Principle of faunal succession

William Smith (1769–1839), the English geologist who defined the principle of family succession.

The principle of faunal succession or correlation establishes that the fossil content of sedimentary rocks varies vertically in a certain order and that each set can be identified horizontally at considerable distances. That is, in rocks of different ages, fossils corresponding to different paleobiological organisms are preserved (vertical variation) and for each age fossils of the same taxa can be identified in remote areas (horizontal correlation). The basis of this principle is the irreversibility of biological evolution, once a species has become extinct, it does not appear again.

History

This principle was developed by geologist William Smith, who worked in Wales (c. 1800), where he developed the idea that each period of earth's history has its own unique fossil record. During his work for the construction of a canal, he verified that the successions of rocks were always found in the same order of superposition and that each group of strata contained the same distinctive fossils throughout the country. He found that although the lithological composition of the strata varied, those of the same age contained the same fossils. With this he created the bases for the application of paleontology to geology, that is, “stratigraphic paleontology” and also concluded that organisms had changed over time. He established the bases of biostratigraphy, using fossils as a tool to characterize, subdivide and correlate strata from different regions and, mainly, order them in a relative temporal sequence. He is considered the creator of modern stratigraphy.

Precautions in applying the principle

Taphonomic reworking

Portrait with reworked fossils (naranja). Although they are together, the organisms from which they come did not live at the same time, the fossils yellow, reworked, are older than the oranges.

The direct application of the principle of faunal succession to fossils has been criticized due to advances in taphonomic studies. The chronological order of a stratigraphic series, in which fossils are found, may not respond to the chronological order of when the organisms from which these fossils come lived. In other words, the order established by the principle of the superposition of strata does not have to correspond to the same order of succession of the fossils they contain, nor, therefore, to the order of biological succession.

Taphonomic reworking is a process that can result in us finding fossils that are older in age than their containing layers, and even mixed assemblages of different ages. Therefore, before addressing the temporal succession of the organisms (biological succession) that gave rise to the fossils found in a stratigraphic series, it is necessary to establish the "corrected" temporal succession of the fossils themselves, discriminating the different "families" of fossils with common taphonomic history and establish the order of temporal succession between them (registered succession).

Lazarus Taxa

There may be cases in which the fossil record of a certain taxon presents a significant temporary hiatus, as if it had become extinct and reappeared ("Lazarus effect"). Explanations for why a taxon can disappear from a region are usually ecological, due to variations in the environmental conditions of the environment, which it reoccupies once the causes of its removal have disappeared, or due to variations in the conditions of production of remains or their fossilization, which can bias the fossil record to the detriment of certain taxa.