Novel origin for the Acanthodii

The so-called ‘spiny sharks’ (clade: Acanthodii)
are not clearly associated with any other fish clade in the current literature.

Using 230+ generalized (not designed for fish) characters,
the large reptile tree (LRT, 1460 taxa) recovers the tiny (3.5cm, Early Devonian) Brachyacanthus (uniquely preserving a mosaic of skull bones;  Fig. 1) with the giant (up to 6m, Late Cretaceous) Xiphactinus (Fig. 2). Both arise from Strunius and Gogonasus, a sister pairing we looked at earlier here.

This is a novel hypothesis of relationships
that needs to be confirmed or refuted. At present it is the most parsimonious solution and the images present a gradual accumulation of derived traits, not more closely shared by any other tested taxa.

Brachyacanthus scutiger (originally Climatius scutiger, Egerton 1860; Hunterian Museum and Art Gallery specimen # 298251; 3.5 cm in length) is a tiny acanthodian or ‘spiny shark’ here arising from Cheirolepis and giving rise to Xiphactinus (Fig. 2). The maxilla is short and posterior to the orbit. The jugal is in two parts. The postorbital is a crescent. Teeth are absent. The ‘scapulocoracoid’ is the cleithrum. Distinct from other acanthodians, the bones split up, rather than fuse together. The fin rays coalesce to form spines.

Xiphactinus audax (Leidy 1870; Late Cretaceous; up to 6m in length) was a large traditional ray-fin fish. In the LRT it is a sister to tiny and toothless, Brachyacanthus (Fig. 1), both derived from Cheirolepis and Gogonasus. The teeth are fewer but longer and stronger. The lacrimal and jugal are broken into several parts. The frontal and parietal are fused. The postparietal is a tall sagittal crest. The ‘open’ cheek is composed of vertically rotated/expanded palate bones, the palatine and pterygoid. The tall quadrate is exposed. The skull is taller than wide.

What about those spine-like fins?
Some spiny sharks (Acanthodii) have a single spine framing a soft tissue fin. Others, like Brachyacanthus, have a bundle of spines creating a pectoral fin. The same trait is found in Xiphactinus (Fig. 3), but apparently misjudged until now. Given the 200 million year interval, perhaps this is what evolution came up with, following basic acanthodian and earlier blueprints. Perhaps this represents an atavism.

In evolution, as we’ve learned several times,
if your ancestors had a trait, it is buried in your genetic code, waiting to come out.

References
Egerton P de MG 1860. Report of the British Association for Science for 1859.
Transactions of the Sections. 116.
Leidy J 1870. [Remarks on ichthyodorulites and on certain fossil Mammalia]. Proceedings of the Academy of Natural Sciences, Philadelphia 22:12–13.
Newman M and Davidson B 2010. Early Devonian fish from the Midland Valley of Scotland. National Palaentological Congress London.14–15.

wiki/Acanthodii
wiki/Xiphactinus