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.

Figure 1. Subset of the LRT focusing on the clade of free-swimming lobefin fish that gave rise to acanthodians (spiny sharks) and ray fin fish.

Figure 1. Subset of the LRT focusing on the clade of free-swimming lobefin fish that gave rise to acanthodians (spiny sharks) and ray fin fish. Note: some acanthodians revert to a ray fin morphology.

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, .

Figure 1. Ischnacanthus in situ. Note the tail is right side up here, the torso is ventrally exposed and the skull is upside down (relative to the tail). So this fossil is twisted.

Figure 1. Ischnacanthus in situ. Note the tail is right side up here, the torso is ventrally exposed and the skull is upside down (relative to the tail). So this fossil is twisted. One pectoral spine is split at the root. 

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.

Figure 2. Ischnacanthus skull in situ and reconstructed. Note the remnants of ray fins posterior to the large pectoral spine.

Figure 2. Ischnacanthus skull in situ and reconstructed. Note the remnants of ray fins posterior to the large pectoral spine. The ventral squamosal is displaced over the maxilla. The circumorbital bones could be confused with a sclerotic ring. Some interesting tooth-like projections emerge from the dorsal squamosal and ventral mandible.

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.

Figure 1. Thoracopterus is a Triassic flying fish unrelated to extant flying fish, but a sister to giant Xiphactinus.

Figure 3. Thoracopterus is a Triassic flying fish unrelated to extant flying fish, but a sister to giant Xiphactinus.

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.

FIgure 2. Thoracopterus skull colorized from Tintori and Sassi 1992. Tetrapod nomenclature added.

FIgure 4. Thoracopterus skull colorized from Tintori and Sassi 1992. Tetrapod nomenclature added.

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

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