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.