Introduction
The coelacanths (Order Coelacanthiformes, Class Coelacanthi) are the modern representatives of a large group of fishes, the Actinistia, which are widespread in the fossil record. Only two species of Actinistia exist at the present day. Their phylogenetic context is illustrated below:
We will now consider the evolution of the stem lines of the Coelacanthi and of the higher-order clade to which they belong, the Sarcopterygii. These stem lines are depicted as red and blue lines respectively in the above phylogenetic tree.
The stem group of the Sarcopterygii
There is currently a lot of debate about the phylogeny of the stem-Sarcopterygii. The discussion centers around a related group of fossil fishes (Achoania, Guiyu, Psarolepis and Sparalepis) that have been placed by some researchers (e.g. Lu et al, 2017; MondéJar-Fernández, 2018) in the stem-Euteleostei and by others (e.g. Broughton et al , 2013; Zhu et al, 2013; Giles et al, 2015; Lu et al, 2016a; Qiao et al, 2016; Choo et al, 2017; Cui et al, 2019) in the stem-Sarcopterygii. Others suggest that this difference in placement results from the application of different methods of phylogenetic analysis (Clement et al, 2018; King, 2019). Given the preponderance of articles, some of which are quite recent, placing these fishes in the Sarcopterygian stem group, that interpretation will be presented here. A representative phylogenetic tree of the stem-Sarcopterygii is shown below:
The oldest known stem-Sarcopterygii are Guiyu oneiros and Sparalepis tingi, which are of the same age. They are both found in the mid-Silurian (Ludlow) Kuanti Formation near Xiaoxiang Reservoir, Qujing, Yunnan, China (Zhu et al, 2009; Choo et al, 2017). Life restorations of Guiyu oneiros and Psarolepis romeri are shown below (for a larger view, click on the image of interest). Unfortunately, no other images of stem-group sarcopterygians are available in the public domain.
* after name indicates that the image represents a life restoration.
The oldest-known crown-group Sarcopterygian is Youngolepis sp., a member of the crown-Rhipidistia from the Early Devonian (Early Lochkovian) of China (Zhu and Fan, 1995; Benton et al, 2015). No public-domain image is available.
The time-frame of appearance of the stem-Sarcopterygii is illustrated below:
The time-frame of appearance of the stem-Sarcopterygii is illustrated below:
The above plot indicates that the duration of the stem-to-crown transition for the Sarcopterygii was only about 7 million years.
The stem group of the Coelacanthi
Several phylogenetic trees have been published for the stem-Coelacanthi. One of the trees with the greatest number of species is shown below:
The oldest known stem-group Coelacanthi 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, 2010; Lu et al, 2016a). It is not included in the above phylogenetic tree, but is illustrated below together with other stem-Coelacanthi for which public-domain images are available and which are numbered in the order they appear in the tree (click on image for a larger view):
* 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 crown-Coelacanthimorpha are represented only by the two extant species of coelacanth, Latimeria chalumnae (the West Indian Ocean coelacanth) and Latimeria menadoensis (the Indonesian coelacanth). No fossils are known from either species, so the crown group is entirely modern.
The time frame of the evolution of the stem group is shown below:
The crown-Coelacanthimorpha are represented only by the two extant species of coelacanth, Latimeria chalumnae (the West Indian Ocean coelacanth) and Latimeria menadoensis (the Indonesian coelacanth). No fossils are known from either species, so the crown group is entirely modern.
The time frame of the evolution of the stem group is shown below:
Given that no fossils are known from the crown-Coelacanthi and that the youngest known member of the stem group (Axelrodichthys megadromos) is around 80 million years old, the above plot indicates that the stem-to-crown transition for the Coelacanthi lasted at least 320 million years. This is more than an order of magnitude longer than the duration of the corresponding transition for the Sarcopterygii.
References
Benton, M. J., Donoghue, P. C., Asher, R. J., Friedman, M., Near, T. J., & Vinther, J. (2015). Constraints on the timescale of animal evolutionary history. Palaeontologia Electronica, 18(1), 1-106.
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.
Cavin, L., Mennecart, B., Obrist, C., Costeur, L., & Furrer, H. (2017). Heterochronic evolution explains novel body shape in a Triassic coelacanth from Switzerland. Nature Scientific reports, 7 (1), 1-7.
Cavin, L., Cupello, C., Yabumoto, Y., Fragoso, L., Deesri, U., & Brito, P. M. (2019). Phylogeny and evolutionary history of mawsoniid coelacanths. Bulletin of the Kitakyushu Museum of Natural History and Human History, Series A (Natural History), 17, 3-13.
Cavin, L., Buffetaut, E., Dutour, Y., Garcia, G., Le Loeuff, J., Méchin, A., ... & Valentin, X. (2020). The last known freshwater coelacanths: new Late Cretaceous mawsoniid remains (Osteichthyes: Actinistia) from southern France. Plos one, 15(6), e0234183.
Choo, B., Zhu, M., Qu, Q., Yu, X., Jia, L., & Zhao, W. (2017). A new osteichthyan from the late Silurian of Yunnan, China. PloS one, 12(3), e0170929.
Clement, A. M., King, B., Giles, S., Choo, B., Ahlberg, P. E., Young, G. C., & Long, J. A. (2018). Neurocranial anatomy of an enigmatic Early Devonian fish sheds light on early osteichthyan evolution. Elife, 7, e34349.
Cui, X., Qiao, T., & Zhu, M. (2019). Scale morphology and squamation pattern of Guiyu oneiros provide new insights into early osteichthyan body plan. Nature Scientific reports, 9(1), 4411.
Giles, S., Darras, L., Clément, G., Blieck, A., & Friedman, M. (2015). An exceptionally preserved Late Devonian actinopterygian provides a new model for primitive cranial anatomy in ray-finned fishes. Proceedings of the Royal Society B: Biological Sciences, 282 (1816), 20151485.
King, B. (2019). Which morphological characters are influential in a Bayesian phylogenetic analysis? Examples from the earliest osteichthyans. Biology letters, 15(7), 20190288.
Lu, J., & Zhu, M. (2010). An onychodont fish (Osteichthyes, Sarcopterygii) from the Early Devonian of China, and the evolution of the Onychodontiformes. Proceedings of the Royal Society B: Biological Sciences, 277(1679), 293-299.
Lu, J., Zhu, M., Ahlberg, P. E., Qiao, T., Zhu, Y. A., Zhao, W., & Jia, L. (2016a). A Devonian predatory fish provides insights into the early evolution of modern sarcopterygians. Science advances, 2(6), e1600154.
Lu, J., Giles, S., Friedman, M., & Zhu, M. (2017). A new stem sarcopterygian illuminates patterns of character evolution in early bony fishes. Nature communications, 8(1), 1932.
MondéJar-Fernández, J. (2018). On cosmine: its origins, biology and implications for sarcopterygian interrelationships. CYBIUM, 42(1), 41-65.
Qiao, T., King, B., Long, J. A., Ahlberg, P. E., & Zhu, M. (2016). Early gnathostome phylogeny revisited: multiple method consensus. PLoS One, 11(9), e0163157.
Renesto, S., & Stockar, R. (2018). First record of a coelacanth fish from the Middle Triassic Meride Limestone of Monte San Giorgio (Canton Ticino, Switzerland). Rivista Italiana di Paleontologia e Stratigrafia, 124, 639-653.
Zhu, M., & Fan, J. (1995). Youngolepis from the Xishancun Formation (Early Lochkovian) of Qujing; China. Geobios, 28, 293-299.
Zhu, M., Zhao, W., Jia, L., Lu, J., Qiao, T., & Qu, Q. (2009). The oldest articulated osteichthyan reveals mosaic gnathostome characters. Nature, 458(7237), 469-474.
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.
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.
Cavin, L., Mennecart, B., Obrist, C., Costeur, L., & Furrer, H. (2017). Heterochronic evolution explains novel body shape in a Triassic coelacanth from Switzerland. Nature Scientific reports, 7 (1), 1-7.
Cavin, L., Cupello, C., Yabumoto, Y., Fragoso, L., Deesri, U., & Brito, P. M. (2019). Phylogeny and evolutionary history of mawsoniid coelacanths. Bulletin of the Kitakyushu Museum of Natural History and Human History, Series A (Natural History), 17, 3-13.
Cavin, L., Buffetaut, E., Dutour, Y., Garcia, G., Le Loeuff, J., Méchin, A., ... & Valentin, X. (2020). The last known freshwater coelacanths: new Late Cretaceous mawsoniid remains (Osteichthyes: Actinistia) from southern France. Plos one, 15(6), e0234183.
Choo, B., Zhu, M., Qu, Q., Yu, X., Jia, L., & Zhao, W. (2017). A new osteichthyan from the late Silurian of Yunnan, China. PloS one, 12(3), e0170929.
Clement, A. M., King, B., Giles, S., Choo, B., Ahlberg, P. E., Young, G. C., & Long, J. A. (2018). Neurocranial anatomy of an enigmatic Early Devonian fish sheds light on early osteichthyan evolution. Elife, 7, e34349.
Cui, X., Qiao, T., & Zhu, M. (2019). Scale morphology and squamation pattern of Guiyu oneiros provide new insights into early osteichthyan body plan. Nature Scientific reports, 9(1), 4411.
Giles, S., Darras, L., Clément, G., Blieck, A., & Friedman, M. (2015). An exceptionally preserved Late Devonian actinopterygian provides a new model for primitive cranial anatomy in ray-finned fishes. Proceedings of the Royal Society B: Biological Sciences, 282 (1816), 20151485.
King, B. (2019). Which morphological characters are influential in a Bayesian phylogenetic analysis? Examples from the earliest osteichthyans. Biology letters, 15(7), 20190288.
Lu, J., & Zhu, M. (2010). An onychodont fish (Osteichthyes, Sarcopterygii) from the Early Devonian of China, and the evolution of the Onychodontiformes. Proceedings of the Royal Society B: Biological Sciences, 277(1679), 293-299.
Lu, J., Zhu, M., Ahlberg, P. E., Qiao, T., Zhu, Y. A., Zhao, W., & Jia, L. (2016a). A Devonian predatory fish provides insights into the early evolution of modern sarcopterygians. Science advances, 2(6), e1600154.
Lu, J., Giles, S., Friedman, M., & Zhu, M. (2017). A new stem sarcopterygian illuminates patterns of character evolution in early bony fishes. Nature communications, 8(1), 1932.
MondéJar-Fernández, J. (2018). On cosmine: its origins, biology and implications for sarcopterygian interrelationships. CYBIUM, 42(1), 41-65.
Qiao, T., King, B., Long, J. A., Ahlberg, P. E., & Zhu, M. (2016). Early gnathostome phylogeny revisited: multiple method consensus. PLoS One, 11(9), e0163157.
Renesto, S., & Stockar, R. (2018). First record of a coelacanth fish from the Middle Triassic Meride Limestone of Monte San Giorgio (Canton Ticino, Switzerland). Rivista Italiana di Paleontologia e Stratigrafia, 124, 639-653.
Zhu, M., & Fan, J. (1995). Youngolepis from the Xishancun Formation (Early Lochkovian) of Qujing; China. Geobios, 28, 293-299.
Zhu, M., Zhao, W., Jia, L., Lu, J., Qiao, T., & Qu, Q. (2009). The oldest articulated osteichthyan reveals mosaic gnathostome characters. Nature, 458(7237), 469-474.
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.
Image credits - Coelacanths
- Header (modern coelacanth, Latimeria sp.) By Mordecai 1998, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Guiyu oneiros By Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Psarolepis romeri By Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Qingmenodus yui From Figure 1E of Lu et al, 2016a, under terms of Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)
- Miguashaia spp. By DiBgd [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Diplocercides heiligostockensis By Robert Gess, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Hadronector donbairdi By Muséum de Toulouse Attribution-NoDerivs 2.0 Generic (CC BY-ND 2.0)
- Rebellatrix divaricerca By Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Allenypterus montanus (7a) By James St. John Attribution 2.0 Generic (CC BY 2.0)
- Allenypterus montanus (7b) 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 woodwardia By Muséum de Toulouse [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Heptanema paradoxum By Giulio de Alessandri (died in 1921), Public domain, via Wikimedia Commons
- 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
- Trachymetopon liassicum By Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Skull of Mawsonia sp. By Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Mawsonia sp. By DiBgd, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Axelrodichthys megadromos From an Open Access article by Cavin et al, 2020, distributed under the terms of a Creative Commons license Attribution 4.0 International (CC BY 4.0)
- Axelrodichthys araripensis By Daderot, CC0, via Wikimedia Commons
- Libys superbus By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Skull of Megalocoelocanthus dobiei By MCDinosaurhunter [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Foreyia maxkuhni (22a) By Lionel Cavin, Bastien Mennecart, Christian Obrist, Loïc Costeur & Heinz Furrer [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)]
- Foreyia maxkuhni (22b) 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)]
- Macropoma mantelli By Gideon Algernon Mantell (1790-1852), Public domain, via Wikimedia Commons