EVOLUTION - THE TRANSITIONAL FOSSILS
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chondrichthyan stem group

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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).

Synapomorphies have been difficult to define for the Chondrichthyes. According to Brazeau and Friedman (2014), the only undisputed chondrichthyan synapomorphy is tessellate prismatic calcified cartilage; the tessellated skeleton is composed of small blocks (tesserae) of calcified cartilage overlying a core of unmineralized cartilage (Dean and Summers, 2006).
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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:
Picture
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 Elegestolepis conica, found in Early Silurian (Llandovery) sediments in the Angara-Jlim area (Niuya River outcrops and Niuya-Berresova area) of the Siberian Platform in Russia (Broughton et al, 2013). It is known only from fossilized scales, and no public-domain images are available.

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).
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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.

The transition from the stem-Chondrichthyes to the crown-Chondrichthyes, illustrated partially by the fossils in Figures 2 and 3, took place over a period of at least 47 million years, from the Early Silurian to the Middle Devonian (Figure 1). However, the highest rate of appearance of new species occurred during the Early Devonian, as illustrated below:
Picture
Figure 4. Rate of appearance of stem-Chondricthyes  (predating the crown group and including only the genera shown in Figure 1)
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References

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.

Broughton, R. E., Betancur-R, R., Li, C., Arratia, G., & Ortí, G. (2013). Multi-locus phylogenetic analysis reveals the pattern and tempo of bony fish evolution. PLoS currents, 5.

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.

​Dean, M. N., & Summers, A. P. (2006). Mineralized cartilage in the skeleton of chondrichthyan fishes. Zoology, 109(2), 164-168.

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
  • 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)]
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  • Evolution of life
    • Overview
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    • Animals >
      • Vertebrate stem group >
        • Cyclostome stem group
        • Hagfish stem group
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        • Chondrichthyan stem group
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        • Actinopterygian stem group
        • Bichir and reedfish stem group
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          • Hyrax stem group
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