Did we ever figure out what Tullimonstrum was?

[Strepsodus]

Tullimonstrum lived in the Carboniferous swamps of the state of Illinois in the US. Ever since its discovery, there has been a debate over whether it was a vertebrate or an invertebrate, but was that debate ever resolved?

[bmathison1972]

I believe it is established as an invertebrate, but its relationships therein I am not sure.

[stargatedalek]

It is still theoretically possible it was a primitive vertebrate, but there is nothing in particular supporting that and a lot more options among invertebrates.

Last I knew the "leading" theory is that it was a gastropod.

[dinofigurehobby]

Probably an invertebrate.

[Halichoeres]

The traits that can be established from Tullimonstrum remains overlap poorly with remains of other animals, so this is likely to remain a hard problem. It was proposed as a vertebrate in large part because a team of researchers found evidence for melanin in the eyes, which they said was a vertebrate thing. Having established that to their own satisfaction, they analyzed it with a matrix containing only chordates. When you do a phylogenetic analysis, everything has to end up somewhere, and in this case Tullimonstrum grouped with the lampreys. Later melanin was discovered in trilobite eyes, showing that this isn't special to vertebrates, unsurprising since protostomes certainly use melanin in other tissues. A later study looked for signatures of chitin decay and found none for Tullimonstrum, which is interesting because very very few animal lineages have lost chitin and vertebrates are the main one. It could still be a deeply weird vertebrate of a type that has not fossilized elsewhere because Mazon Creek conditions are so strange. But it could just as easily be a deeply weird member of some other group.