First evidence of Scottish mammals producing milk

Palaeontologists from National Museums Scotland and the University of Oxford researching a new Scottish mammal fossil from the Isle of Skye have discovered these Jurassic ancestors had milk teeth (like humans), and so probably produced milk for their young.

The jaw of the small mammal from Jurassic Scotland is just two centimetres long and scientists used micro-CT scanning to reveal the incredible detail of teeth from inside this tiny fossil. Called Wareolestes rex, this species was previously only known from a few single molar teeth from England.

The fossil is from a mammal which lived over 165 million years ago in the Jurassic, when dinosaurs roamed the earth. These animals are our ancient ancestors – the origins of mammals stretch right back into the Triassic. Wareolestes rex was a large mammal for this period, around the size of a guinea pig, whereas most other mammals at that time were smaller.

During this period the Isle of Skye was an environment dominated by lagoons and filled with turtles, crocodiles, pterosaurs and dinosaurs. Mainland Scotland was an island, surrounded by a semi-tropical sea filled with marine reptiles and ammonites.

The newly discovered Scottish fossil is the most complete fossil of Wareolestes rex yet known. It includes an almost complete jaw (dentary), with several teeth. The position of some of these teeth, still inside the jaw and not yet erupted through the gums, is what makes this find so special – it shows that Wareolestes replaced its teeth once, like humans and other modern mammals. It had a set of milk teeth, followed by a set of adult teeth. This pattern of tooth replacement was an important step in the evolution of mammals and is linked to the production of milk to feed young.

Elsa Panciroli, the PhD student at the University of Edinburgh and National Museums Scotland, who led the research commented,

“This is such an exciting discovery. It’s one of the most complete Middle Jurassic mammal fossils described from Scotland. This was a juvenile animal that was losing its milk teeth and the permanent teeth were just breaking through the gums. Tooth replacement like this tells us this early mammal fed on milk provided by the parent until it grew to adult size. Wareolestes would have cared for its young, which is a behaviour we associate with modern mammals.”

Co-author Roger Benson, Associate Professor of Vertebrate Palaeontology in the Department of Earth Sciences at the University of Oxford, said,

 “The fossil belongs to one of the most ancient mammal groups, the morganucodonts. But it was actually one of the last survivors of this group, living alongside a huge diversity of other early mammals during the age of dinosaurs”.

It is unlikely the mammal had breast tissue or nipples, the milk was more likely secreted onto a bare patch of skin (as is the mode of delivery for a platypus) and the young then lapped the milk off the mother’s skin. Suckling as seen in other modern mammals is unlikely to have evolved until much later, but here can be seen evidence of the evolutionary building blocks that led to suckling in later animal groups.

This fossil resolves a long-term debate about the position of the teeth of Wareolestes in the jaw. It was uncertain whether the original tooth used to describe Wareolestes (a single molar tooth) was from an upper or lower jaw, and whether it was a left or right tooth. The Scottish fossil finally answers this question: it confirms that original tooth was a lower left molar.

Until recently fossil mammal finds from this time have largely consisted of isolated teeth, some jaws, and the occasional, rare skeleton. However, in the last ten to twenty years examples of complete skeletons have been found in China. This has transformed the study of these mammals revealing evidence of greater diversity than previously understood. The majority of these Chinese fossils come from the Late Jurassic and into the Cretaceous period.

The Scottish mammal fossils currently being found on Skye are amongst the more complete mammal material found outside of China. Dating from the Middle Jurassic, these fossils predate many of the Chinese fossils. Globally, very few fossils exist from the Middle Jurassic, making these of outstanding scientific and international importance.

The Wareolestes rex fossil was found in 2015 on a Site of Specific Scientific Interest (SSSI) on the Isle of Skye on land owned by The John Muir Trust and under permit from Scottish Natural Heritage. The discovery was made during ongoing field work by a team from National Museums Scotland, the University of Oxford and the University of Edinburgh.

Colin MacFadyen from Scottish Natural Heritage said:

“This latest discovery is incredibly exciting as it increases our knowledge concerning the evolution of mammals.  We are incredibly lucky having rare Middle Jurassic rocks in which such fossils are found. Once again the geological heritage of Scotland has yielded something very special of global significance. It is exciting to consider that more outstanding fossil material awaits discovery.” 

The findings were first published online in Papers in Palaeontology in a paper entitled ‘The dentary of Wareolestes rex (Megazostrodontidae): a new specimen from Scotland and implications for Morganucodontan tooth replacement.’ Authored by Elsa Panciroli, PhD student at the University of Edinburgh/National Museums Scotland, Dr Stig Walsh, National Museums Scotland and Dr Roger Benson, University of Oxford.

Paper: Panciroli, E., Benson, R. B. and Walsh, S. (2017) The dentary of Wareolestes rex (Megazostrodontidae): a new specimen from Scotland and implications for Morganucodontan tooth replacement. doi: 10.1002/spp2.1079

The story also featured in the Guardian Science section.