Until now, only two mammal species were known from the Middle Jurassic of Scotland. This month, my colleagues and I added a third one to the list: Wareolestes rex. This fossil from the Isle of Skye isn’t a new species, but until now only a few of its teeth had been found in England. We now have a lower jaw, not only with permanent molar teeth, but with replacement teeth pushing up through the gumline. This confirms the little ancestor replaced its teeth like a modern mammal – and probably fed its young on milk.

The history of Scottish Mesozoic mammals – the first mammals that lived alongside the dinosaurs in the Triassic, Jurassic, and Cretaceous – doesn’t take long to recount. Scotland undoubtedly crawled with these little creatures, but unfortunately there are very few Mesozoic rocks in the country to tell the tale. Scottish deposits that date from these times, between 252 and 66 million years ago, are mostly marine; laid down under salt water. As I’m sure you can imagine, there weren’t a lot of small furry mammals living on the seabed, not now, nor in the Mesozoic.

Old Man of Storr at dawn, Isle of Skye, Highland, Scotland. Photograph: Alamy Stock Photo

The Isle of Skye is an exception. Not only are there Mesozoic rocks nestled under the volcanic blanket of the Cuillin mountain range, but rocks from the Middle Jurassic. This was a time when the first mammals (and many other groups, including dinosaurs) underwent an explosion in diversity. However, it is also a time with a poor fossil record globally, so palaeontologists are keen to find more of these rare fossils to figure out what the causes of the diversity increase could be.

Skye in the Middle Jurassic was part of a great seaway, cutting between the high ground of mainland Scotland to the East, and what are now the Outer Hebrides to the West. In some ways it was similar to today’s geography, but the landscape was radically different. Sitting in a warm sea closer to the equator, the climate was semi-tropical. The landscape was open and lagoonal: river deltas wove their way to the sea, populated by crocodiles, turtles and the occasional wandering dinosaur. In the skies, pterosaurs soared on thermals. In the sea, marine reptiles like ichthyosaurs were plentiful. On land, small mammals darted underfoot.

Mammals evolved from ‘reptile-like’ ancestors. Whatever a creationist may tell you, there are no substantial gaps to cast doubt: the fossil evidence for this in unequivocal. It is one of the most beautiful illustrations of evolutionary change over time. However, the details of early mammal evolution are still in need of fleshing out. Many important soft features don’t preserve in stone. When did we grow our fur? How did marsupials and placental mammals, with their squirmy pink babies, evolve from egg-laying ancestors? And at what point did we start oozing milk, and why?

The jaw of Wareolestes rex from the Isle of Skye. Photograph: Elsa Panciroli

There are a whole load of characteristics we associate with mammals, but one of the most obvious is that we produce milk to feed our young. Because of this, our babies don’t need ‘adult’ teeth until they are being weaned. At this point, mammal offspring replace their ‘baby’, or ‘milk’, teeth (incisors, canines, premolars) with more complex, permanent ones in order to process adult food. As they grow, additional molar teeth appear at the back of the tooth row as well. Hey presto: those gnashers are ready for business.

We began work on our Scottish Wareolestes rex jaw last year, microCT-scanning it so we could reconstruct it in more detail. That was when we discovered it held a secret. Although one side of the jaw was worn down by waves and many teeth were missing, the adult replacement teeth were still nestled inside the jaw like spring bulbs waiting to sprout. This little creature was what we call a sub-adult. It had just begun to replace its baby teeth with adult ones. Their position told us that Wareolestes replaced its teeth in the modern mammalian way, and therefore probably fed its young on milk. This is the earliest evidence for milk-producing mammals in Scotland.



The jaw of Wareolestes rex, digitally reconstructed from microCT scans and showing replacement teeth. Photograph: Panciroli et al. 2017

Wareolestes rex means ‘Ware’s brigand king’. The species name - rex – was not just for comic contrast to the more famous T. rex. It was given because this mammal was relatively large for a Jurassic mammal. Most mammals at this time were mouse to rat-sized animals. In the Triassic they had specialised for miniaturisation, and they did it so successfully they’ve survived to this day. Our Scottish fossil jaw is over 2cm long, so Wareolestes was unusually large, closer to a guinea pig in stature. ‘Ware’s brigand king’ probably ate insects, albeit bigger, possibly harder-carapaced ones. It adds to a growing understanding that mammals were more variable in size and ecology in the Jurassic than scientists previously suspected.

While it is sad to think our Scottish Wareolestes rex never made it to adulthood like its siblings, it is thanks to this wee beastie’s fossil that we can learn more about mammal evolution. This discovery, and many more to come, not only add to the list of Scottish Mesozoic mammals, but suggest we’re only just beginning to uncover the true diversity of life on the Isle of Skye during the Middle Jurassic.

References

Panciroli E., Benson RB., and Walsh S. 2017 The dentary of Wareolestes rex (Megazostrodontidae): a new specimen from Scotland and implications for Morganucodontan tooth replacement. Papers in Palaeontology. Published online 4th May. doi: 10.1002/spp2.1079