Tully Monster: Vertebrate or Enigma?

Tully Monster

© Sean McMahon

Scott Lidgard is the MacArthur Associate Curator of Fossil Invertebrates and Paul Mayer is the Collections Manager, Fossil Invertebrates in the Gantz Family Collections.

Last year, two papers in the journal Nature sought to resolve what group of animals Tully Monster fossils belong to; both papers concluded the vertebrates. Last week, Dr. Lauren Sallan and colleagues published a paper in the journal Palaeontology, "The Tully Monster is Not a Vertebrate..." And that's okay; it is part of how science works.

It's quite normal for new ideas, based on new scientific information and analyses, to raise questions. You might think of scientific discourse as a marketplace of ideas, backed up by data, analyses, and interpretations. Scientists sometimes disagree about one or more of these three components, and that's where the marketplace concept comes in. Other scientists (not just the ones who wrote the papers) are part of the marketplace, and they scrutinize the different perspectives, sometimes doing more research, critiquing, or adding to the discussion and building on the base of knowledge about whatever controversy is at hand. Most often, some sort of consensus is reached in the broader scientific community, though it might take years of back-and-forth discussions.

tully-comparison.jpg

Tully Monster illustration on top of a Tully Monster fossil
Illustration by Monica Jurik, photo by Paul Mayer.    
We are co-authors on one of the papers in Nature, and we took part in a long meticulous slog examining more than 1,000 Tully monster fossils in The Field Museum’s collection. We were looking for biologically informative characters to add to the ones that had been reported in the 50-plus year interval since the Tully monster was described as a bizarre beast that didn't fit into any known animal group. We also examined and looked for characters of other 307-million-year-old Mazon Creek fossils preserved in the same unusual ironstone concretions as the soft-bodied Tully monster.

We then took all these characters and compared them to ones present in living and fossil groups to which the Tully monster had been compared. Our preliminary conclusion: Tully monster wasn't close to the combination of characters found in Arthropods or Molluscs, but was closer to Chordates, the phylum of animals that includes vertebrates. Then, using characters of different living and fossil chordates, especially fishes, we conducted a quantitative analysis with statistical tools from biological systematics to reach our hypothesis. Given the data available, and the analytical methods we used, the Tully monster was categorized as a stem group vertebrate, likely related to the lineage containing lampreys.

Sallan and colleagues disagreed with our exclusion of Arthropods and Molluscs in the last analysis, and also with the characters we had chosen, and how we interpreted them. Sallan et al. also disagreed with the interpretation of characters in the other Nature paper, which had largely (but not entirely) confirmed our results. However, Sallan et al. seemingly didn't add any new evidence, nor did they re-examine the specimens, nor perform an analysis showing any convincing alternative to where the Tully monster fits into the evolution of animals. For now, our hypothesis remains the best-supported estimate of Tully monster's actual relationships.

The good news is that Sallan’s study focused attention on the Tully monster,understudied groups in the early evolution of fishes, as well as how fossils are preserved. More research will be done, and the marketplace of ideas that is science will grow and move forward.