This page deals with the stem group of the sturgeons and paddlefishes (Class Chondrostei, clade Actinopteri). Much of the internal skeleton of modern sturgeons (family Acipenseridae, 27 living species known) is made of cartilage, and for this reason the group to which they belong is called Chondrostei, which means “cartilage bone” (Encyclopedia Britannica). The skeleton of the paddlefishes (family Polyodontidae, 2 living species known), like that of the sturgeons, has lost much of its ossification (Encyclopedia Britannica).
Note that this website does not include a page on the stem group of the clade to which the Chondrostei belong (the Actinopteri). This clade is based primarily on molecular phylogenetic analysis (Near et al, 2012). Few morphological studies provide support for the clade (Betancur-R et al, 2017), and there are no fossil species for which there is consensus in the literature for their assignment to the stem-Actinopteri. Two candidates have been proposed: Birgeria groenlandica (Giles et al, 2017) and Pteronisculus sp. (Xu et al, 2018), but other studies places these in the stem-Actinopterygii (Argyriou et al (2018) for Birgeria and Ren and Xu (2021) for Pteronisculus) or possibly, in the case of Birgeria, in the stem-Chondrostei (Figueroa et al, 2019). In view of these uncertainties, we will not attempt to say more about the stem-to-crown transition of the Actinopteri.
Only a few stem-Chondrostei have been identified. Their phylogeny, based on a modified version of a tree published by Hilton and Forey (2009), is shown in the following time tree:
Note that this website does not include a page on the stem group of the clade to which the Chondrostei belong (the Actinopteri). This clade is based primarily on molecular phylogenetic analysis (Near et al, 2012). Few morphological studies provide support for the clade (Betancur-R et al, 2017), and there are no fossil species for which there is consensus in the literature for their assignment to the stem-Actinopteri. Two candidates have been proposed: Birgeria groenlandica (Giles et al, 2017) and Pteronisculus sp. (Xu et al, 2018), but other studies places these in the stem-Actinopterygii (Argyriou et al (2018) for Birgeria and Ren and Xu (2021) for Pteronisculus) or possibly, in the case of Birgeria, in the stem-Chondrostei (Figueroa et al, 2019). In view of these uncertainties, we will not attempt to say more about the stem-to-crown transition of the Actinopteri.
Only a few stem-Chondrostei have been identified. Their phylogeny, based on a modified version of a tree published by Hilton and Forey (2009), is shown in the following time tree:
Figure 1. Time tree of the stem-Chondrostei
The oldest known member of the stem-Chondrostei is Platysomus superbus, found in the Glencartholm Volcanic Beds Formation of Early Carboniferous (Visean) age at Glencartholm, Eskdale, Scotland (Dineley & Metcalf, 1999; Giles et al, 2017). A simple life restoration of this species is shown below, together with other members of the genus and other stem-group fossils for which public-domain images are available are shown below (for a larger view, click on image):
Figure 2. Images of stem-group Chondrostei
The oldest known representative of the crown-Chondrostei is Protopsephurus liui, found in the Early Cretaceous (Barremian - Aptian) Yixian Formation in Lingyuan County, Liaoning Province, northeastern China (Lu, 1994; Benton et al, 2015). A fossil and a life restoration of this species are shown below (click on image for a larger view):
Figure 3. Images of oldest known crown-group chondrostean
The above time tree (Figure 1) indicates that the chondrostean stem group developed from Early Devonian to Early Cretaceous time, representing a stem-to-crown transition of at least 200 million years.
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), 161.
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.
Betancur-R, R., Wiley, E. O., Arratia, G., Acero, A., Bailly, N., Miya, M., ... & Orti, G. (2017). Phylogenetic classification of bony fishes. BMC evolutionary biology, 17(1), 1-40.
Dineley, D. & Metcalf, S. (1999). Fossil Fishes of Great Britain, Geological Conservation Review Series, No. 16, Joint Nature Conservation Committee, Peterborough, 675 pp.
Figueroa, R. T., Friedman, M., & Gallo, V. (2019). Cranial anatomy of the predatory actinopterygian Brazilichthys macrognathus from the Permian (Cisuralian) Pedra de Fogo Formation, Parnaíba Basin, Brazil. Journal of Vertebrate Paleontology, 39(3), e1639722.
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.
Hilton, E. J., & Forey, P. L. (2009). Redescription of †Chondrosteus acipenseroides Egerton, 1858 (Acipenseriformes, †Chondrosteidae) from the lower Lias of Lyme Regis (Dorset, England), with comments on the early evolution of sturgeons and paddlefishes. Journal of Systematic Palaeontology, 7(4), 427-453.
Lu, L. (1994). A new paddlefish from the Upper Jurassic of northeast China. Vertebrata Pal Asiatica, 32(2), 134-142.
Merck, J. (2021). Osteichthyes and Actinopterygii. Course notes for GEOL 431 Vertebrate Paleobiology, Spring Semester 2021, University of Maryland.
Near, T. J., Eytan, R. I., Dornburg, A., Kuhn, K. L., Moore, J. A., Davis, M. P., ... & Smith, W. L. (2012). Resolution of ray-finned fish phylogeny and timing of diversification. Proceedings of the National Academy of Sciences, 109(34), 13698-13703.
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.
Xu, G. H., Ma, X. Y., & Zhao, L. J. (2018). A large peltopleurid fish (Actinopterygii: Peltopleuriformes) from the Middle Triassic of Yunnan and Guizhou, China. Vertebrata PalAsiatica, 56(2), 106-120.
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.
Betancur-R, R., Wiley, E. O., Arratia, G., Acero, A., Bailly, N., Miya, M., ... & Orti, G. (2017). Phylogenetic classification of bony fishes. BMC evolutionary biology, 17(1), 1-40.
Dineley, D. & Metcalf, S. (1999). Fossil Fishes of Great Britain, Geological Conservation Review Series, No. 16, Joint Nature Conservation Committee, Peterborough, 675 pp.
Figueroa, R. T., Friedman, M., & Gallo, V. (2019). Cranial anatomy of the predatory actinopterygian Brazilichthys macrognathus from the Permian (Cisuralian) Pedra de Fogo Formation, Parnaíba Basin, Brazil. Journal of Vertebrate Paleontology, 39(3), e1639722.
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.
Hilton, E. J., & Forey, P. L. (2009). Redescription of †Chondrosteus acipenseroides Egerton, 1858 (Acipenseriformes, †Chondrosteidae) from the lower Lias of Lyme Regis (Dorset, England), with comments on the early evolution of sturgeons and paddlefishes. Journal of Systematic Palaeontology, 7(4), 427-453.
Lu, L. (1994). A new paddlefish from the Upper Jurassic of northeast China. Vertebrata Pal Asiatica, 32(2), 134-142.
Merck, J. (2021). Osteichthyes and Actinopterygii. Course notes for GEOL 431 Vertebrate Paleobiology, Spring Semester 2021, University of Maryland.
Near, T. J., Eytan, R. I., Dornburg, A., Kuhn, K. L., Moore, J. A., Davis, M. P., ... & Smith, W. L. (2012). Resolution of ray-finned fish phylogeny and timing of diversification. Proceedings of the National Academy of Sciences, 109(34), 13698-13703.
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.
Xu, G. H., Ma, X. Y., & Zhao, L. J. (2018). A large peltopleurid fish (Actinopterygii: Peltopleuriformes) from the Middle Triassic of Yunnan and Guizhou, China. Vertebrata PalAsiatica, 56(2), 106-120.
Image credits – Stem-Chondrostei
- Header (White sturgeon, Acipenser transmontanus) at the Oregon Department of Fish and Wildlife's Sturgeon Center at Bonneville Dam on the Columbia River in January 2010: Oregon Department of Fish & Wildlife, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
- Figure 2 (Platysomus spp.): 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.
- Figure 2 (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.
- Figure 2 (Chondrosteus acipenseroides, fossil): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 2 (Chondrosteus acipenseroides, life restoration): British Museum (Natural History). Dept. of Geology;Woodward, Arthur Smith, 1864-1944, No restrictions, via Wikimedia Commons
- Figure 2 (Peipiaosteus pani): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 3 (Protopsephurus liui, fossil): Zhangzhugang, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 3 (Protopsephurus liui, life restoration): PaleoEquii, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons