A new paper, co-authored by Paul Mayer of the Field Museum, in Nature sheds light on an age old mystery. Tullymonstrum gregarium, commonly known as the Tully Monster, is the official state fossil of Illinois, designated in 1989. It's a soft bodied animal found in the late Carboniferous Mazon Creek biota (approximately 309-307 million years ago) of Illinois, USA. It's phylogenic position has been uncertain from the beginning. It's been proposed to belong with nemerteans, polychaetes, gastropods, conodonts, and even arthropods. After analysing more than 1200 specimens, this paper places T. gregarium among vertebrates, more specifically with the lampreys.
Not coincidentally, our September general meeting program will be by Paul Mayer of the Field Museum, speaking on this very subject!
Problematic fossils, extinct taxa of enigmatic morphology that cannot be assigned to a known major group, were once a major issue in palaeontology. A long-favoured solution to the ‘problem of the problematica’1, particularly the ‘weird wonders’2 of the Cambrian Burgess Shale, was to consider them representatives of extinct phyla. A combination of new evidence and modern approaches to phylogenetic analysis has now resolved the affinities of most of these forms. Perhaps the most notable exception is Tullimonstrum gregarium3, popularly known as the Tully monster, a large soft-bodied organism from the late Carboniferous Mazon Creek biota (approximately 309–307 million years ago) of Illinois, USA, which was designated the official state fossil of Illinois in 1989. Its phylogenetic position has remained uncertain and it has been compared with nemerteans4, 5, polychaetes4, gastropods4, conodonts6, and the stem arthropod Opabinia4. Here we review the morphology of Tullimonstrum based on an analysis of more than 1,200 specimens. We find that the anterior proboscis ends in a buccal apparatus containing teeth, the eyes project laterally on a long rigid bar, and the elongate segmented body bears a caudal fin with dorsal and ventral lobes3, 4, 5, 6. We describe new evidence for a notochord, cartilaginous arcualia, gill pouches, articulations within the proboscis, and multiple tooth rows adjacent to the mouth. This combination of characters, supported by phylogenetic analysis, identifiesTullimonstrum as a vertebrate, and places it on the stem lineage to lampreys (Petromyzontida). In addition to increasing the known morphological disparity of extinct lampreys7, 8, 9, a chordate affinity for T. gregarium resolves the nature of a soft-bodied fossil which has been debated for more than 50 years.
