The beauty of the beast

The enigmatic Tully Monster has become something of a palaeontological icon, yet there is much to learn about this mysterious beast. Here Thomas Clements takes us along for the ride as he and his Professor begin to understand where the Tullymonstrum fits on the tree of life.   

I stand, hoe in hand, at the summit of a steeply banked island looking out over the water on a cold spring morning in central Illinois, watching the clouds of steam roll across the lake. This isn’t a natural body of water, but a huge cooling lake for a nuclear power plant which is visible in the hazy distance. The wind is blowing in my face and the noise of several irritated herons can be heard, bothered by the four humans who have landed and set up camp on their small island. Below me, my supervisor, Professor Sarah Gabbott and our two local guides are carefully traversing the crumbling, slippery shale that makes up the island. We are searching for a rare and mysterious creature, discovered over six decades ago, but no living human has ever seen one. This beast has an eel-like body, the tail of a fish, eyes on stalks like a slug and a crab-like claw lined with tiny teeth on the end of an elephantine trunk. We are hunting for the infamous ‘Tully Monster’.

[caption id="attachment_54471" align="alignnone" width="620"] Thomas Clements at work in Illinois.[/caption]

To understand the Tully Monster we have to go back in time. Six decades ago, the island I am standing on was part of a huge strip mine. The Peabody Company and many other smaller companies were excavating huge swathes of the Illinois countryside, mining coal to fuel American industry. Relatively shallow but vast pits were dug to find this vital resource and subsequently the overlying shales, which had no monetary value, were dumped in huge spoil heaps. Since the 19th century these grey crumbly shales were known to contain rounded brown nodules of a hard rocky mineral called siderite which, when split open, often contain exquisitely preserved fossils. These nodules were considered waste by the coal miners and so private collectors were allowed to scour the heaps looking for fossils.

In the subsequent six decades since its discovery, Tullimonstrum has gained renown and notoriety for remaining such an enigmatic animal.

Some contain nothing, most contain plant fragments but sometimes, very rarely, they contain beautifully preserved animals. When these sediments were deposited, 300 million years ago in the Carboniferous period, Illinois was just slightly north of the equator. It was tropically warm but wet – perfect conditions for huge forests of primitive trees, known as lycopods, which teemed with large insects, millipedes, arachnids and amphibians. Through these forests a large river delta, similar to the Mississippi or the Mekong delta, discharged its water and sediment into large estuaries and embayments. The manmade island where I am standing presently, would have been under several feet of seawater. If I could have snorkelled along the ancient coast, I’d have seen that it was swarming with life: jellyfish, sharks, fish, cephalopods, molluscs, polychaetes, horseshoe crabs, bivalves, crinoids and many other types of creature lived in this prehistoric sea. We know this because these organisms have been preserved in the fossil record. While we aren’t sure what catastrophic event caused these animals to be buried, they were inundated rapidly and subsequently became entombed in siderite. Over a geological timescale of millions of years they would turn into stone.

Fast-forward to the 1950’s, Eugene Richardson, the invertebrate curator at the Field Museum of Natural History in Chicago, was visited by a prolific local fossil collector named Francis Tully. Tully had brought fossil bearing nodules to the Field Museum before, but this time he had discovered something which was unlike anything else he had ever found before. Richardson was amazed – the creature was so bizarre that he couldn’t decide what type of animal it was. Over the next few years hundreds of specimens of this odd organism were found and so in 1966 he formally described it in Nature designating it Tullimonstrum gregarium (Tully’s common monster). In the subsequent six decades since its discovery, Tullimonstrum has gained renown and notoriety for remaining such an enigmatic animal. Such is its fame it has become the state fossil of Illinois. Several studies have tried to ascertain where Tullimonstrum sits in the ‘tree of life’, with the majority believing it to be some form of invertebrate – perhaps a mollusc or a segmented worm, but there has been no agreement between scientists. But Sarah and I are determined to change that and solve the mystery of the Tully monster.

[caption id="attachment_54470" align="alignnone" width="620"] An artist's impression of how the Tully monster may have looked 300mya. Credit:Sean McMahon / Yale University[/caption]

After my first trip to Canada to visit the Royal Ontario Museum, which has one of the largest collection of fossils found in Illinois, we head back to the University of Leicester. Here, in the Palaeobiology Research Group’s labs, we begin to unpack the specimens and plan how we can investigate the processes these organism have gone through to become fossils. As part of my PhD, I have been investigating the vertebrate fossils from the same area – fish, sharks and amphibians. By using a scanning electron microscope (SEM) I can ascertain what parts of the animal have been lost through decay and what has survived to become fossilised. I then compare fossil remains against tissues in modern animals to build up an understanding of how different body parts fossilise. Whilst studying the fossil fish, I made an observation that startles me. The many bones that form the skulls of these animals have preserved beautifully and so the orbits, where the eyes once were, are clearly visible. Within the orbits, a dark black film can be seen by the naked eye but under the scanning electron microscope, magnified over 2000 times, the texture of this film is revealed as a carpet of tiny ‘meatball’ and ‘sausage’ shaped structures, each 50 times smaller than a width of a human hair. In an excited panic I checked another fossil fish eye and another, then a fossil shark. They all have the same dense mass of tiny structures.

I had seen these structures before – in 2008 a Danish palaeontologist, Dr Jakob Vinther (now at The University of Bristol), had been studying 108 million year old fossil feathers which had preserved black and white banding. Interested to know whether the colouration was an artefact of the fossilisation process or the actual colour of the original feather, he discovered that the black stripes were packed with the same ‘sausage’ shaped granules I had seen in the fish from Illinois. He compared these structures to those seen in modern feathers and noticed that they were identical to the organelles within cells that synthesise and store the pigment melanin, known as melanosomes – he had discovered the first known example of pigment in the fossil record. This discovery revolutionised the way we look at fossils. Since their discovery, scientists have found fossil melanosomes in some of the earliest birds, such as Archeopteryx, Microraptor and Confuciusornis, and been able to infer their original plumage colour. Melanosomes have been even been discovered in the downy ‘proto-feathers’ that covered the skin of some dinosaurs, such as Sinosauropteryx which has the dubious honour of being the first ginger dinosaur!

Within the orbits, a dark black film can be seen by the naked eye but under the scanning electron microscope, magnified over 2000 times, the texture of this film is revealed as a carpet of tiny ‘meatball’ and ‘sausage’ shaped structures, each 50 times smaller than a width of a human hair.

My observation of ancient fish melanosomes and thus ocular pigmentation in animals 300 million years old is far older than dinosaurs or primitive birds, in fact the oldest fossil pigment ever described. With this exciting news, I burst into my supervisor’s office. Professor. Gabbott looks at the pictures I thrust in front of her and agrees with my interpretation. Gently, she tells me that in her studies on primitive vertebrates, such as hagfish and lampreys from the same rock bed as my fossil fish, she too has observed melanosomes in their eyes. Slightly deflated I return to my work studying the fish. Several weeks pass and we travel to Illinois and collect the rock samples needed to do a full investigation of the depositional palaeo-environment that my fossils were buried in. I spend hours up to my waste in freezing fast flowing water collecting nodules from river beds by using a technique taught to me by my local guides, with a garden hoe, to dig them out and scoop them up before the river can steal them back. Hopefully some of these can be split open to reveal beautiful fossil ferns or animals. We also travel around Illinois visiting local museums and collectors houses, looking to borrow as many of fossils we can get our hands on including many examples of the famous Tullimonstrum.

[caption id="attachment_54468" align="alignnone" width="600"] Melanosomes discovered in the Tully monster.[/caption]

One specimen from the Burpee Museum of Natural History catches my eye – it’s the only complete Tully Monster specimen we had been able to loan and it is beautiful. One striking feature is that it has two jet black ‘blobs’ on the ends of the stalks protruding from its head. This is quite normal for the Tully Monster, with many of the more complete specimens having these dark blobs on the end of the stalks. Previous studies have suggested that they could by hydrodynamic stabilisers, ‘feelers’ or even paired copulatory organs – but I have another idea… Upon arrival back to Leicester, the first thing I do is get Tullimonstrum into the SEM. The dark blobs, as I suspected, are packed with ocular pigment granules of both the sausage and meatball morphologies, arranged in distinct layers – exactly like we see in the fossil fish, sharks and amphibians. Some samples even preserve a lens, but the original material has been replaced by a form of weathered clay called kaolinite. We also use Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) to identify the chemical signature of the fossil melanosomes and compare it to modern examples of melanosomes, to validate our hypothesis, and it shows a clear match. Tullimonstrum’s black blob structures on the ends of the stalks are, as Eugene Richardson suggested in 1966, definitely eyes.

I begin to wonder if the discovery of these pigments can tell us about what type of animal Tullimonstrum is. I contact Dr. Vinther and with Professor. Gabbott we scour the literature for all the types of eye pigment found across the whole animal kingdom. Our investigation reveals that most organisms can synthesise melanin and that the pigment is utilised by nearly every animal group from jellyfish to snails to us. Soon though, we stumble upon a vital piece of evidence; only vertebrates can synthesise the two different morphologies of melanosomes – invertebrates can only produce the ‘meatball’ shaped granules. This unequivocal evidence shows that the Tullimonstrum must be a vertebrate. In other words, Tully Monster is, in fact, a type of weird fish.

Tullimonstrum’s black blob structures on the ends of the stalks are, as Eugene Richardson suggested in 1966, definitely eyes.

For the first time, our research group has shown that fossil pigments can give a signal to help place an enigmatic organism into the tree of life. But Tullimonstrum’s eye pigments can tell us even more about the animal. In the vertebrate eye, only the retinal pigmented epithelium (RPE) has both types of melanosome morphology, which are distinctly layered. This unique melanosome arrangement is preserved in the Tullimonstrum fossils. Therefore, by finding evidence of an RPE we can infer that Tullimonstrum could form a clear visual image, a useful adaptation to spot prey or predators in the murky ancient seas. I am still amazed that such a delicate and decay prone tissue can be preserved and survive for 300 million years.

Our study goes to show that you shouldn’t judge a book by its cover but you can judge a vertebrate by its eye pigments.

Author: Thomas Clements is a NERC funded PhD Student at the University of Leicester’s Department of Geology. He is also Winner of ‘I’m A Scientist, Get Me Out Of Here!’ Evolution Zone 2015.

Contact:  tc195@le.ac.uk