Origin of fins from acanthodian spines

Friedman and Brazeau 2010 reappraised
the origin and basal radiation of Osteichthyes, the clade of bony fish that includes Tetrapoda and Primates. They wrote, “There has been minimal documentation of the pattern of character acquisition leading to the osteichthyan crown. We review the synapomorphies proposed for various levels within osteichthyan phylogeny (total group; Acanthodes + crown group; crown group; Sarcoptergyii; Actinopterygii), confirming some, rejecting others, and making new additions. This distribution of characters is used to interpret the placement of problematic Siluro-Devonian genera traditionally assigned to Actinopterygii, and suggests these taxa are stem osteichthyans. Earlier placements of these forms within the crown are symptomatic of taxonomies based on unpolarized similarities rather than synapomorphies.”

Figure 1. Nerepisacanthus, Peltopleurus, Ticinolepis and Saurolepis, taxa at the spiny to ray-fin and lobe-in transition in the LRT. Of these taxa, none were mentioned by Friedman and Brazeau 2010. Note the ‘scaled’ pectoral and pelvic fins in late-surviving Peltopleurus from the Middle Triassic, a taxon that must have originated in the Late Silurian given itsEarly Devonian descendants.

Schaeffer 1968
also attempted to describe osteichthyan radiation in the pre-computer era.

Friedman and Brazeau concluded,
“Many aspects of early vertebrate phylogeny have been enriched and resolved since Schaeffer (1968), but the osteichthyan stem is not among these. The problem, it seems, is not limited to bony fishes: recent phylogenetic analyses of Chondrichthyes have suggested a similarly naked or depauperate stem. These apparent gaps in the early records of osteichthyans and chondrichthyans spur appeals to discover new, ever-older fossils that might plug the holes. But such fossils, when discovered, rarely deliver, because they are inserted into an old scheme of relationships with persistent roots in pre-cladistic taxonomy. The problem is not solely a lack of fossils. It is also the lack of adequate documentation of the fossils we already have, and a rigorous framework in which we might interpret them.”

“Systematic analyses inevitably evolve over time, but phylogenetic studies of different groups display remarkably similar ontogenies. First-generation treatments seek synapomorphies of ‘established’ groups. Rarely is assumed monophyly seriously challenged in such analyses. Only later do weak foundations, if present, begin to crumble under the weight of more exacting scrutiny.”

By contrast the large reptile tree (LRT, 2222 taxa) documents the origin of all included taxa back to pre-Cambrian worms. Key basal taxa, like Entelognathus and Shenacanthus, were described after 2010.

Currently
the LRT presents a hypothesis of interrelationships that requires confirmation, refutation or modification from independent research using a similar taxon list. In house modification continues at present.

References
Friedman M and Brazeau MD 2010. A reappraisal of the origin and basal radiation of
the Osteichthyes. Journal of Vertebrate Paleontology, 30:1, 36-56, DOI:
10.1080/02724630903409071
Schaeffer B 1968. The origin and basic radiation of the Osteichthyes; pp. 207–222 in T. Ørvig (ed.), Current Problems of Lower Vertebrate Phylogeny. Almqvist & Wiksell, Stockholm.