This page deals with the stem group of the ray-finned fishes (Superclass Actinopterygii, clade Osteichthyes). The actinopterygians are a very diverse group, consisting of some 42 orders containing more than 480 families, but share the characteristic of having fins that are supported by rays of dermal bone rather than by cartilage (Encyclopedia Britannica).
Many stem-group actinopterygian fossils have been found. Their phylogenetic relationships have been elucidated in many morphological analyses (e.g. Argyriou et al (2018), Wilson et al (2018), Ren and Xu (2021)), of which that by Giles et al (2017), shown in the phylogenetic time tree below, is a representative example:
Many stem-group actinopterygian fossils have been found. Their phylogenetic relationships have been elucidated in many morphological analyses (e.g. Argyriou et al (2018), Wilson et al (2018), Ren and Xu (2021)), of which that by Giles et al (2017), shown in the phylogenetic time tree below, is a representative example:
Figure 1. Time tree of the stem-Actinopterygii
The oldest known member of the stem-Actinopterygii is Meemannia eos, described from the middle part of the Xitun Formation, of Early Devonian (Late Lochkovian) age, at a locality close to Xitun village in the suburb of Qujing, Yunnan, southwestern China (Zhu et al, 2010; Giles et al, 2017). No images of this species are available in the public domain. However, the other stem-group fossils for which public domain images are available (unfortunately rather few relative to the number of species shown in Figure 1 above) are shown below (click on image for a larger view):
Names in red indicate that the fossil is younger than the oldest known crown-group fossil.
Figure 2. Images of stem-group Actinopterygii, with the crown-group example Platysomus sp.
The images shown above are placed in left-to-right order from most basal towards the crown group, but no obvious trends can be seen.
The actinopterygian stem line includes a ghost lineage (shown as a blue bar in Figure 1), because the oldest known stem-group sarcopterygian (of Late Silurian age; see page on the Sarcopterygii) is somewhat older than the stem-group Actinopterygii, which are of Early Devonian (Lochkovian) and younger age. Given that the two stem groups must have appeared at the same time, the actinopterygian stem-group transition must also have begun in the Late Silurian. Comparing this age with that of the oldest member of the crown-Actinopterygii indicates that the stem-to-crown transition lasted at least 76 million years, from the Late Silurian to the Early Carboniferous (Figure 1). A histogram of the rates of appearance of the known stem-group genera is shown below:
The actinopterygian stem line includes a ghost lineage (shown as a blue bar in Figure 1), because the oldest known stem-group sarcopterygian (of Late Silurian age; see page on the Sarcopterygii) is somewhat older than the stem-group Actinopterygii, which are of Early Devonian (Lochkovian) and younger age. Given that the two stem groups must have appeared at the same time, the actinopterygian stem-group transition must also have begun in the Late Silurian. Comparing this age with that of the oldest member of the crown-Actinopterygii indicates that the stem-to-crown transition lasted at least 76 million years, from the Late Silurian to the Early Carboniferous (Figure 1). A histogram of the rates of appearance of the known stem-group genera is shown below:
Figure 3. Rate of appearance of stem-Actinopterygii (predating the crown group and including only the genera shown in Figure 1)
Final transition to the crown group
The actinopterygian crown group is characterized by the following synapomorphies (Argyriou et al, 2018):
Unfortunately, the article by Argyriou et al (2018) does not explicitly state the condition of these morphological characters in the stem group, but in any case these characteristics must have developed during the time represented by the fossil gap of 22 million years or less between the youngest stem-group actinopterygian that predates the crown group (Fouldenia ischiptera) and the oldest member of the crown-Actinopterygii (the stem Chondrostean Platysomus superbus, illustrated below in Figure 4).
The following figure illustrates some examples of crown actinopterygians:
The actinopterygian crown group is characterized by the following synapomorphies (Argyriou et al, 2018):
- Bone carrying otic portion (near the ear) of lateral line canal (a system of sensory organs used to detect movement, vibration, and pressure gradients in the surrounding water) extends past posterior margin of parietals (bones forming part of the side and top of the head)
- Anterior ceratohyal (a bone that forms part of the hyoid arch, one of the arches that support the gills) has no groove
- Dorsal margin of palate is flat rather than having a high posterior extension
- Expanded anterior dorsal fontanelle (junction between parietal bones) is absent
Unfortunately, the article by Argyriou et al (2018) does not explicitly state the condition of these morphological characters in the stem group, but in any case these characteristics must have developed during the time represented by the fossil gap of 22 million years or less between the youngest stem-group actinopterygian that predates the crown group (Fouldenia ischiptera) and the oldest member of the crown-Actinopterygii (the stem Chondrostean Platysomus superbus, illustrated below in Figure 4).
The following figure illustrates some examples of crown actinopterygians:
Figure 4. Examples of crown-Actinopterygii
References
Argyriou, T., Giles, S., Friedman, M., Romano, C., Kogan, I., & Sánchez-Villagra, M. R. (2018). Internal cranial anatomy of Early Triassic species of †Saurichthys (Actinopterygii: †Saurichthyiformes): implications for the phylogenetic placement of †Saurichthyiforms. BMC evolutionary biology, 18(1), 1-41.
Dineley, D. & Metcalf, S. (1999). Fossil Fishes of Great Britain, Geological Conservation Review Series, No. 16, Joint Nature Conservation Committee, Peterborough, 675 pp.
Giles, S., Xu, G. H., Near, T. J., & Friedman, M. (2017). Early members of ‘living fossil’ lineage imply later origin of modern ray-finned fishes. Nature, 549(7671), 265-268.
Lu, J., Giles, S., Friedman, M., den Blaauwen, J. L., & Zhu, M. (2016). The oldest actinopterygian highlights the cryptic early history of the hyperdiverse ray-finned fishes. Current Biology, 26(12), 1602-1608.
Ren, Y., & Xu, G. H. (2021). A new species of Pteronisculus from the Middle Triassic (Anisian) of Luoping, Yunnan, China, and phylogenetic relationships of early actinopterygian fishes. Vertebrata PalAsiatica, 59(3), 169-199.
Wilson, C. D., Pardo, J. D., & Anderson, J. S. (2018). A primitive actinopterygian braincase from the Tournaisian of Nova Scotia. Royal Society open science, 5(5), 171727.
Zhu, M., Wang, W., & Yu, X. (2010). Meemannia eos, a basal sarcopterygian fish from the Lower Devonian of China–expanded description and significance. Morphology, phylogeny and paleobiogeography of fossil fishes. München: Verlag Dr. Friedrich Pfeil, 199-214.
Dineley, D. & Metcalf, S. (1999). Fossil Fishes of Great Britain, Geological Conservation Review Series, No. 16, Joint Nature Conservation Committee, Peterborough, 675 pp.
Giles, S., Xu, G. H., Near, T. J., & Friedman, M. (2017). Early members of ‘living fossil’ lineage imply later origin of modern ray-finned fishes. Nature, 549(7671), 265-268.
Lu, J., Giles, S., Friedman, M., den Blaauwen, J. L., & Zhu, M. (2016). The oldest actinopterygian highlights the cryptic early history of the hyperdiverse ray-finned fishes. Current Biology, 26(12), 1602-1608.
Ren, Y., & Xu, G. H. (2021). A new species of Pteronisculus from the Middle Triassic (Anisian) of Luoping, Yunnan, China, and phylogenetic relationships of early actinopterygian fishes. Vertebrata PalAsiatica, 59(3), 169-199.
Wilson, C. D., Pardo, J. D., & Anderson, J. S. (2018). A primitive actinopterygian braincase from the Tournaisian of Nova Scotia. Royal Society open science, 5(5), 171727.
Zhu, M., Wang, W., & Yu, X. (2010). Meemannia eos, a basal sarcopterygian fish from the Lower Devonian of China–expanded description and significance. Morphology, phylogeny and paleobiogeography of fossil fishes. München: Verlag Dr. Friedrich Pfeil, 199-214.
Image credits – Stem-Actinopterygii
- Figure 2 (Cheirolepis trailli, fossil): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 2 (Cheirolepis trailli, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 2 (Cheirolepis canadensis): Placoderm2, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2 (Howqualepis rostridens): Copyright Museums Victoria / CC BY (Licensed as Attribution 4.0 International)
- Figure 2 (Raynerius splendens): Open Access article Giles, S., Darras, L., Clément, G., Blieck, A., & Friedman, M. (2015). An exceptionally preserved Late Devonian actinopterygian provides a new model for primitive cranial anatomy in ray-finned fishes. Proceedings of the Royal Society B: Biological Sciences, 282(1816), 20151485.
- Figure 2 (Moythomasia nitida): Oilshale, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2 (Woodichthys bearsdeni): British Geological Survey under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
- Figure 2 (Boreosomus gillioti): Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Figure 2 (Cyranorhis bergeraci): Ryan Somma, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
- Figure 2 (Australosomus merlei): Emőke Dénes, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2 (Saurichthys sp., fossil): Ghedoghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2 (Saurichthys sp., life restoration): Bildflut [CC0]
- Figure 4 (Platysomus superbus): Open Access article Wilson, C. D., Mansky, C. F., & Anderson, J. S. (2021). A platysomid occurrence from the Tournaisian of Nova Scotia. Scientific reports, 11(1), 1-12.