Diversification of Amniotes

by

Neil Brocklehurst

• LAST REVIEWED: 24 April 2023
• LAST MODIFIED: 24 April 2023
• DOI: 10.1093/obo/9780199941728-0146

Introduction

The amniotes are a clade represented today by the mammals, reptiles, and birds. Together they include more than 25,000 species living today, and a fossil record stretching back to the Carboniferous, 315 million years ago. Amniotes are united by their reproduction; unlike non-amniote tetrapods (hereafter referred to as “amphibians”), they lack a larval stage and protect the embryo by the amniotic egg. The amniotic egg includes extra-embryonic membranes such as the amnion, and structures that permit an exchange of gases and waste between the embryo and the atmosphere. The evolution of this egg gave their reproduction complete independence from water, allowing a fully terrestrial lifestyle to evolve. Along with this adaptation, the shift to a fully terrestrial lifestyle included evolutionary modifications for breathing, feeding, locomotion, and musculature. Amniotes originated during the latter stages of the Carboniferous, between 315 and 330 million years ago according to molecular clock estimates. Their earliest divergence represents the split into two lineages: the Synapsida (represented in the present by the mammals) and the Reptilia or Sauropsida (represented today by the reptiles and birds). Subsequently these two lineages rapidly diversified into a great diversity of species and ecologies that is seen in the fossil record and the present day. Vertebrate terrestrialization is a classic example of adaptive zone invasion followed by phenotypic diversification, and provides a natural laboratory for studying patterns of diversification during such adaptive radiations. Study of these topics has included examinations of species richness and diversification, morphological diversity, as well as in-depth examination of the key innovations that have made amniotes so successful. All this work has been underpinned by more than three decades of analyses of phylogeny and more than a century of work on the fossil record. These researches have given an extensive literature into the diversification of amniotes.

General Overviews

Probably the earliest detailed overview of amniotes and their evolution may be found in Romer’s Osteology of the Reptiles (1956). It should be noted that the title’s reference to “Reptiles” represents pre-cladistic classifications, where mammals were judged to have evolved from reptiles; as such this book includes descriptions of mammal-line amniotes outside the crown group Mammalia. While primarily an anatomical book, it does represent the first detailed summary of the opinions of their evolution and relationships at that time. Summaries of morphology and taxonomy can also be found in the book series Handbuch der Paläoherpetologie (Encyclopedia of Paleoherpetology) edited by Kuhn et al. since 1969. Each volume details the evolutionary history of a particular taxonomic group and provides complete list of extinct taxa (although again it should be noted these groups are pre-cladistic). A more up-to-date summary incorporating the cladistic thinking on origins of amniotes may be found in the collated volume Sumida and Martin 1997. The book contains contributions documenting numerous perspectives on the evolution of terrestriality in amniotes, including phylogenetic, biogeographic, feeding, the amniotic egg, and morphological changes. The same year, Reisz 1997 summarized early amniote evolution in the review journal Trends in Ecology and Evolution, briefly detailing hypotheses on the origin of terrestriality and the amniotic egg, the early fossil record of the group, and their phylogeny. Another key edited volume is Carrano et al. 2006. Although detailing amniote diversification from a purely paleontological perspective, the book does include multiple aspects, including phylogeny, evolutionary processes, functional morphology, and ontogeny. More recent summaries include Vitt and Caldwell 2014, a chapter summarizing the relationships and biology of the tetrapods (the limbed vertebrates, the larger group containing amniotes). While this is a broad chapter, detailing the many topics outside the scope of amniote evolution, the subsection “Evolution of Early Amniotes” details the early members of this lineage, their relationships, and the key innovations that characterized their origins. The entry Sues 2016 in the Encyclopedia of Evolutionary Biology gives a general assessment of many aspects of amniote evolution and diversification, including brief overviews of the major lineages, as well as more general topics such as the skull fenestration features that characterize the major lineages, the nature of the amniotic egg, and other features such as of the skin and breathing that separate amniotes from their “amphibian” ancestors.

• Carrano, M. T., T. J. Gaudin, R. W. Blob, and J. R. Wible, eds. 2006. Amniote paleobiology: Perspectives on the evolution of mammals, birds, and reptiles. London: University of Chicago Press.

  An edited volume containing fifteen papers intended to represent, at that time, the most up-to-date opinions and theories regarding evolution derived from the amniote fossil record. Provides perspectives on four aspects: New Fossils and Phylogenies; Large-Scale Evolutionary Patterns; Functional Morphology; Ontogeny and Evolution. These aspects are discussed with examples and case studies from a range of groups within amniotes.

• Kuhn, O., H.-D. Sues, P. Wellnhofer, R. M. Appleby, eds. 1969–. Handbuch der Paläoherpetologie. Stuttgart: Gustav Fischer Verlag.

  A collection of encyclopedias documenting the fossil record of reptiles, nonmammalian synapsids, and amphibians. There are currently twenty-four volumes, each detailing the evolution of a particular group, although the groups are in some cases out of date. Each includes sections devoted to morphology, evolutionary history, and phylogeny, as well as documenting the complete (at the time of writing) fossil record of the group. These are invaluable references for researchers, even if they are somewhat out of date. Most volumes are in English, some in German.

• Reisz, R. R. 1997. The origin and early evolutionary history of amniotes. Trends in Ecology and Evolution 12:218–222.

  DOI: 10.1016/S0169-5347(97)01060-4

  A brief introduction to the early evolution of amniotes, summarizing the hypotheses on the origins of terrestriality, as well as their early fossil record and phylogeny.

• Romer, A.S. 1956. Osteology of the reptiles. Chicago: Chicago Univ. Press.

  An extremely detailed and comprehensive review of the anatomy of all the major amniote groups, excluding birds and mammals. Also contains a thorough overview of their evolution, as understood at the time of publishing. Evolutionary interpretations somewhat out of date, but anatomical detail is unsurpassed. The go-to resource for amniote morphology.

• Sues, H.-D. 2016. Amniotes, diversification of. In Encyclopedia of evolutionary biology. Vol. 1. Edited by R. M. Kliman, 56–62. Oxford: Academic Press.

  A thorough and up-to-date overview of numerous aspects of amniote evolution, including the morphological aspects of terrestrialization, early amniote phylogeny, and their initial fossil record. Detailed, but understandable by a non-expert.

• Sumida, S., and K. L. Martin. 1997. Amniote origins: Completing the transition to land. San Diego, CA: Academic Press.

  Twelve chapters representing six aspects of amniote evolution: phylogenetics, biogeography, feeding, the amniotic egg, morphological changes, and integrative biology. Detailed summaries of past research intended for experts.

• Vitt, L. J., and J. P. Caldwell. 2014. Tetrapod relationships and evolutionary systematics. In Herpetology: An introductory biology of amphibians and reptiles. Edited by G. R. Zug, L. J. Vitt, and J. P. Caldwell, 5–32. San Diego, CA: Academic Press.

  The chapter details the evolution of tetrapods, including a section on the origin of amniotes. Despite the section of relevance being a subsection of a single chapter, there is considerable scope regarding the morphological changes in amniotes needed for terrestriality. Can be understood by non-experts.