The phylogenetic comings and goings of lobe fins

The distribution of lobe fins, spiny fins and ray fins 
in the large reptile tree (LRT, 1756+ taxa, subset Fig. 1) indicate that each type of fin came and went and sometimes came back again.

So, contra traditional paleontology,
each type of fin does not represent a monophyletic clade. That would be “Pulling a Larry Martin” by setting up clades based on just a few characters.

The LRT provides a more holistic approach,
looking at 238 character traits from nose to toes and letting the software decide without tradition or bias. The LRT documents the multiple evolution of ray fins by convergence.

Figure 1. Subset of the LRT focusing on basal vertebrates and highlighting ray fins, spiny sharks and lobe fins. Catfish retain spines on their ray-like pectoral fins.

Figure 1. Subset of the LRT focusing on basal vertebrates and highlighting ray fins, spiny sharks and lobe fins. Catfish retain spines on their ray-like pectoral fins.

The basalmost lobefin in the LRT has gone unrecognized until now,
perhaps because Ticinolepis longaeva (Fig. 2) has such a little lobe on its pectoral fin and it is only known from Middle Triassic fossils. Ticinolepis longaeva nests at the base of all lobefins in the LRT (subset Fig. 1) so it would have had a Silurian genesis.

Figure 2. Ticinolepis longaeva in situ and reconstructed. Note the pectoral fin has a small lobe and this taxon nests at the base of all lobefins in the LRT.

Figure 2. Ticinolepis longaeva in situ and reconstructed. Note the pectoral fin has a small lobe and this taxon nests at the base of all lobefins in the LRT.

The resemblance of Ticinolepis longaeva to the next most basal lobefin,
Miguashaia (Middle Devonian; Fig. 3) is also instructive. (As a side note, Ticinolepis crassidens nests with Perleidus, not with Ticinolepis longaeva in the LRT, contra López-Arbarello and Sferco 2018).

Figure 2. The lobefin Miguashaia. Compare to the spiny shark, Diplacanthus, figure 1.

Figure 3. The lobefin Miguashaia. Compare to the spiny shark, Diplacanthus, figure 1.

Ticinolepis longaeva
(López-Arbarello and Sferco 2018; 12cm; Middle Triassic; MCSN 8072) nests at the base of the lobefin fishes. Note the tiny lobe in the middle of the ray fin. Compare that pectoral fin to the one in figure 3.

Miguashaia bureaui
(Schultze 1973, Cloutier 1996; Middle Devonian; 45cm) was considered the sister group (outgroup) of the Actinista (coelocanths). Notably Miguashaia reverses to a heterocercal tail. That’s why it looks a little odd. The dentary is short and the teeth are small.

Figure 2. Sturgeon swimming in a test tank from Wilga and Lauder 1999.

Figure 4. Sturgeon swimming in a test tank from Wilga and Lauder 1999.

Final notes to be covered in more detail later:
Basal pectoral fins are rather inflexible and extend horizontally (Fig. 4). Ratfish hold their pectoral fins vertically, against the torso. Iniopterygians raise the pectoral fin to the dorsal margin. Moray eels lose their fins. So there is more variety here yet to explore.


References
López-Arbarello A and Sferco E 2018. Neopterygian phylogeny: the merger assay. Royal Society open sci. 5: 172337. http://dx.doi.org/10.1098/rsos.172337
Schultze H-P 1973.
 Crossopterygier mi heterozerker Schwanzfloss aus dem Oberdevon Kanadas, nebst einer Beschreibung von Onychodontida-Resten aus dem Middledevon Spaniens und aus dem Karbon der USA. Palaeontograhica A 143:188–208.

the large reptile tree

wiki/Miguashaia

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