A Review on Why Some of the Reptilian Lineages are Relict Taxa?

Debanik Mukherjee, M. Vijay

Abstract: The Earth has never remained environmentally or geologically stable since its origin. Plate tectonics, continental drift, climatic oscillations, sea-level fluctuations, geological catastrophes and associated ecological changes have repeatedly altered the distribution, diversification and extinction of several organisms. These processes have profoundly influenced the evolutionary history and biogeography of the so-called Class Reptilia. Nevertheless, several extant reptilian lineages represent highly distinctive remnants of formerly more diverse and geographically widespread evolutionary radiations. Such taxa are commonly referred to as evolutionary or phylogenetic relicts. The present review reveals why some reptilian lineages have remained comparatively conservative in their fundamental morphology while other reptilian taxa have undergone extensive diversification and innovation of morphologically novel traits. Particular attention is given to the tuatara (Sphenodon punctatus), phylogenetically primitive monitor lizard (lack of exposed external tympanum), Lanthanotus borneensis and the Gharial (Gavialis gangeticus) as examples of evolutionarily distinctive and unique surviving monotypic lineages, taxonomically known as monotypic taxa. The review discusses phylogenetic conservatism and ecological constraints with respect to niche segregation, ecological specialization, stabilizing selection, developmental as well as functional constraints, geographic isolation, extinction filtering, climatic change, ecological opportunity and historical contingency as possible factors contributing to relictuality. Importantly, morphological conservatism should not be interpreted as evolutionary inactivity. Fossil evidence demonstrates that Rhynchocephalians, for example, were once morphologically and ecologically diverse, and Sphenodon represents only the surviving component of that former adaptive radiation. Similarly, extant crocodilians constitute a small surviving fraction of a much more diverse crocodylomorph evolutionary history. Therefore, the apparent evolutionary conservatism of some living reptiles may partly reflect differential extinction rates and the survival of particular ecological and most adaptive phenotypes rather than an absence of evolutionary change. The term "living fossil" is consequently problematic when it is used to imply evolutionary stagnation. The concept of an evolutionary relicts or phylogenetic relicts provides a more appropriate framework for discussing surviving representatives of ancient and formerly more diverse lineages. Understanding reptilian relicts requires much better integration of evolutionary biology, palaeontology, ecology, biogeography, phylogenetics and finally conservation biology.

Keywords: Reptilia, evolutionary relict, phylogenetic relict, phylogenetic niche conservatism, evolutionary stasis, morphological conservatism, extinction filtering, natural selection, macroevolution, Rhynchocephalia, Squamata, Crocodylia

How to Cite?: Debanik Mukherjee, M. Vijay, "A Review on Why Some of the Reptilian Lineages are Relict Taxa?", Volume 15 Issue 8, August 2026, International Journal of Science and Research (IJSR), Pages: 1737-1742, https://www.ijsr.net/getabstract.php?paperid=SR26825114241, DOI: https://dx.doi.org/10.21275/SR26825114241

Download Citation: APA | MLA | BibTeX | EndNote | RefMan