Evolution of mammals

The cynodontian Thrinaxodon , a very close relative of the mammals from the early Triassic.

The evolution of mammals is a gradual process that began with the separation of the Sauropsid and Synapsid lines sometime in the Upper Carboniferous more than 300 million years ago and continues to this day. Representatives who looked very similar to mammals already existed in the Middle Triassic . However, the first "real" mammals did not appear until the Upper Triassic or Lower Jurassic .

Sinodelphys , the earliest known marsupial mammal , lived in the Lower Cretaceous around125 million years ago atabout the same time as Eomaia , the first known higher mammal . Only two million years later the first monotonous Teinolophos appeared . After the non-avian dinosaurs fell victim tothe great mass extinction on the Cretaceous-Tertiary border (the birds are commonly considered to be descendants of the dinosaurs), the marsupial and the higher mammals were able tosplit into many new taxa and new ecological nichesduring the Tertiary take in. By the end of the tertiary all modern orders had developed.

From a phylogenetic perspective, mammals are the only surviving synapsids . The evolutionary line of the synapsids had separated from the Sauropsida (today's reptiles and birds) in the Upper Carboniferous at the latest 306 million years ago . During the Permian , the synapsids were the most common and at the same time the largest land vertebrate. In the Triassic, however, the archosaurs , a previously relatively insignificant group of the sauropsids, became the predominant group of vertebrates. A group of archosaurs - the dinosaurs - would then be the dominant group of terrestrial vertebrates for the rest of the Mesozoic Era. Due to these faunistic changes, the Mesozoic mammals were forced to resort to nocturnal ways of life. This niche change was likely to have been decisive in the development of typical mammalian characteristics such as endothermia , hairiness and large brain volume . It was not until the later Mesozoic era that some mammal species succeeded in penetrating other ecological niches. Examples include Castorocauda , who lived in the water, Volaticotherium , a glider, and Repenomamus , who even stalked small dinosaurs.

Most of our knowledge is based on fossil finds . For a long time, Mesozoic fossils of mammals and their immediate ancestors were very rare and fragmentary, but many important new finds have been made since the mid-1990s, particularly in China. Relatively new techniques of molecular phylogenetics could for their part elucidate some aspects of the mammalian evolution by contributing estimated ages for important points of divergence of modern species. If used carefully, these techniques tend to agree with the fossil record.

Although mammary glands are the defining characteristic of mammals, knowledge about the evolution of lactation is still poor. Practically nothing is known about the evolution of another characteristic, the neocortex . Much of the research in the field of mammalian evolution focuses on the development of the middle ear , which was formed from the temporomandibular joint of previous amniotes . Other areas of research are the placenta , upright limbs, ossified secondary palate , hair and body temperature.

Definition of "mammal"

Temporomandibular joints of mammals and non-mammals. In mammals, quadratum and articulars are much smaller and are part of the middle ear. In mammals, the lower jaw consists only of the dental .

Living mammals can be recognized in females by the milk-secreting mammary gland.

However, other criteria must be used to classify fossils, since mammary glands and other soft body features are usually not preserved in fossils. Paleontologists therefore make use of a distinguishing feature that is common to all living mammals (including the marsupial mammals), but does not occur in the Therapsids ("mammal-like reptiles") from the Lower Triassic: the middle ear . Two jaw bones of the amniotes have migrated into the middle ear in mammals in order to achieve better transmission of the acoustic signal. The earliest amniotes had a temporomandibular joint made up of the articular (a small bone at the rear end of the lower jaw) and the quadratum (a small bone at the rear end of the upper jaw). All non-mammalian amniotes ( lizards , crocodiles , dinosaurs with their descendants, birds , and therapsids) are characterized by this jaw joint. However, mammals use a different temporomandibular joint system, which is composed only of the cover bones Dentale (the tooth-bearing lower jawbone) and Squamosum (small skull bone). In mammals, the quadratum and the articular became the anvil (incus) and the hammer (malleus) in the middle ear.

In contrast to the other vertebrates with a simple occipital condyle , mammals have a double one: at their skull base there are two cones that fit into the uppermost cervical vertebra. To uniquely identify a mammal, paleontologists only use the temporomandibular joint in connection with the middle ear as criteria. Conceivable mixed forms (e.g. a mammalian jaw joint and middle ear with a non-mammalian occipital condyle) would only make the decision-making process unnecessarily difficult.

Because of mostly only minor, gradual changes in so-called transitional forms , it is often difficult to draw a precise line between reptiles and mammals.

The descent of the mammals

Then a somewhat simplified cladogram of the terrestrial vertebrates is shown, followed by more detailed descriptions of possible uncertainties and points of contention.

Land vertebrates (Tetrapoda)	Amniotes    Sauropsida (reptiles + birds)   Synapsids     Pelycosauria    Therapsids  Mammals      Amphibians

Amniotes

The first permanent land-dwelling vertebrates were the amniotes . Their eggs had internal membranes that allowed the growing embryo to breathe while still retaining moisture. Amniotes could therefore lay their eggs on land, whereas amphibians usually spawn their eggs in water (some amphibians, such as comb toads, have found other ways to circumvent this limitation). It seems that the first amniotes split off from their ancestors, the Reptiliomorpha , in the Upper Carboniferous .

Within only a few million years there were already two distinct evolutionary lines within the amniotes: that of the synapsids , from which the precursors of mammals and finally mammals emerged, and that of the sauropsids , from which lizards , snakes , crocodiles , dinosaurs and birds emerged . The first known fossil finds of synapsids ( e.g. Archaeothyris ) and sauropsids ( e.g. Hylonomus ) are between 320 and 315 million years old. It is difficult to determine their exact evolutionary period, since vertebrate finds from the Upper Carboniferous are extremely rare and their first appearance may have been much earlier ( Signor Lipps effect ).

Synapsids

The original synapsid skull has a temporal window in a relatively deep position behind each eye (lower right in this illustration).

Synapsid skulls are determined based on their characteristic arrangement of the temporal windows. The temporal windows did the following:

• They made the skull lighter without sacrificing its stability.
• They were energy efficient because not as much bone material had to be removed.
• They probably served as attachment points for the jaw muscles.

Attachment points further away from the jaw meant longer jaw muscles that could pull hard when the jaw was moved without being overstretched or overly contracted.

Fossil finds from rural dwellers from the Unterperm suggest that the Pelycosauria , an order of the synapsids, were the most common and largest terrestrial vertebrates in their time.

Therapsids

The Therapsids emerged from the Pelycosauria in the Middle Permian and now became the dominant rural dwellers. They differ from the Pelycosauria in several features on the skull and the jaw, such as: B. the larger temporal window and the incisors of equal length . The Therapsids went through several stages of development, whereby the original forms were still very similar to the Pelycosauria, but their terminal links can already be confused with mammals. They are characterized by the following features:

• An ossified secondary roof of the mouth is gradually formed.

Mostly this fact is interpreted to mean that it is a prerequisite for a high metabolic rate , since the animals could now eat and breathe at the same time. However, some scientists point out that modern cold-blooded animals have a secondary palate made of connective tissue to separate the mouth and airway and that a palate made of bone material for the tongue is an abutment for moving the food and thus supports the chewing process much more than independent breathing . The interpretation of the ossified secondary roof of the palate as facilitating the chewing process also suggests an increased metabolism, since chewing allows food to be digested more quickly. In mammals, the palate consists of two very specific bones. The Permian Therapsids only had a slightly different bone arrangement for the function of the palate.

• The dental gradually becomes the main bone of the lower jaw.
• The gradual straightening of the limbs is accompanied by an increase in endurance (avoidance of the carrier restriction ). This was a very slow and erratic conversion process. So all herbivorous Therapsids still have bent limbs. (Some late forms also had more straight extremities.) In the carnivorous Therapsids from the Permian, the front extremities were still bent and even in the Upper Permian the hind legs were still half curved. Even in the modern bag mammals , slightly curved limbs can still be found.
• In the Triassic period , the transition to the mammalian jaw and the middle ear takes place.
• The hairiness in Triassic Therapsids is entirely plausible, but there is no evidence of this in the Permian Therapsids (see below).
• Some scientists even bring signs of lactation in Permian Therapsids into play (see below).

Pedigree of the Therapsids

Cladogram taken over from Palaeos in simplified form; only those taxa relevant to the evolution of mammals are listed:

Therapsids	NN    Dinocephalia   Neotherapsida     Anomodontia    Dicynodontia   Theriodontia    Gorgonopsia   Therocephalia    Cynodontia ... mammals      Biarmosuchia

Only Dicynodontier, Therocephalia and Cynodontier still lived in the Triassic, all other taxa had previously become extinct (Perm-Triassic border).

Biarmosuchia

The Biarmosuchia were the most original Therapsiden and still had a great resemblance to the Pelycosauria.

Dinocephalia

The dinocephalia ("terrible heads") could reach the size of a rhinoceros, among them both herbivores and carnivores. Some of the carnivores had fairly upright hind legs, but the front limbs of all representatives of the Dinocephalia were still curved. In many respects they were very original Therapsids, for example they lacked a secondary roof of the mouth and their jaw structure was still very "reptile-like".

Anomodontia

Lystrosaurus , one of the few dicynodontic animals tosurvivethe mass extinction on the Permian-Triassic border

The anomodontia ("abnormal teeth") were the most successful of the herbivorous therapsids. The Dicynodontians , one of their subordinates, survived almost to the end of the Triassic . Anomodontia, however, looked very different from modern herbivorous mammals. Their only teeth were two large tusks on their upper jaw, and they also had a horn-like beak, not unlike that of birds or ceratopsids .

Theriodontia

The Theriodontia ("animal teeth") and their descendants were characterized by jaw joints, in which the articular of the lower jaw enclosed the small square of the skull very closely. This allowed the jaw to be opened further. This made z. B. the Gorgonopsia ("Gorgon faces"), a carnivorous suborder of the Theriodontia, to develop "saber teeth". But the temporomandibular joint of the Theriodontia had a much more far-reaching meaning - the large size reduction of the square was an important step in the development of the temporomandibular joint and the middle ear of mammals. The Gorgonopsia also still had primitive features:

• no ossified secondary roof of the mouth (in its place there were other bones with the same function),
• curved front legs,
• Hind legs that could be used both curved and upright.

The Therocephalia ("animal heads"), which appeared around the same time, had more mammalian-like features, for example their finger and toe bones were composed of the same number of limbs as in mammals (the same limb formula is still found today in primates and consequently also in hominids ) .

Cynodontia

The Cynodontier are a suborder of the Theriodontia, which also appeared in the Upper Permian . It includes the ancestors of all mammals and mammals. The trithelodontia family is generally considered to be the group of animals that included the ancestors of mammals. The very mammalian characteristics of the cynodontics include:

• progressive reduction in the number of mandibular bones,
• a secondary ossified palatal plate,
• Molars with a complex crown pattern,
• complete filling of the cranial cavity by the brain.

Multi -chambered animal burrows were discovered that contained up to 20 skeletons of the Lower Triassic Cynodontias Trirachodon . Presumably the animals perished during a flash flood . The rather complexly constructed chambers, which were shared by several individuals, suggest that the animals have a very pronounced social behavior.

Triassic faunal change

The catastrophic mass extinction on the Permian-Triassic border destroyed around 70 percent of all terrestrial vertebrate species and most of the land plants. The immediate result was the collapse of ecosystems and food chains . The recovery process took 6 million years. Surviving groups of animals were forced to fight a new contest for their traditional ecological niches . This also applied to the cynodontic animals, which very probably made up the dominant group of animals towards the end of the Permian.

The Cynodontier lost the battle of assertion in favor of a previously inconspicuous group of sauropsid animals, the Archosauria (includes crocodiles , dinosaurs and birds and their ancestors). The sudden change of fauna is often referred to as the "Triassic fauna change". Several hypotheses have been considered as an explanation. Most likely this may be that in the mostly arid Lower Triassic, the archosaurs were better equipped against evaporation problems and therefore it contributed a significant advantage over their competitors (all known sauropsids have a skin without glands and excrete uric acid from mammals and probably also Therapsid contrast, urine - to Keeping uric acid fluid requires less water than urine). The change of fauna in Triassic was slow. At the very beginning of the Triassic, the cynodonts were the main carnivores and Lystrosauria the most important herbivores, but by the Middle Triassic, the archosaurs had occupied all important niches of carnivores and herbivores.

However, it is possible that the Triassic fauna change was a decisive factor in the evolutionary development from the Cynodontians to the mammals. The descendants of the cynodontics could only survive as small nocturnal insectivores . This had the following effects:

• The development process that began with the Therapsids to differentiate teeth with precise jaw closure accelerated, as captured arthropods had to be held and their shells had to be broken open .
• A nocturnal way of life called for improved thermal insulation and thermoregulation , as the mammalian precursors now hunted for prey during the cool night.
• Improved hearing and smell became essential for survival.
  • The development of the mammalian middle ear accelerated and, as a result, of course that of the temporomandibular joint, as bones migrated from the jaw to the ear.
  • The increased brain praise for the auditory and smell senses in turn caused an overall percentage increase in the weight of the brain compared to the total body weight. Brain tissue also has an above-average energy requirement. The increased need for food due to the larger brain increased the evolutionary pressure for improved heat insulation, temperature control and food utilization.
• A side effect of the nocturnal lifestyle was a decrease in the quality of color vision. Two of the four opsins were lost - most mammals therefore only have relatively poor color vision , including demi-monkeys such as B. the lemurs .

From cynodontics to real mammals

Lots of uncertainties

Even if the Triassic fauna change may have accelerated the evolution of mammals, it did not necessarily make the work of paleontologists easier. Good mammal-like fossil finds are extremely rare, as they were usually not much larger than rats. Other reasons are:

• Mammal-like ones were tied to ecotopes, which rarely leave good fossils . Among terrestrial ecotopes, floodplains are best suited for fossil conservation, as seasonal floods very quickly embed dead animal bodies in a protective silt layer , which later condenses into sedimentary rock . Meadows are mostly dominated by medium-sized to large animals - an order of magnitude in which the Triassic Therapsids and mammal-like ones could not compete with the Archosauria .
• Before a possible fossilization, their delicate bone structure usually fell victim to destruction - caused by fungi and bacteria or by purely physical influences.
• Small fossils are more difficult to find and are also more susceptible to weathering and other natural processes before they are discovered. As recently as the early 1980s, paleontologists believed that all Mesozoic and mammalian mammals could fit in a couple of shoeboxes because they were essentially dental material, the most durable of all types of tissue. Since then, however, the number of Mesozoic mammal finds has increased sharply, from 116 genera in 1979 to 310 genera in 2007. At the same time, the quality of the finds has increased, as 18 Mesozoic mammals have almost completely preserved their skeleton.

It follows:

• It is often difficult to assign a Mesozoic mammal or mammal-like animal to a genus .
• All fossil finds of a single genus rarely result in a complete skeleton. It is therefore usually seldom possible to weigh up the degree of relationship between the individual genres, let alone to make a cladistic classification.

The evolution of mammals in the Mesozoic is therefore still full of uncertainties. The only thing that remains indubitable is the fact that the actual mammals first appeared in the Mesozoic Era.

Mammals or mammals (Mammaliaformes)?

One result of these uncertainties was the redefinition of the term "mammal" in paleontology. For a long time, a fossil was assigned to a mammal if it met the criteria of the temporomandibular joint / middle ear. More recently, however, paleontologists have been defining a "mammal" as the last common ancestor of monotremes , marsupial, and higher mammals and all of their descendants. They were therefore forced to introduce another clade of Mammaliaformes , which included all the animal groups that were closer to mammals than the more primitive Cynodontians, but which were nonetheless different from monotons, marsupials and higher mammals. Even if the majority of palaeontologists take this cladistic new approach, there are lateral thinkers; in their opinion, most problems are simply moved to a new clade without getting any closer to a solution. The clade Mammaliaformes now contains groups of animals with a typical mammalian jaw joint and groups of animals with a typical reptile joint. In addition, the redefinition of "mammal" and "mammalian" depends on the last common ancestors of both groups, which have not yet been found. Despite these objections, this post follows the prevailing view and regards most of the Mesozoic descendants of the Cynodontics as mammalian (Mammaliaformes) .

Cladogram - From Cynodontics to Real Mammals

(based on the article Mammaliformes - Palaeos )

Mammaliaformes	NN    Morganucodontidae   NN    Docodonta   NN    Hadrocodium   Symmetrodonta    Kuehneotheriidae      Mammals      Allotheria , Multituberculata

Multituberculata

Skull of Ptilodus , a Multituberculata

The Multituberculata (named after the many tubercles on their molars ) are often referred to as the "rodents of the Mesozoic Era". But this is more an example of convergent evolution than an actual kinship relationship. At first glance, the multituberculata also look very similar to mammals: Their temporomandibular joint consists of the dental and the squamosum, while the quadratum and articular are components of the middle ear. Their teeth are differentiated, show occlusion and have mammalian cusps. They have a zygomatic arch . The shape of their pelvis suggests that, like the marsupial mammals, they only gave birth to tiny young. The multituberculates were a very long-lived group of animals that survived 120 million years (from the Middle Jurassic around 160 million years ago to the Lower Oligocene around 35 million years ago). They are therefore likely to have been the most successful group of mammals.

On closer inspection, however, there are significant differences to modern mammals:

• Their "molars" have two parallel rows of tubercles, not to be compared with the tribosphenic (three-humped) molars of earlier mammals.
• They are chewed in a completely different way. Mammals chew with a sideways grinding movement, with the molars only closing tightly on one side of the cheek. The multituberculates could not perform a lateral chewing movement, rather they had to guide the teeth of the lower jaw in a backward movement against the teeth of the upper jaw while chewing.
• The forward part of the zygomatic arch consists of the maxilla (upper jawbone) and not the zygomatic bone ; rather, the cheekbone is just a small bone that engages in a small indentation in a protuberance of the maxillary bone.
• The squamosum does not belong to the brain skull .
• The rostrum ( snout ) appears completely different, it is more similar to the rostrum of Pelycosauria such as Dimetrodon . The snout of the Multituberculata is box-shaped - the two broad, flattened upper jaw bones form the sides, the nasal the lid and the large premaxilla the front cover.

In Morganucodontidae and other transitional forms, both temporomandibular joint types were present at the same time: Dentale - Squamosum (front) and Articulare - Quadratum (back).

Morganucodontidae

The Morganucodontidae first appeared in the fossil record in the Upper Triassic 205 million years ago. They are a prime example of a transitional form , as the dental squamosum jaw joint and the articular quadratum jaw joint are formed at the same time. They are among the earliest finds among the Mammaliaformes. They have been extensively studied as numerous individuals have been discovered from them.

Docodonta

Replica of Castorocauda . The fur and the adaptations to a swimming (broad, flattened tail; webbed) and burrowing way of life (stocky limbs and robust claws) are remarkable.

The most prominent member of the Docodonta is Castorocauda ("beaver tail"), which lived in the Middle Jurassic about 164 million years ago. Castorocauda was discovered in 2004 and scientifically described for the first time in 2006. It was neither a typical docodonta nor a real mammal, but it is an extremely important find for research into mammalian evolution, as the skull was completely preserved and the stereotypical image of mammals as "small nocturnal insectivors" was broken:

• Castorocauda is much larger than previously known mammals - about 43 centimeters from the nose to the tip of the tail (with a tail length of 13 centimeters) and should have weighed around 500 to 800 grams.
• It provides the first irrevocable evidence of hair and fur. Previously this was Eomaia , a real mammal about 125 million years old.
• It shows adaptations to the aquatic environment such as flattened tail vertebrae and tissue remnants between the toes of the hind legs, which can be interpreted as webbed feet. Before Castorocauda was found, the first mammal-like species with a semi-aquatic way of life came from the Eocene , i.e. around 110 million years younger.
• The strong front legs seem to be predisposed for a digging activity. This feature together with the appendages on the ankles are reminiscent of Platypus , who also practices a swimming and digging way of life.
• The teeth are obviously adapted to fish food: the first two molars had cusps arranged in a straight line, an arrangement that was more suitable for grabbing and parting than for chewing; they are also bent backwards in order to better hold slippery prey.

Hadrocodium

The cladogram shown above shows Hadrocodium only as the "aunt" of real mammals, whereas Symmetrodonta and Kuehneotheriidae are shown as sister groups of real mammals. We have only a few and very fragmentary fossils of the Symmetrodonta and the Kuehneotheriidae, so the two groups are not yet exactly known and perhaps also paraphyletic . However, there is good fossil material from Hadrocodium (around 195 million years old from the Lower Jurassic ) that has some very important characteristics:

• The temporomandibular joint is made up of the squamosum and the dental. In contrast to the Therapsids, the rear end of the lower jaw no longer has any other small bones.
• In Therapsids and in most mammalian forms , the eardrum spans a depression in the lower jaw. Hadrocodium no longer had this arrangement, which suggests that its ear belonged completely to the skull , as in real mammals , and that the articular and quadratum had migrated into the middle ear and became a hammer and anvil . However, the lower jaw still has an "indentation" at its rear end that has been lost in real mammals. This suggests that the lower jaw of Hadrocodium retained the same shape in spite of the lack of articulare and quadratums and that Hadrocodium or a very close relative was therefore the first animals with a fully developed, mammalian middle ear.
• In Therapsiden and the early Mammaliaformes the temporomandibular joint sat very far back on the skull. The ear was also at the back of the jaw, but at the same time it had to remain near the brain. This arrangement prevented expansion of the brain skull. In addition, the jaw muscles were forced to run across the skull. Since the supporting function for the ear ceased to exist with Hadrocodium, the brain skull and jaw were no longer tied to each other and the temporomandibular joint could move further forward. In the successors of Hadrocodium or in taxa with a similar structure, the brain skull could then expand further without being hindered by the jaw. The jaw, for its part, was now capable of changes, as it no longer had a supporting function for the ear - conclusion: the way for the mammal-like was cleared to develop a larger brain and to redesign the jaws and teeth purely for a specialized, more efficient feeding function.

The first real mammals

This stage of development brings with it further complications, since the real mammals are the only animal group with still living taxa:

• A distinction must therefore be made between extinct groups and groups with still living taxa.
• Attempts are often made to explain the evolution of non-fossilizable characteristics. This venture makes use of molecular phylogenesis , a technique that has been gaining popularity since the mid-eighties but is still controversial because of its premises, especially when it comes to the reliability of the molecular clock .

Cladogram of the early real mammals

(based on Mammalia: Overview - Palaeo ; a † indicates extinct taxa)

Mammals	Australosphenida    Ausktribosphenidae †      Monotremes   NN    Triconodonta †   NN    Spalacotheroidea †   Cladotheria    Dryolestoidea †   Theria    Marsupial (Metatheria)      Higher mammals or placenta animals (Eutheria)

Australosphenida and Ausktribosphenidae

With the taxon Ausktribosphenidae a group of strange fossils has been identified, which is characterized as follows:

• The group obviously has tribosphenic molars , which are otherwise known only in higher mammals and marsupials .
• These fossils were discovered in sites from the Middle Cretaceous Australia.

At that time, Australia was only attached to the Antarctic. The higher mammals had their origin in the northern hemisphere and were bound to it until the continental drift led to the formation of land bridges between North and South America, between Asia and Africa and between Asia and India.

• There are only skull and jaw fragments.

The Australosphenida taxon was introduced to accommodate the Ausktribosphenidae and the monotremes . Asfaltomylos from the Middle to Upper Jura of Patagonia is regarded as an original australosphenida that is said to have spread over large parts of Gondwana , the former southern hemisphere supercontinent. (Basal australosphenidae have characteristics that are shared by both the Ausctriboshenidae and the monotremes. However, they do not share the presence or absence of taxon-specific characteristics of the Ausktribosphenidae or the monastery.)

A recent study of the 115-million-year-old Teinolophos came to the conclusion that it was an advanced and relatively specialized cloister and that the original cloister must therefore be much older. In addition, some alleged Australosphenida ( e.g. Steropodon ) may also have been advanced monotremes. Other alleged Australosphenida (such as Ausktribosphenos , Bishops , Ambondro and Asfaltomylos ) are probably more closely related to the Theria (see below) or possibly belong to them.

Monotremes

The oldest known monastery is Teinolophos , who lived in Australia around 123 million years ago . However, recent studies have come to the conclusion that this is not a primitive monotony, but an already fully developed platypus . The evolutionary lines of platypus and ant urchins must have split off earlier and the original monotremes are even much older.

Monotremes are distinguished by features that may have been inherited from the original amniotes :

• As in lizards and birds, the same body opening is used for excreting urine and feces as well as for sexual reproduction. (The Latin term "monotremata" means "sole opening".)

• Like lizards, turtles and crocodiles, they lay leathery and non-calcified eggs .

In contrast to other mammals, female monotons do not have teats . They breastfeed their young by "sweating out" milk in several places in their stomach area.

These characteristics are irrelevant in fossils and paleontologists consequently use other characteristics:

• The lower jaw with a small or nonexistent crown process is slender.
• The outer opening of the ear is at the posterior base of the jaw.
• The cheekbone is small or absent.
• The shoulder girdle is originally reinforced with raven bones , collarbones and interclavicles . These do not occur with the Theria .
• The front legs are bent or half-bent.

Theria

The Theria ("wild animals") are a hypothetical group of animals from which both the Metatheria (including marsupials ) and the Eutheria (including higher mammals ) arose. Even if no unambiguous fossil finds of the original Theria have been made (only a few teeth and jaw fragments have been discovered), Metatheria and Eutheria still share some characteristics that suggest a common ancestral group of animals:

Ankle joint in the Theria. Slightly changed representation, with permission from Palaeos

• They have no interclavicles .
• Raven bones are absent or fused to the shoulder blade and form raven beak processes .
• In the ankle , the main joint lies between the shin and the ankle . The calcaneus has no contact with the shin, but it forms a heel-like protrusion with muscle attachments. (The other type of ankle joint is easy to recognize in crocodiles . In his case, the main joint lies between the heel bone and the ankle bone.)
• The molars are tribosphenic.

Tribosphenic molars appear in fossil finds from Madagascar , which suggest an age of the Theria of at least 167 million years BP.

Metatheria

The living Metatheria all belong to the marsupials . Some fossil genera such as Asiatherium from the Upper Cretaceous Mongolia are either marsupials or members of other animal groups among the Metatheria.

The oldest known marsupial mammal is Sinodelphys , which was found in 125 million year old shale clays from the Lower Cretaceous in northeastern Liaoning (People's Republic of China). The fossil is almost completely preserved and provided with remains of the fur and tissue impressions.

The Didelphimorphia (common possums of the western hemisphere ) appeared for the first time in the Upper Cretaceous and have still living descendants, the reason for this is likely to be in their less specialized way of life - they are omnivores and partly tree-dwelling.

Probably the best known characteristic of the marsupial mammals is their mode of reproduction:

• The mother animal develops a kind of yolk sac in its abdomen , which supplies the embryo with nutrients. Australian nasal bags , koalas and wombats also form a placenta-like organ that keeps the germ in contact with the uterine wall; however, this organ is smaller than in the higher mammals (actual placentates) and it is not yet clear whether it will transfer nutrients from the mother to the embryo at all.
• The pregnancy is very short, usually between 4 and 5 weeks. The germ is born at a very early stage of development and is usually smaller than two inches. The short pregnancy is supposed to undermine an onset of an immune defense reaction in the mother animal.
• The newborn uses its relatively well-developed forelimbs (with strong grasping hands) to climb to one of the teats , which are usually hidden in a pouch on the mother's belly. Through muscle contractions on her mammary glands , the mother feeds the newborn, which is still too weak to suckle on its own. This climbing process of the newborn is said to have prevented the marsupial from developing flippers or wings - which would explain the absence of swimming or active flight-capable marsupials (some marsupials, however, can do glide).

While some marsupials sometimes look very similar to higher mammals (a good example is the thylacine ), they differ from the higher mammals in some details of their skeleton:

• Some marsupials (such as the thylacine) have four molars, no higher mammal has more than three.
• All bag mammals have (in addition to nostrils) paired palatal windows in the lower section of the skull.

Bag mammals also have bag bones arranged in pairs , which support the bag of the female animals. Pouch bones, however, are not characteristic of marsupial mammals, namely they are also found in multituberculata, monotremes and even some Eutheria. Pouch-bones are believed to be an original feature that disappeared after the ancestors of today's higher mammals branched off from the marsupial. Some researchers believe that the original function of the pouch bones is to support forward movement by providing an abutment for the contracting muscles of the thigh.

Eutheria

All living Eutheria ("real animals") are higher mammals (placenta animals). The oldest known Eutheria fossil Eomaia , which is 125 million years old and was found in the People's Republic of China, still has features that come much closer to the marsupial (the surviving Metatheria):

• Pouch-bones directed forward from the pelvis . These no longer occur in modern higher mammals, but are found in all other mammals - in the rest of the Eutheria, marsupials, monotones and Mammaliaformes . Yes, even with therapsids close to mammals such as B. the cynodontia they occur. The purpose of the pouch bones is to stabilize the body when moving. This stiffening would be detrimental in pregnant placentals whose abdomen must be able to expand.
• A narrow pelvic outlet. This suggests that the boys must have been very young when they were born. The pregnancy was likely to be short-lived. The formation of a placenta is therefore a later evolutionary step.

Formation of the secondary villi in the placentalia . The chorion of the amniotic egg becomes the pars fetalis of the placenta in the course of phylogenesis . In the sauropsids , the chorion is used for gas exchange in connection with the allantois .

• Five incisors on each side of the upper jaw. This number is typical for Metatheria, modern mammals have a maximum of 3 with the exception of homodonts such as the armadillo . The ratio of the molars to the premolars ( Eomaia has more premolars than molars) is again more typical for Eutheria (including placenta animals) than for marsupials.

Eomaia also had a Meckel's pit , an original lower jaw feature that no longer occurs in higher mammals.

All of these transitional features are consistent with molecular phylogenesis , which estimates suggest that the higher mammals begin to spread around 110 million years ago - 15 million years after Eomaia .

Several features in Eomaia , particularly on the feet and toes, suggest a climbing lifestyle. For example, the attachment points for the muscles required for climbing were well developed and the tail was twice as long as the rest of the spine.

Most characteristic of higher mammals is their mode of reproduction:

• The embryo is connected to the uterus by a large placenta , which is responsible for supplying food, oxygen and removing waste .
• Pregnancy lasts a relatively long time, so that the boys are relatively well developed at birth. With some species (especially steppe-dwelling herbivores) the young can often get up and start walking an hour after birth.

It was believed that the step towards the mode of reproduction of the higher mammals was only made possible by retroviruses :

• Retroviruses form the interface between placenta and uterus into a syncytium , i. H. a thin layer of cells with a common outer membrane. This allows oxygen, food and waste products to pass through, but stops blood cells and other cells that could activate the mother's immune system and attack the fetus .
• Retroviruses reduce the aggressiveness of the mother's immune system - an advantage for the fetus, but a disadvantage for the mother due to the increased risk of infection.

From a paleontological point of view, Eutheria can mainly be characterized by various features on teeth, joints and feet.

Expansion of ecological niches in the Mesozoic

The stereotypical image of mammals as "small, nocturnal insectivore" still has a certain justification, but recent finds (mainly in the People's Republic of China) show that some Mammaliaformes and some real mammals were much larger and led different ways of life. The following may serve as examples:

• Castorocauda from the Middle Jurassic around 164 million years ago was 42.5 centimeters tall and weighed 500 to 800 grams. His limbs were suitable for swimming and digging, the teeth showed adaptations for consumption by fish.
• Multituberculata survived for more than 125 million years (from the Middle Jurassic around 160 million years ago to the Lower Oligocene around 35 million years ago). They are often referred to as "rodents of the Mesozoic Era" because, like modern rodents, they had incisors that wereconstantly growing back.

Repenomamus sometimes preyed on young dinosaurs

• Fruitafossor from the Upper Jurassic around 150 million years ago was roughly the size of a chipmunk . Teeth, forelegs and back indicate an insect eater that broke open the nests of insect colonies (most likely termites , ants appeared later).
• Volaticotherium from the Lower Cretaceous around 125 million years ago is the first mammal capable of gliding. Its flight membrane stretched between the limbs, somewhat comparable to the flying squirrel . Volaticotherium was very likely diurnal.
• Repenomamus , also from the Lower Cretaceous about 130 million years ago, was a squat, badger-like predator who alsohuntedyoung dinosaurs . Two species are known, one a little more than a meter tall and weighing 12 to 14 kilograms, the other smaller than 50 centimeters and weighing only 4 to 6 kilograms.

Evolution of important existing groups of mammals

Traditional paleontologists and molecular geneticists are currently debating the timing and nature of diversification in real mammals, and particularly in the higher mammals. Usually paleontologists date the appearance of a particular group of animals from the oldest fossil record, which has all the characteristics of the group of animals in question. Molecular phylogeneticists argue, however, that the individual evolutionary lines separated much earlier and that the oldest representatives of a certain group of animals looked anatomically very similar to the oldest representatives of the other groups of animals and differed only in terms of their genes . These debates also extend to definitions and relationships among the major groups of higher mammals - the Afrotheria controversy serves as a good example .

Cladogram of the Upper Mammals

To establish a cladogram, molecular genetics uses species-specific genes , roughly comparable to the methodology used in paleontology, which is based on the characteristics of fossils: if the genes of two organisms show more agreement with one another than with those of a third, then these two organisms must consequently also be more closely related to each other.

The following cladogram shows the internal systematics of the higher mammals:

Note: The cladogram does not contain extinct branches as it is not possible to extract DNA from fossils.

Higher mammals  (Eutheria)

Afrotheria    Elephant jerk  (Macroscelidea)      Aardvark  (Tubulidentata)      Tenrecus  (Afrosoricida)   Paenungulata    Schliefer  (Hyracoidea)      Russell animals  (Proboscidea)      Manatees  (Sirenia)      Sub-articulated animals  (Xenarthra)   Boreoeutheria     Laurasiatheria    Insect  eater (Eulipotyphla)      Bats  (chiroptera)      Unpaired ungulate  (Perissodactyla)      Cetartiodactyla  ( cloven-hoofed animals  (Artiodactyla) and  whales  (Cetacea))   Ferae    Predators  (Carnivora)      Pangolins  (Pholidota)   Euarchontoglires     Glires    Hare-like  (Lagomorpha)      Rodents  (Rodentia)   Euarchonta    Pointed Squirrel  (Scandentia)      Giant Glider  (Dermoptera)      Primates  (Primates)

The cladogram has a few surprises in store:

• The main division of real mammals is a dichotomy of Afrotheria on the one hand and Xenarthra and Boreoeutheria on the other.
• The elephants' closest living relatives are the aquatic manatees and hyrax, which are more like guinea pigs.
• Two groups are separated from the insectivores, which today belong to the Afrotheria, the tenre-kitties and the elephants.
• The closest living relatives of the Cetacea (whales, dolphins, porpoises) are the even-toed ungulates, almost all of them pure vegetarians.
• Sister group of predators are the pangolins.

The merging of Afrotheria has geographical justifications. All recent members of Afrotheria come from either South American or largely African lineages - even the Indian elephant descends from an African lineage, which it left about 7.6 million years ago. When Pangea broke up, Africa and South America separated from the other continents less than 150 million years ago, and the final break between Africa and South America only occurred 100 to 80 million years ago. The oldest known fossil of Eutheria is Eomaia , which was dated to 125 million years BP. It would therefore not be surprising if the oldest immigrants of the Eutheria to Africa or South America were isolated there and then spread into all available ecological niches there by means of adaptive radiation .

Nonetheless, these proposals remain controversial. Understandably, paleontologists insist that fossil evidence takes precedence over conclusions drawn from the DNA of recent animal groups. Even molecular phylogeneticists have attacked the new cladograms, sometimes very vehemently, with reference to the following points:

• The mutation rate of from mitochondria -derived DNA of mammals does not proceed uniformly. Certain sections of the sequence hardly change, while other regions are subject to extremely rapid changes. There can be differences even between members of the same species.
• DNA from the mitochondria of mammals can mutate so quickly that the problem of "saturation" arises and statistical background noise obscures any information that may be available. Assuming a particular segment of mitochondrial DNA mutates indiscriminately every few million years, it has undergone countless changes since the main evolutionary lines among mammals split up between 60 and 75 million years ago.

Age determination of the evolutionary steps of higher mammals

New molecular phylogenetic studies come to the conclusion that most mammalian orders diverged around 100 to 85 million years ago BP, but the first recent families appeared much later in the Upper Eocene and Lower Miocene .

Some paleontologists suggest that fossil higher mammals older than the KT limit have not yet been found. The 75 million year old Maelestes gobiensis belongs to the Eutheria, but is not yet a higher mammal. Many Cretaceous fossil sites contain well-preserved lizards, salamanders, birds and mammals, but no higher mammals. It is quite possible that they simply did not exist at this point in time or that the molecular clock begins to run faster with larger evolutionary radiations . Nevertheless, fossil remains of 85 million year old ungulates are known, which may represent the ancestors of the recent ungulata .

The earliest fossils of representatives of modern groups of mammals mostly go back to the Paleocene , some are a little younger and only very few date back to the Cretaceous , i.e. before the disappearance of the dinosaurs . Some paleontologists influenced by molecular phylogenetic work have used statistical methods to interpolate back into the past from fossil finds of modern groupings and have come to the conclusion, for example, that the group of primates originated in the Upper Cretaceous. Statistical studies of the fossil record confirm, however, that mammals were subject to severe restrictions in terms of size and diversity right up to the end of the Cretaceous and that they only grew rapidly in the Lower Paleocene and that their biodiversity increased rapidly at the same time.

Evolution of mammalian traits

Mammals have a number of evolutionary innovations that make their eponymous property possible. These mainly included the more or less parallel evolution of the placenta, breast milk, the mammary gland and the mother-child relationship .

placenta

The placenta is a key innovation and thus an evolutionary leap in mammals, from their beginnings until today. It represents a new organ that the egg-laying animals did not have before. Among other innovations, a nutritional connection had to evolve from the egg of the embryo in the womb to the mother in order to allow the embryo to grow in the womb. That growth was a critical selective advantage. The placenta increased the chances of survival of the unborn in the time of the dinosaurs about 160 million years ago. The earliest known representative of the placenta animals from this period was Juramaia sinensis .

The gene retrotransposon-derived protein or paternally expressed gene ( Peg10 ) has been identified as an important gene for the formation of the placenta . This gene was likely encoded in the DNA of the germ cells of early mammals by a retrovirus , a viral invasion and a process similar to that observed in the recent koala with the pathogenic gene KORV . The knockout of Peg10 in the mouse in the laboratory leads to a halt in the growth of the placenta and the early death of the embryo.

So Peg10 is responsible for forming the placenta. The gene suppresses the mother's immune defense and thus prevents the embryo from being rejected when the physical mother-child connection is established. Only for the further course of evolution is it assumed that the food supply of the embryo by means of the placenta was added to protect the immune system. In the course of evolution, the placenta became larger and the embryo's gestation period was extended. Longer pregnancies helped the mother to be more independent from predators, and animals began to carry live mammals to term.

Secretion of milk (lactation)

The original task of the milk secretion was on the one hand to keep the clutch of monotones (egg-laying mammals) moist and on the other hand to act as a bactericidal protective function of the milk. The following speaks for the function of moisturizing:

• Monotremes do not have teats, the milk is secreted by glands in a hairy area on their belly.
• During the breeding season, the eggs are covered in a sticky substance whose origin is unknown. Before laying, the eggshells consist of a three-layer layer. Then a fourth layer of different composition is added. The sticky substance and the fourth layer of eggs may be produced by the mammary glands.
• If this is the case, then the hairiness of the milk-secreting region could be explained: it should be easier to distribute moist substances over the egg from a broad, hairy area than from a small, bare teat.

The discovery of the protein lysozyme as a component of milk speaks for the protective function of milk secretion, even before the function of nutrition appeared evolutionarily . Lysozyme has a bactericidal effect. The embryo ingests the protein outside of the womb and is thereby protected. This thesis is supported by the almost identical molecular structure of lysozyme and lactalbumin , a key protein in breast milk that is responsible for the biosynthesis of milk sugar . It is therefore believed that it was easy that lysozyme was evolutionarily converted into lactalbumin and thus the important nutritional function of milk was made possible.

Jaw and middle ear

Hadrocodium , whose fossils come from the Lower Jurassic , provides the first clear evidence of a fully developed, typical mammalian temporomandibular joint with dentals and squamosum and a mammalian middle ear, in which articulars and quadratum had migrated from the jaw and were there transformed into an anvil and hammer . Oddly enough, Hadrocodium is still considered a member of the Mammaliaformes , rather than a real mammal.

An investigation of the monotonous Teinolophos came to the conclusion that its temporomandibular joint consisting of angulare and quadratum was a precursor to the mammalian temporomandibular joint and that the middle ear typical of mammals developed independently of each other twice in monotones and in mammals. However, this view is being challenged. In a follow-up study, the same facts were interpreted to mean that Teinolophos is supposed to have been a real beak hedgehog with the typical mammalian jaw joint and middle ear.

Hair and fur

The first clear finding of hair or fur comes from the 164 million year old Castorocauda fossil from the Middle Jurassic .

Since 1955, some scientists have been interpreting the foramina in cynodontics in the upper jaw and in the premaxillary as channels for blood vessels and nerve tracts that were connected to vibrissae (whiskers). You see in it the evidence of hair or fur. The presence of foramina does not automatically indicate whiskers - the best counterexample is the recent Tupinambis rail lizard , whose foramina are almost identical to the foramina of the non-mammalian cynodontal Thrinaxodon , but which has no whiskers.

Upright limbs

The development of upright limbs in mammals is not yet complete, as fossil and recent monotremes have bent extremities. Some scientists are even of the opinion that the parasagittal (non-flexed) posture represents a synapomorphism (characteristic feature) in the Boreosphenida (this group contains the Theria and thus the last common ancestor of the modern marsupial and higher mammals) and that consequently all older mammals as well had bent limbs.

Sinodelphys (the first known marsupial mammal) and Eomaia (the oldest Eutheria ) lived around 125 million years ago, so the development into erect limbs must have started earlier.

Heat balance

The term "warm-bloodedness" is ambiguous, it should rather be divided into the following more specific sub-terms:

• Endothermia, or the ability of an organism to generate heat internally through metabolic activity without relying on external sources (such as sunbathing) or muscle movements (microvibration).
• Homoiothermia or the ability to maintain a constant body temperature (see also animal at constant temperature).
• Tachymetabolism or the ability to ensure a high metabolic rate when resting. This requires a very high and stable body temperature, since biochemical processes only run half as fast when the body temperature drops by 10 ° C. Most enzymes are only reactive in an optimal temperature range, outside of which their efficiency drops rapidly.

Since we know little or nothing about internal processes in extinct animal groups, only homoothermia and tachymetabolism can be taken into account in the discussion about the heat balance.

Recent monotons have a slightly lower body temperature than marsupial mammals and higher mammals, and their metabolic activity is also subject to greater fluctuations. The question arises as to when this cloak-like metabolism was established in mammals. The fact that even cynodontals already had a fairly high metabolic rate is still controversial.

Turbinates

Recent mammalian groups have turbinates, twisted structures made up of thin bones in the nasal cavity. They are covered by a mucous membrane , the task of which is to warm and humidify inhaled air and, in return, to remove heat and moisture from the exhaled air. Animals with turbinates can maintain increased respiratory activity without the risk of their lungs drying out. As a result, you can also achieve a high metabolic rate. The turbinate bones are very fragile and are not even preserved in fossil form. Rudimentary inguinal and supporting bones of the turbinates, however, were found in Triassic therapsids such as B. Thrinaxodon and Diademodon discovered, their metabolic rate should therefore have already increased.

Ossified secondary palate

Mammals have an ossified secondary palate that separates the airways from the mouth, so they can eat and breathe at the same time. Secondary ossified palates are already found in the more advanced cynodontics and have also been interpreted as an indication of an increased metabolic rate. But some cold-blooded vertebrates such as crocodiles and some lizards are also equipped with secondary ossified palates. The birds of the same temperature do not have them.

diaphragm

A muscular diaphragm helps mammals breathe, especially during strenuous physical activity. It can only work sensibly if the abdomen remains free of obstructing ribs and thus the bellows-like interplay between the chest and abdomen can fully develop. Advanced cynodontals already had a very mammal-like chest with greatly reduced lumbar vertebrae. It follows that these animals had a diaphragm, were able to maintain physical exertion over a longer period of time and consequently also had an increased metabolic rate. It is possible that the mammalian rib cage has brought greater mobility with it at the same time. The mode of locomotion even of advanced Therapsids is likely to have been "wheelbarrow-like", with the hind legs doing the entire thrust and the front legs only exercising a control function. Advanced therapsids are therefore likely to have been far less agile than recent mammals or even early dinosaurs . It is therefore doubtful that the main function of the mammalian chest was to increase agility.

Position of the limbs

The Therapsids had bent front legs and semi-upright hind legs. This means that because of the carrier restriction, they should have found it relatively difficult to breathe while moving. The lizards, whose limbs are fully flexed and are forced to take longer breaks after physical exertion, find it even more difficult. The cynodontics wore ribbed plates to stiffen the rib cage, which may restrict the torso from bending to the side while moving and this may make it easier for them to breathe. Everything indicates that advanced therapsids were much more restricted in their freedom of movement than recent groups of mammals of comparable size and that their metabolism was also much slower.

Thermal insulation (hair and fur)

Thermal insulation is the simplest solution to ensure a reasonably constant body temperature. Hair and fur are good indicators of homeothermia, but less of a high metabolic rate .

The first fossil evidence of hair or fur comes from the 164 million year old Castorocauda from the Middle Jurassic . Indications of hairiness in advanced therapsids are not conclusive.

See also

• Mammals - Systematics and History of Development
• Therapsids
• Synapsids

swell

• Robert L. Carroll : Vertebrate Paleontology and Evolution. WH Freeman and Company, New York 1988, ISBN 0-7167-1822-7 , Chapters XVII to XXI
• Nicholas Hotton III, Paul D. MacLean, Jan J. Roth, E. Carol Roth (Eds.): The Ecology and Biology of Mammal-like Reptiles. Smithsonian Institution Press, Washington / London 1986, ISBN 0-87474-524-1 .
• TS Kemp: The Origin and Evolution of Mammals. Oxford University Press, New York 2005, ISBN 0-19-850760-7 .
• Zofia Kielan-Jaworowska, Richard L. Cifelli, Zhe-Xi Luo: Mammals from the Age of Dinosaurs: Origins, Evolution, and Structure. Columbia University Press, New York 2004, ISBN 0-231-11918-6 . (Comprehensive representation from the first mammals to mass extinction at the KT border)
• Zhe-Xi Luo: Transformation and diversification in early mammal evolution. In: Nature. volume 450, number 7172, December 13, 2007, pp. 1011-1019. doi: 10.1038 / nature06277 . (A review with 98 individual references to scientific literature)

Web links

• The Cynodontia deals with various aspects of the evolutionary transition between cynodonts and mammals - with many individual references.
• Springer Link: Relationships of the Liassic Mammals Sinoconodon, Morganucodon oehleri, and Dinnetherium

Individual evidence

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34. ↑ Drawing of the anatomical relationships of the reproductive organs in monotremata after David Peters: From the Beginning: The Story of Human Evolution. William Morrow & Co (1876) [1]
35. ↑ For comparison, the drawing of the anatomical relationships of the reproductive organs in Marsupialia and Placentalia . [2]
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49. ↑ While in the higher mammals the chorionic villi sink into the uterine lining ( decidua as part of the " archimetra ") and thus form the fetal part of the placenta, through which the substance and gas exchange between the child and maternal organism takes place.
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58. ^ Scientists map elephant evolution. Retrieved August 11, 2008 . 
59. ^ Historical perspective (the Dynamic Earth, USGS)
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73. ↑ Ryuichi Ono, Kenji Nakamura, Kimiko Inoue, Mie Naruse, Takako Usami, Noriko Wakisaka-Saito, Toshiaki Hino, Rika Suzuki-Migishima, Narumi Ogonuki, Hiromi Miki, Takashi Kohda, Atsuo Ogura, Minesuke Yokoyko-Isokane & Fumitoshi Kino Ishino. Deletion of Peg10, an imprinted gene acquired from a retrotransposon, causes early embryonic lethality. Nature Genetics 38, 101-106 (2006)
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