Rhincodon, the whale shark, enters the LRT basal to sharks and placoderms

I think you’ll be pleasantly surprised
how key this taxon is.

The largest fish in the sea,
Rhincodon typus (Smith 1829; up to 18.8m; Fig 1) is the extant whale shark. It is also the most primitive tested shark, sharing a large number of traits with jawless Thelodus, the walking catfish Clarias, and the early jawed placoderm, Entelognathus. Superficial similarities to catfish are real, including the small barbels near the mouth. The primitive terminal jaws are transverse, not underslung as in derived sharks. The jaws contain 300-350 rows of tiny carpet-like teeth and 20 filter pads, similar to the fewer and smaller tooth rows in catfish and similar to the skin scales in Thelodus. The orbits occupied the front corners of the poorly ossified skull. Open spiracles were posterior to the orbits. Although an open seas swimmer, the flattened shape of Rhincodon is a legacy from its bottom-dwelling ancestry. Related carpet sharks, like Squatina, are all bottom dwellers.

Based on its phylogeny,
smaller sisters to Rhincodon had their genesis in Silurian waters alongside tiny Thelodus (Fig. 1). This is not the first time such relationships, revealed by the wide gamut feature of the large reptile tree (LRT, 1503 taxa), have been completely overlooked by prior workers.

Even so,
I think it is surprising that no one has ever imagined that Silurian sisters to both tiny Thelodus and giant Rhincodon were in the lineage of sharks, catfish, tetrapods and humans.

References
Smith A 1829. Descriptions of new, or imperfectly known objects of the animal kingdom, found in the south of Africa. South African Commercial Advertiser 3: 2.

wiki/Whale_shark