Among the earliest known crown group amniotes, the oldest known sauropsid is Hylonomus and the oldest known synapsid is Asaphestera, both of which are from Nova Scotia during the Bashkirian age of the Late Carboniferous around 318 million years ago.[1][16] Basal amniotes resembled small lizards and evolved from semiaquatic reptiliomorphs during the Carboniferous period.[17] After the Carboniferous rainforest collapse, amniotes spread around Earth's land and became the dominant land vertebrates,[17] and soon diverged into the synapsids and sauropsids, whose lineages both still persist today. Older sources, particularly before the 20th century, may refer to amniotes as "higher vertebrates" and anamniotes as "lower vertebrates", based on the antiquated idea of the evolutionary great chain of being.
^Benton, Michael J. (1997). Vertebrate Palaeontology. London: Chapman & Hall. pp. 105–109. ISBN978-0-412-73810-4.
^Cieri, R.L., Hatch, S.T., Capano, J.G. et al. (2020). Locomotor rib kinematics in two species of lizards and a new hypothesis for the evolution of aspiration breathing in amniotes. Sci Rep10. 7739. https://doi.org/10.1038/s41598-020-64140-y
^Janis, C. M., Napoli, J. G., & Warren, D. E. (2020). Palaeophysiology of pH regulation in tetrapods. Philosophical Transactions of the Royal Society B: Biological Sciences, 375 (1793), 20190131. https://doi.org/10.1098/rstb.2019.0131
^Hickman, Cleveland P. Jr (17 October 2016). Integrated principles of zoology (Seventeenth ed.). McGraw-Hill. pp. 563–567. ISBN978-1-259-56231-0.
^ abcKardong, Kenneth V. (16 February 2011). Vertebrates: Comparative Anatomy, Function, Evolution. McGraw-Hill. ISBN978-0-07-352423-8.
^Clack, Jennifer A. (27 August 2023). Gaining Ground: The Origin and Evolution of Tetrapods. Indiana University Press. p. 370. ISBN978-0-253-35675-8.
^Mann, Arjan; Gee, Bryan M.; Pardo, Jason D.; Marjanović, David; Adams, Gabrielle R.; Calthorpe, Ami S.; Maddin, Hillary C.; Anderson, Jason S. (5 May 2020). Sansom, Robert (ed.). "Reassessment of historic 'microsaurs' from Joggins, Nova Scotia, reveals hidden diversity in the earliest amniote ecosystem". Papers in Palaeontology. 6 (4). Wiley: 605–625. Bibcode:2020PPal....6..605M. doi:10.1002/spp2.1316. ISSN2056-2802.