By around 340mya, amphibian life was flourishing, as evolution from fishes for life out of water was advancing rapidly. However these animals were still dependant on water, as amphibian eggs had to be laid in water in order to avoid them drying out. Terrestrial life was therefore restricted only to areas near water sources, leaving vast inland areas unoccupied by vertebrate life.
This all changed when a group of animals known as the reptilomorphs evolved in the Carboniferous period. These animals were, put simply, half way between amphibians and reptiles, and were the precursors to reptilian life. But at what critical point did an amphibian become a reptile?The most important feature that defines a reptile, besides its scaly skin to cope with water loss on land, is the ‘amniote egg’. The first true reptiles could lay this type of egg, which allowed gas exchange with air through a robust membrane (the shell) whilst avoiding drying out. This meant that reptiles could thrive without need to return to water to reproduce, allowing colonisation of the rich and vast inner land masses in a time where the climate was becoming more arid.
It will probably never be known at what specific time this transition first occurred, but there have been a number of fossil discoveries which give us a general idea. One of these was found close to home, at the East Kirkton quarry in Scotland, and is called Westlothiana lizziae. The reptilomorph species was present around 335mya, and resembled a small lizard. Another Scottish fossil, Casineria, is very similar, with skeletal structures suggesting a very terrestrial lifestyle. It is therefore possible that the first reptiles were small creatures like these, whose pioneering steps into fully terrestrial life would lead to the evolution of all amniotes, including dinosaurs, birds and mammals.
References:Paton, R.L., Smithson, T.R and Clack, J.A. (1999). An amniote-like skeleton from the Early Carboniferous of Scotland. Nature, vol. 398 pp. 508-513.
Smithson, T.R, Carroll, R.L, Panchen, A.L and Andrews, S.M. (1993). Westlothiana izziae from the Viséan of East Kirkton, West Lothian, Scotland, and the amniote stem. Transactions of the Royal Society of Edinburgh: Earth Sciences, vol. 84, issue 3-4, pp. 383-412.
What you can do:Reptiles are believed to be under recorded in some parts of Scotland. Why not help increase local records by using the free Dragon Finder App to get your records to Froglife.