In the summer of 1955, amateur paleontologist Francis Tully was exploring the spoil piles of a coal mine south of Chicago, searching for the 308 million years old Mazon Creek nodules that often contain fossils. He spotted one that had split open, revealing its contents, and knew that he had found something strange and new. Inside was a soft-bodied organism that had an unusual suite of anatomical traits that did not fit clearly into any animal group: a proboscis ending in a claw-shaped apparatus, a horizontal bar at the front end of the body, and spade-shaped fins at the rear. This strange fossil creature, which would eventually be named Tullimonstrum gregarium (common Tully Monster) and become the State Fossil of Illinois, has confounded paleontologists’ efforts to identify its position on the tree of life for more than 50 years. The Tully Monster has been described as a nematode, polychaete word, mollusc, and stem arthropod, but until this year, its identity has remained a mystery.
In two recent papers, the Tully Monster has been reinvestigated using new methods and analytical approaches in hopes of finally solving this paleontological problem. Victoria McCoy and colleagues examined the anatomy and preservation of more than 1200 Tully Monster fossils, and compared them to other animals found within the Mazon Creek fauna. Thomas Clements and colleagues took a more specific approach, analyzing dark spots at the end of the Tully Monster’s horizontal bar, which have previously been interpreted as eye-spots.
The extensive comparative study conducted by McCoy and co-authors found several new anatomical characteristics that pointed toward a vertebrate identity – most notably, the presence of a notochord, the flexible rod-like structure that is a defining feature of all chordates. Previous interpretations of Tully Monster fossils identified this feature as a gut trace. However, gut traces in the Mazon Creek fossils are normally preserved as dark, three-dimensional features; in the Tully Monster, the linear feature that begins behind the transverse bar and runs down the body is normally white and has no significant topography. Moreover, some Tully Monster fossils have a dark, raised linear feature that extends from the clawed proboscis to just before the fins in addition to the lighter character, providing further evidence for the presence of a notochord. McCoy and colleagues also noted similarities between the teeth of the Tully Monster and those of lampreys and hagfish.
Clements and colleagues focused on a specific portion of the Tully Monster in their study: the horizontal bar. Using scanning electron microscopy, a method that enables paleontologists to examine fossils at micrometer scales, Clements and his co-authors found that the dark spots at the end of many Tully Monsters’ transverse bars are comprised of small spherical and cylindrical structures arranged in well-ordered layers. They analyzed the chemical composition of these structures, and found them to be the fossilized remains of pigment-bearing organelles called melanosomes. These layered melanosomes confirm the Tully Monster is a vertebrate, for while melanosomes are known from several groups of animals, only chordates have both spherical and cylindrical melanosomes in their eyes.
Why has it taken so long for paleontologists to unravel the Tully Monster mystery? The Tully Monster did not have any mineralized tissues, like a skeleton. Soft-bodied organisms require certain conditions to be fossilized, and even in localities that promoted soft-tissue preservation such as Mazon Creek, preservation of every specimen is imperfect. Each individual fossil may only preserve a fraction of useful traits, and thus analyses of hundreds of fossils are necessary to determine how the living animal was built. Our understanding of how fossilization processes act on different tissues has increased in the past 30 years, and these studies were able to use this knowledge to differentiate between the different portions of the fossil that have been previously overlooked or misinterpreted, such as the notochord and gut traces. Additionally, advances in imaging technology, microscopy, and chemical analyses that have been developed in the last 20 years provided the tools necessary to reveal some of the otherwise obscure anatomical traits that helped to nail down the Tully Monster’s identity, like the melanosomes at the ends of the transverse bar that confirm a vertebrate-affinity.
The strange puzzle of the Tully Monster that has intrigued paleontologists for so long has been solved, but many new questions now arise. How did the Tully Monster come to evolve such an unusual body-plan? Why is the Tully Monster known from only a single locality? Are there other ancient animals like the Tully Monster that we have yet to discover? What role did the Tully Monster play in the ancient Mazon Creek environment? With the vertebrate identity now well-established, paleontologists will continue to discover new aspects of Francis Tully’s monster for decades to come.
To learn more:
McCoy, V. E., , (20., Saupe, E.E., Lamsdell, J.C., Tarhan, L.G., McMahon, S., Lidgard, S., Mayer, P., Whalen, C.D., Soriano, C., Finney, L., Vogt, S., Clark, E.G., Anderson, R.P., Petermann, H., Locatelli, E.R., & Briggs, D. E. G16). The ‘Tully monster’ is a vertebrate. Nature, 532, 496–499 (28 April 2016) doi:10.1038/nature16992
Emily Graslie interviews Scott Lidgard and Paul Mayer of the Field Museum, other study co-authors, in this BrainScoop video (Note: Interesting technique involving your home freezer is revealed.)
This video from the Field Museum shows the technology used to aid in the discovery.