An unexpected resolution to the ‘spiny shark’ problem

Short summary for those who are skimming:
Rather than being odd sideshow characters, acanthodians (spiny-sharks; Fig. 1) were key players in the evolution of stem tetrapods, placoderms and several clades of ray-fin fish within the stem tetrapod lineage as today recovered by the The Large Reptile Tree (LRT, 1683+ taxa, subset Fig. 2). Acanthodians have nothing to do with sharks, ratfish or most other bony fish (e.g. sea horses, tuna, flounders, etc. in the bowfin clade).

The problem:
Earlier here, here, here and here the LRT nested a few acanthodians in the stem tetrapod branch. Back then the few tested acanthodians nested far from sharks with similar spines on their dorsal fins only. These include Cladoselache, Hybodus, and Gregorius (Fig. 1). Back then the few tested acanthodians also nested far from other fish with spines on their pectoral fins, but not on the dorsal fins, like Doliodus and the catfish, Clarias.

Were these examples of spine convergence?
Or homology? Or are we looking for a third answer? As usual, in order to get to a phylogenetic solution that minimizes taxon exclusion, we add taxa to the LRT (Figs. 1, 2).

Asterisk*
Some readers may remember that the extant Amazonian bronze featherback, Notopterus, (Fig. 1) earlier nested with the few tested acanthodians, despite having ray fins everywhere, but spines for pelvic fins. So that pattern (Fig. 1) has been hinted at. Today an added taxon, Ptomacanthus (Fig. 1), is a closer transitional spiny shark taxon to Notopterus. Today additional spiny sharks likewise nest basal to ray-fin fish and others.

Figure 1. Click to enlarge. Acanthodians and their spiny and non-spiny relatives in the LRT (subset Fig. 2), not to scale.

Figure 1. Click to enlarge. Acanthodians and their spiny and non-spiny relatives in the LRT (subset Fig. 2), not to scale. Earlier I provisionally named ‘Proacanthodes’ ‘Proamia’ when it nested between Gregorius and Amia. Today that mistake is corrected.

Backstory:
According to Wikipedia“Acanthodii or acanthodians (sometimes called spiny sharks) is an extinct paraphyletic class of teleostome fish, sharing features with both bony fish and cartilaginous fish. In form they resembled sharks, but their epidermis was covered with tiny rhomboid platelets like the scales of holosteans (gars, bowfins). They represent several independent phylogenetic branches of fishes leading to the still extant Chondrichthyes.”

Based on the earlier nesting of a few acanthodians deep in the bony fish clade, phylogenetic bracketing indicates that the apparent lack of a bony skeleton in acanthodians is likely based on a skeletal reversal or decay based on a slow-moving lifestyle and perhaps a deep-sea niche.

Wikipedia continues:
“In a study of early jawed vertebrate relationships, Davis et al. (2012) found acanthodians to be split among the two major clades Osteichthyes (bony fish) and Chondrichthyes (cartilaginous fish). The well-known acanthodian Acanthodes was placed within Osteichthyes, despite the presence of many chondrichthyan characteristics in its braincase.

In the LRT Acanthodes is also a basal member of the bony fish (Osteichthys).

“However, a newly described Silurian placodermEntelognathus, which has jaw anatomy shared with bony fish and tetrapods, has led to revisions of this phylogeny: acanthodians were then considered to be a paraphyletic assemblage leading to cartilaginous fish, while bony fish evolved from placoderm ancestors.”

The LRT does not support those hypotheses of interrelations.

“Burrow et al. 2016 provides vindication by finding chondrichthyans to be nested among Acanthodii, most closely related to Doliodus and Tamiobatis. A 2017 study of Doliodus morphology points out that it appears to display a mosaic of shark and acanthodian features, making it a transitional fossil and further reinforcing this idea.”

The LRT nests Doliodus with spiny sharks and close to the base of stem bony fish.

“Many palaeontologists originally considered the acanthodians close to the ancestors of the bony fishes. Although their interior skeletons were made of cartilage, a bonelike material had developed in the skins of these fishes, in the form of closely fitting scales (see above). Some scales were greatly enlarged and formed a bony covering on top of the head and over the lower shoulder girdle. Others developed a bony flap over the gill openings analogous to the operculum in later bony fishes. However, most of these characteristics are considered homologous characteristics derived from common placoderm ancestors, and present also in basal cartilaginous fish.”

The LRT nests acanthodians as basal bony fish. Highly derived placoderms are likewise in the stem tetrapod clade of bony fish, close to catfish. Both are derived from spiny sharks (Figs. 1, 2). When workers ‘consider characteristics’ they are relying on a few traits, which may converge or reverse. So that puts us on shaky ground. It is much better to let software decide placement in a tree topology, based on hundreds of traits and taxa to minimize the ever-present problem of pertinent taxon exclusion.

Figure 3. Distribution of spiny sharks and other fish with dorsal spines in the LRT.

Figure 2. Distribution of spiny sharks and other fish with dorsal spines in the LRT. Will adding taxa link catfish to traditional acanthodians? Add taxa to find out. Lunaspis and Dicksonosteus have a structurally different sort of pectoral fin, but a connection to acanthodians may someday be revealed.

Traditionally
fish workers thought of spiny sharks as enigmatic, odd and somewhat apart from the rest of fish evolution, leaving no living descendants with similar spine fins (see above).

By contrast,
In the LRT (subset Fig. 2), which tests a wider gamut of taxa, spiny sharks appear at the genesis of stem tetrapods (Fig. 1) and give rise to a wide variety of ray fin fish, placoderms and lobe fin fish. A wide-gamut cladogram is a powerful tool for resolving enigma taxa and clades. This problem was resolved today without guesswork—despite this clade leaving no living descendants with spine fins.

This is probably the best cautionary tale yet
for not “Pulling a Larry Martin” (i.e. depending on a few traits to define a clade). Always define a clade based on a wide-gamut cladogram that includes a last common ancestor and all of its descendants. Based on that methodology, spiny sharks are not extinct. Several extant fish are derived from spiny shark ancestors, and so are all tetrapods—all without spine fins.

This presentation appears to be
a novel hypothesis of interrelations. If not, please provide an earlier citation so I can promote it here.


References
Brazeau MD 2012. A revision of the anatomy of the Early Devonian jawed vertebrate Ptomacanthus anglicus Miles. Palaeontology. 55 (7227): 355–367.
Davis SP, Finarelli JA and Coates MI 2012. Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes. Nature. 486 (7402): 247–50.
Maisey JG et al. 2017. Pectoral morphology in Doliodus : bridging the ‘acanthodian’-chondrichthyan divide. American Museum Novitates 3875:1–15.
Miles R 1973. Articulated acanthodian fishes from the Old Red Sandstone of England, with a review of the structure and evolution of the acanthodian shoulder-girdle. Bulletin of the British Museum (Natural History), 24, 111–213.

wiki/Acanthodii

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