This page covers the stem group of the sharks, skates and rays (Class Elasmobranchii, superclass Chondrichthyes). The Elasmobranchii are divided into two clades, the Batoidea (the rays and skates) and the Selachimorpha (the sharks). The Selachimorpha are also known as the Selachii.
Owing to their cartilaginous skeleton, elasmobranchs are generally not well preserved in the fossil record and their phylogeny has been controversial (Marramà and Kriwet, (2017). Indeed, as discussed by Brazeau and Friedman (2014), distinguishing the synapomorphies of the elasmobranchs from those of the higher clade Chondricthyes has proved difficult.
Earlier phylogenies were based largely on teeth, but the discovery of articulated skeletons has allowed phylogenetic trees to be constructed with more confidence. Recent examples are Coates et al (2018), Frey et al (2019) and Frey et al (2020). A phylogenetic time tree based on Frey et al (2020) is shown below:
Owing to their cartilaginous skeleton, elasmobranchs are generally not well preserved in the fossil record and their phylogeny has been controversial (Marramà and Kriwet, (2017). Indeed, as discussed by Brazeau and Friedman (2014), distinguishing the synapomorphies of the elasmobranchs from those of the higher clade Chondricthyes has proved difficult.
Earlier phylogenies were based largely on teeth, but the discovery of articulated skeletons has allowed phylogenetic trees to be constructed with more confidence. Recent examples are Coates et al (2018), Frey et al (2019) and Frey et al (2020). A phylogenetic time tree based on Frey et al (2020) is shown below:
Figure 1. Time tree of the stem-Elasmobranchii
The choice of the above tree, rather than those in the other papers cited above, for presentation here is simply owing to the fact that it is the one that includes most genera. It is not necessarily the most accurate.
The oldest known member of the stem-Elasmobranchii is Phoebodus sophiae, found in Middle to Late Devonian (Givetian to Famennian) sediments over almost the entire world (Frey et al, 2019). It is known only from isolated teeth, and no images are avialble in the public domain. However, one later species of the genus is illustrated in Figure 2 below.
Of the genera included in the above tree, those with images available in the public domain are shown below (for a larger view, click on image):
The oldest known member of the stem-Elasmobranchii is Phoebodus sophiae, found in Middle to Late Devonian (Givetian to Famennian) sediments over almost the entire world (Frey et al, 2019). It is known only from isolated teeth, and no images are avialble in the public domain. However, one later species of the genus is illustrated in Figure 2 below.
Of the genera included in the above tree, those with images available in the public domain are shown below (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 elasmobranchs
The images are numbered from most basal to most crownward, based on Figure 1 above. With the exception of the rather eel-like Phoebodus and Thrinacodus, all the fossils display a generally shark-like form. Nearly all have prominent fin spines (for good examples see Acronemus, Onychoselache, Hamiltonichthys and Egertonodus).
The oldest known member of the crown-Elasmobranchii is the stem shark Rhomaleodus budurovi, described from the Middle Triassic (Anisian) of Bulgaria (Andreev and Cuny, 2012). No images are available in the public domain.
The fossils illustrated above can be considered as a set of transitional fossils that led to the appearance of the elasmobranch crown group over a period of between 138 and 144 million years from the Middle Devonian to the Early Triassic.
The oldest known member of the crown-Elasmobranchii is the stem shark Rhomaleodus budurovi, described from the Middle Triassic (Anisian) of Bulgaria (Andreev and Cuny, 2012). No images are available in the public domain.
The fossils illustrated above can be considered as a set of transitional fossils that led to the appearance of the elasmobranch crown group over a period of between 138 and 144 million years from the Middle Devonian to the Early Triassic.
References
Andreev, P. S., & Cuny, G. (2012). New Triassic stem selachimorphs (Chondrichthyes, Elasmobranchii) and their bearing on the evolution of dental enameloid in Neoselachii. Journal of Vertebrate Paleontology, 32(2), 255-266.
Brazeau, M. D., & Friedman, M. (2014). The characters of Palaeozoic jawed vertebrates. Zoological journal of the Linnean Society, 170(4), 779-821.
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.
Frey, L., Coates, M., Ginter, M., Hairapetian, V., Rücklin, M., Jerjen, I., & Klug, C. (2019). The early elasmobranch Phoebodus: phylogenetic relationships, ecomorphology and a new time-scale for shark evolution. Proceedings of the Royal Society B, 286(1912), 20191336.
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.
Marramà, G., & Kriwet, J. (2017). Principal component and discriminant analyses as powerful tools to support taxonomic identification and their use for functional and phylogenetic signal detection of isolated fossil shark teeth. PloS one, 12(11), e0188806.
Brazeau, M. D., & Friedman, M. (2014). The characters of Palaeozoic jawed vertebrates. Zoological journal of the Linnean Society, 170(4), 779-821.
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.
Frey, L., Coates, M., Ginter, M., Hairapetian, V., Rücklin, M., Jerjen, I., & Klug, C. (2019). The early elasmobranch Phoebodus: phylogenetic relationships, ecomorphology and a new time-scale for shark evolution. Proceedings of the Royal Society B, 286(1912), 20191336.
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.
Marramà, G., & Kriwet, J. (2017). Principal component and discriminant analyses as powerful tools to support taxonomic identification and their use for functional and phylogenetic signal detection of isolated fossil shark teeth. PloS one, 12(11), e0188806.
Image credits - stem-Elasmobranchii
- Header (Blue shark (Prionace glauca) off southern California): Mark Conlin/NMFS, Public domain, via Wikimedia Commons
- Figure 2 (Diplodoselache woodi): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 2 (Orthacanthus senckenbergianus, fossil): Ghedoghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2 (Orthacanthus senckenbergianus, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 2 (Triodus sessilis, fossil): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 2 (Triodus sessilis, life restoration): Dmitry Bogdanov, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 2 (Phoebodus saidselachus): DiBgd, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2(Thrinacodus gracia): DiBgd, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2 (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)]
- Figure 2 (Acronemus tuberculatus): Ghedoghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2 (Onychoselache traquairi): GB3D Type Fossils, licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
- Figure 2 (Hamiltonichthys mapesi): Ghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2 (Egertonodus fraasi, fossil): Haplochromis [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Figure 2 (Egertonodus fraasi, life restoration): Nobu Tamura (http://spinops.blogspot.com) [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Figure 2 (Synechodus jurensis): Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]