Monday, 8 May 2017

Reconstructing Wareolestes

I recently blogged about the paper I published with colleagues on a little lower jaw from the Isle of Skye, belonging to a Middle Jurassic mammaliaform called Wareolestes rex. Like many people, I believe palaeoart is a vitally important part of palaeontology, particularly as it forms a quick visual bridge between palaeontologists and the public (and other scientists!) So I'd like to share the palaeoart reconstruction I did for Wareolestes, and talk about the science behind it, and the informed speculations.

My reconstrction of Wareolestes rex. The only bit we have is the jaw, so much of the rest is informed speculation - informed by scientific evidence and living relatives. It was done in pencil and watercolour.
Mesozoic mammal art is often not especially inspiring. This is partly because, until recently, most Mesozoic mammal fossil material comprised mostly teeth, which doesn't make for exciting reconstruction. As a result, artists often choose to have their early mammals snarling to show off their teeth - the only certain part of the art! Also, there is a strange notion that small mammals are boring; but a quick google search will soon show you that they come in a huge range of sizes, shapes, colours and behaviours: inspiration for great art. However, palaeoartists have shied away from speculating about fur, whiskers, ears and eyes, erring on the side of caution and producing identikit snarling, splayed mice.

I wanted to try and do a reconstruction in an informed, but moderately speculative way. So I started work on reconstructing Wareolestes while I was still writing the paper, hoping to make informed scientific decisions based on the fossil record, and then spice it up by examining the appearance of small modern mammals.

There is a bit of a tradition among Mesozoic mammal workers of comparing our fossil beasties to the modern American opossum (Didelphis). I therefore decided to use them as the inspiration for the reconstruction. Google 'opossum', and you'll find this animal has a penchant for bearing its teeth. While I shy away from those snarling Mesozoic mammals on the whole, our Wareolestes fossil is a lower jaw with teeth, so this open-mouthed expression was fitting in this instance. Forgive me for breaking my own rules; in future reconstructions of fossils (especially those with known postcranial skeletons) I'll avoid teeth-bearing if I can help it.
The opposum Didelphis is a wonderfully charismatic creature, and often photographed bearing its teeth - handy if your fossil is a jaw with teeth and nothing else.

Our digital reconstructions (from microCT scans) were the base for drawing the mouth and teeth. As not all of the teeth were present in the Scottish fossil, I modelled the other premolars, canines, and incisors on two closely related genera: Dinnetherium and Megazostrodon. On reflection, I think the front of the snout should have been a little longer... but at this point we don't know for sure. I based the rest of the skull on Morganucodon, as Wareolestes is a morganucodontid. However, I made it more robust, because Wareolestes was larger and chunkier than it's geologically older relative.

Amazing skull and muscle reconstruction of Morganucodon by Lautenschlager et al 2016

So far, so good. But from here on, things get more speculative. We don't have much in the way of preserved fur or skin, except for a few exceptional specimens from China (but not of this genus). Undoubtedly Wareolestes and other Mesozoic mammals had fur, inherited from their non-mammal ancestors. They almost certainly had whiskers: we know this thanks to evidence in the fossil record for innervation in the snout. Whiskers probably developed in earlier non-mammalian cynodonts, which would have used them to sense their way through burrows. Therefore, whiskers are very likely to have existed in the earliest insectivorous, nocturnal mammals, being used to sense their environment and hunt for food.

I chose the shrew and the Solenodon (see below) as inspiration for the whiskers. Their whiskers extend quite far up the face, and point in multiple directions.

Beautiful shrew, showing off those sensitive whiskers. (By David Chapman, from the Cornwall Mammal Group)

The nose and ears of Mesozoic mammals, being entirely composed of soft tissue, are also impossible to reconstruct without speculation. Mesozoic mammals had well-developed olfactory bulbs, so they had a good sense of smell. I went for an opossum-like nose, simply because I liked the look of it.

This sleeping opposum's nose might be the cutest thing EVER. Will the squeeing ever stop? (I got this off pintrest, contact me if you can ID the source)
At this point in their evolution, mammals still had their post-dentary bones attached to the inside of the jaw. These bones would later reduce and detach, becoming incorporated into the middle ear. This allowed mammals to develop exquisite hearing, especially at higher frequencies. So what did they hear when the bones were still attached to the jaw? The answer is: we don't know. The postdentary bones were certainly used in hearing though. I decided a small proto-ear was fitting, based on a slightly crumpled version of a Solenodon ear. I kept them simple, small, and placed low on the head.

Close up of a Hispaniolan Solenodon (Source)
The Solenodon is a small, nocturnal insectivorous mammal found on some Caribbean Islands. It is weird on so many levels, not just because it has venomous saliva, but also as it is the only genus surviving in its family, the Solenodontidae. Phylogeneticists trace their origins back to the Cretaceous, making this an altogether unique animal and a good analogue to find inspiration for the life appearance of Mesozoic mammals. Because of this, I chose the dark back and upper face of the Solenodon as inspiration for the colour pattern on Wareolestes. However, I then decided to add a little cheek and eye patterning.

Finally, the eyes. It's hard to say how large the eye would have been relative to the head. I decided to go for something I know and love: the eye of a rat. This is a total bias on my part, because as a many-time rat-owner, I was always delighted by those dark chocolate beady eyes greeting me each morning, staring with demented twitchiness and tiny black pupils pointing in opposite directions. Mental. However it does leave me open to all the "aren't Mesozoic mammals just a bunch of rats anyway?" comments people just can't restrain themselves from making... sigh*.

So there you have it. This was my process in creating a Wareolestes rex reconstruction. I played it kind of safe - I could have speculated about the rest of the body, but decided it was a step too far. This is my first proper go at palaeoart, so theres still a lot to learn. Hopefully in the coming field seasons we'll find more of the skeleton and I'll be able to revise the image based on more evidence. In the meantime, I'd love to hear you opinions, ideas, and comments - get in touch on twitter: @gsciencelady

*Wareolestes is of course not a rodent, as rodents didn't evolve for another 100 million years. The resemblence is superficial, anatomically they are totally different.


References
 
Benoit J., Manger P. and Rubidge BS. 2016 Palaeoneurological clues to the evolution of defining mammalian soft tissue traits. Scientific Reports.

Lautenschlager S., Gill P, Luo Z-X., Fagan MJ., and Rayfield EJ. 2016 Morphological evolution of the mammalian jaw adductor complex. Biological Reviews.

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

2 comments:

  1. It was interesting to read, thanks for the explanation :-)

    ReplyDelete
  2. No problem, thank you for reading it :)

    ReplyDelete