The best fossil hagfish

[Halichoeres]

I posted my interpretation in my art thread, but I never posted the paper! Meet Tethymyxine tapirostrum, the best hagfish fossil yet found. As you might imagine, hagfish are hard to fossilize, as they have no bone and not even much cartilage. This is an extraordinarily lucky find, even preserving outlines of the branchial pouches and slime glands.



I think the paper is paywalled but I'm happy to share the pdf. https://www.pnas.org/content/early/2019/01/14/1814794116

A decent write-up at phys.org: https://phys.org/news/2019-01-fossilized-slime-million-year-old-hagfish-vertebrate.html

[Faelrin]

I had a look at that phys article. I'll be honest I thought these were related to lamprey's to begin with, but that goes to show how little I know of these creatures. Really well preserved fossil though. Quite a long looking one too.

[Ravonium]

Wow, this is a pretty nice specimen! Just goes to show how prehistoric some of the deep sea's events and life is. Nice to see that even some slime was preserved    I could easily picture them scavenging on the corpse of a pliosaur.

I'll be honest I thought these were related to lamprey's to begin with, but that goes to show how little I know of these creatures. 

The taxonomic placement of hagfish and lampreys, both in the chordates and relative to eachother has, to my knowledge, always been a debated subject. The generally accepted consensus prior to this paper (at least I would assume based on the phys article) was that they occupied two seperate lineages in the vertebrate family tree, seperate from the one containing all other fish (and hence vertebrates). I also recall reading somewhere that it was disputed whether hagfish were even 'vertebrates' (in the taxonomic sense), rather than non-vertebrate chordates in the same sense that lancelets and sea squirts are.

I'm pretty sure @Halichoeres could explain this better than I just did.

[Halichoeres]

Yeah, Ravonium has it pretty much right. For a long time, hagfish were interpreted as the living sister group of [lampreys + jawed vertebrates], in part because lampreys appeared to have a partial skeleton, and hagfish a really rudimentary one. Lampreys have, for example, a braincase and vertebrae, whereas the skull of a hagfish is basically a little shelf, and it has the merest hints of vertebrae that don't really form a proper spine (though of course they have a notochord, which is homologous with our intervertebral discs). This skeletal simplicity has been interpreted as the result of hagfish descending from a lineage that split from other vertebrates (or craniates, if you prefer) before a complete skull and vertebral column were acquired. So far every molecular (DNA-based) study I know of has supported a sister relationship between hagfish and lampreys instead, in a clade called Cyclostomata ("circle-mouthed ones"). The cyclostome hypothesis would imply that some of the simplicity of hagfish is a consequence of secondary loss of characteristics, possibly associated with life at the bottom of the sea, where bone or even firm cartilage is less useful. However, the DNA studies are mainly from the mitochondrion, with only a single nuclear gene, so there is definitely room for their falsification given that mitochondrial DNA becomes less reliable the further back in time you go because of its relatively high rate of fixed mutation. The analysis conducted in this paper, including this fossil in a data set that already existed for other chordates, is the first I'm aware of to find morphological support for the cyclostome hypothesis, although the hypothesis that lampreys are sister to jawed vertebrates is only a little bit less parsimonious. I personally favor the cyclostome hypothesis, but it could still prove to be wrong. We will probably need a good molecular phylogeny with lots of nuclear genes from multiple hagfish and lamprey species to resolve it.

[Faelrin]

@Ravonium @Halichoeres Thanks for the much in depth explanation about this stuff. Hopefully some day they get it figured out. They are getting closer at least.