The Tully Monster, or Tullimonstrum gregarium, has long been one of the most popular Mazon Creek fossil specimens. The reasons are unclear... maybe it's the strange shape of the animal, or that it's the State Fossil of Illinois, or the questions and controversy as to what type of animal it is. It is fairly rare, but not as much as most people believe. Oh, and by the way, it's only found in Illinois.
Francis Tully is credited with discovering it in a strip mine near Braidwood, IL, about 50 miles southwest of Chicago, in 1955. Mr. Tully was an amateur fossil collector. He brought it to the Field Museum in Chicago. There at the museum Eugene Richardson found the fossil interesting and described and named it in 1969. After a long effort, Tullimonstrum gregarium became the state fossil of Illinois in 1989. You can find more about him here.
An invertebrate, (an animal having no backbone), the beast had a dirigible-like body. On one end was a spade shaped tail which must have provided propulsion. At the opposite end of its body was a long trunk tipped by a toothy claw. Protruding from the torso were stalks that scientists suspect were its eyes. An indentation under these eye stalks may indicate the location of its mouth. Until the last few years, scientists did not know where to classify the Tully Monster.
Paul Mayer, Collections Manager of Fossil Invertebrates at the Field Museum, wrote a great blog post about the Tully Monster. It's now thought to be a vertebrate.
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.
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.
Here are a couple videos for more information.
The Brain Scoop - "The tully monster SOLVED!"
PBS Eons - "The Tully Monster & Other Problematic Creatures"
""What Was The Tully Monster?"
