Solving The Mystery Of Mazon Creek

An ironstone nodule containing
a fossil of the Tully Monster

Mazon Creek is one the most famous Carboniferous fossil localities in the world. Formed in a system of deltas on the Pangaea coast, the Mazon Creek shales contain nodules of iron stone. When split, these can be found to contain the exceptionally well preserved remains of a plethora of extinct species.

Plants are common, but vertebrates can also be found along with arthropods. Such creatures have mineralised body parts and so are easily preserved. Yet the preservation of soft tissues means that a host of soft-bodied creatures can be found at Mazon Creek, giving profound insight into the ancient ecosystem. Some species, however, have defied classification.

One such species is the Tully Monster, Tullimonstrum gregaricum. Bearing an odd blend of features, it has been banded between groups as diverse and distantly related as the molluscs, arthropods and conodonts. A recent study, however, lays the matter to rest.

Discovered more than 60 years ago, the type-specimen is still the most complete discovery. 1000s of other specimens, however, exist in the collections of the Field Museum in Chicago. This gave researchers, led by Victoria McCoy from the University of Leicester, enough material to make a detailed reconstruction of its anatomy, and in turn, its placement in the tree of life.

A reconstruction of Tullimonstrum, based on the new data from the study

Many specimens show a light coloured band running from snout to tail. This was previously interpreted as a gut. The apparent lack of a backbone, but distinct digestive system, made palaeontologists think that it had affinities with worm, mollusc or arthropod clades.

Closer examination of the structure, however, showed that it had a notochord - the defining feature of chordates. Its presence in the adult form helps narrow it down to the non-vertebrate chordates, namely the hagfish and lampreys. Several other features, including the structure of its teeth and a crescent-shaped pore in its mid-section, show that it belonged to the latter. As a stem group lamprey, the morphology of Tullimonstrum was highly unusual, with stalked eyes and a proboscis-like claw mouth.

'It would be fascinating to watch it swim around,' said Paul Mayer from the Field Museum. 'How does a Tully Monster make its living? We don’t know.' The next step for the researchers is to determine its lifestyle and ecology. Despite its bizarre morphology, the rich nature of the fossil locality will help to reconstruct its interactions with other organisms in its habitat.