Strunius: transitional between lobe fins and ray fins

Not sure why this one was overlooked for so long…
… but then again, so many phylogenetic relationships have been overlooked by taxon exclusion.

Earlier the large reptile tree (LRT, 1155 taxa, subset Fig. 4) indicated a novel origin for many (not all) ray-fin fish arising from the lobe fin, Gogonasus (Fig. 1). Today another transitional taxon was added to the LRT, Strunius (Fig. 1). Described as the sarcopterygian with ray fins, Strunius cements that earlier hypothesis.

Figure 1. Transitional taxa from the lobe fin Osteolepis to the ray fin Xiphactinus, including tiny Strunius at the transition.

Figure 1. Transitional taxa from the lobe fin Osteolepis to the ray fin Xiphactinus, including tiny Strunius at the transition. Note, Cheirolepis retains lobe fins on the pectoral set, not the pelvic set.

Once again,
phylogenetic miniaturization played a part in clade origins as Strunius is much smaller than both its ancestors and descendants (Fig. 2).

Figure 2. Tiny Strunius to scale with Cheirolepis.

Figure 2. Tiny Strunius to scale with Cheirolepis.

The earlier question about
the second origin of the dual (in-out) external naris is answered with Strunius (Fig. 3). The naris appears to be split in two (at least in this drawing), creating a new dual naris. The palate remains unknown, so whether Strunius retained a choana or not is not yet known.

Figure 3. Strunius enlarged to show detail. Inset shows the second origin of the dual external naris as the original apparently splits by the addition of a skin bridge creating two openings. A reminder, this is a late-survivor of an earlier radiation.

Figure 3. Strunius enlarged to show detail. Inset shows the second origin of the dual external naris as the original apparently splits by the addition of a skin bridge creating two openings. A reminder, this is a late-survivor of an earlier radiation.

A cladogram of tested taxa
(Fig. 4) shows three separate origins for ray-fin fish:

  1. sturgeons and spoonbills arising from placoderms;
  2. bichirs arising from lungfish;
  3. the rest of the ray-fins arising from Strunius
Figure 4. Subset of the LRT focusing on the three origins of ray-fin fish.

Figure 4. Subset of the LRT focusing on the three origins of ray-fin fish.

Including the outgroup taxon
Entelognathus (Zhu et al. 2013) might make all the difference between traditional cladograms (Fig. 5) and the LRT (subset Fig. 4). In the basal fish cladogram by Bemis, Findels and Grande 1997 (Fig. 5) Cheirolepis nests with the distinctly different Polypterus at the base. This does not show a gradual accumulation of derived traits.

Figure 5. Traditional cladogram of sturgeon origins from Bemis, Findels and Grande 1997. They did not have Entelognathus as an outgroup, which might make all the difference.

Figure 5. Traditional cladogram of sturgeon origins from Bemis, Findels and Grande 1997. They did not have Entelognathus as an outgroup, which might make all the difference. Note the huge morphological gap between the first two taxa.

Some authors
have championed the lungfish clade as tetrapod ancestors. Others have championed the rhipidistian clade (Osteolepis and kin). The present cladogram indicates both were offshoots with convergent traits. Here (Fig. 4), the tetrapod lineage arose more directly from basalmost bony fish, before the Devonian radiation of lungfish and rhipidistians.

So
Eusthenopteron and Osteolepis turn out to have a different set of living representatives than earlier workers once thought — IF this hypothesis of relationships pans out. I will keep adding taxa, but the topology is not changing, so far.

Strunius rolandi (Jessen 1966; originally Glyptomus rolandi Gross 1956; 10 cm in length; Late Devonian) was considered a lobe-fin fish with ray fins. Here it nests with Cheirolepis, a traditional and transitional ray fin fish. The origin of the double naris in this lineage appears here as a split dividing the original single in two. The palate and possible choana are not known. The maxilla and quadratojugal are fused relative to more primitive taxa.


References
Gross W 1956. Über Crossopterygier und Dipnoer aus dem baltischen Oberdevon im Zusammenhang einer vergleichenden Untersuchung des Porenkanalsystems paläozoischer Agnathen und Fische. Stockholm Almqvist & Wiksell.
Jessen H 1966. Die Crossopterygier des Oberen Plattenkalkes (Devon) der Bergisch-Glabach-Paffrather Mulde (Rheinisches Schiefergebirge) unter Berücksichtigung von amerikanischem und europäischem Onychodus-Material. Arkiv für Zoolgi 18:305–389.

wiki/Gogonasus
wiki/Cheirolepis
wiki/Strunius

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