Anapsids, Synapsids, and Diapsids

Introduction

One of the most important events in the evolution of vertebrate life was the development of the amniotic egg. The evolution of this type of egg was a primary difference between the ancient amphibians and primitive reptiles. The development of this type of egg was also instrumental in the successful radiation of these reptiles across the terrestrial landscape as water was no longer required for reproduction.

The evolutionary split of amniotes (from amphibians) occurred during the Carboniferous Period some 310 million years ago. There are three basic types of amniotes: the synapsids, many of which are referred to as Therapsida and consisting of all mammal-like species and their ancestors, and the sauropsids, which are divided into anapsids and diapsids and consist of all reptiles and their ancestors. There is still a great deal of paleontological debate regarding the classification of anapsids, synapsids, and diapsids due to the lack of fossils of sufficient diversity and relevant geological age.

Fast Facts

Pronunciation: The term “anapsid” (a-NAP-sid) is derived from modern Latin and means “no-arch”. “Synapsid” (sin-AP-sid) is derived from modern Latin and means “fused-arch.” “Diapsid” (di-AP-sid) is derived from modern Latin and means “two-arch.”

Time Period: Late Carboniferous (310 million years ago) to Current

Size: Synapsids — 30 mm to 33 m (1 in–108 ft) in length, Anapsids — 8 cm to 3 m (3 in–10 ft) in length, Diapsids — 16 mm to 60 m (0.6 in–200 ft) in length

Weight: Synapsids — 1.2 g to 180 ton (0.04 oz–397,000 lbs), Anapsids — 140 g to 900 kg (4.9 oz–2000 lbs), Diapsids — 1.6 g to 120 tons (0.06 oz–265,000 lbs)

Diet: Carnivorous, Omnivorous, Insectivorous and/or Herbivorous

Location: Worldwide

Lifespan: Up to several hundred years

Homologous Traits

Anapsids, diapsids, and synapsids share all traits that define the amniotes, which evolved as adaptations to the difficulty of life away from water. Amniotes are defined as tetrapod vertebrates that can lay “amniotic” eggs on land. These eggs are adapted to survive in water-free environments due to the waterproof shell and amniotic sac for gas regulation.

Amniotes evolved from primitive, thin-skulled amphibians, and are also noted for possessing two or more sacral rib pairs, an astragalus ankle bone, thick skins and strong nails, enlarged lungs, a four-legged gait, and a strong sternum. From these ancient species, the first reptiles evolved and split rapidly into different groups based on their number of skull openings (fenestrae).

The Sauropsida anapsids have no fenestrae, and the diapsids have two fenestrae. Other synapomorphies of the Sauropsida include a long femoral groove and a single coronoid bone on the lower jaw. Anapsids are composed of basal forms and the turtles, diapsids are comprised of the lizards, snakes, crocodilians, dinosaurs, pterosaurs, and birds. Synapsid mammal-like species and their descendents are characterized by the single fenestrae on each side of their heads as well as the bony bridge (the zygomatic arch) underneath which the chewing muscles pass.

Evolutionary Divergences

The origin of the amniotes is thought to have occurred as far back as the Middle Pennsylvanian epoch of the Carboniferous, some 310 million years ago. The oldest known species were found in Canada, and only in the last 10 or so years have paleontologists discovered that amniote species existed outside of North America, with the discovery of early stem-amniotes in Scotland and Europe. Very early after their first appearance, amniotes were divided into two main clades—– the sauropsids, consisting of anapsids and diapsids, and the synapsids. These amniote evolutionary divisions are generally based on temporal skull openings (fenestrae), which are thought to be an adaptation to reduce the weight of the skull and to have provided surfaces for jaw muscle attachment.

Fossil evidence is relatively strong for the synapsid species, and shows that this group diversified earlier than the other amniotes. After their appearance during the Carboniferous, synapsids rapidly became the most varied, abundant, and widely dispersed terrestrial animal. The mammal-like synapsids dominated the terrestrial landscape until the end of the Permian (252 million years ago) when a massive extinction event wiped out 95 percent of all animal life. It was this event that saw the sauropsids begin to dominate the significantly reduced synapsid species due to sauropsid's rapid recovery and radiation after the mass extinction.

The close of the Permian also heralded the start of the Mesozoic Era, otherwise known as the age of dinosaurs and the rise of the Sauropsida. The most basal sauropsids were the anapsid protorothyridids and the diapsid araeoscelids from the Late Carboniferous period (305 million years ago). These small animals shared the Carboniferous landscape with other abundant and successful tetrapods and synapsid (mammal-like) species. Rapid expansion in the sauropsid clade occurred in the Permian (299–252 million years ago). Although basal species were small, by the end of the Permian, sauropsids developed into quite large species (although they did not rival the later dinosaur relatives). The early sauropsid species, such as Proterosuchidae, gave rise to the highly successful archosaurs during the Triassic period, who dominated until the end of the Mesozoic (66 million years ago). Basal sauropsids are thought to have possessed a primitive and simple anapsid type skull, in which no fenestrae were present. Paleontologists believe that lighter, more complex diapsid skulls with two fenestrae evolved from these basal and unmodified heads. Development of these skull adaptations was an evolutionary advantage as they increased jaw strength by allowing muscles to attach to the fenestrae openings and at the same time decreased skull weight. These improvements not only increased feeding efficiency, but also enhanced speed and locative ability.

The close of the Cretaceous was witness to another mass extinction event that wiped out more than 50 percent of all animal life and significantly reduced the reptilian sauropsids, particularly evident with the demise of all non-avian dinosaurs. Although some sauropsid species survived this event, the close of the Mesozoic saw the arrival of the Cenozoic Era, and once again the synapsid-derived mammals became the dominant species of land-living vertebrates.

Creatures in This Group

Amniotes are tetrapod vertebrate that first appeared about 310 million years ago during the Carboniferous period and evolved from reptile-like amphibians. Amniotes are divided into the sister clades of Synapsida, which include all mammals and their extinct relatives more closely related to mammals than reptiles, and Sauropsida, which include all reptiles, dinosaurs and birds and their extinct relatives. Synapsida and Sauropsida last shared a common ancestor about 320 million years ago.

Synapsida are currently the dominant and largest life-form on land (and in the oceans). This group consists of the earliest and most primitive pelycosaurs, such as the “sail-back” Dimetrodon and herbivorous Caseidae, and the more advanced therapsids, which include all the modern-day mammals and their ancestors. The most basal synapsid was a Canadian ophiacodontid dating back almost 320 million years, while the earliest known therapsid is thought to be Tetraceratops insignis, which dates to the Early Permian, about 280 million years ago.

After their appearance during the Carboniferous, most sauropsids remained as small primitive lizard-like creatures. Most researchers believe that basal sauropsids were small lizard-like animals, although much more morphologically primitive than lizards seen today. Sauropsida is comprised of the amniote classes Anapsida and Diapsida, which are differentiated by their different number of temporal skull openings. The anapsids includes the Testudines (turtles), as well as many extinct groups, such as the millerettids, procolophonoids, pareiasaurs, Acleistorhinus, lanthanosuchids, and nyctiphruretians. The diapsids include Lepidosauromorpha (snakes and lizards) and Archosauromorpha (crocodilians, pterosaurs, dinosaurs, and birds), with the earliest diapsid represented by the tiny lizard-like Petrolacosaurus.

Ecology

Based on current classification, sauropsids and synapsid are very varied and include species of turtles, lizards, snakes, crocodilians, dinosaurs, pterosaurs, birds, and mammals. Such a wide array of species are ecologically, morphologically, anatomically, and behaviorally diverse and can be found in various aquatic, terrestrial and aerial habitats and ecological niches. Reproductive behavior is also varied, with most Sauropsida (the anapsids and diapsids) being oviparous, meaning they lay eggs, and most synapsids being viviparous, meaning they give birth to live young (with the exception of the monotreme mammals).

The development of the amniotic egg was not just instrumental in terms of evolutionary advancement and greater sophistication; it was also responsible for the successful dispersal of sauropsids and synapsids into a diverse range of habitats. Wide distribution into various landscapes also reduced competition for limited food resources and space.

The evolutionary move into the terrestrial landscape also influenced the adaptation of new feeding mechanisms and diets. Synapsid, in particular, developed highly advanced specialization of their teeth to accommodate the varied diets (from carnivorous to herbivorous and everything in-between). It is thought by some paleontologists that this highly efficient dental differentiation helped the synapsids eventually out-compete the anapsids and diapsids.

In addition to efficient and diverse feeding behavior, perhaps the most remarkable synapsid divergence from the sauropsids was the development of mammalian auditory adaptations. The modification of the synapsid lower jaw not only improved feeding ability, but also coincided with the evolutionary modification of tiny jawbones into the three bones of the mammalian inner ear, specifically the malleus, incus, and stapes. These bones improved hearing substantially and had a significant impact on mammalian life and survival.

Paleontology News

Although synapsids and sauropsids were introduced as one of the main subclasses of reptiles a number of years ago, phylogenetic classification remains unsettled and in many cases unstable.

The paucity of the fossil record has meant that understanding the evolutionary relationships between the early members of the Sauropsida, specifically the anapsids and diapsids, remains difficult and controversial. Currently, however, basal sauropsids are thought to have possessed a primitive and simple anapsid type skull, in which no fenestrae were present, and paleontologists believe that lighter, more complex diapsid skulls with two fenestrae evolved from these basal and unmodified skulls.

Classification of synapsids has changed significantly in recent years, with many of the historical groups found to be paraphyletic. Currently, most researchers state that pelycosaurs are almost certainly paraphyletic and are the most basal synapsid form.

Bibliography

Books

Benton, Michael J. Vertebrate Palaeontology. Oxford: Wiley-Blackwell, 2004.

  • This text provides detailed information about amniote divisions, including sauropsids and synapsids, and early vertebrate evolution and radiation.

Cloudsley-Thompson, John L. Ecology and Behaviour of Mesozoic Reptiles. London: Springer, 2010.

  • This insightful text provides useful information on the reptiles of the world, particularly focusing on ecology, physiology and behavior.

De Iuliis, Gerardo, and Pulerà Dino. Dissection of Vertebrates. Oxford: Academic Press, 2010.

  • This text is a great resource on all vertebrate life, including diapsids and anapsids (that is, Sauropsida species), with a particular focus on vertebrate anatomy.

Fastovsky, David E., and David B. Weishampel. Dinosaurs: A Concise Natural History. London: Cambridge University Press, 2009.

  • A book aimed at non-specialists and those with little understanding or background in dinosaurs, it focuses on many aspects of the natural sciences and how they relate to dinosaur biology, evolution, life history, and classification.

Kemp, T. S. Origin and Evolution of Mammals. Oxford: Oxford University Press, 2005.

  • This text details the evolution, adaptation, and radiation of the synapsids, including a thorough examination of the basal pelycosaurs and the more advanced therapsids.

Laurin, Michel. How Vertebrates Left the Water. Berkeley: University of California Press, 2010.

  • This book provides a well illustrated look at the paleontology, geology, physiology, and comparative anatomy of the amniotes and vertebrates.

Martin, Anthony J. Introduction to the Study of Dinosaurs. Malden, MA: Blackwell, 2006.

This book is exactly as the title claims, a comprehensive and up-to-date introduction to dinosaurs, providing scientific-based chapters on the major dinosaur clades, as well as anatomical, physiological and behavioral information.

Weishampel, David B., Peter Dodson, and Halszka Osmólska. Dinosauria. Berkeley: University of California Press, 2007.

This book is an in-depth text providing resources and scientific papers on dinosaurs, especially looking at the saurichia and ornithisia orders in regards to their evolution, distribution, and ecology.

Journals

Laurin, Michel, and Robert R. Reisz. “A Reevaluation of Early Amniote Phylogeny.” Zoological Journal of the Linnean Society 113 (1995): 165-223.

Modesto, Sean P., and Jason S. Anderson. “The Phylogenetic Definition of Reptilia.” Systematic Biology 53.5(2004): 815-21.