Hagfish from the Cretaceous Tethys Sea and a reconciliation of the morphological-molecular conflict in early vertebrate phylogeny

– One slimy fossil fish and a change of viewpoint led to major reorganization of early vertebrate phylogeny –

Holotype of Tethymyxine tapirostrum.  Above: false-color composite of chemical element distribution on the surface of the specimen, as revealed by X-ray fluorescence using a synchrotron radiation scan. Color codes: calcium (blue). iron (green), phosphorus (magenta).  Middle: photograph.  Bottom: interpretive drawing.

From Miyashita et al. (2019)

Significance Statement from Miyashita et al. (2019)

Jawless, boneless, and virtually without fossil record, hagfish have long escaped systematists' grip on their place among other fish. Yet their systematic resolution is critical to define vertebrates as a clade.

We report an unequivocal fossil hagfish from the Cretaceous Mediterranean. Using this fossil to calibrate the evolutionary history of the group, our analysis supports hagfish and lampreys as sister groups, which likely diverged from one another in early Paleozoic times.

As a result, vertebrates have a deep dichotomy, where some fossil jawless vertebrates sit closer to hagfish and lampreys than to jawed vertebrates.

We showed that morphology-based analysis converged onto molecular inferences when characters are coded non-independently, providing a case study for morphological-molecular conflicts in animal phylogeny.

— Background —

Fig. 1 from Miyashita et al. (2019) – *open access* – showing previous and new phylogenetic schemes for major vertebrate lineages.  Note that it supports the molecular inference for cyclostome monophyly, but also proposes new relationships for 'ostracoderms' (extinct lineages of jawless fishes).

  • Hagfish represent one of the two surviving lineages of jawless fishes.
  • Hagfish have acted as a wildcard in vertebrate phylogeny. Linnaeus classified them as invertebrates; morphology-based phylogenies placed hagfish outside true vertebrates (lampreys + jawed vertebrates); molecular phylogenies typically supported hagfish within vertebrates, sister to lampreys.
  • Both methods have their own problems. Morphological data are complex, and simple presence/absence of a character has cascading effects on the dataset, depending on how the character is formulated and what other characters it interacts with. Molecular phylogeny only works with tips (living taxa), and cannot inform relationships of fossil forms.
  • Even if molecular inferences turned out to be correct, early vertebrate phylogeny would remain unstable and of little use unless reconciled with morphological evidence, because the diverse assemblage of fossil jawless vertebrates can only be placed on tree with morphological data.
  • Entirely soft-bodied, hagfish have extremely poor fossil record (the only putative fossil hagfish was known from the Carboniferous times). The long ghost lineage (missing information) presents a formidable challenge to any phylogenetic analysis of early vertebrate lineages.

— Highlights from the Paper —

  • We described a fossil hagfish from the 109-million-year-old mid-Cretaceous limestone of Lebanon, named Tethymyxine tapirostrum.  This breaks up the minimum 310-million-year-long hagfish ghost lineage.
  • Exquisitely preserved soft tissues reveal evidence of crown group-like features in Tethymyxine, including slime glands, specialized gills, and barbels.
  • These features place Tethymyxine within the living group of hagfish, close to the genus Rubicundus, which is known for a tubular nasal opening.
  • For this phylogenetic analysis, we also implemented a series of changes to the morphological dataset for early vertebrate phylogeny.  Instead of treating every character equally and independently, we incorporated contingency of characters.  That is, if state 0 of character A assumes state 1 of character B, A is contingent on B.1 and was coded as such.

Supplementary Fig. 4 from Miyashita et al. (2019) – *open access* – shows interpretation of SRS-XRF data on Tethymyxine tapirostrum. Slime glands are shown in panels B–E by red arrows. It is also possible to differentiate organically versus inorganically bound sulphur, distinguishing keratinous tooth plates and slime glands from glue or paint used to prepare the specimen (panel F).  This is potentially a useful method to detect evidence of forgery.

  • This revision changed results of our morphology-based phylogenetic analysis. Instead of letting the hagfish lineage slip outside the true vertebrates, our morphology-based phylogeny converged on the vertebrate status of hagfish supported by molecular phylogeny. Now both morphological and molecular phylogenies place hagfish as sister to lampreys, deeply nested within vertebrates.
  • Our new phylogeny not only reconciled morphological and molecular evidence, but also generates a new hypothesis about relationships of fossil jawless vertebrates. Anaspids and conodonts now fall into the stem of the hagfish+lamprey clade (cyclostomes).
  • This means that early members of both cyclostome and gnathostome (jawed vertebrate) clades had bony skeletons; by extension, so did the last common ancestor of all living vertebrates.

Fig. 3 from Miyashita et al. (2019) – *open access* – shows a time-calibrated graphic summary of phylogenetic analysis.  Hagfish and lampreys form the monophyletic Cyclostomi, united by two unambiguous synapomorphies (keratinous tooth plates motored by the protractor-retractor complex in hypobranchial position; perioptic muscles derived from the trunk).  Anaspids and conodonts fall into the cyclostome stem.

  • Both hagfish and lampreys appear primitive for lacking features characteristic of jawed vertebrates, but they are in fact derived. Their ancestors secondarily lost features shared among early vertebrates, many of which were retained in the jawed vertebrate stem.
  • The hagfish and lamprey lineages split from one another sometime between 536 and 428 million years ago; within each of these lineages, the living group originated sometime around the Jurassic period. The last common ancestor of all living vertebrates probably existed during the Ediacaran period (approximately 550 million years ago).

Comparison of snout morphology in fossil and living hagfishes.

Top: Tethymyxine tapirostrum, a fossil hagfish from mid-Cretaceous Lebanon (approx. 100 million years ago).  It has barbels originating from behind the nasal aperture, and a tapering snout.  

Bottom left: Eptatretus stoutii, representing general morphology of living hagfish.  

Bottom right: Rubicundus eos, representing a living hagfish genus that potentially forms a clade with Tethymyxine. This genus is characterized by having a tubular snout.  Note similarly posterior position of the barbels.  All in ventral view.

— Additional Information —

  • We scanned the fossil hagfish using a Synchrotron radiation method called SRS-XRF.  This allowed us to identify chemical elements on the surface of the fossil, leading to identification of slime filling the glands.
  • A bony skeleton in the last common ancestor of all living vertebrates seems counter to the idea that vertebrates originated as a filter-feeding form similar to an ammocoete (larval form of modern lampreys).  Although an ammocoete provided a convenient model for vertebrate ancestry, I followed up on this hypothesis in a subsequent study.  That paper rejected the ammocoete model: Miyashita et al. (2021) "Non-ammocoete larvae of Palaeozoic stem lampreys".