Tiny Triassic Boreosomus and its larger Early Jurassic descendants

Again,
once the solution comes about it appears obvious. When you start to analyze fish you’ll find yourself becoming more familiar with the hundreds of tested fish taxa the more they are studied.

Figure 1. Boreosomus, Pachycormus, Euthynotus and Ohmdenia are related to one another in the LRT, here 3 of 4 of them are shown to scale. Currently this is the sister clade to the Porolepiformes, which gave rise to lungfish and tetrapods.

This time
tiny, speedy, streamlined Boreosomus (Fig 1) from Triassic Spitzbergen to Madagascar gives rise to a larger taxon attributed to Early Jurassic Pachycormus (BRLSI M1332, Fig 1), then Early Jurassic Euthynotus (Fig 1) and finally the rather large but left scattered in situ, Early Jurassic Ohmdenia (Fig 1) shown here reduced relative to the others in order to get more than just the nose in.

Boreosomus arcticus
(aka Diaphorognathus, Woodward 1912; Priem 1924; Early Triassic; 250 mya, 10-20cm) nests with two pachycormids including Ohmdenia. Note the procumbent dentary teeth and small anterior dorsal fin.

References
Cawley JJ, Kriwet J, Kug S and Benton MJ 2019. The stem group teleost Pachycormus (Pachycormiformes: Pachycormidae) from the Upper Lias (Lower Jurassic) of Strawberry Bank, UK. PalZ 93(2):285–302.
Hauff B 1953. Ohmdenia multidentata nov. gen. et nov. sp. Ein neuer grober Fischfund aus den Posidonienschiefern des Lias e von Ohmden/Holzmaden in Wü rttemburg. Neues Jahrb. Geol. P.-A. 97, 39–50.
Priem F 1924. Paleontologie de Madagascar. XII. Les Poissons fossiles. Ann. Paleont., vol. 13: 105–132.
Woodward AS 1912. Notes on some fish-remains from the Lower Trias of Spitzbergen. Bulletin of the Geological Institution of the University of Upsala 11:291-297.

wiki/Ohmdenia
wiki/Pachycormus
wiki/Boreosomus