Introduction
The saurians are the sister clade to the mammals and as such comprise all non-mammalian amniotes (see above section on the amniotes). In some classification schemes (e.g. in Benton et al, 2015), they are synonymous with the reptiles (class Reptilia). They represent a crown group comprising all living lepidosaurs and archelosaurs, together with all extinct species descended from the last common ancestor of all living saurians. Their phylogeny is illustrated below:
The first division in the saurian crown group (node shown as red dot above) took place between the Lepidosauria and the Archelosauria. The lepidosaurs may be distinguished from the archelosaurs in having overlapping scales, but there are many other differences that are only apparent in comparison of the genomes (Crawford et al, 2012).
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:
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:
The stem group
The stem-group and crown-group saurians taken together, the total-group Sauria, are known as the Sauropsida. One possible phylogenetic tree showing relationships between the stem saurians is shown below:
The above tree divides the stem saurians into two groups: one comprising the direct stem line to the crown saurians, and one forming a separate group that diverged from the direct stem line. The latter group is known as the Parareptilia, and represents an evolutionary dead end that did not lead to any living species.
The oldest known stem-group saurian fossil is Hylonomus lyelli, found in Bashkirian (Late Carboniferous) sediments in the sea cliffs at Joggins, Novia Scotia, Canada (Carroll, 1964; Benton et al, 2015). Owing to its poor degree of preservation, no attempt has een made in the literature to assign it a place in the phylogeny of the other stem saurians. It is illustrated as a life restoration below, together with other stem group fossils for which images are available in the public domain (for larger versions click on the images):
The oldest known stem-group saurian fossil is Hylonomus lyelli, found in Bashkirian (Late Carboniferous) sediments in the sea cliffs at Joggins, Novia Scotia, Canada (Carroll, 1964; Benton et al, 2015). Owing to its poor degree of preservation, no attempt has een made in the literature to assign it a place in the phylogeny of the other stem saurians. It is illustrated as a life restoration below, together with other stem group fossils for which images are available in the public domain (for larger versions click on the 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.
* after name indicates that the image represents a life restoration.
These images are numbered in order from the base to the branches of the tree. Images numbered 1 through 8b represent the parareptiles; there seems to be a tendency for these animals to become less lizard-like and more sturdy in their body form. However, no obvious trends can be seen in the slender forms of the direct stem line to the crown saurians (images 9a through 12).
The saurian stem group evolved mainly from Late Carboniferous to Middle Permian time, as shown in the following figure:
The saurian stem group evolved mainly from Late Carboniferous to Middle Permian time, as shown in the following figure:
However, note that many of the stem species appeared after the crown group; these represent branches of the stem line that continued to evolve after the crown-group had appeared. It is noteworthy that the parareptile branch, despite representing the most basal stem-Sauria, kept evolving until the Early Triassic.
The crown group
The crown-Sauria appeared when the stem lines of the lepidosaurs and the archelosaurs separated from one another:
The oldest known member of the saurian crown group is Eunotosaurus africanus, a stem turtle of Middle Permian (Capitanian) age. This represents only a minimum age for the appearance of the total-group Archelosauria, because Eunotosaurus belongs to the crown group of the archelosauria; no stem-Archelosauria are known from the fossil record. The earliest known stem lepidosaur, Paliguana whitei, appeared slightly later, in the earliest Triassic (Induan). The relationship between these ages and that of the earliest stem saurian 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 saurian crown node. This uncertainty represents the maximum period of time for the stem-to-crown transition; the time between the origin of the saurian stem group and the initiation of the crown group could have been as long as 59 million years:
The figure below depicts the total uncertainty in the age of the saurian crown node. This uncertainty represents the maximum period of time for the stem-to-crown transition; the time between the origin of the saurian stem group and the initiation of the crown group could have been as long as 59 million years:
Image credits - Saurians
- Header (Mangrove hummingbird (Amazilia boucardi), Costa Rica) By Jorge Obando Nature Photo (Mangrove Hummingbird ♂) [CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons
- Hylonomus lyelli By MUSE [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
- Milleretta 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
- Macroleter poezicus By Dmitry Bogdanov [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
- Lanthanosuchus watsoni (3A) By Radim Holiš, Wikimedia Commons [CC BY-SA 3.0 cz (https://creativecommons.org/licenses/by-sa/3.0/cz/deed.en)], from Wikimedia Commons
- Lanthanosuchus watsoni (3B) 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
- Milleropsis pricei By Conty [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)], from Wikimedia Commons
- Bradysaurus baini (5A) By Masur [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY-SA 2.5 (https://creativecommons.org/licenses/by-sa/2.5)], from Wikimedia Commons
- Bradysaurus baini (5B) 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
- Procolophon trigoniceps 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
- Anthodon serrarius By C.L. Griesbach [Public domain], via Wikimedia Commons
- Anthodon gregoryi By Nobu Tamura (http://spinops.blogspot.com) Creative Commons Atribución 3.0 Unported
- Scutosaurus karpinskii (8A) Radim Holiš, Wikimedia Commons [CC BY-SA 3.0 cz (https://creativecommons.org/licenses/by-sa/3.0/cz/deed.en)], from Wikimedia Commons
- Scutosaurus karpinskii (8B) 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
- Captorhinus aguti (1A) By Didier Descouens [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons
- Captorhinus aguti (1B) 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
- Araeoscelis grandis By Smokeybjb [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or FAL], from Wikimedia Commons
- Claudiosaurus germaini By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons
- Youngina sp. By Nkansahrexford [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)], from Wikimedia Commons
- Icarosaurus siefkeri 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
- Eunotosaurus africanus By Nobu Tamura (http://spinops.blogspot.ca/) [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons