EVOLUTION - THE TRANSITIONAL FOSSILS
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TETRAPODS

Introduction

The tetrapod clade (superclass Tetrapoda) represents the four-limbed vertebrates and comprises the amniotes and the amphibians. The former lay eggs that are adapted to be laid on land, while the eggs of the latter do not have that adaptation and thus need to be laid in water. The crown tetrapods consist of all living amniotes and amphibians, together with all extinct species descended from the last common ancestor of all living amniotes and amphibians.

More than 30,000 species of tetrapods exist in the world today, distributed among the taxa shown in the following phylogenetic tree:
Picture
The crown node of the Tetrapoda is shown as a black dot. The evolution of the animals that have descended from that latest common ancestor will be examined in the following pages, which can be accessed from the above menu bar under “Tetrapods” in the ”Evolution of Life” tab.
​
This remainder of this page will deal with the transition from fishes to land animals represented by the stem group of the tetrapods.  ​

The stem group of the Tetrapoda

The evolution of the stem-Tetrapoda is a topic that is undergoing active research at the present time, and new phylogenetic trees continue to be published. Those published since around 2008 are broadly in agreement, although the placement of some fossil species does vary significantly. A representative tree, constructed by combining two recent publications, is shown below:
Picture
The above tree represents a trend from species that are clearly fishes to those that are clearly four-legged animals. The more basal species are classified as tetrapodomorph fish by Hu et al (2019). There is disagreement about whether Tiktaalik, for example is a fish (Ruta et al, 2019; Molnar et al, 2020) or a limbed tetrapod (Dickson et al, 2021), but by all accounts that species is a key representative of the transition from fishes to land animals (Shubin et al, 2006; Hohn-Schulte et al, 2013).

The oldest known member of the stem group is Tungsenia paradoxa, from the Posongchong Formation of Pragian (Early Devonian) age near Zhaotong in northeast Yunnan, China (Lu et al, 2012). No public-domain image is available, but some other stem-Tetrapoda are illustrated in the following figures (click on image to see a larger version):
Names in    red indicate   that the fossil is younger  than the oldest known crown-group fossil.
​* after name indicates that the image represents a life restoration. 
This set of images illustrates the transition from fishes to tetrapods. The pictures are numbered in order from most basal to most crownward in the stem group. Note the progressive change from fish-like (up to Eusthenodon, numbers 12a and 12b) to increasingly lizard-like form (from Panderichthys to Greererpeton) with a change from fins to limbs with digits (e.g. Acanthostega, numbers 17a and 17b).  However, this record is undoubtedly incomplete, because footprints interpreted to have been left by limbed tetrapods have been found in the Middle Devonian  Valentia Slate Formation on Valentia Island in southwestern Ireland (Lucas, 2019); these are significantly older than the oldest known limbed tetrapods  (e.g. Elginerpeton), which are of Late Devonian age (Ahlberg, 2018).

Some of the fossils illustrated above (with labels in red) are younger than the oldest crown group fossil; they represent descendants of undiscovered ancestors that would have separated from the stem line before the crown group appeared.

Much research is actively going on to understand the nature of the changes in the skeleton and musculature that allowed the stem-group tetrapods to move on land (see Ahlberg, 2018; Molnar et al, 2018, 2020 and 2021;  Esteve-Altava et al, 2019; Ruta et al, 2019; Cloutier et al, 2020; Dickson et al, 2021). The main developments are nicely summarized in the following figure from Ahlberg (2018):
Picture
The earliest known member of the crown-Tetrapoda is Lethiscus stocki, a member of the Aïstopoda, a group belonging to the Lepospondyli, which are assigned to the stem-Amniota by Anderson et al (2013), from the Early Carboniferous (mid-Visean) Lower Oil Shale Group at Wardie shore, north of Edinburgh, Scotland (Anderson et al, 2003; Benton et al, 2015). No public-domain image is available.
​
The time frame of the evolution of the stem group is shown below:
Picture
​The above plot indicates that the tetrapod stem group developed from Early Devonian to Early Carboniferous time, representing a stem-to-crown transition of at least 65 million years.

References

Ahlberg, P. E. (2018). Follow the footprints and mind the gaps: a new look at the origin of tetrapods. Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 109(1-2), 115-137.

​Ahlberg, P. E., & Clack, J. A. (2020). The smallest known Devonian tetrapod shows unexpectedly derived features. R. Soc. Open Sci.

7: 192117. http://dx.doi.org/10.1098/rsos.192117.

​Anderson, J. S., Carroll, R. L., & Rowe, T. B. (2003). New information on 
Lethiscus stocki    (Tetrapoda: Lepospondyli: Aistopoda) from high-resolution computed tomography and a phylogenetic analysis of Aistopoda. Canadian Journal of Earth Sciences, 40(8), 1071-1083.

Anderson, P. S. L., Friedman, M., & Ruta, M. (2013). Late to the Table: Diversification of Tetrapod Mandibular Biomechanics Lagged Behind the Evolution of Terrestriality.   Integrative and Comparative Biology
, Volume 53, Issue 2, 1 August 2013, p. 197–208.

​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.

Cloutier, R., Clement, A. M., Lee, M. S., Noel, R., Bechard, I., Roy, V., & Long, J. A. (2020). 
Elpistostege   and the origin of the vertebrate hand. Nature, 579(7800), 549-554.

​Dickson, B. V., Clack, J. A., Smithson, T. R., & Pierce, S. E. (2021). Functional adaptive landscapes predict terrestrial capacity at the origin of limbs. Nature, 589(7841), 242-245.

​Esteve-Altava, B., Pierce, S. E., Molnar, J. L., Johnston, P., Diogo, R., & Hutchinson, J. R. (2019). Evolutionary parallelisms of pectoral and pelvic network-anatomy from fins to limbs. Science advances, 5(5), eaau7459.

​Hohn-Schulte, B., Preuschoft, H., Witzel, U., & Distler-Hoffmann, C. (2013). Biomechanics and functional preconditions for terrestrial lifestyle in basal tetrapods, with special consideration of 
Tiktaalik roseae. Historical Biology, 25(2), 167-181.

Hu, Y. Z., Young, G. C., & Lu, J. (2019). The Upper Devonian tetrapodomorph 
Gogonasus andrewsae from Western Australia: Reconstruction of the shoulder girdle and opercular series using X-ray Micro-Computed Tomography. Palaeoworld, 28(4), 535-542.

​Lu, J., Zhu, M., Long, J. A., Zhao, W., Senden, T. J., Jia, L., & Qiao, T. (2012). The earliest known stem-tetrapod from the Lower Devonian of China. Nature Communications, 3, 1160.

Lucas, S. G. (2019). An ichnological perspective on some major events of Paleozoic tetrapod evolution. Bollettino della Società Paleontologica Italiana, 58(3), 224.​

Molnar, J. L., Diogo, R., Hutchinson, J. R., & Pierce, S. E. (2018). Reconstructing pectoral appendicular muscle anatomy in fossil fish and tetrapods over the fins‐to‐limbs transition. Biological Reviews, 93(2), 1077-1107.


​Molnar, J. L., Diogo, R., Hutchinson, J. R., & Pierce, S. E. (2020). Evolution of hindlimb muscle anatomy across the tetrapod water‐to‐land transition, including comparisons with forelimb anatomy. The Anatomical Record, 303(2), 218-234.

​Molnar, J. L., Hutchinson, J. R., Diogo, R., Clack, J. A., & Pierce, S. E. (2021). Evolution of forelimb musculoskeletal function across the fish-to-tetrapod transition. Science Advances, 7(4), eabd7457.

​Ruta, M., Krieger, J., Angieczyk, K. D., & Wiils, M. A. (2019). The evolution of the tetrapod humerus: Morphometrics, disparity, and evolutionary rates. Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 109(1-2), 351.

​Shubin, N. H., Daeschler, E. B., & Jenkins, F. A. (2006). The pectoral fin of 
Tiktaalik roseae and the origin of the tetrapod limb. Nature, 440(7085), 764-771.

Image credits   -   Tetrapods
  • Header (Bison bison at the Wichita Mountain Wildlife Refuge in Oklahoma)   By katsrcool from Edmond, OK, USA (Majestic Bison) [CC BY 2.0  (https://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons
  • Tungsenia paradoxa  By Nobu Tamura, Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license 
  • Gooloogongia loomesi  By ДиБгд, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Barameda decipiens  By Apokryltaros, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Fin of Sauripterus taylori  Courtesy: National Science Foundation
  • Tooth of Strepsodus sp.  By the paleobear from Lontananza, Loreto, Peru, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
  • Rhizodus hibberti By DiBgd, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Gyroptychius agassizi  By Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Gogonasus andrewsae  By Nobu Tamura, Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Scales of Megalichthys hibberti (9a)  By CC-by-SA, Museum of Natural History, Neuchâtel / FR: CC-by-SA, Musée d'histoire naturelle de Neuchâtel
  • Megalichthys hibberti (9b)  By ДиБгд, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Ectosteorhachis sp.  By Ghedoghedo, Public domain, via Wikimedia Commons
  • Osteolepis microlepidotus  By Ghedoghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Osteolepis panderi  By Nobu Tamura,  Creative Commons Attribution- ShareAlike (CC BY-SA)  license
  • Eusthenopteron foordi (12a)  By Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Eusthenopteron foordi (12b)  By Nobu Tamura, Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Skull of Panderichthys rhombolepis (13a)  By Ghedoghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Panderichthys rhombolepis (13b)  By Nobu Tamura, Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Tiktaalik roseae (14a)  By Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Tiktaalik roseae (14b)  Courtesy: National Science Foundation
  • Elpistostege watsoni  By SOCIÉTÉ DES MUSÉES DU QUÉBEC under Creative Commons licence
  • Ventastega curonica  By Nobu Tamura, Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Model of Acanthostega gunnari (17a)  By Ryan Somma, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
  • Acanthostega gunnari (17b)  By Nobu Tamura, Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Skull of Ymeria denticulata  By FunkMonk (Michael B. H.), CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Skull of Ichthyostega sp. (19a)  By FunkMonk, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons 
  • Ichthyostega stensioei (19b) By Nobu Tamura, Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license 
  • Elginerpeton pacheni  By Nobu Tamura (http://spinops.blogspot.com), CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
  • Pederpes finneyae  By DiBgd, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons
  • Greererpeton burkemorani (22a)  By Tim Evanson, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
  • Greererpeton burkemorani (22b)  By Nobu Tamura (http://spinops.blogspot.com), CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons
  • Crassigyrinus scoticus  By Nobu Tamura (http://spinops.blogspot.com), CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons
  • Skull of Megalocephalus pachycephalus (24a)  By Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Megalocephalus pachycephalus (24b)  By Dmitry Bogdanov, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
  • Home
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