​The clade Sauria, which comprises the reptiles and birds (Class Reptilia, superclass Tetrapoda) is the sister clade to the mammals and as such comprise all non-mammalian amniotes. The Sauria is actually a crown group (Ezcurra et al, 2014); the corresponding total group is known as the Reptilia or the Sauropsida, which are synonymous terms (Modesto and Anderson, 2004).

The saurians are diapsids, which means that they have two openings, or fenestrae, in the skull behind each eye in contrast to the synapsids (a clade that contains the mammals), which have only one opening. This distinction seems clear, but a problem arises with the turtles, which have no such fenestra and thus resemble the stem-Sauria known as anapsids because of their lack of fenestrae. This resemblance has led to the suggestion that the turtles fall outside the crown-Sauria, but it is now commonly thought that the fenestrae in ancestral turtles closed up during the evolutionary process (Schoch and Sues, 2016).

There is still disagreement over the phylogeny of the stem-Sauria. One recent and widely cited study is that by Ford and Benson (2020), on which the following phylogenetic time tree is based:
Figure 1.  Phylogenetic time tree of the stem-Sauria
​The above tree contains several clades that died out and thus represent evolutionary dead ends (the Captorhinidae, Varanopidae and Parareptilia). We will start by excluding those clades (except for their most basal representative in each case) so that we can consider only the direct stem line that led to the crown group.
​
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). It is illustrated below, together with other fossils (for which public-domain images are also available) on the direct stem line (for a larger view, click on image):
Figure 2. Images of saurians in the direct stem line
The images are placed in order from most basal to most crownward on the direct stem line. The fossils all appear lizard-like, and there is no obvious change from the most basal to the most crownward. However, two of the more crownward species, Claudiosaurus germaini and Acerosodontosaurus piveteaui, have been interpreted to have had an aquatic lifestyle (Carroll, 1981 and Bickelmann et al, 2009, respectively).

For comparison with the direct stem line, we will now consider the side branches. The figure below illustrates the fossils of the Captorhinidae for which public-domain images are available (click on image for larger view):
Figure 3. Images of stem saurians in the   family Captorhinidae
​These images are placed in order from most basal (a duplication of the image for Euconcordia cunninghami) to those closest to the termination of the clade. Again, no clear differences can be seen. They are all lizard-like, but they are characterized by skulls with a triangular shape when viewed from above. They also have peg-like teeth, as can be seen above in the image of Captorhinus laticeps.

​The next set of images, which includes a duplication of the image for Ascendonanus nestleri, are from the Varanopidae:
Figure 4. Images of stem saurians in the   family  ​ Varanopidae
These also show no clear development from a basal to a terminal position in the tree. However, they differ from the fossils seen above in having generally longer legs and, commonly, a stouter body. They have been interpreted to have been active and agile in their habits (Benton, 2015).
​
The final side-branch clade to be considered is the Parareptilia:
Names in   red indicate   that the fossil is younger  than the oldest known crown-group fossil.
Figure 5. Images of stem saurians in the  clade Parareptilia
The fossils shown above (including a duplicated image of Erpetonyx arsenaultorum) are variable in shape and size, and no clear trends can be seen. Again, some species are interpreted to have been aquatic; the mesosaurs, exemplified above by Mesosaurus tenuidens and Stereosternum tumidum, are generally considered to have been the earliest fully aquatic amniotes (Nuñez Demarco et al, 2018). Known morphologies within the clade vary dramatically, ranging from smaller, superficially lizard-like forms, to large, armoured herbivores (MacDougall et al, 2019).

Some idea of the nature of the transition from the stem group to the crown group of the saurians can be derived from a  comparison of the above images with the examples of crown-Sauria shown below:
Figure 6. Examples of crown-Sauria
As indicated in the above time tree (Figure 1), the time between the origin of the saurian stem group and the initiation of the crown group was at least 58 million years, from the Late Carboniferous to the Late Permian.

References

Benton, M. J. (2015). Vertebrate Palaeontology - Fourth edition. John Wiley & Sons, 468 pages.

Benton, M. J., Donoghue, P. C., Asher, R. J., Friedman, M., Near, T. J., & Vinther, J. (2015). Constraints on the timescale of animal evolutionary history. Palaeontologia Electronica, 18(1), 1-106.

Bickelmann, C., Müller, J., & Reisz, R. R. (2009). The enigmatic diapsid Acerosodontosaurus piveteaui (Reptilia: Neodiapsida) from the Upper Permian of Madagascar and the paraphyly of “younginiform” reptiles. Canadian Journal of Earth Sciences, 46(9), 651-661.

Carroll, R. L. (1964). The earliest reptiles. Zoological Journal of the Linnean Society, 45(304), 61-83.

Carroll, R. L. (1981). Plesiosaur ancestors from the Upper Permian of Madagascar. Philosophical Transactions of the Royal Society of London. B, Biological Sciences, 293(1066), 315-383.

Ezcurra, M. D., Scheyer, T. M., & Butler, R. J. (2014). The origin and early evolution of Sauria: reassessing the Permian saurian fossil record and the timing of the crocodile-lizard divergence. PloS one, 9(2), e89165.

Ford, D. P., & Benson, R. B. (2020). The phylogeny of early amniotes and the affinities of Parareptilia and Varanopidae. Nature Ecology & Evolution, 4(1), 57-65.

Lichtig, A. J., & Lucas, S. G. (2021). Chinlechelys from the Upper Triassic of New Mexico, USA, and the origin of turtles. Palaeontologia Electronica, 24(1):a13. https://doi.org/10.26879/886.

MacDougall, M. J., Brocklehurst, N., & Fröbisch, J. (2019). Species richness and disparity of parareptiles across the end-Permian mass extinction. Proceedings of the Royal Society B, 286(1899), 20182572.

Modesto, S. P., & Anderson, J. S. (2004). The phylogenetic definition of Reptilia. Systematic biology, 53(5), 815-821.

Nuñez Demarco, P., Meneghel, M., Laurin, M., & Piñeiro, G. (2018). Was Mesosaurus a fully aquatic reptile?. Frontiers in Ecology and Evolution, 6, 109.

Schoch, R. R., & Sues, H. D. (2016). The diapsid origin of turtles. Zoology, 119(3), 159-161.
Image credits – Stem-Sauria
  • Figure 2 (skull of Hylonomus lyelli):  Gretarsson, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Hylonomus lyelli, life restoration):  Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Figure 2 (Euconcordia cunninghami):  Fanboyphilosopher (Neil Pezzoni), CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
  • Figure 2 (Anthracodromeus longipes):  Arjan Mann, Thomas W. Dudgeon, Amy C. Henrici, David S Berman, Stephanie E. Pierce1, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
  • Figure 2 (Paleothyris acadiana, fossil):  https://www.si.edu/object/nmnhpaleobiology_3441090, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 2 (Paleothyris acadiana, life restoration):  Conty, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 2 (Protorothyris archeri):  Smokeybjb, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Araeoscelis gracilis):  Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Figure 2 (Petrolacosaurus kansensis, fossil):  Fanboyphilosopher (Neil Pezzoni), CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
  • Figure 2 (Petrolacosaurus kansensis, life restoration):  Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Figure 2 (Ascendonanus nestleri):  Museum für Naturkunde Chemnitz, Namensnennung - Nicht-kommerziell - Weitergabe unter gleichen Bedingungen 3.0 Deutschland (CC BY-NC-SA 3.0 DE)
  • Figure 2 (Erpetonyx arsenaultorum):  Fanboyphilosopher (Neil Pezzoni), CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
  • Figure 2 (Claudiosaurus germaini, fossils):  Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Claudiosaurus germaini, life restoration):  Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Figure 2 (skull of Acerosodontosaurus piveteaui):  Fanboyphilosopher (Neil Pezzoni), CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
  • Figure 2 (Acerosodontosaurus piveteaui, life restoration):  Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Figure 2 (juveniles of Youngina capensis):  Nkansahrexford, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
  • Figure 2 (Youngina capensis, life restoration):    Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Figure 2 (Lanthanolania ivakhnenkoi):  GleisReis, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 3 (Euconcordia cunninghami):  Fanboyphilosopher (Neil Pezzoni), CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
  • Figure 3 (Thuringothyris mahlendorffae):  karkemish00 under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License
  • Figure 3 (Captorhinus aguti, fossils):  Didier Descouens, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 3 (Captorhinus aguti, life restoration):  Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Figure 3 (Captorhinus laticeps):  Open Access article DeBraga, M., Bevitt, J. J., & Reisz, R. R. (2019). A new captorhinid from the Permian cave system near Richards spur, oklahoma, and the taxic diversity of captorhinus at this locality. Frontiers in Earth Science, 7, 112.
  • Figure 3 (Labidosaurus hamatus, fossil):  Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 3 (Labidosaurus hamatus, life restoration):  Smokeybjb, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 3 (skull of Labidosaurikos meachami):  https://www.si.edu/object/labidosaurikos-meachami:nmnhpaleobiology_12124569, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 3 (Labidosaurikos meachami, life restoration):  ДиБгд, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons
  • Figure 4 (Ascendonanus nestleri):  Museum für Naturkunde Chemnitz, Namensnennung - Nicht-kommerziell - Weitergabe unter gleichen Bedingungen 3.0 Deutschland (CC BY-NC-SA 3.0 DE)
  • Figure 4 (Archaeovenator hamiltonensis, fossil):  Fanboyphilosopher (Neil Pezzoni), CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
  • Figure 4 (Archaeovenator hamiltonensis, life restoration): Theropsida under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License
  • Figure 4 (skull of Aerosaurus wellesi):  Patricia Holroyd (University of California Museum of Paleontology), CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
  • Figure 4 (Aerosaurus wellesi, life restoration): Theropsida under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License
  • Figure 4 (Varanops brevirostris, fossil):  Jonathan Chen, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 4 (Varanops brevirostris, life restoration):  ДиБгд, Public domain, via Wikimedia Commons
  • Figure 4 (Varanodon agilis):  Dmitry Bogdanov, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
  • Figure 4 (Mycterosaurus longiceps):  Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA)  license
  • Figure 4 (Mesenosaurus romeri, fossil):  Ghedoghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 4 (Mesenosaurus romeri, life restoration):  Theropsida under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License
  • Figure 4 (Elliotsmithia longiceps):  Theropsida under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License
  • Figure 4 (Heleosaurus scholtzi, fossils):  Original work: Robert L. Carrollmodified by Gretarsson, Copyrighted free use, via Wikimedia Commons
  • Figure 4 (Heleosaurus scholtzi, life restoration):  Theropsida under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License
  • Figure 5 (Erpetonyx arsenaultorum):  Fanboyphilosopher (Neil Pezzoni), CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
  • Figure 5 (Belebey vegrandis):  Dmitry Bogdanov, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 5 (Eudibamus cursoris, fossil):  Open Access article Berman, D. S., Sumida, S. S., Henrici, A. C., Scott, D., Reisz, R. R., & Martens, T. (2021). The Early Permian Bolosaurid Eudibamus cursoris: Earliest Reptile to Combine Parasagittal Stride and Digitigrade Posture During Quadrupedal and Bipedal Locomotion. Frontiers in Ecology and Evolution, 469.
  • Figure 5 (Eudibamus cursoris, life restoration):  Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Figure 5 (Mesosaurus tenuidens, fossils):  Ghedoghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 5 (Mesosaurus tenuidens, life restoration):  Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Figure 5 (Stereosternum tumidum, fossil):  Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 5 (Stereosternum tumidum, life restoration):  Smokeybjb, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 5 (skull of Colobomycter pholeter):  Fanboyphilosopher (Neil Pezzoni), CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
  • Figure 5 (Colobomycter pholeter, life restoration):  Nixillustration under a Creative Commons Attribution-NonCommercial license (CC BY-NC 4.0)
  • Figure 5 (Milleretta rubidgei):  Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Figure 5 (skull of Macroleter poezicus):  Johannes Müller, Linda A. Tsuji, CC BY 2.5 <https://creativecommons.org/licenses/by/2.5>, via Wikimedia Commons
  • Figure 5 (Macroleter poezicus, life restoration):  Dmitry Bogdanov, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 5 (skull of Deltavjatia rossicus):  Ghedoghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 5 (Deltavjatia rossicus, life restoration):    Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Figure 5 (Emeroleter levis):  Dmitry Bogdanov, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 5 (skull of Nyctiphruretus acudens):  Ghedoghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 5 (Nyctiphruretus acudens, life restoration):  Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Figure 5 (Procolophon trigoniceps):  Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Figure 5 (skull of Candelaria barbouri):  Open Access article Müller, R. T. (2021). An additional specimen of owenettid procolophonoid from the Middle Triassic of Southern Brazil. Palaeontologia Polonica, 66(4).
  • Figure 5 (Candelaria barbouri, life restoration):  JohnnyMingau, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 5 (Owenetta kitchingorum):  https://www.si.edu/object/owenetta-kitchingorum:nmnhpaleobiology_3449755, CC0, via Wikimedia Commons
  • Figure 6 (Planocephalosaurus robinsonae):  Virginia Museum of Natural History, licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License.
  • Figure 6 (Microsphenodon bonapartei):   Open Access article Chambi-Trowell, S. A., Martinelli, A. G., Whiteside, D. I., Vivar, P. R. R. D., Soares, M. B., Schultz, C. L., ... & Rayfield, E. J. (2021). The diversity of Triassic South American sphenodontians: a new basal form, clevosaurs, and a revision of rhynchocephalian phylogeny. Journal of Systematic Palaeontology, 19(11), 787-820.
  • Figure 6 (Protorosaurus speneri):    Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Figure 6 (Aenigmastropheus parringtoni):  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