Tujiaaspis: a complete galeaspid, not just a head shield

Tuesday, January 23, 2024 10:31

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Gai et al 2024 described
a Silurian galeaspid, Tujiaaspis vividus, with three dorsal fins, a symmetrical hypochordal tail and and a pair of continuous, branchial-to-caudal ventrolateral fins (Figs. 1,2). The authors reported, “our computed fluid-dynamics experiments show [finfolds] passively generated lift.”

That may be so. Tujiaaspis was no doubt occasionally mobile, but there may be another reason for finfolds for this taxon. After all, the skull itself has much larger hydrodynamic ‘wings’ passively generating much more lift than in the ventrolateral fins aligned parasagittally.

Figure 1. Tujiaaspis in situ in ventral view. Colors added here based on Dunyu in figure 2. Much tesselation and therefore much guesswork. The dorsal view is cut and pasted from figure 2. The cloaca is shown as a red ellipse. This is an excellent specimen!

Like modern rays, galeaspids likely buried themselves in sand,
filter feeding while breathing through the immobile vertical slit (= oral + olfactory + incurrent) located between its eyes. Since the bony dorsal skull was immobile and the fish was often immobile, the unossified and flexible throat expanded, like a frog’s throat, to draw in water. The excurrent of water was expelled through a series of lamprey-like ventral gill serial openings visible at the flexible throat margins.

When buried, where did the expelled water go?
Here’s where galeaspid finfolds came in handy, channeling expelled water back to the tail.

In this case, finfolds were not used for propulsion, guidance or elevation.
In galeaspids finfolds were used as excurrent water gutters.

Figure 2. A model of Tujiaaspis from extended data figure 5 in Gai et al 2024. Colors added here. Another galeaspid, Dunyu, also shown not to scale. In this clade the oral and olfactory openings are permanently open and dorsal in location and orientation. This was not a predator, but a filter feeder. The ventral oral opening on this model is imagined and incorrect. See figure 3 for the evolution of the oral opening in galeaspids.

Gai et al identified
an ‘oralobranchial fenestra’ locate generally in the throat skin between the gills. Their model (Fig 2) shows a ventral mouth (= oral opening) between the flexible throat and ventral bony rim. The model ‘mouth’ is located anterior to the ‘fenestra’ they identified in their figure 1.

A ventral oral opening in galeaspids is incorrect.

When we look at the evolution of galeaspsids
(Fig 3) it becomes clear that the wide anterior almost immobile oral opening evolves in certain taxa. It migrates to become a narrow immobile dorsal opening. The throat area remains mobile (= not ossified with large immobile elements). The eyes migrate dorsally at the same time.

That makes galeaspids different. That makes galeaspids buried filter feeders, not predators.

Figure 3. Evolution and dorsal migration and narrowing of the oral opening in galeaspids. This chart first appeared in this blogpost in February 2021.

SciNews reported,
“The paired fins of Tujiaaspis vividus act as hydrofoils, passively generating lift for the fish without any muscular input from the fins themselves,” said Dr. Humberto Ferron, a researcher at the University of Bristol.”

“It is amazing to think that the evolutionary innovations seen in Tujiaaspis vividus underpin locomotion in animals as diverse as birds, whales, bats and humans.

Figure 4. Early Cambrian Haikouicthys is close to the last common ancestor of all extant fish with fins. Here it is compared to a young sturgeon, a close living relative. Note the emergence of finfolds below the serial gill openings.

In the LRT
paired fins did not evolve just once, but several times by convergence,contra Gai et al.

Early Cambrian Haikouichthys (Fig 4, not mentioned in the text) also had fin folds. They were less distinct, but appeared much earlier.

References
Gai et al (6 co-authors) 2024. Galeaspid anatomy and the origin of vertebrate paired appendages. Nature 609. https://doi.org/10.1038/s41586-022-04897-6

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