Introduction
The coelacanths (order Coelacanthiformes, superclass Sarcopterygii) are the modern representatives of a large group of fishes, the Coelacanthimorpha, which are widespread in the fossil record. Only two coelacanthimorph species exist at the present day.
The stem group
Several phylogenetic trees have been published for the stem-Coelacanthimorpha. One of the trees with the greatest number of species is shown below:
The oldest known stem-group coelacanthimorph is Qingmenodus yui , found in the Early Devonian (Pragian) Posongchong Formation in an outcrop near the Qingmen reservoir in the suburb of Zhaotong, northeastern Yunnan, China (Lu and Zhu, 2009; Lu et al, 2016a). A life restoration is shown below:
Other stem-group examples from the above phylogenetic tree are shown below (click on any image for a larger version):
* after name indicates that the image represents a life restoration.
The above images are numbered in order from the most basal to those closest to the crown group, but no obvious trends can be seen except perhaps the establishment of a body form quite similar to that of modern coelacanths in the Jurassic and younger examples.
The time frame of the evolution of the stem group is shown below (click on image to see a larger version):
The time frame of the evolution of the stem group is shown below (click on image to see a larger version):
The crown group
The crown-Coelacanthimorpha are represented only by the two extant species of coelacanth, Latimeria chalumnae (the West Indian Ocean coelacanth. illustrated in the header above) and Latimeria menadoensis (the Indonesian coelacanth). No fossils are known from either species, so the crown group is entirely modern. The time between the origin of the coelacanthimorph stem group and the initiation of the crown group could have been as long as 411 million years, given that the oldest known stem fossil, Qingmenodus yui, has an age of between 408 and 411 million years (Lu and Zhu, 2009; Cascales-Minana et al, 2019).
Image credits - Coelacanths
- Header (preserved specimen of West Indian Ocean coelacanth, Latimeria chalumnae) By Alberto Fernandez Fernandez [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]
- Life restoration of Qingmenodus yui From Figure 1E of Lu et al, 2016a, under terms of Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)
- Miguashaia sp. By DiBgd [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Hadronector donbairdi By Muséum de Toulouse Attribution-NoDerivs 2.0 Generic (CC BY-ND 2.0)
- Rebellatrix divaricerca By Apokryltaros [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Allenypterus montanus (4a) By James St. John Attribution 2.0 Generic (CC BY 2.0)
- Allenypterus montanus (4b) By DiBgd [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Caridosuctor populosum By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Piveteauia madagascariensis By Daderot [CC0]
- Coccoderma suevicum By Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Coelacanthus granulatus By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Whiteia woodwardi By Muséum de Toulouse [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Diplurus sp. By Tim Evanson Attribution-ShareAlike 2.0 Generic (CC BY-SA 2.0)
- Chinlea sorensoni By Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Skull of Mawsonia sp. By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Mawsonia sp. By DiBgd [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Axelrodichthys araripensis By Daderot [CC0]
- Garnbergia ommata By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Libys superbus By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Skull of Megalocoelacanthus dobei By MCDinosaurhunter [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Megalocoelacanthus dobei By Apokryltaros [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Foreyia maxkuhni (17a) By Lionel Cavin, Bastien Mennecart, Christian Obrist, Loïc Costeur & Heinz Furrer [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)]
- Foreyia maxkuhni (17b) By Alain Bénéteau [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)]
- Ticinepomis peyeri By Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Undina penicillata By Gunnar Creutz [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Holophagus sp. By Chillibilli [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Macropoma willemoesi By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)].