Introduction
The lepidosaurs (clade Lepidosauria, class Reptilia) form a sister group to the Archelosauria. They differ from the latter mainly in the structure of their skull, their type of dentition and in their male reproductive organs (Evans, 2014). The lepidosaurs are a crown group comprising all living snakes, lizards and tuatara, together with all extinct species descended from the last common ancestor of all living lepidosaurs. Their phylogeny is illustrated below:
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 stem group
Few interpretations of the phylogeny of the stem lepidosaurs have been published, and they generally agree. The tree shown below is fairly representative:
The total-group Lepidosauria are known as the Lepidosauromorpha (Evans and Jones, 2010). The oldest-known fossil representative of the stem-Lepidosauria is Paliguana whitei, found in the Early Triassic (Induan) Katberg Formation of Donnybrook, Queenstown District, Eastern Cape, South Africa (Broom, 1903; Evans, 2016). The skull of this species is illustrated below, together with the few stem group fossils for which public-domain images are available (click on images to see larger versions):
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.
While there are too few images above to draw any inferences about development within the lepidosaurian stem group, the literature focuses on changes in skull size and morphology (Evans and Jones, 2010). Note that the winged species, which all belong to the family Kuehneosauridae, were either gliders (Kuehneosuchus, Icarosaurus) or parachutists (Kuehneosaurus) (Stein et al, 2008; Colbert, 1970); they were not capable of powered flight.
Note that two of the stem group fossils shown above (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 elapsed time between the appearance of the earliest stem-group fossil and that of the oldest crown-group lepidosaur was relatively short, being confined to the Early and Middle Triassic, but some stem branches continued to develop until the Middle Jurassic. This is illustrated in the figure below:
Note that two of the stem group fossils shown above (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 elapsed time between the appearance of the earliest stem-group fossil and that of the oldest crown-group lepidosaur was relatively short, being confined to the Early and Middle Triassic, but some stem branches continued to develop until the Middle Jurassic. This is illustrated in the figure below:
The crown group
The crown-Lepidosauria appeared when the stem lines of the squamates and the rhynchocephalians separated from one another:
The earliest-known fossil representing the squamate stem group is Megachirella wachtleri, of Middle Triassic (Anisian) age, while the stem-Rhynchocephalia appeared somewhat later, in the Late Triassic (Carnian) in the form of Brachyrhinodon taylori. The relationship between these ages and that of the earliest stem lepidosaur 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 lepidosaur crown node. This uncertainty represents the maximum period of time for the stem-to-crown transition; the time between the origin of the lepidosaur stem group and the initiation of the crown group would have been no longer than 8.5 million years:
The figure below depicts the total uncertainty in the age of the lepidosaur crown node. This uncertainty represents the maximum period of time for the stem-to-crown transition; the time between the origin of the lepidosaur stem group and the initiation of the crown group would have been no longer than 8.5 million years:
Image credits - Lepidosaurs
- Header: Large-eyed pit viper ( Trimeresurus macrops), Khao Yai National Park, Thailand By Rushenb [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Skull of Paliguana whitei By Martín D. Ezcurra, Torsten M. Scheyer, Richard J. Butler. Drawing by Emilio López-Rolandi. [CC BY 2.5 (https://creativecommons.org/licenses/by/2.5)], via Wikimedia Commons
- Kuehneosuchus spp. 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
- 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
- Huehuecuetzpalli mixtecus By Apokryltaros [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons
- Brachyrhinodon taylori 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