Tiny Early Silurian Shenacanthus reconstructed and reidentified

This is a follow-up to
yesterday’s post on several tiny Early Silurian fish from China. Among the specimens was one plate and counterplate of tiny Shenacanthus vermiformis (Zhu et al 2022, Figs 1, 2).

Figure 1. Shenacanthus vermiformis images from Zhu et al 2022. Not a chondrichthyan, but an arthrodire placoderm, as suspected yesterday.
Figure 1. Shenacanthus vermiformis traced and reconstructed using DGS techniques. Colors added here.
Figure 1. Shenacanthus vermiformis traced and reconstructed using DGS techniques. Colors added here.

From the Zhu et al diagnosis:
“Small chondrichthyan, approximately 22 mm from the rostrum to the anal fin. Fusiform body shape; small cranium (~3.5 mm from the rostrum to the shoulder girdle) with blunt rostrum; dentition absent; branchial region posteriorly positioned in relation to the cranium; branchiostegal and hyoidean plates absent, shoulder girdle covered in large dermal plates both dorsally and ventrally, two median dorsal plates, the anterior one smaller and oblate, the posterior one larger and teardrop-shaped, with vermiform ornament; paddle-like pectoral fins lacking fin spine; Anal fin also lacking fin spine. Small, diamond-shaped scales. Small scutes or dermal plates with linear ornament along the dorsal and ventral midlines.”

The reconstruction (Fig 1) confirms an earlier suspicion that Shenacanthus was a tiny arthrodire placoderm, not a chondrichthyan (= sharks, rays and ratfish). Apparently the ‘dorsal spine’ is instead part of the dorsal shield. There is no anal fin. Claspers are identified here.

Figure 3. The iconic spiny shark, Climatius. Shown about 2x life size.

Zhu et al. nested Shenacanthus
between the spiny sharks Paucicanthus, Gladbachus, Climatius (Fig 3) and Doliodus + crown-group chondrichthyans in an unresolved clade. Taxon exclusion is a problem.

Gladbachus is an extinct whale shark.
Doliodus is a primitive deep-sea, big-mouth actinopterygian.
Spiny sharks are not chondrichthyans, but basal bony fish in the lobefin clade.

Figure 4. Early Silurian Shenacanthus demonstrates extreme phylogenetic miniaturization at the origin of arthrodire placoderms. That’s the headline here.

Here
in the large reptile tree (LRT, 2158 taxa) tiny Shenacanthus nested with tiny Millerosteus with such a similar trait list that these two are likely congeneric. Only size and time separate them. Millerosteus is not mentioned in the Zhu et al text. Their cladogram includes only a suprageneric clade ‘Arthrodira’.

This appears to be a novel hypothesis of interrelationships.

References
Zhu Y-A et al (10 co-authors) 2022. The oldest complete jawed vertebrates from the early Silurian of China. Nature 609:954–958. online

wiki/Shenacanthus – not yet posted

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