Wareolestes Bears its Teeth

I've been lucky enough to see my first scientific paper come out this year (first of many I hope!) with my wonderful colleagues, Stig Walsh (National Museum of Scotland) and Roger Benson (Oxford). We describe a lower jaw found in the last couple of years during our ongoing palaeontological fieldwork on the Isle of Skye, Scotland. It is from the Kilmaluag Formation: which is Bathonian (Middle Jurassic) in age. This lower jaw, or dentary, is from a relatively large Mesozoic mammal living around the lagoons of Jurassic Skye. What’s more, it retains replacement teeth inside the jaw. This is the first evidence for mammalian milk-production in the Scottish fossil record.

The surface of the jaw was somewhat abraded by wave action, but microCT scanning revealed exceptional details (see below). The fossil is now part of the collection at the National Museum of Scotland: NMS G.2016.34.1

The Middle Jurassic was a pivotal time for most major groups of animals: the dinosaurs exploded in diversity, as did Mesozoic mammals. Unfortunately for palaeontologists, globally there is a real paucity of Middle Jurassic rocks. This makes the sediments of Skye internationally important. On the island, myself and my colleagues are uncovering the secrets of Middle Jurassic mammals and other small vertebrates from this elusive time period.

We identified the jaw as belonging to Wareolestes rex. Scampering between the copious turtles and crocodiles near the water’s edge, or perhaps living further inland and being washed by river into one of these lagoons upon death, Wareolestes is one of the many mammals that lived alongside their infamous dinosaurian cousins at this time in Earth's history.

Wareolestes was previously only known from a couple of isolated molar teeth from England. Palaeontologists disagreed about whether the holotype (the tooth that defines this species) was an upper or lower molar, and whether it was left or right. Now that we have a dentary with teeth in it, we can compare it with the holotype and say that the holotype is a lower left.

The holotype of Wareolestes rex, NHMUK PV M36525.

Wareolestes rex means 'Ware's brigand king', and was named in 1978 by Eric Freeman as one of several species recovered from productive fossil mammal beds in Oxfordshire. Productive is a relative term for mammal palaeontologists. We aren’t talking about whole skeletons, or even skulls. We’re talking teeth – lots and lots of them. Sifting through them, many new species of mammal were discovered and named. You can flick through hundreds of them in the collections at Oxford Museum of Natural History and the London Natural History Museum, each mounted on a pinhead and placed carefully in its own miniature test tube.

This Scottish jaw is not only the most intact fossil of this species yet found, but only the third species of Mesozoic mammal described from Scotland. (There are others: watch this space, as I'll be publishing on them in the near future).

The jaw was microCT scanned, revealing beautiful detail. This is a lingual view. Inside, several unerupted adult teeth tell us this animal was a sub-adult, and that it had the modern pattern of mammal tooth replacement.

Our Scottish Wareolestes rex fossil would have once held five premolar teeth and three molars. Only the three molars remain in place above the gumline. The front of the jaw with canines and incisors (the puncturing and biting teeth) is missing. The part of the jaw that hinged against the skull is also missing at the back. It may seem quite paltry: only 2cm long, missing lots of bits, worn on one side by the crashing waves... Yet this is unusually complete for a Mesozoic mammal, let alone one from the UK, or the Middle Jurassic.

When we microCT-scanned it, we discovered the replacement teeth were still sitting inside the jaw. They are all more or less level, suggesting the teeth were replaced at approximately the same time. Sadly, the wave-damage means we can’t be more definitive about the sequence of replacement, but hopefully future finds will confirm this.

This is my palaeoartistic reconstruction - check out tomorrow's blog for more on how it was made.

This is my palaeoartistic reconstruction of Wareolestes rex, based on our jaw fossil from Skye. (Come back soon for my blog on Reconstructing Wareolestes, where I discuss the science and speculation behind my reconstruction of our Jurassic Scottish beastie). While it is sad to think it never made it to adulthood like its siblings, because this wee beastie was preserved in the fossil record, we can learn more about Mesozoic mammal evolution. It also adds to our picture of great diversity of Middle Jurassic ecosystems on the Isle of Skye.

My 3D print out of Wareolestes rex.

All of the microCT scan data, plus 3D models of the dentary, are available on Data Dryad - DOI: doi:10.5061/dryad.5n36j - so you can download a model and print your own copy. Mesozoic mammals: every home should have one.

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References 

Freeman EF. 1979 A Middle Jurassic mammal bed from Oxfordshire. Palaeontology, 22, 135-166.

Panciroli E., Benson RBJ., 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