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.

Its phylogeny is summarized in the following tree:
The nodes shown as black dots in the above figure represent crown nodes of the successive crown groups into which the tetrapod crown group (represented by a red dot) has divided. 

The terminal taxa shown above represent the limit of subdivision to be considered in this website; generally speaking, the origin of ranks lower than the order will not be examined.

This website contains a separate page for each successive crown group and the terminal clades (as shown above)  of the tetrapods; the pages can be reached  through the following links:

The stem group

A recent phylogenetic tree for the total-group Tetrapoda, also known as the Tetrapodamorpha, is shown below:
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, number 12) to increasingly lizard-like form (from Panderichthys to Eucritta) with a change from fins to limbs with digits (e.g. Ichthyostega, number 17a).​

The tetrapod stem group developed mainly during the Devonian and the Early Carboniferous, but continued evolving into the Early Permian, as illustrated below (click on plot to see a larger version):

The crown group

The crown-Tetrapoda appeared when the stem lines of the amniotes and the amphibians  separated from one another:
The stem-Amniota and the stem-Amphibia both appeared almost at the same time, in the Early Carboniferous. The oldest known stem amniote is Casineria kiddi, of Early Carboniferous (Visean; Asbian) age, while the  oldest stem amphibian, Balanerpeton woodi, is slightly younger (Visean; Early Brigantian). The relationship between these ages and that of the earliest stem tetrapod 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 tetrapod crown node. This uncertainty represents the maximum period of time for the stem-to-crown transition; the time between the origin of the  tetrapod stem group and the initiation of the crown group could have been as long as 78 million years:
To put the appearance of the tetrapod crown group in the context of the evolution of the entire crown group, the following figure depicts the appearance times of all the tetrapod clades discussed in this website:
The above figure shows that the  two major subgroups of the tetrapods, the amniotes and the amphibians, had appeared by the end of the Paleozoic (i.e. the end of the Permian). Furthermore, the saurians,  and possibly the mammals too, had also appeared by that time.
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
  • Gooloogongia loomesi     By ДиБгд [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons
  • Rhizodus sp.     By   DiBgd [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons
  • Gogonasus andrewsae     By Nobu Tamura (http://spinops.blogspot.com) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0        (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY 2.5 (https://creativecommons.org/licenses/by/2.5)], from Wikimedia Commons
  • Osteolepis sp.     By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons
  • Ectosteorhachis  sp.     By   Ghedoghedo [Public domain], from Wikimedia Commons
  • Megalichthys sp.     By ДиБгд [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons
  • Canowindra grossi     By   Age of Fishes Museum, Canowindra, NSW, Australia
  • Gyroptychius sp.     By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons
  • Eusthenopteron foordi     (9a)By   Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons
  • Eusthenopteron foordi    (9b)   By Nobu Tamura (http://spinops.blogspot.com) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]
  • Jarvikina  wenjukovi     By  DiBgd [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons 
  • Skull of Platycephalichthys  sp.     By   Eduard Solà Vázquez [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons
  • Platycephalichthys  bischoffi     By   DiBgd [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons
  • Eusthenodon  sp.     By   DiBgd [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons
  • Skull of Panderichthys sp.    By Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons
  • Panderichthys  sp.     By Nobu Tamura (http://spinops.blogspot.com) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY 2.5 (https://creativecommons.org/licenses/by/2.5)], from Wikimedia Commons
  • ​Tiktaalik roseae   (14a)   By Experiencing Life Trips, https://creativecommons.org/licenses/by-nc-sa/2.0/legalcode
  • Tiktaalik roseae   (14b)   By Obsidian Soul [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
  • Ventastega curonica   By   Nobu Tamura (http://spinops.blogspot.com) [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)]
  • Skull of Ymeria denticulata  By   FunkMonk (Michael B. H.) [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
  • Ichthyostega sp.   (17a)     By OlegTarabanov [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons
  • Ichthyostega  sp.   (17b)      By Günter Bechly, Creative Commons Credit-Share Alike 3.0 Unpatched
  • Skull of Acanthostega gunnari       By Ghedoghedo, GNU Free Documentation License version 1.2
  • Acanthostega gunnari      By Nobu Tamura (http://spinops.blogspot.com) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY 2.5 (https://creativecommons.org/licenses/by/2.5)], from Wikimedia Commons
  • Coloraderpeton brilli  By Smokeybjb [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
  • Pederpes finneyae   By DiBgd [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]
  • Crassigyrinus scoticus By Nobu Tamura (http://spinops.blogspot.com) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]
  • Greererpeton burkemorani (22a) By   Tim Evanson [CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0)]
  • Greererpeton burkemorani (22b)   By   Nobu Tamura (http://spinops.blogspot.com) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]
  • Proterogyrinus sp.  By   DiBgd at English Wikipedia [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]
  • Archeria crassidisca  By Nobu Tamura (http://spinops.blogspot.com) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]
  • Eucritta melanolimnetes  By   Dmitry Bogdanov [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
  • Casineria kiddi      By   ДиБгд [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]  
  • Balanerpeton woodi     By Nobu Tamura (http://spinops.blogspot.com) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)], from Wikimedia Commons