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.

Killing-off Scottish Mammaliamorphs

With all that’s been going on in the last couple of months (including attending #2017SVP, Friends of  Hugh Miller’s conference, and #SVPCA2017), I’ve dropped the ball on this blog a little. But this week past, I had a new paper out with my colleagues, and I’d like to tell you a little more of the story behind it.

When I started my PhD on the fossil material of the Isle of Skye, one thing really surprised me. There is this whole group of cynodonts (the group that includes mammals and their nearest relatives) that no one ever talks about. They are called tritylodontids (Tritylodontidae), and despite being one of the most successful clades of cynodont – lasting from the Late Triassic to the Early Cretaceous - they are often overlooked by palaeontologists. We find one of them, called Stereognathus, on the Isle of Skye.

Perhaps the best piece of tritylodontid palaeoart, Stereognathus by Mark Witton - find out about him and support his amazing work.

Why should we care about these extinct animals? Well, they’re considered by most to be the sister-group to stem-mammals, and share many similarities in their skeleton. They would have had fur and whiskers, and you might mistake one for a mammal if you didn’t look too closely. But there are some interesting differences. Tritylodontids kept the so-called “reptilian” jaw joint, between the articular and quadrate bones (not the dentary squamosal contact as in mammals). They also didn’t go through the same extreme reduction in body size we see in the first mammals. In fact some tritylodontids were pretty big, like Kayentatherium from the Early Jurassic, which was about a metre long. Unlike the earliest mammals, tritylodontids appear to have been almost exclusively herbivorous. 

All in all, they were occupying quite a unique ecological space in the Mesozoic. 

The first tritylodontid to be described was the Middle Jurassic genus, Stereognathus, named by Charlesworth in 1855 and figured by Richard Owen shortly after. Unfortunately though, this genus is represented by only a few molar teeth in a fragment of jaw, a bunch of isolated teeth, and some individual bits of limb that might belong to Stereognathus, but it’s uncertain. Most of this material came from sites in Oxfordshire, England.

Owen's original figures from 1857, scanned courtesy of BHL

Charlesworth called this first Stereognathus species, S. ooliticus, after the rocks they were found in, the Great Oolite Group. More than 100 years later, Robert Savage and Michael Waldman found teeth they identified as belonging to Stereognathus on the Isle of Skye. They named it S.hebridicus, after the Inner Hebrides islands, to which Skye belongs. Unfortunately, their only diagnostic feature was that S. hebridicus was bigger than S. ooliticus. More recently, field work has recovered more S. hebridicus teeth. I decided to set out with my co-authors to test this size difference, and see if we could find other details of the shape of the teeth that would support S. hebridicus as a separate, Scottish species.

A photo of the most intact Stereognathus specimen, found and photographed by Andrzej Wolniewicz during fieldwork on the Isle of Skye.

We microCT scanned all the new material, and the two holotypes of S. ooliticus and S. hebridicus. We measured every tooth of Stereognathus we could, many being too broken up or worn to give reliable measurements. Even the original S. ooliticus specimen figured by Owen was in a poor state: comparing it to his original drawings, it seems over 150 years of handling hasn’t been kind. 

We didn’t find any good evidence that S. hebridicus is a separate species from S. ooliticus. Although the holotypes are radically different in size, when you take all of the rest of the teeth we have and plot them (below), they fall along a line of size change you’d see from baby to adult. In other words, size differences are explained by ontogeny: development from earliest stage of life to maturity.

All of the measureable upper postcanines of Stereognathus. Empty diamonds and squares indicate less certain measurements (due to breakage or wear). I expect field work over the coming years will fill this out to show a full growth series of size.

To my horror, I’ve sunk the only Scottish tritylodontid! Unless new fossils tell us differently, it looks like S. hebridicus is no more. This makes it a junior synonym to S. ooliticus. It’s a shame to see our Scottish species go, but I’m glad that it was the new, more physically intact fossil material from Skye that allowed us to make a proper reassessment of the genus. 

Who knows, perhaps there is still another species of tritylodontid to be found on the rocky shores of the misty Isle? I'll keep looking and get back to you...

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Panciroli E, Walsh S, Fraser NC, Brusatte SL, and Corfe I. 2017. A reassessment of the postcanine dentition and systematics of the tritylodontid Stereognathus (Cynodontia, Triltylodontidae, Mammaliamorpha) from the Middle Jurassic of the United Kingdom. Journal of Vertebrate Paleontology.

Waldman M, and Savage RJG. 1972. The first Jurassic mammal from Scotland. Journal of the Geological Society of London.

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