EVOLUTION - THE TRANSITIONAL FOSSILS
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lobe-finned fishes

Introduction

The lobe-finned fishes (superclass Sarcopterygii, infraphylum Gnathostomata) comprise a clade that includes the tetrapods, as well as the modern lungfishes and coelacanths. Their phylogeny is summarized below:
Picture
The sarcopterygian crown group, represented above as a red dot, is divided into two clades, the Dipnotetrapodomorpha and the Coelacanthimorpha, also known as the Rhipidistia and the Actinistia, respectively.

Each of the terminal clades shown at the right-hand side of the above tree has a separate page that can be reached through the menu bar at the top of the page (under Evolution of Life/Animals/Vertebrates). The Dipnotetrapodomorpha can be reached through this link:​
  • Dipnotetrapodomorpha​   ​ ​​​

The stem group

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 sp. are shown below (for a larger view, click on the image of interest). Unfortunately, no other images are available in the public domain.
* after name indicates that the image represents a life restoration. 
​The time frame of the evolution of the stem group is shown below:
Picture

The crown group

The sarcopterygian crown group appeared when the stem lines of the Dipnotetrapodomorpha and the Coelacanthimorpha separated from one another:
Picture
The oldest known fossil representative of the crown-Sarcopterygii is Youngolepis sp., a crown dipnotetrapodomorph from the Early Devonian (Early Lochkovian) of China (Zhu and Fan, 1995; Benton et al, 2015). This is older than the oldest known member of the stem-Dipnotetrapodomorpha, Styloichthys changae, which is of Late Lochkovian age (Zhu and Yu, 2002). Youngolepis is slightly older than the oldest stem coelacanthimorph, Qingmenodus yui, which is of Pragian age (Lu and Zhu, 2009: Lu et al, 2016a), resulting in a ghost lineage of about 4 million years on the coelacanthimorphan stem line. The relationship between these ages and that of the earliest stem sarcopterygian is shown in the figure below. Also shown are representative images of species from the respective stem groups.
 
The figure below depicts the total uncertainty in the age of the Sarcopterygii crown node. This uncertainty represents the maximum period of time for the stem-to-crown transition; the time between the origin of the sarcopterygian stem group and the initiation of the crown group was no more than 11 million years.
Picture

Image credits - Lobe-finned fishes
  • Header (Queensland lungfish, Neoceratodus forsteri)  By Mitch Ames [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
  • Guiyu oneiros  By Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Psarolepis sp.  By Nobu Tamura (http://spinops.blogspot.com) [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
  • Gooloogongia loomesi     By ДиБгд [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons
  • Qingmenodus yui  From Figure 1E of Lu et al, 2016a, under terms of Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)  
  • Home
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  • Time scale
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        • Chimaeras
        • Bichirs and reedfish
        • Sturgeons and paddlefishes
        • Teleostei
        • Holostei
        • Lobe-finned fishes
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