This time last year, two teams of paleontologists announced that they had finally solved one of the most inscrutable mysteries in the history of life on Earth. The Tully Monster – a flat, bug-eyed, nozzle-nosed animal truly worthy of the title Monster – went from a 300 million year old Rorsarch test to a vertebrate, a strange lamprey rather than a pincer-faced invertebrate. Fossil fans sent up a cheer as the unusual critter was welcomed into the fold. But now a different group of researchers has once again raised uncertainty about what, exactly, the Tully Monster is.

Paleontologist Lauren Sallen and colleagues write that the idea Tullimonstrum was a nightmarishly aberrant fish runs into “biological, functional, and taphonomic” challenges. In their view, the Tully Monster is no vertebrate but something more akin to a mollusk, arthropod, or some strange chordate. Disagreement is no surprise when you’re dealing with a species that looks like the star of an early Roger Corman film.

One of the pro-vertebrate studies, published by Thomas Clements and collaborators, concluded that the Tully Monster is one of our family based on its eye. The shape as the eye, as well as two organelles inside it, appeared to align the Tully Monster with vertebrates more than other animals. Sallen and coauthors regard these same characteristics as equivocal, possibly having more to do with convergence than true relationships.

As for the other pro-vertebrate study, Victoria McCoy and colleagues announced what was thought to be the gut of the Tully Monster was actually a notochord similar to that seen in early lampreys. But Sallen and colleagues dispute this identification, pointing out that the structure runs past the eye stalks towards the front, far forward of where a true notochord should stop. On top of that, Sallen and coauthors point out, other definite vertebrates found in the same rock as the Tully Monster show no sign of the notochords they must have had. It’d be strange for fossils of the enigmatic Tully Monster alone to preserve the feature.

Once you change your expectations of what a Tully Monster is, the critical paper suggests, the entire nature of the beast changes. What seem to be W-shaped vertebrate muscles start to look like muscle blocks found in ancient arthropods. Other traits, such as the eyes and side fins, seem to fit the weird anomalocaridids best. Other traits of the brain, eyes, and mouth might pin the Tully Monster among the mollusks. Sallen and coauthors don’t definitively recategorize the beastie. It’s still a strange mishmash of traits. But, in their view, the Tully Monster is unlikely to be a vertebrate.

Naturally, some of the scientists who forwarded the vertebrate hypothesis aren’t convinced by the new argument. Field Museum experts Paul Mayer and Scott Lidgard, who were coauthors on one of the Nature paper, shrug off the criticism, pointing out that the new position piece didn’t perform any new analysis or offer new evidence. It was a critique, and, as the Field’s invertebrate experts conclude, the vertebrate hypothesis “remains the best-supported estimate of the Tully monster’s actual relationships.”

This is how science works. It’s not a matter of collecting facts and pinning them to a board as if they were butterflies. Hypotheses go back and forth, contested and checked and debated, hopefully moving us closer and closer to a more accurate and precise view of Nature. The Tully Monster is just an especially squirmy customer to get a grip on.

[This post was supported by my generous backers on Patreon. For more details on how you can get an early view of new blog posts and exclusive natural history essays, click here.]

References:

Clements, T., Dolocan, A., Martin, P., Purnell, M., Vinther, J., Gabbott, S. 2016. The eyes of Tullimonstrum reveal a vertebrate affinity. Nature. doi: 10.1038/nature17647

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