Diapsids are a large group of reptiles and birds, distinguished by a particular skull architecture. This shared anatomical trait, characterized by specific skull openings, has enabled their diverse adaptations and widespread presence across various environments.
The Skull’s Defining Fenestrae
The defining characteristic of diapsids is the presence of two temporal fenestrae, or openings, on each side of the skull, located behind the eye socket. These openings are positioned both above and below the post-orbital bone. This unique arrangement provides space for the attachment and bulging of larger, more powerful jaw muscles.
The openings also contribute to reducing the overall weight of the skull while maintaining structural integrity. This allows for a more efficient jaw mechanism, enabling diapsids to exert stronger bite forces and consume a wide range of food sources.
Diverse Diapsid Lineages
Diapsids encompass an extensive array of animals, both living and extinct, that share this distinctive skull structure. Modern reptiles and birds are classified within the Neodiapsida subclade, Sauria. This broad group is primarily divided into two main branches: Archosauromorpha and Lepidosauromorpha.
Archosauromorpha includes crocodilians, suchigators, and the extinct pterosaurs and dinosaurs, from which birds evolved. Lepidosauromorpha consists of lizards, snakes, and tuataras. While some diapsids, such as modern birds, snakes, and turtles, have highly modified or secondarily lost these skull openings, they are still classified as diapsids based on their ancestral skull anatomy.
An Evolutionary Success Story
Diapsids emerged approximately 300 million years ago during the late Carboniferous period, with the tiny, lizard-like Petrolacosaurus representing one of the earliest known diapsids. This group diversified significantly, occupying various ecological niches, including swimming, slithering, crawling, running, and even flying forms.
Diapsids dominated the Mesozoic Era, often referred to as the “Age of Dinosaurs.” Their continued prevalence is evident today with over 17,000 extant species of diapsid animals. This includes approximately 9,159 bird species and around 7,925 species of snakes, lizards, tuataras, turtles, and crocodiles.
Diapsids Among Other Amniotes
Diapsids are part of a larger group called amniotes, which are vertebrates characterized by an egg adapted for terrestrial environments. Amniotes diverged into two main lineages: synapsids and sauropsids. Synapsids, which include mammals and their extinct ancestors, are distinguished by having a single temporal opening low on each side of the skull.
Sauropsids, encompassing reptiles and birds, are further divided into anapsids and diapsids. Anapsids traditionally included early reptiles and turtles, characterized by skulls lacking temporal openings. However, molecular and fossil evidence indicates that turtles arose within the diapsid lineage and later lost their temporal fenestrae.