EVOLUTION - THE TRANSITIONAL FOSSILS
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mammals

Introduction

The mammals (class Mammalia, clade Amniota) comprise animals that feed their young on milk produced by mammary glands. Mammals form a crown group that consists of all living placental, marsupial and egg-laying mammals, together with all extinct species descended from the last common ancestor of all living mammals.

Their phylogeny is illustrated in the following tree:
Picture
The mammalian crown group (node shown as red dot) consists of the Theria and the Prototheria. The primary difference between these clades is that the therians give birth to live young without a shelled egg, while the prototherians lay eggs.

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 Theria can be reached through this link:​
  • Theria  

The stem group

The following phylogenetic tree, based on a combination of two recently published trees (Huttenlocker and Farmer, 2017; Lautenschlager et al, 2017), illustrates an interpretation of the phylogenetic relationships of the mammal stem group:
Picture
The earliest-known member of the mammalian stem group is Sphenacodon ferox, found in the Late Carboniferous (Gzhelian) Cutler Formation of Rio Arriba County, New Mexico, USA (Romer, 1937; Benson, 2012). This species, together with other genera included in the above tree, is shown below (click on image for larger version):
* after name indicates that the image represents a life restoration.
The fossils are numbered in order from the most basal to the most crownward in the tree. The obvious change is from a lizard-like form to one more like that of mice or rats, but this is accentuated by the interpretation of hair or fur in the more crownward genera. However, hair has been found in coprolites (fossilized faeces) of Late Permian age (Bajdek et al, 2016), and Benoit et al (2016) present indirect evidence for the existence of hair by the end of the Early Triassic, so the restorations might not be unreasonable.

But many of the changes that occurred through the mammalian stem line are not obvious from pictures such as those above. The main developments had to do with  transformations of the mammalian middle ear and the jaw hinge  (Luo, 2007) and also in the morphology of the shoulder (Luo, 2015), but there were other modifications to elements such as brain size, nasal cavity elaboration, secondary palate and evolution of a diaphragm (Kemp, 2007).

The mammalian stem group evolved from Late Carboniferous to Early Jurassic time, as shown in the figure below:
Picture

The crown group

The crown-Mammalia appeared when the stem lines of the therians and the prototherians  separated from one another:
Picture
The oldest known fossil representative of the crown-Mammalia is Asfaltomylos patagonicus, a Late Jurassic (Toarcian) stem prototherian, while the earliest known stem therian is Amphibetulimus krasnolutskii, of Middle Jurassic (Bathonian) age. The relationship between these ages and that of the earliest stem mammal 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 mammalian crown node. This uncertainty represents the maximum period of time for the stem-to-crown transition; the time between the origin of the  mammal stem group and the initiation of the crown group could have been as long as 130 million years:
Picture

Image credits - Mammals
  • Header (Panthera leo in Kruger National Park, South Africa)    By Bernard DUPONT from FRANCE (Lion (Panthera leo)) [CC BY-SA 2.0  (https://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons ​
  • Mycterosaurus  longiceps     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
  • Sphenacodon ferox     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
  • Sphenacodon ferocior     By Дибгд, public domain
  • Biarmosuchus  sp.     DiBgd at English Wikipedia [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)], via Wikimedia Commons
  • Diictodon  sp.     By Bernard Dupont, https://creativecommons.org/licenses/by-sa/2.0/legalcode
  • Moschorhinus  sp.      [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
  • Ictidosuchoides  intermedium     By Rept0n1x [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons
  • Procynosuchus    sp.     By Lutz Benseler [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons
  • Procynosuchus delaharpeae     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
  • Thrinaxodon   liorhinus     By http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0064978 [CC BY-SA 2.5 (https://creativecommons.org/licenses/by-sa/2.5)], via Wikimedia Commons
  • Cynognathus   crateronotus     By Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons
  • Massetognathus  sp.     By Raymond Rogers [CC0], via Wikimedia Commons
  • Diademodon tetragonus     By FunkMonk [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
  • Diademodon   sp.     By Mojcaj [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], from Wikimedia Commons
  • Probelesodon lewisi     By Smokeybjb, Creative Commons Attribution-Share Alike 3.0 Generic license
  • Probainognathus jenseni      By Nobu Tamura (http://spinops.blogspot.ca/) [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons
  • Brasilitherium​ riograndensis     By Smokeybjb [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons
  • Morganucodon watsoni     By FunkMonk (Michael B. H.) [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
  • Fruitafossor   windscheffeli     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​
  • Pseudotribos robustus     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
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