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:
Their phylogeny is illustrated in the following tree:
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:
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:
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:
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:
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:
The crown group
The crown-Mammalia appeared when the stem lines of the therians and the prototherians separated from one another:
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:
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:
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