A PALEOECOLOGICAL ASSESSMENT OF THE TULLY MONSTER (TULLIMONSTRUM GREGARIUM) USING GEOMETRIC MORPHOMETRICS

The Tully monster (Tullimonstrum gregarium) is a small and enigmatic aquatic chordate known only from the Pennsylvanian Mazon Creek fossil assemblage. This animal possessed an unusual suite of characteristics, including eyes mounted on a stiff cranial bar and a jointed proboscis terminating in a flexible, jaw-like feeding structure. Because of these bizarre anatomical features its life history and taxonomic affinities are difficult to assess. The purpose of this study was to establish the paleoecology of the Tully monster via morphometric comparison with modern fish. A comparative dataset consisting of photographs of modern, fluid-preserved marine actinopterygian fish was compiled. This dataset included 490 distinct species belonging to 260 families and 56 orders, representing nearly all shape diversity among extant fish. Because body shape is an ecologically informative characteristic, the placement of the Tully monster in morphospace using landmark-based geometric morphometrics (GM) allowed for quantitative inferences regarding the paleoecology of this animal. Landmark-based GM is a technique in which biologically informative landmarks are placed on structurally or functionally homologous points across a series of samples in order to statistically compare shape. Principal components analysis (PCA), sequential agglomerative hierarchical non-overlapping (SAHN) clustering, and a multi-response permutation procedure (MRPP) were used to compare the modern taxa and the Tully monster. Three versions of the Tully monster were incorporated, representing three hypothetical neutral proboscis positions: tucked underneath the body, hanging straight underneath the body, and extended straight in front of the body. These images were taken from 3-dimensional models of the animal. Preliminary analyses found morphometric affinities between the Tully monster and members of Ostraciidae, making a demersal lifestyle feeding on benthic invertebrates a reasonable paleoecological interpretation. While superficially strange in appearance, the Tully monster falls within the range of natural shape variation among modern fish. This implies the paleoecological role of the Tully monster is not unique, and is likely represented within modern aquatic and marine ecosystems.