Almaco jack + phylogenetic miniaturization = Plectocretacicus

Plectocretacicus clarae (Sorbini 1996, Fig. 1; early Late Cretaceous; about 3cm) is traditionally known as the earliest known tetraodontiform, the clade that includes queen trigger fish, ocean sunfish and pufferfish. Earlier the large reptile tree (LRT, 1565 taxa) nested the extant amberjack, Seriola rivoliana (Fig. 2) basal to this clade, a novel nesting.

Tiny Plectocretacicus is about 3m long,
about 1/30 the length of the 90 cm Almaco jack. We’ve already seen phylogenetic miniaturization at the genesis of many, many vertebrate clades. This appears to be one more case of that.

Distinguishing traits,
such as the spine-like pectoral fins, show that Plectocretacicus had a more ancient sister with more plesiomorphic traits. The radiation of current sisters would also have preceded the Late Cretaceous. Acanthodians (spiny sharks) also reduce their ray fins into spikes by convergence.

Seriola rivoliana (Valenciennes 1833; 90cm) is the extant Almaco jack or high-fin amberjack. This sister to Seriola zonata nests at the base of the Tetraodontiformes. Subtle differences separate these two species. Note the tall dorsal and anal fins, further exagerated in Mola(below). Pelvic fins are lost in derived taxa.

References
Sorbini L 1979. Segnalazione di un plettognato Cretacico Plectocretacicus nov. gen. Bollettino del Museo Civico di Storia Naturale di Verona, 6:1–4.
Tyler JC and Sorbini L1996. New Superfamily and Three New Families of Tetraodontiform Fishes from the Upper Cretaceous: The Earliest and Most Morphologically Primitive Plectognaths. Smithsonian Contributions to Paleobiology. 82: 1–59.