Phylogeny is the study of evolutionary relationships, creating a family tree that illustrates how different species are connected through shared ancestors. Within this tree, one major group is the amniotes, which includes all mammals, reptiles, and birds. The feature that unites these animals is the amniotic egg, a development that allowed their ancestors to reproduce on land and colonize diverse terrestrial environments.
The Amniotic Egg
The evolution of the amniotic egg freed vertebrate life from dependency on water for reproduction. This structure provided a self-contained aquatic environment, allowing the embryo to develop safely on dry land. Its design incorporates four specialized membranes that support the growing organism.
The amnion forms a fluid-filled sac that cushions the embryo, while the yolk sac contains a rich supply of nutrients. The chorion and allantois work together to manage gas exchange and waste. This complete life-support system permitted amniotes to lay their eggs in a much wider range of environments.
Major Branches of the Amniote Tree
Shortly after amniotes first appeared around 340 million years ago during the Carboniferous period, they split into two distinct lineages. This divergence created the two major branches of the amniote family tree: the Synapsida and the Sauropsida. Every mammal, reptile, and bird alive today belongs to one of these two ancient groups.
The synapsid lineage is the branch that led to modern mammals and their extinct relatives. The sauropsid lineage gave rise to all living reptiles—such as lizards, snakes, crocodiles, and turtles—as well as birds and extinct dinosaurs.
Classifying Amniotes by Skull Structure
For many years, the primary method for classifying amniotes relied on skull anatomy, specifically the presence and number of openings behind the eye socket known as temporal fenestrae. These openings allowed jaw muscles to expand and lengthen, enabling more powerful biting. Based on this feature, amniotes were traditionally sorted into three main groups.
The anapsid skull, which has no temporal openings, is considered the ancestral condition for amniotes. This solid skull structure was historically used to classify turtles, placing them as a very early and distinct offshoot of the reptile lineage.
The synapsid skull features a single lower temporal opening behind each eye and is the defining characteristic of the Synapsida lineage. The third type is the diapsid skull, which has two temporal openings—an upper and a lower one—behind the eye socket. This structure is characteristic of the Sauropsida, which later diverged into groups such as the lepidosaurs (modern lizards and snakes) and the archosaurs (crocodiles and birds).
The Evolving Understanding of Amniote Relationships
The classic understanding of amniote phylogeny based on skull morphology has been updated with new evidence. Genetic and molecular analysis has provided a new tool for mapping evolutionary relationships, challenging long-held classifications based on fossil anatomy.
The most prominent example of this shift is the re-evaluation of the turtle’s place in the amniote tree. Traditionally, turtles were classified as anapsids because their skulls lack temporal fenestrae. However, a wealth of molecular data has overturned this view, showing that turtles are not primitive anapsids but are instead highly modified diapsids.
This evidence indicates that the ancestors of turtles originally possessed the two temporal openings characteristic of diapsids but lost them over the course of their evolution. This means the anapsid-like skull of a turtle is a secondarily evolved trait, not an ancestral one. Modern phylogenetic consensus places turtles firmly within the diapsid group, most closely related to archosaurs like birds and crocodiles.