“Tullimonstrum, colloquially known as the Tully Monster, is an extinct genus of soft-bodied bilaterian that lived in shallow tropical coastal waters of muddy estuaries during the Pennsylvanian geological period, about 300 million years ago.”
Taxonomists cannot yet agree on where it belongs in the animal taxonomy, or even as to whether it's a vertebrate or not. See: Wikipedia
Its fossils are an example of a large group of as yet unidentified remains collectively known as 'probematica' or Incertae sedis (Latin for “of uncertain placement”).
McCoy, V., Saupe, E., Lamsdell, J., Tarhan, L., McMahon, S., Lidgard, S., Mayer, P., Whalen, C., Soriano, C., Finney, L., Vogt, S., Clark, E., Anderson, R., Petermann, H., Locatelli, E., Briggs, D. 2016. The ‘Tully Monster’ is a vertebrate. Nature. doi: 10.1038/nature16992
Sallan, L., Giles, S., Sansom, R., Clarke, J., Johanson, Z., Sansom, I., Janvier, P. 2017. The ‘Tully Monster’ is not a vertebrate: characters, convergence and taphonomy in Palaeozoic problematic animals. Palaeontology. doi: 10.1111/pala.12282
Also see: Number of species