The evolution of mammals has passed through many stages since the first appearance of their synapsid ancestors in the Pennsylvanian sub-period of the late Carboniferous period. By the mid-Triassic, there were many synapsid species that looked like mammals. The lineage leading to today's mammals split up in the Jurassic; synapsids from this period include Dryolestes, more closely related to extant placentals and marsupials than to monotremes, as well as Ambondro, more closely related to monotremes. Later on, the eutherian and metatherian lineages separated; the metatherians are the animals more closely related to the marsupials, while the eutherians are those more closely related to the placentals. Since Juramaia, the earliest known eutherian, lived 160 million years ago in the Jurassic, this divergence must have occurred in the same period.
After the Cretaceous–Paleogene extinction event wiped out the non-avian dinosaurs (birds being the only surviving dinosaurs) and several mammalian groups, placental and marsupial mammals diversified into many new forms and ecological niches throughout the Paleogene and Neogene, by the end of which all modern orders had appeared.
The synapsid lineage became distinct from the sauropsid lineage in the late Carboniferous period, between 320 and 315 million years ago. The only living synapsids are mammals, while the sauropsids gave rise to today's reptiles; to the dinosaurs, hence in turn to birds; and to all the extinct amniotes more closely related to them than to mammals. Primitive synapsids were traditionally called "mammal-like reptiles" or "pelycosaurs", but both are now seen as outdated and disfavored paraphyletic terms, since they were not reptiles, nor part of reptile lineage. The modern term for these is "stem mammals", and sometimes "protomammals" or "paramammals".
Throughout the Permian period, the synapsids included the dominant carnivores and several important herbivores. In the subsequent Triassic period, however, a previously obscure group of sauropsids, the archosaurs, became the dominant vertebrates. The mammaliaforms appeared during this period; their superior sense of smell, backed up by a large brain, facilitated entry into nocturnal niches with less exposure to archosaur predation. (Conversely, mammaliaforms' success in these niches may have prevented archosaurs from becoming smaller or nocturnal themselves.) The nocturnal lifestyle may have contributed greatly to the development of mammalian traits such as endothermy and hair. Later in the Mesozoic, after theropod dinosaurs replaced rauisuchians as the dominant carnivores, mammals spread into other ecological niches. For example, Castorocauda became aquatic, Volaticotherium were gliders, and Repenomamus even fed on juvenile dinosaurs.
Most of the evidence consists of fossils. For many years, fossils of Mesozoic mammals and their immediate ancestors were scarce and fragmentary. However, since the mid-1990s, numerous significant discoveries particularly in China have greatly expanded knowledge in this area. The relatively new techniques of molecular phylogenetics have also shed light on some aspects of mammalian evolution by estimating the timing of important divergence points for modern species. When used carefully, these techniques often, but not always, agree with the fossil record.
Although mammary glands are a signature feature of modern mammals, little is known about the evolution of lactation as these soft tissues are not often preserved in the fossil record. Most research on mammalian evolution focuses on tooth morphology, as teeth are among the most durable parts of the tetrapod skeleton. Other important research characteristics include the evolution of the middle ear bones, erect limb posture, a bony secondary palate, fur, hair, and endothermy.
Definition of "mammal"
While living mammal species can be identified by the presence of milk-producing mammary glands in the females, other features are required in classifying fossils, because mammary glands and other soft-tissue features are not visible in fossils.
One such feature available for paleontology, shared by all living mammals (including monotremes), but not present in any of the early Triassic therapsids, is shown in Figure 1 (on the right), namely: mammals use two bones for hearing that all other amniotes use for eating. The earliest amniotes had a jaw joint composed of the articular (a small bone at the back of the lower jaw) and the quadrate (a small bone at the back of the upper jaw). All non-mammalian tetrapods use this system including amphibians, turtles, lizards, snakes, crocodilians, dinosaurs (including the birds), ichthyosaurs, pterosaurs and therapsids. But mammals have a different jaw joint, composed only of the dentary (the lower jaw bone, which carries the teeth) and the squamosal (another small skull bone). In the Jurassic, their quadrate and articular bones evolved into the incus and malleus bones in the middle ear.Mammalia: Overview – Palaeos Mammals also have a double occipital condyle: they have two knobs at the base of the skull that fit into the topmost neck vertebra, while other tetrapods have a single occipital condyle.
In a 1981 article, Kenneth A. Kermack and his co-authors argued for drawing the line between mammals and earlier synapsids at the point where the mammalian pattern of molar occlusion was being acquired and the dentary-squamosal joint had appeared. The criterion chosen, they noted, is merely a matter of convenience; their choice was based on the fact that "the lower jaw is the most likely skeletal element of a Mesozoic mammal to be preserved." Today, most paleontologists consider that animals are mammals if they satisfy this criterion.
Ancestry of mammals
Amniotes The first fully terrestrial vertebrates were reptilian amniotes — their eggs had internal membranes that allowed the developing embryo to breathe but kept water in. This allowed amniotes to lay eggs on dry land, while amphibians generally need to lay their eggs in water (a few amphibians, such as the common Suriname toad, have evolved other ways of getting around this limitation). The first amniotes apparently arose in the middle Carboniferous from the ancestral reptiliomorphs.Carroll R.L. (1991): The origin of reptiles. In: Schultze H.-P., Trueb L., (ed) Origins of the higher groups of tetrapods — controversy and consensus. Ithaca: Cornell University Press, pp 331–353.
Within a few million years, two important amniote lineages became distinct: synapsids, from which mammals are descended, and sauropsids, from which lizards, snakes, turtles/tortoises, crocodilians, dinosaurs, and birds are descended. The earliest known fossils of synapsids and sauropsids (such as Archaeothyris and Hylonomus, respectively) date from about 320 to 315 million years ago. The times of origin are difficult to know, because vertebrate fossils from the late Carboniferous are very rare, and therefore the actual first occurrences of each of these types of animal might have been considerably earlier than the first fossil.
Synapsids
Synapsid skulls are identified by the distinctive pattern of the holes behind each eye, which offered the following advantages: made the skull lighter without sacrificing strength. saved energy by using less bone. probably provided attachment points for jaw muscles. Having attachment points further away from the jaw made it possible for the muscles to be longer and therefore to exert a strong pull over a wide range of jaw movement without being stretched or contracted beyond their optimum range.
A number of creatures often – and incorrectly – believed to be dinosaurs, hence part of the reptile lineage and sauropsids, were in fact synapsids. This includes the well-known Dimetrodon.
Terms used for discussing non-mammalian synapsids When referring to the ancestors and close relatives of mammals, paleontologists also use the following terms of convenience:
Pelycosaurs — all synapsids, and all of their descendants, except for therapsids – the eventual ancestor of mammals. The pelycosaurs included the largest land vertebrates of the Early Permian, such as the 6 metre (20 foot)-long Cotylorhynchus hancocki. Among the other large pelycosaurs were Dimetrodon grandis and Edaphosaurus cruciger. Stem mammals (sometimes called protomammals or paramammals, and previously called mammal-like reptiles) — all synapsids, and all of their descendants, except for mammals themselves. Stem mammals therefore include all pelycosaurs, and also all non-mammalian therapsids. Traditionally these were known as "mammal-like reptiles", but this is incorrect; terms such as "stem mammal" are preferred instead, because these synapsids were neither reptiles nor even part of reptile lineage.
Therapsids
Therapsids descended from sphenacodonts, a group of primitive synapsids, in the middle Permian, and took over from them as the dominant land vertebrates. They differ from earlier synapsids in several features of the skull and jaws, including larger temporal fenestrae and incisors that are equal in size.
The therapsid lineage then went through several stages, leading to the evolution of cynodonts in the late Permian, some of which had begun to resemble early mammals: gradual development of a bony secondary palate. Most books and articles interpret this as a prerequisite for the evolution of mammals' high metabolic rate, because it enabled these animals to eat and breathe at the same time. But some scientists point out that some modern ectotherms use a fleshy secondary palate to separate the mouth from the airway, and that a bony palate provides a surface on which the tongue can manipulate food, facilitating chewing rather than breathing. The interpretation of the bony secondary palate as an adaptation for chewing also implies the evolution of a faster metabolism, as chewing reduces food particle size, thereby accelerating digestion. In mammals, the palate is formed by two specific bones; however, various Permian therapsids possessed different combinations of bones positioned to serve a similar function. the dentary gradually becomes the main bone of the lower jaw.
Therapsid family tree A simplified phylogenetic tree showing only what is most relevant to the evolution of mammals is shown below:
Only the dicynodonts, therocephalians, and cynodonts survived into the Triassic.
Biarmosuchia The Biarmosuchia were the most primitive and pelycosaur-like of the therapsids.
Dinocephalians Dinocephalians ("terrible heads") included both carnivores and herbivores. They were large; Anteosaurus was up to long. Some of the carnivores had semi-erect hindlimbs, but all dinocephalians had sprawling forelimbs. In many ways they were very primitive therapsids; for example, they had no secondary palate and their jaws were rather "reptilian".
Anomodonts
The anomodonts ("anomalous teeth") were among the most successful of the herbivorous therapsids — one sub-group, the dicynodonts, survived to the end of the Triassic. But dicynodonts were very different from modern herbivorous mammals, as their only teeth were a pair of fangs in the upper jaw (lost in some derived kannemeyeriiformes) and it is generally agreed that they had beaks like those of birds or ceratopsians.
Theriodonts The theriodonts ("beast teeth") and their descendants had jaw joints in which the articular bone of the lower jaw tightly gripped the very small quadrate bone of the skull. This allowed a much wider gape and allowed one group, the carnivorous gorgonopsians ("gorgon faces"), to develop "sabre teeth". However, the jaw hinge of the theriodont had a longer term significance — the much reduced size of the quadrate bone was an important step in the development of the mammalian jaw joint and middle ear.