EVOLUTION - THE TRANSITIONAL FOSSILS
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archosaurs

Introduction

Archosauria is a clade (within the archelosaurs, in class Reptilia) that forms the sister group to the turtles and comprises the birds and the crocodylians, the two clades that comprise the archosaur crown group (node shown as red dot):
Picture
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). The page for the Aves can be reached through the following link:​
  •  Aves​

The stem group

Many stem-group archosaurs have been identified and several phylogenetic trees of the stem group have been published. One of these is shown below:
Picture
The earliest known fossil representing stem-Archosauria is Protorosaurus speneri, found in the Kupferschiefer Formation of Late Permian (Wuchiapingian) age at the Mansfeld and Sangerhausen fossil sites, Saxony-Anhalt, Germany. This species is illustrated below, together with many of the other species that appear in the above tree (to see larger versions, click on images):
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 images illustrate the ancestry of the archosaur crown group. Note that most of the stem species (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 species are numbered in order from the most basal to the most crownward in the phylogeny, and a general transition can be seen from lizard-like to crocodile-like forms. For a detailed discussion of the skeletal changes that occurred through the archosaur stem line, see Sookias (2016).

​The fossil data indicate that the transition from earliest stem-Archosauria to earliest crown-Archosauria was relatively rapid, occupying only the Late Permian and Early Triassic. However, many stem branches continued evolving after the appearance of the earliest crown-group fossil:
Picture

The crown group

The archosaur crown group appeared when the stem lines of the birds (Aves) and the crocodylians separated from one another:
Picture
The oldest known crown-group archosaur fossil is the stem crocodylian Ctenosauriscus koeneni. This appeared in the Early Triassic (Olenekian), slightly before the oldest known stem bird, Teleocrater rhadinus, which is of Middle Triassic (Early Anisian) age. The relationship between these ages and that of the earliest stem archosaur 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 archosaur crown node. This uncertainty represents the maximum period of time for the stem-to-crown transition; the time between the origin of the  archosaur stem group and the initiation of the crown group would have been no more than 13 million years:
Picture

Image credits - Archosaurs
  • Header (Crocodiles (Crocodylus) in the Oasis Park, Fuerteventura, Canary Islands, Spain)    By Norbert Nagel, Mörfelden-Walldorf, Germany [CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons​
  •  Protorosaurus speneri     By Ghedoghedo, Creative Commons Genérica de Atribución/Compartir-Igual 3.0
  • Trilophosaurus buettneri (2a)   By Daderot (Daderot) [CC0 or CC0], via Wikimedia Commons
  • Trilophosaurus buettneri  (2b)     By Petrified Forest from Petrified Forest, USA (File:Trilophosaurus buettneri (1).jpg) [CC BY 2.0  (https://creativecommons.org/licenses/by/2.0) or Public domain], via Wikimedia Commons
  • Mesosuchus browni     By Nobu Tamura, GNU Free Documentation License, Version 1.2
  • Prolacerta broomi     By Ghedoghedo, GNU Free Documentation License, Version 1.2
  • Proterosuchus fergusi   (5a)     By Jaime A. Headden ( User: Qilong ),  Creative Commons Attribution 3.0 Unported license 
  • Proterosuchus fergusi    (5b)   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
  •  Sarmatosuchus otschevi     By Dmitry Bogdanov, GNU Free Documentation License, Version 1.2
  • Shansisuchus shansisuchus     By   DiBgd [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
  • Erythrosuchus africanus     By Foth et al. [CC BY-SA 4.0  (https://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons
  • Garjainia prima     By Creator:Dmitry Bogdanov (dmitrchel@mail.ru) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY 3.0  (https://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
  • Vancleavea campi     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
  • Proterochampsa barrionuevoi     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
  • Archeopelta arborensis     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
  • Chanaresuchus bonapartei     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
  • Euparkeria capensis     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
  • ​Doswellia kaltenbachi     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
  • Halazhaisuchus qiaoensis     By Sookias RB, Sullivan C, Liu J, Butler RJ. [CC BY-SA 4.0  (https://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons
  • Parasuchus hislopi     By Martin D. Ezcurra [CC BY 4.0  (https://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons
  • Smilosuchus gregorii (18a)     By Tim Evanson, Attribution-ShareAlike 2.0 Generic (CC BY-SA 2.0)
  • Smilosuchus gregorii  (18b)    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
  • Pseudopalatus  sp.     By Lee  Ruk from North Tonawanda [CC BY-SA 2.0  (https://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons 
  • Teleocrater rhadinus     By Fanboyphilosopher [CC BY 4.0  (https://creativecommons.org/licenses/by/4.0)], from Wikimedia Commons​
  • Ctenosauriscus koeneni     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
  • Home
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    • Eukaryotes
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      • Vertebrates >
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        • Chimaeras
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          • Eutheria
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          • Crocodylia
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    • Land plants >
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