This page deals with the stem group of the chimaeras (Class Holocephali, superclass Chondrichthyes). The extant holocephalans, which belong to the order Chimaeriformes, comprise around 47 species in three families, the Rhinochimaeridae, Chimaeridae and Callorhinchidae (Encyclopedia Britannica).
A recent interpretation of the phylogeny of the stem-Holocephali (Frey et al, 2020) is shown in the following phylogenetic time tree:
A recent interpretation of the phylogeny of the stem-Holocephali (Frey et al, 2020) is shown in the following phylogenetic time tree:
Figure 1. Time tree of the stem-Holocephali
The above tree includes a clade called Symmoriiformes, which is distinguished from other stem-group holocephalans by certain characteristics of the braincase (Coates et al, 2017), the existence of a “spine-brush complex”, features of the postcranial endoskeleton and lack of body squamation (Maisey, 2007). The spine-brush complex is a peculiar dorsal protuberance immediately behind the head (Maisey, 2009). We will consider the stem group in two parts: first the Symmoriiformes and then the rest of the stem-Holocephali.
Symmoriiformes
The symmoriiform clade contains the oldest known stem-group holocephalan: Ferromirum oukherbouchi, described from the Thylacocephalan Layer of the Lahfira Member of the Ibaouane Formation of Late Devonian (Middle Famennian) age at Madene el Mrakib, Maïder Basin, southeastern Anti-Atlas, Morocco (Frey et al, 2020). This species is illustrated below, together with other symmoriiform fossils for which images are available in the public domain (click on image for a larger view):
Symmoriiformes
The symmoriiform clade contains the oldest known stem-group holocephalan: Ferromirum oukherbouchi, described from the Thylacocephalan Layer of the Lahfira Member of the Ibaouane Formation of Late Devonian (Middle Famennian) age at Madene el Mrakib, Maïder Basin, southeastern Anti-Atlas, Morocco (Frey et al, 2020). This species is illustrated below, together with other symmoriiform fossils for which images are available in the public domain (click on image for a larger view):
Figure 2. Images of stem-group Holocephalans: Symmoriiformes
The images shown above are placed in left-to-right order from most basal towards the crown group. A trend can be seen from fishes that look quite shark-like (e.g. Cladoselache) to those with a forward-directed, rodlike spine (e.g. Damocles) or a spine-brush complex (e.g. Akmonistion).
Other stem-Holocephali
The few available public-domain images of these fishes are shown below (for a larger view, click on image):
Other stem-Holocephali
The few available public-domain images of these fishes are shown below (for a larger view, click on image):
Figure 3. Images of non-symmoriiform stem-group Holocephalans
The fossils shown above show a clear resemblance to crown-group holocephalans, as may be seen by comparing these images with the extant chimaera shown in the header to this page (pointy-nosed blue chimaera, Hydrolagus trolli).
If we follow Janvier and Pradel (2015) in assigning the sub-order Chimaeroidei to the crown-Holocephali, the oldest member of the crown-Holocephali is the chimaeroid Eomanodon simmsi, found in "Middle Lias" strata of Early Jurassic (Pliensbachian) age at Stanley Hill, Gretton, Gloucestershire, England (Ward and Duffin, 1989). No public-domain image is available.
If we follow Janvier and Pradel (2015) in assigning the sub-order Chimaeroidei to the crown-Holocephali, the oldest member of the crown-Holocephali is the chimaeroid Eomanodon simmsi, found in "Middle Lias" strata of Early Jurassic (Pliensbachian) age at Stanley Hill, Gretton, Gloucestershire, England (Ward and Duffin, 1989). No public-domain image is available.
References
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.
Didier, D. A. (1992). Phylogenetic systematics of extant chimaeroid fishes (Holocephali, Chimaeroidei). University of Massachusetts Amherst.
Didier, D. A. (2004). Phylogeny and classification of extant Holocephali. Biology of sharks and their relatives. CRC Press, Boca Raton, 4, 115-138.
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.
Janvier, P., & Pradel, A. (2015). Elasmobranchs and their extinct relatives: diversity, relationships, and adaptations through time. In Fish Physiology (Vol. 34, pp. 1-17). Academic Press.
Maisey, J. G. (2007). The braincase in Paleozoic symmoriiform and cladoselachian sharks. Bulletin of the American Museum of Natural History, 2007(307), 1-122
.
Maisey, J. G. (2009). The spine-brush complex in symmoriiform sharks (Chondrichthyes; Symmoriiformes), with comments on dorsal fin modularity. Journal of Vertebrate Paleontology, 29(1), 14-24.
Pradel, A., Tafforeau, P., Maisey, J. G., & Janvier, P. (2011). A new Paleozoic Symmoriiformes (Chondrichthyes) from the Late Carboniferous of Kansas (USA) and cladistic analysis of early chondrichthyans. Plos one, 6(9), e24938.
Ward, D., & Duffin, C. J. (1989). Mesozoic chimaeroids. 1. A new chimaeroid from the Early Jurassic of Gloucestershire, England. Mesozoic Res, 2(2), 45-51.
Didier, D. A. (1992). Phylogenetic systematics of extant chimaeroid fishes (Holocephali, Chimaeroidei). University of Massachusetts Amherst.
Didier, D. A. (2004). Phylogeny and classification of extant Holocephali. Biology of sharks and their relatives. CRC Press, Boca Raton, 4, 115-138.
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.
Janvier, P., & Pradel, A. (2015). Elasmobranchs and their extinct relatives: diversity, relationships, and adaptations through time. In Fish Physiology (Vol. 34, pp. 1-17). Academic Press.
Maisey, J. G. (2007). The braincase in Paleozoic symmoriiform and cladoselachian sharks. Bulletin of the American Museum of Natural History, 2007(307), 1-122
.
Maisey, J. G. (2009). The spine-brush complex in symmoriiform sharks (Chondrichthyes; Symmoriiformes), with comments on dorsal fin modularity. Journal of Vertebrate Paleontology, 29(1), 14-24.
Pradel, A., Tafforeau, P., Maisey, J. G., & Janvier, P. (2011). A new Paleozoic Symmoriiformes (Chondrichthyes) from the Late Carboniferous of Kansas (USA) and cladistic analysis of early chondrichthyans. Plos one, 6(9), e24938.
Ward, D., & Duffin, C. J. (1989). Mesozoic chimaeroids. 1. A new chimaeroid from the Early Jurassic of Gloucestershire, England. Mesozoic Res, 2(2), 45-51.
Image credits - Stem-Holocephali
- Header (pointy-nosed blue chimaera, Hydrolagus trolli): Citron,licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.
- Figure 2 (fossil, Cladoselache fyleri): Ghedoghedo, Public domain, via Wikimedia Commons
- Figure 2 (life restoration, Cladoselache fyleri): EvolutionIncarnate, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2 (fossil, 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 2 (life restoration, 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 2 (Cobelodus aculeatus): Slate Weasel, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
- Figure 2 (fossil, Damocles serratus): Meghunter99 on Fossil Wiki (https://fossil.fandom.com/wiki/Damocles?file=Damocles_serratus_640.jpg), stated to be in public domain
- Figure 2 (life restoration, Damocles serratus): Mixtix-freeman, Attribution-NonCommercial-NoDerivs 3.0 Unported (CC BY-NC-ND 3.0)
- Figure 2 (fossil, Akmonistion zangerli): Lukas, under Creative Commons Attribution-ShareAlike 2.0 Generic (CC BY-SA 2.0)
- Figure 2 (life restoration, Akmonistion zangerli): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3 (Helodus simplex): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 3 (fossil, Debeerius ellefseni): Open Access presentation by Grogan, Eileen D. and Lund, Richard. "Snorkeling a 323myo Paleozooic Bay Community Structure and Depositional Environment of the Bear Gulch Limestone of Montana". 10-9-2015. VIMS 75th Anniversary Alumni Research Symposium.
- Figure 3 (life restoration, Debeerius ellefseni): Gasmasque, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 3 (Chondrenchelys problematica): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license