This page covers the stem group of the holostean fishes (class Holostei, superclass Actinopterygi). These are represented today by the bowfins (order Amiiformes, clade Halecomorphi) of North America and the gars (order Semionotiformes, clade Ginglymodi) of North and Central America and Cuba (Encyclopedia Britannica).

The stem-Holostei are well represented in the fossil record. An interpretation of their phylogenetic relationships is shown in the time tree below:
​Figure 1.    Time tree of the stem-Holostei
An interesting aspect of the above time tree is that all of the known stem-group fossils are younger than the oldest crown-group fossil. These post-crown stem-group fossils represent descendants of as-yet undiscovered ancestors that would have separated from the stem line during or before the Early Triassic.
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The oldest known member of the stem-Holostei is Sargodon tomicus, described from multiple sites within the late Triassic (Middle Norian) Zorzino Limestone in the Southern Alps of Italy (Tintori, 1983; 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   Holostei
The images shown above are numbered in order from most basal towards the crown group, but no obvious trends can be seen apart from the fact that the most basal species, Hulletia americana, does not have the deep-bodied form of all the other stem-Holostei illustrated above.

The oldest known crown-group holostean is Watsonulus eugnathoides, a member of the stem-Halecomorphi, found in the Early Trassic (Induan) Middle Sakamena Formation, Sakamena Group, Ambilombe Bay, Madagascar (Benton et al, 2015). A fossil of this species is illustrated below:
Figure 3. Image of oldest known member of the crown-Holostei
The time-frame of development of the holostean stem group is only loosely constrained, owing to the lack of any known stem-Holostei older than Watsonulus. All we can say is that the holostean stem group must have appeared no earlier than the Serpukhovian stage of the Early Carboniferous, which is the age of the oldest known member of the neopterygian stem group (Discoserra pectinodon). Thus the duration of the stem-group evolution leading up to the appearance of the crown-Holostei could not have been more than 80 million years (compare Figure 1 above with Figure 1 on the Neopterygian Stem Group page).

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.

López-Arbarello, A., & Sferco, E. (2018). Neopterygian phylogeny: the merger assay. Royal Society Open Science, 5(3), 172337.

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.
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Tintori, A. (1983). Hypsisomatic Semionotidae (Pisces, Actinopterygii) from the Upper Triassic of Lombardy (N. Italy). Rivista Italiana di Paleontologia e Stratigrafia 88(3):417-442.
Image credits – stem-Holostei
  • Header (Shortnose gar (Lepisosteus platostomus), Hellabrunn Zoo, Munich): Rufus46, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Hulettia americana): Isaac T Valtez, Public domain, via Wikimedia Commons
  • Figure 2 (Tetragonolepis semicincta): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Hemicalypterus weiri, fossil): Gibson SZ (2016), CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 2 (Hemicalypterus weiri, life restoration): Gibson SZ (2016), CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 2 (Dapedium noricum): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Sargodon tomicus, fossil): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Sargodon tomicus, life restoration): Apokryltaros, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Dandya ovalis, fossil): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Dandya ovalis, life restoration): Apokryltaros, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Heterostrophus latus): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons'
  • Figure 2 (Dapedium punctatum): Ghedoghedo, licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.
  • Figure 2 (Dapedium caelatum): Daderot, Public domain, via Wikimedia Commons
  • Figure 2 (Dapedium pholidotum): Photographed by Bob James (owner of website) at American Museum of Natural History, New York, May 2024.
  • Figure 2 (Dapedium stollorum): Daderot, CC0, via Wikimedia Commons
  • Figure 3: GleisReis, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons