Scotland's First Jurassic Mammal - our paper is out!

The first mammal fossil ever found from the ‘time of dinosaurs’ was announced almost 200 years ago*. It was a small jaw just a couple of centimetres across, which emerged from Middle Jurassic rocks in Oxfordshire, England. It was 150 years later that the first such beast was found in Scotland. This week my colleagues and I described that Scottish specimen in detail for the first time, along with stunning new material from the same species.

The most complete Mesozoic-aged mammal jaw found in Scotland so far, belonging to Borealestes serendipitus, a small insectivorous animal.

The animal these fossil bones belong to is delightfully called Borealestes serendipitus, the ‘northern rogue, found by chance’. But it was not just luck, its founder Dr Michael Waldman tells me. Mike was leading a group of school pupils on an outdoor skills trip to the Isle of Skye in the 1970s, and he knew from his research that there were fossils in the area. He never imagined they would be so spectacular or rare of course. Upon finding the first jaw, Mike brought the tiny fossil, no bigger than a thumbnail, to his mentor and friend, Professor Robert ‘Bob’ Savage at the University of Bristol. Together they returned to Skye find more fossils from Scotland’s Jurassic heyday.

Since the first publication on Borealestes in 1972, very little work has been done on it. Several jaws, a partial skeleton, and some isolated teeth were found, but only two partial jaws were published, and the descriptions were sparse. This new paper in the Journal of Vertebrate Paleontology is part of my ongoing research on the mammals of Jurassic Scotland, begun while I was at National Museums Scotland (NMS) and the University of Edinburgh. I’m part of a team from NMS and the Universities of Oxford and Birmingham who have been working on the Isle of Skye since 2010, and finding yet more of this animal and the many creatures that lived alongside it.

A new fossil jaw I found in 2016. Only the tooth tips were protruding from the rock.

Our paper is first and foremost anatomical, clarifying the identification (diagnosis) of Borealestes. In other words, how do you recognise Borealestes if you find it? This is predominantly based on the shape of the teeth. In 2003 a close relative, Borealestes mussettae, was named from isolated teeth from the English Jurassic-aged site of Kirtlington. Our new paper sets out the differences between the two species, and re-evaluates all previously known specimens in light of this new information (some were previously misidentified). 

The fossil specimens are quite exceptional. They include the most complete Mesozoic mammal jaw found so far in Scotland: a very long and slender dentary with delicate jaw hinge and perfect insect-munching teeth. There is also a Borealestes specimen I found myself during fieldwork and so feel a personal connection to, as well as the upper tooth row of a partial skeleton that I’m currently completing work on with my colleagues (watch this space!) 

The micro CT-scan of the jaw I found in 2016 revealed the rest of the bone hiding below the surface. Without computed tomography (CT) it is unlikely we could have studied this fossil intact.

This publication is the next in a series of papers on Borealestes. In 2018 my colleagues and I described the ear bones of this Jurassic animal, whereas this paper focuses on the jaw and dentition. With five jaws figured in this publication, all from micro CT-scans, the upper molar row and incisors known for the first time, and countless individual teeth re-evaluated, this paper expands our understanding of this modest docodontan, and adds significantly to the number of Mesozoic mammal specimens known from Scotland. 

*The jaw belonged to Phascolotherium. Other mammal jaws were found previous to this one, but it was the first one recognised for its significance, and was announced (alongside Megalosaurus) during the inaugural address of the Geological Society of London by its first President, William Buckland, in 1824.

References

Panciroli, E, Benson, R, and Luo, Z-X. 2019. The mandible and dentition of Borealestes serendipitus (Docodonta) from the Middle Jurassic of Skye, Scotland. Journal of Vertebrate Paleontology. https://doi.org/10.1080/02724634.2019.1621884

Say hyoid to the latest Jurassic mammaliaform from China

China continues to amaze us with its fossil treasures. The latest is a 165 million year old Jurassic mammal, belonging to my favourite group, the docodontans. Docodontans are one of the earliest branches of mammaliaforms, the wider group that includes Mammalia. They are our cousins, and in the last twenty years they’ve transformed our understanding of mammal ecological diversity in deep time, because they’ve basically done it all before. This group includes the first mammal specialist diggers, swimmers and tree-climbers, occupying these niches long before modern mammals were on the scene. Their teeth were complex, with a triangular arrangement of cusps and troughs that later convergently evolved in modern mammals. In other words, docodontans are the trend-setters of the mammal world.

The docodontans were incredibly ecologically diverse in the Jurassic. They include climbers like Agilodocodon, swimmers like Castorocauda, and diggers like Docofossor. Amazing palaeoart by April I Neander, University of Chicago.

This new genus and species is called Microdocodon gracilis. The name says it all: this creature was tiny and slender. With an estimated body mass of just 5-9 grams, it’s on a par with the modern shrews, Sorex. Unlike them, it has an elongate face and limbs and a long tail, suggesting it may have been a competent tree-climber as well as ground-scurryer (this combination is also known as being scansorial). What’s really stunning about this wee beastie is something common to many of the spectacular fossils of China: it is exquisitely preserved with almost every bone in articulation. This includes bones that are usually too small and fragile to survive fossilisation, such as the hyoid.

So what is a hyoid? You may know it from crime dramas like CSI; it’s the bone in the throat that is often damaged when a murder victim has been strangled. If you locate your larynx (the ‘Adam’s apple’) in your own throat, then feel upward to the area where your chin meets your neck, you can feel it in there. It’s not in contact with other bones, but floats there, anchored in place with muscles and ligaments. The hyoid is where your tongue and the other muscles in the floor of your mouth attach, and it is intrinsic to swallowing and moving the tongue.

Position of the hyoid in humans (red). From Wikipedia

When you’re looking at mammal skeletons in museums, the hyoid bone is often conspicuous by its absence – it’s a tricky wee blighter, and I suppose that it is often lost or too fiddly to make it to the skeletal mount. However, it is an important and unique part of the skeleton, inherited by vertebrates from their common ancestor over 375 million years ago. The hyoid originates from the same embryonic structure that becomes the second gill arch of fishes; those loops of bone that support the fish breathing apparatus. In mammals it has taken on a special significance, because it’s thanks to the hyoid that mammals literally suck.

Mammals use the control provided by the hyoid to suckle and feed on liquefied food, often processed by their complex teeth, but also including milk from their mothers. Mammals are not the only animals that suck of course. There are other vertebrates that use the hyoid to create a vacuum for feeding. The turtle Chelus fimbriata, or mata mata, for example, has a large hyoid and a pair of flappy cheeks, which it opens suddenly to draw-in passing fish by suction. As well as suction, this bone allows for special movement of the tongue: lizards and snakes carry out their characteristic tongue-flicking thanks to their hyoid bone and attachments.

Embryology reveals the origin of the structures of the face. Source

Due to its role in swallowing, the hyoid can provide clues about the kind of food being eaten by the earliest mammals. Microdocodon has a saddle-shaped and complex hyoid that is the clear predecessor of later mammalian hyoid bones. This tells us that this animal had a muscular throat with good control of the ability to swallow. The authors suggest that the very modern structure of Microdocodon’s hyoid is linked to the ability of early mammals with complex teeth to chew their food until it was well liquefied. This required a sophisticated and controlled ability to swallow the food. We also infer from the tooth replacement patterns of the first mammals that they likely fed milk to their young (they had 'milk teeth' followed by adult teeth), which would also require controlled ability to suck and swallow. Therefore it is likely that this complex hyoid structure appeared in the common ancestor of docodontans and the rest of the mammals, and not in earlier mammal relatives, because the latter didn’t have such complex food processing abilities.

More amazing artwork by April I Neander

Unlike modern mammals, docodontans still had their middle ear bones attached to their jaws. In modern mammals these bones have become the malleus and incus, and help provide our extra-sensitive hearing capabilities. This new Chinese fossil gives us previously unknown information about how the hyoid was positioned in ancient mammals – and therefore the larger patterns of its evolution and function. The only other Jurassic hyoid known belongs to Vilevolodon, a haramiyidan mammal. Because there is an ongoing disagreement about whether haramiyidans are an early branch of mammaliaforms, or a much more derived group belonging to crown Mammalia (that’s a blog for another time), the evidence from Vilevolodon comes with baggage. Microdocodon however, is a safe phylogenetic bet, and so uncontroversially clarifies the structure of this feature in the earliest mammals.

Such a great fossil, and continued proof that docodontans are among the most exciting group of mammals from the Mesozoic. 

References

Lemell, P, Beisser, C, Gumpenberger, M, Snelderwaard, P, Gemel, R, Weisgram, J. 2010. The feeding apparatus of Chelus fimbriatus (Pleurodira; Chelidae) – adaptation perfected? Amphibia-Reptilia. 31: 97-107.

Luo, Z-X,  Meng, Q-J, Grossnickle, DM, Liu, D, Neander, AI, Zhang, YG, Ji, Q. 2017. New evidence fo rmammaliaform ear evolution and feeding adaptation in a Jurassic ecosystem. Nature 548, 326–329.

Zhou, C-F, Bhullar, B-AS, Neander, AI, Martin, T, Luo, Z-X. 2019. New Jurassic mammaliaform sheds light on earlyevolution of mammal-like hyoid bones. Science, 365:276-279.