formation in China, and is around 160m years old. The fossil skeleton includes jaws, forelimbs, vertebrae and ribs. Juramaia predates the Purbeck mammals by fifteen million years. This suggests that if eutherian mammals had their origins in Asia in the Jurassic, they had spread across the continent to Europe by the Early Cretaceous.

Juramaia, a Jurassic eutherian mammal ancestor. Fossil found in the Tiaojishan Formation , China. Composite: Zhe-Xi Luo/Mark Klingler/Carnegie Museum of Natural History

“It is clear that the Early Cretaceous of the UK had eutherians,” said Professor Zhe-Xi Luo of the University of Chicago, a world expert on mammals of the Mesozoic. “These fossils are a very interesting hint that eutherian mammals were already present in Western Europe at this time.”

Trying to work out exactly when eutherians originated is not straightforward. Palaeontologists use the anatomical shape and characteristics of fossils to work out how extinct animals are related. By giving these characters numerical scores, they can use computer programs to work out the complex networks of relationships mathematically. So, for example, if a tooth possesses a certain cusp it can be given a score of 1; if it does not, it can be scored as 0. These numbers are fed into programmes that map out different possible family trees based on the scores. It also gives the statistical likelihood of each branch on the tree being close to the “true” relationship, or not. This branch of science is called phylogenetics. It is one of the most important methods used by modern palaeontologists, and a common component of published papers describing new species.

To work out relationships between living mammals, scientists can use molecular data. As there is no molecular data for fossils, palaeontologists often combine the molecular results with those generated from the fossil anatomy. But this can lead to problems when the molecular results don’t match the results found using fossils.

The origins of eutherian mammals is one such problematic case. Sweetman and his colleagues suggest that Juramaia might not be eutherian at all, based on a study which estimated Eutheria had its origins only 91m years ago. But other estimates have placed the origin at up to 193m years ago - a whopping 100myear earlier, and comfortably encompassing Juramaia.

The hypothetical placental mammal ancestor, a small, insect-eating animal. Illustration: Carl Buell/American Museum of Natural History/AP

The discrepancies between these results are caused by differences in the data used to calculate them. For example, an analysis with lots of modern mammals and only a few fossils will tell you a lot about recent branches of the family tree, but not give useful information about ancient relatives. Palaeontologists therefore consider the results of phylogenetic analyses as “theoretical”. They are constantly refining and adding to the data, fuelling continued research and debate. Juramaia, Durlstodon and Durlstotherium all fall inside the range of estimates for the origin of eutherians, so they are currently the three oldest eutherians known.

The new Purbeck mammal teeth are an exciting addition to the impressive list of Mesozoic mammals known from the United Kingdom. Their age makes them the second oldest placental mammals ever found, proving that there is still plenty to learn about the Jurassic and Cretaceous fossils of this small Atlantic island.

Martill added: “The Jurassic Coast is always unveiling fresh secrets and I’d like to think that similar discoveries will continue to be made right on our doorstep.”

References

Luo Z-X, Yuan C-X, Meng Q-J, and Ji Q. 2011. A Jurassic eutherian mammal and the divergence of marsupials and placentals. Nature, 476.

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O’Leary MA, Bloch JI, Flynn JJ, Gaudin TJ, Giallombordo A, Giannini NP, Goldberg SL, Kraatz BP, Luo Z-X, Meng J, Ni X, Novaceck MJ, Pernini FA, Randall ZS, Rougier GW, Sargis EJ, Silcox MT, Simmons NB, Spaulding M, Velazco PM, Weksler M, Wible JR, and Cirranello AL. 2013. The Placental Mammal Ancestor and the Post-K-Pg Radiation of Placentals. Science, 339, 662-667.

Sweetman SC, Smith G, and Martill DM. 2017. Highly derived eutherian mammals from the earliest Cretaceous of southern Britain. Acta Palaeontologica Polonica.