Sometimes evolution produces an organism so weird and wonderfully different to anything that we know of that scientists are stumped. Enter the Tully monster, a soft-bodied sea creature that swam in the muddy estuaries of what’s now Illinois, around 300 million years ago.
It was discovered in 1955, by an amateur fossil collector called Francis Tully, who was picking through the Mazon Creek fossil beds in the northeast of the state.
Torpedo-shaped, with a triangular tail fin and teeth on the end of its long, bendy proboscis, the Tully monster looked as though someone had stabbed it through the back with a skewer, and then stuck eyes on each end.
It was, and I’m using the appropriate scientific term here, absolutely bonkers. Tully took the fossils (below) to the Field Museum of Natural History in Chicago, where experts were bamboozled.
Was it a worm? Was it a slug? Did it have a backbone? Could it be an eel? They had no idea and so they dubbed it Tullimonstrum gregarium, which is Latin for ‘Tully’s common monster.’
In 1989, the Tully monster became the official state fossil of Illinois, but still, no one knew what it actually was. Then in 2016, two papers were published, both suggesting that the Tully monster was indeed a vertebrate.
A structure previously thought to be the animal’s gut was found to be a primitive backbone-like structure called a notochord, while pigments in its eyes were determined to be more vertebrate than invertebrate-like.
With a backbone made of cartilage, teeth made of keratin, a single nostril and a dorsal fin, perhaps, they mooted, the Tully monster was likely to be a distant relative of modern-day lampreys. Taxonomic mystery resolved. But not everyone was convinced.
Sceptics pointed out that the eye pigments were unconvincing and that the notochord extended in front of its eyes, which was weird. Maybe it was an odd squid? Perhaps a bizarre cuttlefish?
Then in 2023, Japanese researchers looked closely at 153 museum specimens. Where previous approaches had been subjective, driven by the researchers’ hunches, this time, a neutral, data-driven approach was used.
A 3D scanner was used to create a colour-coded digital map of the animal’s surface, leading to the conclusion that some features presumed to be vertebrate-like – such as gill pouches and fin rays – were either absent or not vertebrate-like at all. In addition, the Tully monster was shown to have segments, not just in its body, but in the region of its head too.
Vertebrates aren’t known to have this particular arrangement of features, so conclusively the Tully monster couldn’t possibly have been a vertebrate.
Or maybe it could? Today, the jury is still out and the joy of the Tully monster is that it’s a mystery that just keeps on giving. The best guess from the Japanese team is that it's an ‘invertebrate chordate’, which is a category that includes animals such as the eel-like lancelets, but honestly, no one knows for sure.
So, until consensus is reached, the Tully monster remains in taxonomic limbo.
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