Tully Monster: Is this the world’s most mysterious fossil?

James O’Donoghue (UK).

The Tully Monster Mystery

The Tully Monster is a mysterious 307myr-old marine animal known only from the famous Mazon Creek fossil locality in Illinois. Its body plan is unlike any other animal that has ever lived, and it has been subject to wildly different interpretations as to its identity since its discovery in 1955. Last year, Victoria McCoy of Yale University and colleagues identified it as a lamprey, a primitive type of fish, but this has since been challenged by a team of vertebrate palaeontologists.

02. Reconstruction
Fig. 1. Reconstruction of a Tully monster based on the research of McCoy and colleagues. The claw and proboscis are on the right and its eyebar and eyes, gills and tail fin are further back. (Sean McMahon/Yale University.)

Fossil collector Francis Tully knew he had made an extraordinary discovery. Inside a rounded nodule was a bizarre, foot-long animal with a long trunk and claw. But he could never have known quite how extraordinary his 307myr-old fossil would turn out to be. Sixty two years later, scientists are still arguing over the basics as to what sort of creature it really was.

What makes it even stranger is that this is no rarity known only from fragmentary remains. After Tully made his find, word got around among collectors and, before long, hundreds more had been found. Tullimonstrum gregarium, or ‘Tully’s common monster’, is now known from well over a thousand fossils, including many complete specimens. “We’ve got four cabinets of Tully monsters here, each of which has 25 drawers,” says Paul Mayer of the Field Museum in Chicago, where the largest collection is held.

Common, well-preserved and complete fossils should be the very easiest to identify. So why has this one remained so enigmatic? And how did it acquire such notoriety that the MailOnLine recently suggested it was the ‘weirdest animal that ever lived’?

Tully, a pipe-fitter by trade, discovered the fossil in a pile of nodules discarded from a coal mine in the Mazon Creek area of Illinois. “One day in the summer of 1955 I found two rocks that had cracked open due to natural weathering. They held something completely different. I knew right away, I’d never seen anything like it. None of the books had it. I’d never seen it in museums or at rock clubs. So I brought it to Chicago to the Field Museum to see if they could figure out what the devil it was,” said Tully in an interview with the Chicago Tribune in 1987.

Fig. 2. Tully monster fossil in an ironstone nodule. Its head is on the right and its tail is on the left. Actual life-size. (Thomas Clements/Burpee Museum of Natural History.)

Weathering had split the nodule to reveal a creature with an elongated squid-like body and tail fin that was fronted by a long thin snout with a toothed claw at the end. Widely spaced eyes projected from a pole-like structure that stuck out on either side of its body. This body plan is different from any other creature that has ever lived.

Tully showed his fossil to Eugene Richardson, the curator of fossil invertebrates at the Field Museum, who was also baffled by the find. Fossil-rich ironstone nodules were found in shale beds that lay on top of coal deposits. Miners would move the shale and nodules aside to get at the coal which allowed collectors to scour the spoil heaps for fossils. Before long, hundreds more specimens had come to Richardson’s attention, but he was still none the wiser as to what it was.

Fig. 2
Fig. 3. Tully monster photographed in polarized light. At the top, its proboscis has folded back over its body, while the dark spot projecting out is one of its eyes. Muscle blocks run along its body (as identified by McCoy and challenged by Sallan), while its tail fin is at the bottom. This is the ‘holotype’ specimen used when the species was first described. (Nicole Karpus/Field Museum.)

Fig. 3.
Fig. 4. Tully monster in polarized light. This one is folded in two, with its claw lying over the end of the tail at the bottom. Its eyebar and eyes are on the left, while the tail fin spreads out bottom right. Scale bar in inches. (Paul Mayer/Field Museum.)

Measuring between 8cm and 40cm long, the Tully monster has only ever been found at sites around the Mazon Creek, a tributary of the Illinois River, in north-eastern Illinois. The Mazon Creek is world famous for its abundant fossils of soft-bodied animals, which are not normally preserved. The site provides palaeontologists with the most complete snapshot known of the creatures that lived during the Carboniferous period of 359 to 299mya.

Fig. 4.
Fig. 5. Partial Tully monster, with its proboscis folded over. Its eyes project out on an eyebar at the top. The claw is not visible in this specimen. Yellow scale bar equals 1cm. (Paul Mayer/Field Museum.)

Fig. 5.
Fig. 6. Partial Tully monster showing striping that may be muscle segments. The eyes and eyebar are visible at the top. (Paul Mayer/Field Museum.)

The Tully monster lived in a large estuary alongside jellyfish, sea anemones, a wide variety of marine worms, different types of mollusc, shrimps, horseshoe crabs and other arthropods, as well as several kinds of fish – lungfish, ray-finned and spiny-jawed fishes, coelacanths and sharks. Leaves and branches of land plants were also washed by a great river into the estuary, where it flowed into a shallow sea.

Heteropod Mollusk
Fig. 7. This free-swimming sea snail, a type of heteropod mollusc known as Pterotrachea, arguably looks more like a Tully monster than any other living animal. Palaeontologist Merrill Foster proposed in 1979 that the Tully monster was a strange type of sea snail comparable with, but not directly related to, the heteropods. Some scientists still believe this to be the case. In this photo, its black eyes are above its proboscis and mouth on the right. Its rounded swimming fin hangs down centre-left and its internal organs are visible. (Dante Fenolio/Science Photo Library.)

On land, swampy tropical forests dominated by giant clubmoss trees, seed ferns, horsetails and true ferns were home to amphibians and a rich variety of insects and other arthropods. Waterlogged soil in these forests created low oxygen conditions that inhibited the decay of  dead trees and plants,  which eventually resulted in the coal seams that were mined in modern times. The climate was hot and sticky and the site was located just ten degrees from the equator.

The fossils were formed when a colossal flood dumped huge amounts of sediment into the river delta. Animals living on the sea floor were smothered, while swimming creatures were jumbled up with plants and insects from land in a muddy grave. Very low levels of oxygen meant the buried organisms were not subject to rapid decay. Instead, bacteria slowly decomposed the remains, while emitting methane and ammonia, and triggered a series of chemical reactions that eventually resulted in nodules forming around the fossil nuclei.

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