EVOLUTION - THE TRANSITIONAL FOSSILS
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amniotes

Introduction

The amniote clade (within superclass Tetrapoda) represents animals that produce an egg with protective membranes that allow the egg to be laid on land or gestated within the mother. It is a crown group comprising all living mammals and saurians (reptiles and birds), together with all extinct species descended from the last common ancestor of all living mammals and saurians.

Its phylogeny is summarized in the following tree:
Picture
The red dot represents the crown node of the amniotes, while the black dots depict the successive nodes into which the crown-Amniota subdivided. The initial division took place between the Mammalia and the Sauria. Mammals are synapsids, having only one hole in the skull behind each eye, while saurians are diapsids, with two holes (except for the turtles, or Testudines, in which both holes behind the eye are thought to have closed up during the evolutionary process; Schoch and Sues, 2016).

The terminal clades shown ​ at the right-hand side of the above tree each have a separate page that can be reached through the menu bar at the top of the page (under Evolution of Life/Animals/Vertebrates/Tetrapods). Pages for the higher-level crown groups can be reached through one of the following links:​
  •   Mammalia
    • Theria
  • Sauria
    • Lepidosauria 
    • Archelosauria
      • Archosauria
        •  Aves

The stem group

There is little current consensus on the phylogeny of the amniote stem group, such that widely different trees have been published over the past few years. However, the tree below is is a fairly representative example:
Picture
The oldest known member of the amniote stem group is Casineria kiddi, found in the Early Carboniferous (Visean; Asbian) Cheese Bay Shrimp Bed of the Gullane Formation at Cheese Bay, Gullane, East Lothian, Scotland (Paton et al, 1999; Clack et al, 2017). This species is illustrated in the following figure, together with some other stem amniotes for which public-domain images are available (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.
The above series of images illustrates some of the fossils that represent the ancestry of the amniote crown group. Note that most of the fossils (represented by names in red) post-date the appearance of the crown group; these represent branches of the stem line that continued to evolve after the crown-group had appeared.

The images are numbered in order from most basal to most crownward in the stem group. There is no obvious change from the most basal to the most crownward, except for the occasional appearance of species without limbs or with very small legs, but there were in fact significant changes in jaw structure along the amniote stem line (Anderson et al, 2013).

The amniote stem group appeared in the Early Carboniferous and continued evolving until the Early Permian. However, most of the known stem amniotes appeared after the crown group, as illustrated below: 
Picture

The crown group

The crown-Amniota appeared when the stem lines of the mammals  and the saurians separated from one another:
Picture
The earliest known stem saurian, Hylonomus lyelli, appeared around 14 million years before before the oldest known stem mammal, Sphenacodon ferox.  The relationship between these ages and that of the earliest stem amniote 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 amniote crown node. This uncertainty represents the maximum period of time for the stem-to-crown transition; the time between the origin of the  amniote stem group and the initiation of the crown group would have been no more than 18 million years:
Picture

Image credits - Amniotes
  • Header (Mother humpback and calf)    NOAA News July 6, 2012,   National Oceanic and Atmospheric Administration/Department of Commerce
  • Silvanerpeton miripedes      [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
  • Gephyrostegus bohemicus      [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
  • Casineria kiddi   (3a)  By   Xenarachne at English Wikipedia [Public domain]
  • Casineria kiddi    (3b)  By  ДиБгд [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]  
  • Discosauriscus sp.     By Tommy from Arad, Creative Commons Atribución 2.0 Genérica
  • Discosauriscus    austriacus     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
  • Seymouria sp.    By   Sanjay Acharya [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
  • Seymouria baylorensis    By   Nobu Tamura (http://spinops.blogspot.com) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]
  • Westlothiana    lizziae     By Nobu Tamura (http://spinops.blogspot.com) (own aork) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
  • Hyloplesion   longicostatum     By  Smokeybjb [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], from Wikimedia Commons 
  • Microbrachis   pelikani     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
  • Brachydectes    newberryi     By   Smokeybjb [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], from Wikimedia Commons
  • Batropetes   sp.     By   Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons
  • Tuditanus   punctulatus     By   S. W. Williston (https://www.jstor.org/pss/30059773) [Public domain], via Wikimedia Commons
  • Limnoscelis  paludis   By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons 
  • Diadectes phaseolinus     By Ghedoghedo [Public domain, GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
  • Diadectes 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
  • Sphenacodon ferocior     By Дибгд, public domain​
  • Hylonomus lyelli     By   MUSE [CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
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