Tiny Cretaceous stickleback ancestor for the extant giant oarfish

According to Wikipedia, 
“Plectocretacicus is an extinct genus of prehistoric ray-finned fish that lived during the lower Cenomanian. It contains a single species, P. clarae. Plectocretacicus is the earliest known member of the order Tetraodontiformes.”

By contrast,
the large reptile tree nests tiny Plectocretacicus ancestral to the extant giant oarfish, Regalecus glesne (Figs. 2, 3). Both are derived from the stickleback, Gasterosteus.

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. The small size of this taxon follows a pattern seen in other vertebrates of phylogenetic miniaturization at the genesis of new clades. Distinguishing traits, such as the spine-like pectoral fins, show that this taxon had a more ancient sister with more plesiomorphic traits.

Regalecus glesne (Ascanius 1772; typically 3m in length, rarely to 11m) is the extant giant oarfish. Like the seahorse it nests with, the mouth is vertical, the dorsal fin provides the propulsion. The oarfish often swims vertically, sometimes at great depth. It feeds on plankton and small fish. The pelvic fins remind one of oars, but they do not contribute to propulsion. The oarfish is nearly all tail (caudal vertebrae). Distinct from seahorses, the snout is quite short.

References
Ascanius P 1772. Philine quadripartita, et förut obekant sjö-kräk, aftecknadt och beskrifvet. Kongliga Vetenskaps Academiens Handlingar 33 (10-12): 329-331, pl. 10.
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 L 1996. New Superfamily and Three New Families of Tetraodontiform Fishes from the Upper Cretaceous: The Earliest and Most Morphologically Primitive Plectognaths. (PDF). Smithsonian Contributions to Paleobiology. 82: 1–59.

wiki/Plectocretcicus
wiki/Giant_oarfish