Ischnacanthus: basal to spiny sharks and most ray-fin fish

There is a growing consensus
among paleoichthyologists (fish workers) that spiny sharks (clade: Acanthodii) belong closer to bony ray-fin fish, rather than to sharks. The large reptile tree (LRT, 1504 taxa; subset Fig. 1) has been nesting spiny sharks between lobe-fin fish and ray-fin fish since pertinent fish taxa have been added many months ago. That hypothesis of relationships is novel and continues today, even after a few dozen taxa have been added inviting spiny sharks to nest elsewhere.

Today a basal spiny shark, Ischnacanthus,
(Figs. 1, 2) nests at the base of the spiny sharks, a clade that nests between the semi-lobe-fin, Cheirolepis, and the Jurassic ray-fin pre-piranha, Dapedium, .

The Ischnacnathus specimen under study
(Figs. 1) is crushed, articulated, and twisted like a wet towel, so, it’s a perfect specimen to apply DGS (color tracing and reconstruction, Fig. 2) methods. Scoring from the reconstruction nests Ischnacanthus conventionally at the base of the Acanthodii in the LRT. Note: several small (vestigial) rays succeed the large pectoral spine. The skull reconstruction greatly resembles that of Cheirolepis, the outgroup taxon and the traditional basal ray-fin fish, despite the presence of a transitional pectoral lobefin.

At this point
it is worthwhile to take a look back the Triassic flying fish, Thoracopterus (Figs. 3, 4), which was presented earlier here. In the LRT Thoracopterus is a spiny shark (clade: Acanthodii) only this time with spectacular ray fins.

Be careful not to pull a Larry Martin here!
Just because Thoracopterus has ray fins does not mean it is a member of the ray-fin clade Teleostei. We’ve learned not to depend on one or a dozen traits. Use hundreds of traits and let the software decide where taxa nest. It is also a good idea to create a reconstruction from precise color tracings. Bones can be displaced. Freehand reconstructions introduce bias. You may draw what you think a taxon should be, and that’s never the way to go.

Ischnacanthus gracilis (Egerton 1861; Early Devonian, 430 mya; up to 2m in length) is a basal acanthodian with teeth (some spiny sharks are toothless). Traces of ray fins can still be seen posterior to the large pectoral spine. Note the in situ skull is twisted 180º from the tail. Ischnacanthus further cements the nesting of spiny sharks between lobe fins and most ray fins.

References
Egerton P de MG 1860. Report of the British Association for Science for 1859.
Transactions of the Sections. 116.

wiki/Acanthodii
wiki/Doliodus
wiki/Thoracopterus
wiki/Xiphactinus
wiki/Ischnacanthus