Did mammals evolve from reptiles? The truth is they didn’t. Reptiles and mammals both have independent evolutionary histories that separated soon after the apparition of the so-called amniotic egg, which allowed the babies of these animals to be born outside of water. Previously, we talked about the origin of vertebrates and about how they managed to get out of the sea to start walking on land for the first time. In this entry we’ll explain how the ancestors of reptiles and mammals, the AMNIOTES, became independent of the aquatic medium and became the dominant land animals.
THE AMNIOTIC EGG
The characteristic that unites reptiles and mammals in the same group is the amniotic egg. While amphibian eggs are relatively small and only have one inner membrane, the eggs of amniotes are much bigger and present various membranes protecting the embryo and keeping it in an aqueous medium. The outer layer is the eggshell which, apart from offering physical protection to the embryo, prevents water loss and its porosity allows gas interchange. Beneath the eggshell we can find the next membranes:
Diagram of a crocodile egg: 1. eggshell 2. yolk sac 3. yolk (nutrients) 4. vessels 5. amnion 6. chorion 7. air 8. alantois 9. albumin (white of the egg) 10. amniotic sac 11. embryo 12. amniotic fluid. Image by Amelia P.
- Chorion: The first inner membrane, which offers protection and, together with the amnion, forms the amniotic sac. Also, being in contact with the eggshell, it participates in gas interchange, bringing oxygen from the outside to the embryo and carbon dioxide from the embryo to the outside.
- Amnion: Membrane that surrounds the embryo and constitutes a part of the amniotic sac. It offers an aqueous medium for the embryo and connects it with the yolk sac (a structure that brings food and that is also found in fish and amphibians).
- Allantois: The third layer, it is used as a storage for nitrogen waste products, and together with the chorion, helps in gas interchange.
Diagram of an amphibian egg: 1. jelly capsule 2. vitelline membrane 3. perivitelline fluid 4. yolk 5. embryo. Image by Separe3g.
All these different kinds of membranes eliminate the need amphibians had of laying their eggs in water. Also, unlike amphibians, amniotes don’t go through a gilled larval stage, but are instead born as miniature adults, with lungs and legs (at least those that have them). All these made the first amniotes completely independent of the aquatic medium.
The first amniotes evolved around 312 million years ago from reptiliomorph tetrapods. At the end of the Carboniferous period lots of tropical forests where the great primitive amphibians lived disappeared, leaving a colder and drier climate. This ended with many of the big amphibians of that time, allowing the amniotes to occupy new habitats.
Reconstruction of Solenodonsaurus janenschi, one of the candidates in being the first amniote, which lived around 320-305 million years ago in what is now the Czech Republic. Reconstruction by Dmitry Bogdanov.
These early amniotes had a series of characteristics that set them apart from their semiaquatic ancestors:
- Horny claws (amphibians don’t have claws) and keratinized skin that prevents water loss.
- Bigger large intestine and higher density of renal tubules to increase water reabsorption.
- Specialized lacrimal glands and a third membrane in the eye (nictitating membrane) which keep the eye wet.
- Larger lungs.
- Loss of the lateral line (sensory organ present in fish and amphibians).
The skeleton and musculature also evolved offering better mobility and agility on a terrestrial medium. The first amniotes presented ribs that encircled their body converging at the sternum, making their inner organs more secure, and a series of muscular receptors offered them better agility and coordination during locomotion.
Traditionally, the different amniotes were classified based on the structure of their cranium. The characteristic used to classify them was the presence of temporal openings (fenestrae), by which we have three groups:
- Anapsids (“no arches”): No temporal openings (turtles).
Diagram of an anapsid skull, by Preto(m).
- Synapsids (“fused arches”): With only one temporal opening (mammals).
Diagram of a synapsid skull, by Preto(m).
- Diapsids (“two arches”): With two temporal openings (reptiles, including birds).
Diagram of a diapsid skull, by Preto(m).
Previously it was believed that the first amniotes presented an anapsid skull (without openings, like turtles) and that subsequently they separated into synapsids and diapsids (the temporal openings formed “arches” that offered new anchor points for the jaw’s musculature). Yet, it has been discovered that this three-group classification is not valid.
Even though we still believe that the first amniotes were anapsid, it is currently known that these, soon after their apparition, separated into two different lineages: the synapsids (clade Synapsida) and the sauropsids (clade Sauropsida).
This lineage includes mammals and their amniote ancestors. Even though the first synapsids like Archaeothyris looked externally like lizards, they were more closely related to mammals, as they shared one temporal fenestrae where the jaw muscles passed through.
Drawing of the skull of Archaeothyris, which is thougth to be one of the first synapsids that lived around 306 million years ago in Nova Scotia. Drawing by Gretarsson.
The ancestors of mammals were previously known as “mammal-like reptiles”, as it was thought that mammals had evolved from primitive reptiles. Currently it’s accepted that synapsids form a different lineage independent of reptiles, and that they share a series of evolutionary trends that makes them closer to modern mammals: the apparition of different kinds of teeth, a mandible made of one single bone, the vertical posture of their limbs, etc…
Reconstruction of Dimetrodon grandis, one of the better known synapsids, from about 280 million years ago. Reconstruction by Dmitry Bogdanov.
Even though most modern mammals don’t lay eggs and give birth to live offspring, all groups maintain the amniote’s three characteristic membranes (amnion, chorion and allantois) during embryonic development.
Sauropsids include current reptiles and their amniote ancestors. Currently, in many scientific papers the word “sauropsid” is used instead of “reptile” when discussing phylogenies, as the sauropsids also includes birds. The first sauropsids were probably anapsids, and soon after their appearance they separated into two groups: the Parareptilia which conserved anapsid skull, and the Eureptilia which include the diapsids (current reptiles and birds).
Evolutionary tree of current vertebrates, in which green color marks the groups previously included inside reptiles. As you can see, the traditional conception of "reptile" includes the ancestors of mammals and excludes birds. Image by Petter Bøckman.
Diapsids are currently the most diversified group of land vertebrates. They diversified greatly in the late Permian period (about 254 million years ago), just before the Mesozoic (the Age of Reptiles). These can be divided into two main groups: the Lepidsaurs and the Archosaurs, both with representatives in our days.
LEPIDOSAURIA: SMALL AND PLENTIFUL
Lepidosaurs (literally “reptiles with scales”) appeared in the early Triassic (around 247 million years ago) and, even if most of them didn’t grow to big sizes, they are currently the largest group of non-avian reptiles. These are characterized by presenting a transversal cloacal slit, by having overlapping scales and shedding their skin whole or in patches and by other skeletal characters.
Shed skin of a rat snake. Photo by Mylittlefinger.
The current lepidosaurs belong to one of two different orders:
- Order Rhynchocephalia: That includes the two species of tuatara. Currently endangered, they are considered living fossils because they present skulls and characteristics similar to the Mesozoic diapsids.
Photo of a tuatara (Sphenodon punctatus), by Tim Vickers.
- Order Squamata: Current squamates include iguanas, chameleons, geckoes, skinks, snakes and other legless lizards. With more than 9000 living species, squamates are a large group with a wide array of adaptations and survival strategies.
Photos of some squamates, from left to right and from top to bottom: Green iguana (Iguana iguana, by Cary Bass), king cobra (Ophiophaga Hannah, by Michael Allen Smith), Mexican mole lizard (Bipes biporus, by Marlin Harms) and Indian chameleon (Chamaeleo zeylanicus, by Shantanu Kuveskar).
ARCHOSAURIA: ANCIENT KINGS
Archosaurs (literally “ruling reptiles”) were the dominant group of land animals during the Mesozoic. These conquered all possible habitats until the extinction of most groups at the end of the Cretaceous period. Some of the extinct groups were the pseudosuchians (relatives of modern crocodiles, order Crocodylia), the pterosaurs (large flying reptiles) and the dinosaurs (excepting birds, clade Aves).
Drawing of the skull of the dinosaur Massospondylus in which we can see the different characteristic openings of diapsid archosaurs. Image by Steveoc 86.
As you see, both groups of modern archosaurs couldn’t be more different. Yet, crocodiles and birds share a common ancestor, and they are both more closely related with each other than with the rest of reptiles.
Photo of two species of modern arcosaurs: a Nile crocodile (Crocodylus niloticus) and a yellow-billed stork (Mycteria ibis). Photo by Tom Tarrant.
AND WHAT ABOUT TURTLES?
Turtles (order Testudines) have always been a group difficult to classify. Turtles are the only living amniotes with an anapsid skull, without any post-ocular opening. That’s why previously they had been classified as descendants of primitive amniotes (clade Anapsida, currently disused) or as primitive anapsid sauropsids (inside the Parareptilia clade)
Skeleton of the extinct tortoise Meiolania platyceps which lived in New Caledonia until 3000 years ago. In this photo it can be seen the compact cranium without openings. Photo by Fanny Schertzer.
Recent molecular studies have revealed that turtles are actually diapsids that lost their temporal openings secondarily. What still divides the scientific community is if testudines are more closely related to Lepidosauromorphs (lepidosaurs and their ancestors) or to Archosauromorphs (archosaurs and their ancestors).
Individual leopard tortoise (Stigmochelys pardalis) from Tanzania. Photo by Charles J. Sharp.
As you have seen, the evolution of amniotes is an extremely complex matter. We hope that with this entry some concepts have been clarified:
- Mammals (synapsids) come from an evolutionary lineage different from that of reptiles (sauropsids).
- Sauropsids include traditional reptiles (lepidosaurs, archosaurs and turtes) and birds (inside archosaurs).
- There’s still so much to investigate about the placement of turtles (testudines) in the evolutionary tree of sauropsids.
Modified diagram about the evolutionary relationships of the different amniote groups.
During the elaboration of this entry the following sources have been consulted:
- Cover image by Henri Pidoux.
- Halliday & Adler (2007). La gran enciclopedia de los Anfibios y Reptiles. Editorial Libsa.
- Hickman, Roberts, Keen, Larson, l’Anson & Eisenhour (2009). Principios integrales de Zoología (decimocuarta edición). Mc Graw Hill.