An appraisal of the ‘Living Fossil’ Concept

Abstract

Although the term ‘living fossil’ has been around for over 150 years, it remains scientifically undefined and contentious. Generally, it refers to any taxon that is evolutionarily unique, species-poor and exhibits traits closely resembling those of extinct taxa. This interpretation, however, is not universal. Other interpretations of the term include species that are evolutionary dead-ends, taxa that were first discovered in the fossil record and/or lineages that have undergone no (sic) morphological change. In addition to the confusion over definition, many have argued against the concept on the grounds that it is either not feasible, that it may not delineate a true category of biodiversity or that it recalls defunct Victorian scientific ideas. Despite these objections, it is evident that the concept of the living fossil can play an important role in our understanding of evolution over large timescales. In recent years there has been renewed interest in the concept of the living fossil thanks to the development of genomic techniques that are revealing previously hidden rates of molecular change in well-known living fossils. Additionally, many species that have been described as living fossils are threatened with extinction and there exists a risk that their unique evolutionary history is being lost. In this thesis, I attempt a re-appraisal of the living fossil concept. Firstly I test whether the concept of the living fossil can be made scientifically sound. Secondly I develop a new quantitative definition of the living fossil. I show that the concept is robust as it can be used to distinguish lineages that have experienced different rates of evolution. In addition I demonstrate that it is possible to devise a quantitative definition, which I use to identify living fossils and their traits across a wide range of taxonomic groups.