This page covers the stem group of the neopterygian fishes (clade Neopterygii, Superclass Actinopterygii). The clade contains over 30,000 species in 4,914 genera and 509 families (Encyclopedia of Life, eol.org/pages/2775792).

The synapomorphies of the Neopterygii pertain mainly to the jaws and gills (López-Arbarello and Sferco, 2018):
  1. Absence of postrostral bone (in the snout)
  2. Maxilla (upper jawbone) detached from preopercle (a flat membrane bone in the gill cover)
  3. Maxilla elongate and shallow
  4. Presence of supramaxilla (small bone (or bones) lying immediately above or partially overlapping the posterior end of the maxillary bone)
  5. Lower jaw with coronoid process (a muscle attachment point)
  6. Subopercle (bony plate immediately below the opercle in the gill cover) with ascending process (protuberance)
  7. Presence of interopercle (membrane bone between the preopercle and the branchiostegal bones that support the gill membrane)
Clearly, none of the above characteristics, which represent evolutionary novelties that appeared along the neopterygian stem line, will be obvious from casual examination of fossils.

The stem-Neopterygii are quite well represented in the fossil record. Their phylogenetic relationships are shown in the time tree below:
​Figure 1.    Time tree of the stem-Neopterygii
​The oldest known member of the stem-Neopterygii is Discoserra pectinodon, described from the Early Carboniferous (Serpukhovian) Bear Gulch Limestone member of the Heath Formation at a location south of Becket, Fergus County, Montana (Lund, 2000; Latimer and Giles, 2018). This species is illustrated below, together with other members of the stem group for which public-domain images are available (for a larger view, click on image):
Names in   red indicate   that the fossil is younger  than the oldest known crown-group fossil.
​Figure 2. Images of  stem-group  Neopterygii
​The images shown above are numbered in order from most basal towards the crown group, but no obvious trends can be seen.
​
​The stem-to-crown transition of the Neopterygii lasted at least 71 million years, from the Early Carboniferous to the Early Triassic (Figure 1).

References

Benton, M. J., Donoghue, P. C., Asher, R. J., Friedman, M., Near, T. J., & Vinther, J. (2015). Constraints on the timescale of animal evolutionary history. Palaeontologia Electronica, 18(1), 1-106.

Latimer, A. E., & Giles, S. (2018). A giant dapediid from the Late Triassic of Switzerland and insights into neopterygian phylogeny. Royal Society open science, 5(8), 180497.

López-Arbarello, A., & Sferco, E. (2018). Neopterygian phylogeny: the merger assay. Royal Society Open Science, 5(3), 172337.
​
Lund, R. (2000). The new actinopterygian order Guildayichthyiformes from the Lower Carboniferous of Montana (USA).   Geodiversitas,    22(2).
Image credits - Stem-Neopterygii
  • Figure 2 (Ebenaqua ritchei, fossil):  Ghedoghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 2 (Ebenaqua ritchei, life restoration):  Apokryltaros, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 2 (Bobosatrania canadensis):  DiBgd, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 2 (Discoserra pectinodon):  Gyik Toma  (Tommy the paleobear) from Romania, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
  • Figure 2 (Brembodus ridens):  Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Gibbodon cenensis):  Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Mesturus verrucosus):  Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Arduafrons prominoris):  Ghedoghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 2 (Peltopleurus lissocephalus):  Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Dipteronotus ornatus):  Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons​