EVOLUTION - THE TRANSITIONAL FOSSILS
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coelacanths

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:
Picture
​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:
Picture
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:
Picture
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:
Picture
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, 2009; 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:
Picture
​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.

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 &amp; 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
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