This page covers the stem group of the cartilaginous fishes (Superclass Chondrichthyes, infraphylum Gnathostomata), a clade that comprises the Elamosbranchii (sharks, skates and rays) and the Holocephali (the chimaeras).
The stem-Chondrichthyes are fairly well represented in the fossil record. An interpretation of their phylogeny is illustrated in the phylogenetic time tree shown below:
The stem-Chondrichthyes are fairly well represented in the fossil record. An interpretation of their phylogeny is illustrated in the phylogenetic time tree shown below:
Figure 1. Time tree of the stem-Chondrichthyes
The above tree includes a clade with the name “Acanthodians”. This is a group of cartilaginous fishes with spine-bearing paired and median fins (Brazeau and de Winter, 2015). Uncertainty about their phylogenetic placement has existed for a long time, but analysis carried over the last ten years has indicated that they belong to the chondrichthyan stem group (Zhu et al, 2013; Brazeau and de Winter, 2015; Coates et al, 2018). We follow that interpretation here.
The oldest known stem-group chondrichthyan is Fanjingshania renovata, found in the Rongxi Formation of Early Silurian (Aeronian) age in the Shiqian–Tunping section at Leijiatun village (Shiqian county), Guizhou province, China (Andreev et al, 2022). The only image available in the public domain is a skeletal fragment of the pectoral girdle, shown in Figure 3 below.
Relatively few images, especially of fossils rather than life restorations, exist in the public domain for the species shown in the above tree. The available images of acanthodians are reproduced below (click on any image to see a larger version):
The oldest known stem-group chondrichthyan is Fanjingshania renovata, found in the Rongxi Formation of Early Silurian (Aeronian) age in the Shiqian–Tunping section at Leijiatun village (Shiqian county), Guizhou province, China (Andreev et al, 2022). The only image available in the public domain is a skeletal fragment of the pectoral girdle, shown in Figure 3 below.
Relatively few images, especially of fossils rather than life restorations, exist in the public domain for the species shown in the above tree. The available images of acanthodians are reproduced below (click on any image to see a larger version):
Names in red indicate that the fossil is younger than the oldest known crown-group fossil.
Figure 2. Images of stem-Chondrichthyes: Acanthodians
The images shown above are placed in left-to-right order from most basal towards the crown group; no clear trends can be seen, in that they all appear quite shark-like apart from their spiny fins (except for the tail fins, which are not spine-supported).
The following set of images represents the stem-group chondrichthyans that do not belong to the acanthodians (for a larger view, click on image):
The following set of images represents the stem-group chondrichthyans that do not belong to the acanthodians (for a larger view, click on image):
Names in red indicate that the fossil is younger than the oldest known crown-group fossil.
Figure 3. Images of stem-Chondrichthyes: Non-acanthodians
These fishes appear less like sharks than do the acanthodians, with a greater variation in body form. Furthermore, there are less-obvious characteristics that changed through this series; see Coates et al (2018) for details. These authors conclude that Gladbachus displays “morphological incongruence with its phylogenetic position”, from which they suggest that the early stem-Chondrichthyes probably had greater morphological disparity than that seen in the limited pre-Devonian fossil record of the stem group.
Some idea of the nature of the transition from the stem group to the crown group of the chondrichthyans can be derived from a comparison of the above images with the examples of early crown-Chondrichthyes shown below:
Some idea of the nature of the transition from the stem group to the crown group of the chondrichthyans can be derived from a comparison of the above images with the examples of early crown-Chondrichthyes shown below:
Figure 4. Examples of early crown-Chondrichthyes
The transition from the stem-Chondrichthyes to the crown-Chondrichthyes, illustrated partially by the fossils in Figures 2, 3 and 4, took place over a period of between 52 and 55 million years, from the Early Silurian to the Middle Devonian (Figure 1).
References
Andreev, P. S., Sansom, I. J., Li, Q., Zhao, W., Wang, J., Wang, C. C., ... & Zhu, M. (2022). Spiny chondrichthyan from the lower Silurian of South China. Nature, 609(7929), 969-974.
Brazeau, M. D., & de Winter, V. (2015). The hyoid arch and braincase anatomy of Acanthodes support chondrichthyan affinity of ‘acanthodians’. Proceedings of the Royal Society B: Biological Sciences, 282(1821), 20152210.
Coates, M. I., Gess, R. W., Finarelli, J. A., Criswell, K. E., & Tietjen, K. (2017). A symmoriiform chondrichthyan braincase and the origin of chimaeroid fishes. Nature, 541(7636), 208-211.
Coates, M. I., Finarelli, J. A., Sansom, I. J., Andreev, P. S., Criswell, K. E., Tietjen, K., ... & La Riviere, P. J. (2018). An early chondrichthyan and the evolutionary assembly of a shark body plan. Proceedings of the Royal Society B: Biological Sciences, 285(1870), 20172418.
Zhu, M., Yu, X., Ahlberg, P. E., Choo, B., Lu, J., Qiao, T., ... & Zhu, Y. A. (2013). A Silurian placoderm with osteichthyan-like marginal jaw bones. Nature, 502(7470), 188-193.
Brazeau, M. D., & de Winter, V. (2015). The hyoid arch and braincase anatomy of Acanthodes support chondrichthyan affinity of ‘acanthodians’. Proceedings of the Royal Society B: Biological Sciences, 282(1821), 20152210.
Coates, M. I., Gess, R. W., Finarelli, J. A., Criswell, K. E., & Tietjen, K. (2017). A symmoriiform chondrichthyan braincase and the origin of chimaeroid fishes. Nature, 541(7636), 208-211.
Coates, M. I., Finarelli, J. A., Sansom, I. J., Andreev, P. S., Criswell, K. E., Tietjen, K., ... & La Riviere, P. J. (2018). An early chondrichthyan and the evolutionary assembly of a shark body plan. Proceedings of the Royal Society B: Biological Sciences, 285(1870), 20172418.
Zhu, M., Yu, X., Ahlberg, P. E., Choo, B., Lu, J., Qiao, T., ... & Zhu, Y. A. (2013). A Silurian placoderm with osteichthyan-like marginal jaw bones. Nature, 502(7470), 188-193.
Image credits
Stem-Chondrichthyans: Acanthodians
Stem-Chondrichthyans: Acanthodians
- Figure 2 (Nerepisacanthus denisoni): Danielle Dufault [CC BY 2.5 (https://creativecommons.org/licenses/by/2.5)]
- Figure 2 (Ischnacanthus gracilis): Nobu Tamura (http://spinops.blogspot.ca/) [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Figure 2 (Diplacanthus longispinus): FunkMonk [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Figure 2 (Diplacanthus crassissimus): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Figure 2 (Mesacanthus pusillus): Liopleurodon93 at English Wikipedia [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]
- Figure 2 (Cheiracanthus sp.): Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Figure 2 (Cheiracanthus murchisoni): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Figure 2 (Acanthodes sp.): Momotarou2012 [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Figure 2 (Acanthodes bronni): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Figure 3 (Gladbachus adentatus): DiBgd [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Figure 3 (Brochoadmones milesi): DiBgd [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Figure 3 (Climatius reticulatus): Nobu Tamura (http://spinops.blogspot.com) [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)]
- Figure 3 (Parexus recurvus): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Figure 3 (Gyracanthides murrayi): DiBgd [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Figure 4 (Cladoselache fyleri): EvolutionIncarnate, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 4 (Ferromirum oukherbouchi): Open Access article Frey, L., Coates, M. I., Tietjen, K., Rücklin, M., & Klug, C. (2020). A symmoriiform from the Late Devonian of Morocco demonstrates a derived jaw function in ancient chondrichthyans. Communications biology, 3(1), 1-10.
- Figure 4 (Phoebodus saidselachus): DiBgd, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 4 (Tristychius arcuatus): Nobu Tamura email:[email protected] http://spinops.blogspot.com/ http://paleoexhibit.blogspot.com/ [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]