On Traces of Non-Equilibrium States in the Evolution of Terrestrial Vertebrate Communities across the Paleozoic-Mesozoic Boundary

Evolutionary change in a living system starts with a disturbance to its equilibrium and an increase in its internal disorderliness. Typical manifestations of this process are exemplified by the transformation of the tetrapod faunas across the Permian-Triassic boundary. These are seen both at the end of the transition to a new equilibrium (in the pioneer Triassic faunas) and at its beginning (in the terminal Permian communities). In the first case, the most distinct manifestations are: unfinished state of structural changes within some new groups; a high degree of endemism of regional pioneer communities, and the heterogeneity of their change with time. In the second case, as can be seen from analysis of the early-crisis (Vyaznikovian) biota of Eastern Europe, the observed events primarily include the "precocious" appearance of some future dominants; the "return" of a number of relics to the fossil record; and examples of explosive group variability in some taxa close to extinction. Actual or probable traces of non-equilibrium changes, similar to those noted in the Vyaznikovian biota, can also be partly traced in other Late Permian faunas of Eurasia and in coeval communities of southern and northern Gondwana.