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.

International Women's Day: Frances Mussett

I was recently asked by a reviewer to amend the name of one of the animals my colleagues and I were re-describing, Borealestes mussetti. ‘It should be B. mussettae’ the reviewer explained, ‘as it honours Frances Mussett’. I’d encountered Mussett’s name many times in my research, but through my own subconscious gender bias, had assumed Mussett was a man. It turns out that the original description of Borealestes mussetti used the wrong gender when erecting this species, and so Frances Mussett became masculinised both through taxonomy and assumption. 

This is one of the subtler ways in which women can be written out of science, and out of history. With International Women’s Day, 8^th March, on our doorstep yet again (and Women and Girls in Science Day just passed, 11^th February) it feels right not only to amend her namesake species (paper coming soon!), but to share some of the achievements of Mussett’s scientific career, and her role in early mammal palaeontology.

Despite being on so many of the most significant papers on Mesozoic mammals, Frances Mussett is poorly biographed. She is frequently mentioned as the assistant of Kenneth Kermack (1919-2000), one of the big names in Mesozoic mammal palaeontology, but seldom as a researcher in her own right. Mussett was a work colleague and friend of the Kermacks (a married couple and both significant palaeontologists), as well as carrying out field work with them. Her publications included co-authorship of papers on two of the most important early mammaliaforms, Kuehneotherium and Morganucodon, and although she didn’t hold a PhD she was undoubtedly a capable researcher and meticulous writer.

Reconstruction of Morganucodon by Bob Nicholls

After her bachelors degree at Birkbeck, Universoty of London, Mussett joined the thriving team of palaeontologists at University College London (UCL), collecting fossils and publishing in the 1950s-70s. The team included Professor Kenneth Kermack, Dr Doris Kermack, Jackie Papworth, Patricia Ferguson (née Lees) and Mussett, the latter two women employed by the department as Kenneth Kermack’s research technician and assistant, respectively. While Ferguson and Mussett’s roles overlapped, Mussett was more heavily involved in manuscript preparation, as well as some teaching. Mussett suffered from diabetes, and had to be strict in order to manage the condition. Restrictions at the time mean that due to her diabetes Musset was not allowed to drive, and so Lees did any driving necessary during fieldwork. 

In the 1960s the UCL team worked regularly at the Welsh fissure fills – a series of cracks in Carboniferous rocks that are filled with sediment from the Late Triassic to Early Jurassic. This sediment has proven rich in fossil material, and Kermack, Kermack and Mussett named a new genus of one of the very earliest mammaliaforms, Kuehneotherium, from among these finds (1968). 

Kermack, Mussett and Rigney (1973) were the first to realise that the Late Triassic and Early Jurassic mammals with triconodont teeth (where the main cusps are arranged in a line along the tooth axis) were a separate group from mammals with the same tooth arrangement that lived later, in the Middle Jurassic to Early Cretaceous. These were once collectively known as ‘Triconodonta’ (Osborne, 1888), but the team divided them into sub-groups, calling their earlier ones Morganucodonta, and the later ones Eutriconodonta. This was an important shift in our understanding of early mammal relationships, and later it was realised the two suborders were completely distinct (Kielan-Jaworowska et al., 2004).

One of the most famous and pivotal early mammals is Morganucodon. It belongs right at the base of the mammaliaform tree, is well-known in terms of fossil material. Along with Kuehneotherium and Sinoconodon, this genus is a touchstone for Mesozoic mammal palaeontologists. Mussett co-authored two major monographs on Morganucodon’s skull and jaw with Kenneth Kermack (1973,1981). Professor Kielan-Jaworowska, one of the most important Mesozoic mammal palaeontologists of all time, said ‘in addition to this wonderful skull, Kenneth and Frances had at their disposal thousands of isolated bones of Morganucodon watsoni from the fissure fillings in Wales’ (Kielan-Jaworowska, 2013: p80). 

The research Mussett was involved in through her role at UCL spanned much of the synapsid family tree, including examining the jaw articulation of docodonts, erecting the name Eupantotheria (Kermack and Mussett, 1958), and looking at the ears of Permian synapsids (Kermack and Mussett, 1983). In the 1970s the UCL team began work at Kirtlington Cement Quarry, a Middle Jurassic locality in England rich in small vertebrate fossils. This led to publications on the mammals from this site (Kermack, Lee Lees, Musset, 1987), as well as lissamphibians (Evans, Milner and Mussett, 1988, 1990). As an important member of the team, Mussett was part of the fieldwork, as well fullfilling her vital role assisting in manuscript preparation and editing.

After the Kermacks retired, Frances Mussett stayed on at UCL, carrying on teaching and other work, but she never published independently. Upon retirement, she donated many specimens to UCL as well as the Natural History Museum in London. Collected between 1961 and 1994 and unusually well-documented (even including the day of collection in some cases), these finds included material from the Isle of Wight, such as the bones of marine reptiles. There were also specimens of Morganucodon, Kuehneotherium and the many other exceptional mammals and small vertebrates collected during her career working alongside the Kermacks (many of these are part of the Kermack Collection). In 2003, Mussett was acknowledged in the naming of Borealestes mussetti, to recognise the ‘major participation of Dr [sic] Frances Mussett in the accumulation of the Kirtlington fauna’ (Sigogneau-Russell, 2003). 

The holotype lower molar tooth of Borealestes mussettae.

And so we come full circle. Here I am sitting in the Oxford Natural History Museum with specimens of B. mussettae – now emended to the feminine form – glinting up at me from under the lights of the microscope. Mussett herself must have spent uncountable hours this way, gazing down the eyepiece at tiny perfections of anatomy from the dawn of mammals. I wonder if she ever saw her namesake species? Undoubtedly the many hours she and women like her have spent contributing to science deserves recognition, not just on International Women’s Day, but every day.

(Note: this blog was emended as previously Pat Lees and Patricia Ferguson were listed as seperate lab members, but they were one and the same: Patricia 'Pat' Lees married John Ferguson, a senior technician at UCL and took his surname. The university Mussett attended was aldo added, and the information about her diabetes. 10/3/19 Thanks to Pam Gill for this extra information.)

References

Evans SE, Milner AR and Mussett F. 1988. The earliest known salamanders (Amphibia, Caudata): a record from the Middle Jurassic of England. Geobios, 21: 539-552.

Evans S.E, Milner AR and Mussett F. 1990. A discoglossidfrog from the Middle Jurassic of England. Palaeontology, 33: 299-311.

Gabriel N. 2018. Specimen of the Week 337: The Mussett Collection.

Kielan-Jaworowska Z, Cifelli RL and Luo Z-X. 2004. Mammals from the age of dinosaurs: origins, evolution, and structure. Columbia University Press.

Kielan-Jaworowska Z. 2013. In pursuit of early mammals. Indiana University Press.

Kermack KA and Mussett F. 1958. The jaw articulation of the Docodonta and the classification of Mesozoic mammals. Proceedings of the Royal Society of London. Series B-Biological Sciences, 149(935), pp.204-215.

Kermack DM, Kermack KA and Mussett F. 1968. The Welsh pantothere Kuehneotherium praecursoris. Journal of the Linnean Society of London Zoology, 47: 407–423.

Kermack KA, Mussett F and Rigney HW. 1973. The lower jaw of Morganucodon. Zoological Journal of the Linnean Society, 53: 87–175.

Kermack KA, Mussett F and Rigney HW. 1981. The skull of Morganucodon. Zoological Journal of the Linnean Society, 71: 1–158.

Kermack KA and Mussett F. 1983. The ear in mammal-likereptiles and early mammals. Acta Palaeontologica Polonica, 28.

Sigogneau-Russell D. 2003. Docodonts of the British Mesozoic. Acta Palaeontologica Polonica, 43: 357-374.