Origin and early migration of fish fins in the LRT

Today we’re confirming earlier hypotheses.
while adding a little data here and there.

The outgroup
the nematode, Enoplus (Fig. 1) lacks a notochord and fins.

Step one:
Pikaia (Middle Cambrian) has a lancelet-like, hagfish-like (genus: Myxine) dorsal fin from head to tail the wraps around ventrally and terminates at the cloaca/anus (Fig. 1).

Step two:
A caudal fin first appears separate from a now discontinuous dorsal fin first in the lamprey, Pteromyzon.
Lateral fin folds also first appear in the lamprey, Pteromyzon (Fig. 1).

Step three:
Pectoral fins first appear in the pre-sturgeon, Thelodus.
Pelvic fins first appear as low, robust spines in Thelodus, then as typical fins in the sturgeon, Acipenser (Fig. 1).

Chronology
Since Loganellia is from the Early Silurian, phylogenetic predecessors must be Ordovician and earlier. Haikouichthys is from the Early Cambrian, so predecessors, like Myxine, the hagfish, must be from the Ediacaran. Others, like the extant sturgeon, Acipenser, and whale shark (Rhincodon) are extant late survivors of Late Ordovician to Early Silurian radiations.

Figure 1. Sample basal vertebrates from the LRT demonstrating the genesis and evolution of dorsal, pectoral, pelvic and anal fins.

Fish fins were all immobile at first.
Derived taxa evolved stronger girdle anchors and muscles to make various fins the primary locomotory organs, reducing the need for caudal propulsion and locomotion in taxa from Manta to Chelonia to Homo.

Figure 2. Subset of the LRT focusing on basal chordates.

Traditional hypotheses:
Gill arch theory or “Gegenbaur hypothesis,” was posited in 1870 and proposes that the “paired fins are derived from gill structures”.

Lateral fin-fold theory: paired fins budded from longitudinal, lateral folds along the epidermis just behind the gills. “There is weak support for both hypotheses in the fossil record and in embryology.

Andrew Gillis 2016 wrote:
Unfortunately, the fossil record currently tells us relatively little about the stepwise acquisition of paired fins during vertebrate evolution, so we decided to address this question from a developmental perspective.” More on their study here.

One clade of basal chordates lacks any trace of dorsal, anal or lateral fins.
These include naked Metaspriggina, and armored Arandaspis, Poraspis and Drepanaspis (Fig. 3) in which the gill atrium became larger and wider as the taxon became overall larger. These taxa left no extant descendants. Other armored basal chordtes, like the Dunyu and Hemicyclaspis, are derived from Haikouichthys (Fig. 1).

Figure 3. A clade of finless chordates that became armored.
Figure 3. A clade of finless chordates that became armored apart from other chordates. These include naked Metaspriggina to armored Arandaspis, Poraspis and Drepanaspis. The large gill chamber is much larger than the tiny head.

According to results recovered by the LRT
(subset Fig. 1) workers need to continue combining extant and extinct taxa in phylogenetic analyses in their studies on fin origins. It is clear that prior workers understood the origin of fins as they studied shark embryos and fossils. Those studies are confirmed by the LRT so long as they do not include spiny sharks and placoderms.

References:
Diogo R 2020. Cranial or postcranial—Dual origin of the pectoral appendage of vertebrates combining the fin‐fold and gill‐arch theories? Developmental Dynamics https://doi.org/10.1002/dvdy.192
Goodrich ES 1906. Notes on the Development, Structure, and Origin of the Median and Paired Fins of Fish. 50(2):333–371.
Jarvik E 1964. On the origin of girdles and paired fins. Israel Journal of Zoology 14:1–4.
Shubin N 1995. The Evolution of Paired Fins and the Origin of Tetrapod Limbs. In: Hecht M.K., Macintyre R.J., Clegg M.T. (eds) Evolutionary Biology. Evolutionary Biology, vol 28. Springer, Boston, MA. https://doi.org/10.1007/978-1-4615-1847-1_2

thenode.biologists.com: Gills, fins and the evolution of vertebrate paired appendages
notesonzoologycom: click here.
ScienceDaily: Scientists Discover Evolutionary Origin Of Fins, Limbs
PNAS.org: Seeking the origin of paired fins
Wiki/Fish_fin: Evolution of paired fins:

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