The evolution of mammals from synapsids (mammal-like reptiles) was a gradual process which took approximately 70 million years, from the mid-Permian to the mid-Jurassic, and by the mid-Triassic there were many species that looked like mammals.
From the point of view of cladistics, mammals are the only surviving synapsids.
Restoration of Thrinaxodon, a member of the cynodont group which includes the ancestors of mammals
Here is a very simplified "family tree" - the text below describes some of the uncertainties and areas of debate.
--Tetrapods--------------------------------------------------
|
+-- Amphibians ---------------------------------------
|
`--Amniotes-----
|
+--Sauropsids------------------------------------
|
`--Synapsids------
|
`--Pelycosaurs----
|
`--Therapsids-----
|
`--Mammals------------------
Therapsids
Therapsids
Therapsids descended from pelycosaurs in the middle Permian and took over their position as the dominant land vertebrates. They differ from pelycosaurs in several features of the skull and jaws, including: larger temporal fenestrae; incisors which are equal in size.[7]
The therapsids went through a series of stages, beginning with animals which were very like their pelycosaur ancestors and ending with some which could easily be mistaken for mammals:
* gradual development of a bony secondary palate.[8] Most books and articles interpret this as a prequisite 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.[9] The interpretation of the bony secondary palate as an aid to chewing also suggests the development of a faster metabolism, since chewing makes it possible to digest food more quickly. In mammals the palate is formed by two specific bones, but various Permian therapsids had other combinations of bones in the right places to function as a palate.
* the dentary gradually becomes the main bone of the lower jaw.
* progress towards an erect limb posture, which would increase the animals' stamina by avoiding Carrier's constraint. But this process was erratic - for example: all herbivorous therapsids retained sprawling limbs (some late forms may have had semi-erect hind limbs); Permian carnivorous therapsids had sprawling forelimbs, and some late Permian ones also had semi-sprawling hindlimbs. In fact modern monotremes still have semi-sprawling limbs.
* in the Triassic, progress towards the mammalian jaw and middle ear.
* there is plausible evidence of hair in Triassic therapsids, but none for Permian therapsids.
* some scientists have argued that some Triassic therapsids show signs of lactation.
Therapsid family tree
(simplified from [10]; only those which are most relevant to the evolution of mammals are described below)
Therapsids
|
+--Biarmosuchia
|
`--+--Dinocephalia
|
+--Neotherapsida
|
+--Anomodonts
| |
| `--Dicynodonts
|
`--+--Theriodontia
|
+--Gorgonopsia
|
`--+--Therocephalia
|
`--Cynodontia
.
. . . (Mammals, eventually)
Only the dicynodonts, therocephalians and cynodonts survived into the Triassic.
Triassic takeover
The catastrophic Permian-Triassic mass extinction killed off about 70 percent of terrestrial vertebrate species, and the majority of land plants. As a result:[14]
* Ecosystems and food chains collapsed, and the recovery took about 6 million years.
* The survivors had re-start the struggle for dominance of their former ecological niches - even the cynodonts, which had seemed on the way to dominance at the end of the Permian.
But the cynodonts lost out to a previously obscure group of sauropsids, the archosaurs (which include the ancestors of crocodilians, dinosaurs and birds). This reversal of fortunes is often called the "Triassic takeover". Several explanations have been offered for it, but the most likely is that the early Triassic was predominantly arid and therefore archosaurs' superior water conservation gave them a decisive advantage (all known sauropsids have glandless skins and excrete uric acid, which requires less water to keep it sufficiently liquid than the urea which mammals excrete and presumably therapsids excreted).[15] The Triassic takeover was gradual - in the earliest part of the Triassic cynodonts were the main predators and lystrosaurs were the main herbivores, but by the mid-Triassic archosaurs dominated all the large carnivore and herbivore niches.
But the Triassic takeover may have been a vital factor in the evolution of cynodonts into mammals. The cynodonts' descendants were only able to survive as small, mainly nocturnal insectivores.[16] As a result:
* The therapsid trend towards differentiated teeth with precise occlusion accelerated, because of the need to hold captured arthropods and crush their exoskeletons.
* Nocturnal life required advances in thermal insulation and temperature regulation to enable the ancestors of mammals to be active in the cool of the night.
* Acute senses of hearing and smell became vital.
o This accelerated the development of the mammalian middle ear, and therefore of the mammalian jaw since bones which had been part of the jaw joint became part of the middle ear.
o The increase in the size of the olfactory and auditory lobes of the brain increased brain weight as a total percentage of body weight. Brain tissue requires a disproportionate amount of energy.[17][18] The need for more food to support the enlarged brains increased the pressures for improvements in insulation, temperature regulation and feeding.
* As a side-effect, sight became slightly less important, and this is reflected in the fact that most mammals have poor color vision, including the "lower primates" such as lemurs.[19]
From cynodonts to true mammals
Many uncertainties
While the Triassic takeover probably accelerated the evolution of mammals, it made life more difficult for paleontologists because good fossils of the nearly-mammals are extremely rare, mainly because they were mostly smaller than rats:
* They were largely restricted to environments which are less likely to provide good fossils. The best terrestrial environments for fossilization are floodplains, where seasonal floods quickly cover dead animals in a protective layer of silt which is later compressed into sedimentary rock. But floodplains are dominated by medium to large animals, and the Triassic therapsids and near-mammals could not compete with archosaurs in the medium to large size range.
* Their delicate bones were vulnerable to being destroyed before they could be fossilized - by scavengers (including fungi and bacteria) and by being trodden on.
* Small fossils are harder to spot and more vulnerable to being destroyed by weathering and other natural stresses before they are discovered.
In fact it was said as recently as the the 1980s that all the Mesozoic fossils of mammals and near-mammals could be contained in a few shoeboxes - and they were mostly teeth, which are the most durable of all bones.[20]
As a result:
* In many cases it is difficult to assign a Mesozoic mammal or near-mammal fossil to a genus.
* All the available fossils of a genus seldom add up to a complete skeleton, and hence it is difficult to decide which genera are most like each other and therefore most likely to be closely-related. In other words, it becomes very difficult to classify them by means of cladistics, which is the most reliable and least subjective method currently available.
So the evolution of mammals in the Mesozoic is full of uncertainties, although there is no room for doubt that true mammals did first appear in the Mesozoic.
Mammals or mammaliformes?
One result of these uncertainties has been a change in the paleontologists' definition of "mammal". For a long time a fossil was considered a mammal if it met the jaw-ear criterion (the jaw joint consists only of the squamosal and dentary; and the articular and the quadrate bones have become the middle ear's malleus and incus). But more recently paleontologists have usually defined "mammal" as the last common ancestor of monotremes, marsupials and placentals and all of its descendants. So they had to define another clade mammaliformes to accommodate all the animals which are more mammal-like than cynodonts but less closely related to monotremes, marsupials and placentals.[21] Although this now appears to be the majority approach, some paleontologists have resisted it because: it simply moves most of the problems into the new clade without solving them; the clade mammaliformes includes some animals with "mammalian" jaw joints and some with "reptilian" (articular-to-quadrate) jaw joints; and the newer definition of "mammal" and "mammaliformes" depend on last common ancestors of both groups which have not yet been found.[22] Despite these objections, this article follows the majority approach and treats most of the cynodonts' Mesozoic descendants as mammaliformes.
Family tree - cynodonts to mammals
(based on Mammaliformes - Palaeos)
--Cynodonts
|
`--Mammaliformes
|
+--Allotheria
| |
| `--Multituberculates
|
`--+--Morganucodontidae
|
`--+--Docodonta
|
`--+--Hadrocodium
|
`--Symmetrodonta
|
|--Kuehneotheriidae
|
`--Mammals
The earliest true mammals
This part of the story introduces new complications, since true mammals are the only group which still has living members:
* One has to distinguish between extinct groups and those which have living representatives.
* One often feels compelled to try to explain the evolution of features which do not appear in fossils. This endeavor often involves Molecular phylogenetics, a technique which has become popular since the mid-1980s but is still often controversial because of its assumptions, especially about the reliability of the molecular clock.
Family tree of early true mammals
(based on Mammalia: Overview - Palaeos; X marks extinct groups)
--Mammals
|
+--Australosphenida
| |
| +--Ausktribosphenidae X
| |
| `--Monotremes
|
`--+--Triconodonta X
|
`--+--Spalacotheroidea X
|
`--Cladotheria
|
|--Dryolestoidea X
|
`--Theria
|
+--Metatheria
|
`--Eutheria
Evolution of major groups of living mammals
There are currently vigorous debates between traditional paleontologists ("fossil-hunters") and molecular phylogeneticists about how and when the true mammals diversified, especially the placentals. Generally the traditional paelontologists date the appearance of a particular group by the earliest known fossil whose features make it likely to be a member of that group, while the molecular phylogeneticists suggest that each lineage diverged earlier (usually in the Cretaceous) and that the earliest members of each group were anatomically very similar to early members of other groups and differed only in their genes. These debates extend to the definition of and relationships between the major groups of placentals - the controversy about Afrotheria is a good example.
Fossil-based family tree of placental mammals
Here is a very simplified version of a typical family tree based on fossils, based on Cladogram of Mammalia - Palaeos. Paleontologists agree about most of it, especially the major groups, but still have some debates about the details.
For the sake of brevity and simplicity the diagram omits some extinct groups in order to focus on the ancestry of well-known modern groups of placentals - X marks extinct groups. The diagram also shows:
* the age of the oldest known fossils in many groups, since one of the major debates between traditional paleontologists and molecular phylogeneticists is about when various groups first became distinct.
* well-known modern members of most groups.
--Eutheria
|
+--Xenarthra (Paleocene)
| (armadillos, anteaters, sloths)
|
`--+--Pholidota (early Eocene)
| (pangolins)
|
`--Epitheria (late Cretaceous)
|
|--(some extinct groups) X
|
`--+--Insectivora (late Cretaceous)
| (hedgehogs, shrews, moles, tenrecs)
|
`--+--+--Anagalida
| | |
| | +--Zalambdalestidae X (late Cretaceous)
| | |
| | `--+--Macroscelidea (late Eocene)
| | | (elephant shrews)
| | |
| | `--+--Anagaloidea X
| | |
| | `--Glires (early Paleocene)
| | |
| | +--Lagomorpha (Eocene)
| | | (rabbits, hares, pikas)
| | |
| | `--Rodentia (late Paleocene)
| | (mice & rats, squirrels,
| | porcupines)
| |
| `--Archonta
| |
| |--+--Scandentia (mid [Eocene])
| | | (tree shrews)
| | |
| | `--Primatomorpha
| | |
| | +--Plesiadapiformes X
| | |
| | `--Primates (early Paleocene)
| | (tarsiers, lemurs, monkeys,
| | apes, humans)
| |
| `--+--Dermoptera (late Eocene)
| | (colugos)
| |
| `--Chiroptera (late Paleocene)
| (bats)
|
`--+--Ferae (early Paleocene)
| (cats, dogs, bears, seals)
|
`--Ungulatomorpha (late Cretaceous)
|
+--Eparctocyona (late Cretaceous)
| |
| +--(some extinct groups) X
| |
| `--+--Arctostylopida X (late Paleocene)
| |
| `--+--Mesonychia X (mid Paleocene)
| | (predators / scavengers,
| | but not closely related
| | to modern carnivores)
| |
| `--Cetartiodactyla
| |
| +--Cetacea (early Eocene)
| | (whales, dolphins, porpoises)
| |
| `--Artiodactyla (early Eocene)
| (even-toed ungulates:
| pigs, hippos, camels,
| giraffes, cattle, deer)
|
`--Altungulata
|
+--Hilalia X
|
`--+--+--Perissodactyla (late Paleocene)
| | (odd-toed ungulates:
| | horses, rhinos, tapirs)
| |
| `--Tubulidentata (early Miocene)
| (aardvarks)
|
`--Paenungulata ("not quite ungulates")
|
+--Hyracoidea (early Eocene)
| (hyraxes)
|
`--+--Sirenia (early Eocene)
| (manatees, dugongs)
|
`--Proboscidea (early Eocene)
(elephants)
This family tree contains some surprises and puzzles. For example:
* The closest living relatives of cetaceans (whales, dolphins, porpoises) are artiodactyls, hoofed animals which are almost all pure vegetarians.
* Bats are fairly close relatives of primates.
* The closest living relatives of elephants are the aquatic sirenians, while their next relatives are hyraxes, which look more like well-fed guinea pigs.
* There is little correspondence between the structure of the family (what was descended from what) and the dates of the earliest fossils of each group. For example the earliest fossils of perissodactyls (the living members of which are horses, rhinos and tapirs) date from the late Paleocene but the earliest fossils of their "sister group" the Tubulidentata date from the early Miocene, nearly 50M years later. Paleontologists are fairly confidents about the family relationships, which are based on cladistic analyses, and believe that fossils of the ancestors of modern aardvarks have simply not been found yet.
Family tree of placental mammals according to molecular phylogenetics
Molecular phylogenetics uses features of organisms's genes to work out family trees in much the same way as paleontologists do with fossils - if two organisms' genes / fossil features are more similar to each other than to those of a third organism, the two organisms are more closely related to each other than to the third.
Molecular phylogeneticists have proposed a family tree which is very different from the one with which paleontologists are familiar. Like paleontologists, molecular phylogeneticists have different ideas about various details, but here is a typical family tree according to molecular phylogenetics:[49][50] Note that the diagram shown here omits extinct groups, as one cannot extract DNA from fossils.
--Eutheria
|
+--Atlantogenata ("born round the Atlantic ocean")
| |
| +--Xenarthra (armadillos, anteaters, sloths)
| |
| `--Afrotheria
| |
| +--Afroinsectiphilia
| | (golden moles, tenrecs, otter shrews)
| |
| +--Pseudungulata ("false ungulates")
| | |
| | +--Macroscelidea (elephant shrews)
| | |
| | `--Tubulidentata (aardvarks)
| |
| `--Paenungulata ("not quite ungulates")
| |
| +--Hyracoidea (hyraxes)
| |
| +--Proboscidea (elephants)
| |
| `--Sirenia (manatees, dugongs)
|
`--Boreoeutheria ("northern true / placental mammals")
|
+--Laurasiatheria
| |
| +--Erinaceomorpha (hedgehogs, gymnures)
| |
| +--Soricomorpha (moles, shrews, solenodons)
| |
| +--Cetartiodactyla
| | (cetaceans and even-toed ungulates)
| |
| `--Pegasoferae
| |
| +--Pholidota (pangolins)
| |
| +--Chiroptera (bats)
| |
| +--Carnivora (cats, dogs, bears, seals)
| |
| `--Perissodactyla (horses, rhinos, tapirs).
|
`--Euarchontoglires
|
+--Glires
| |
| +--Lagomorpha
| | (rabbits, hares, pikas)
| |
| `--Rodentia (late Paleocene)
| (mice & rats, squirrels, porcupines)
|
`--Euarchonta
|
|--Scandentia (tree shrews)
|
|--Dermoptera (colugos)
|
`--Primates
(tarsiers, lemurs, monkeys, apes, humans)
The most significant of the many differences between this family tree and the one familiar to paleontologists are:
* The top-level division is between Atlantogenata and Boreoeutheria, instead of between Xenarthra and the rest. But some Molecular phylogeneticists have proposed a 3-way top-level split between Xenarthra, Afrotheria and Boreoeutheria.
* Afrotheria contains several groups which are only distantly related according to the paleontologists' version: Afroinsectiphilia ("African insectivores"), Tubulidentata (aardvarks, which paleontologists regard as much closer to odd-toed ungulates than to other members of Afrotheria), Macroscelidea (elephant shrews, usually regarded as close to rabbits and rodents). The only members of Afrotheria which paleontologists would regard as closely related are Hyracoidea (hyraxes), Proboscidea (elephants) and Sirenia (manatees, dugongs).
* Insectivores are split into 3 groups: one is part of Afrotheria and the other two are distinct sub-groups within Boreoeutheria.
* Bats are closer to Carnivora and odd-toed ungulates than to primates and Dermoptera (colugos).
* Perissodactyla (odd-toed ungulates) are closer to Carnivora and bats than to Artiodactyla (even-toed ungulates).
The grouping together of the Afrotheria has some geological justification. All surviving members of the Afrotheria live in South America or (mainly) Africa. As Pangaea broke up Africa and South America separated from the other continents less than 150M years ago, and from each other between 100M and 80M years ago.[51][52] The earliest known eutherian mammal is Eomaia, from about 125M years ago. So it would not be surprising if the earliest eutherian immigrants into Africa and South America were isolated there and radiated into all the available ecological niches.
Nevertheless these proposals have been controversial. Paleontologists naturally insist that fossil evidence must take priority over deductions form DNA samples. More surprisingly, these new family trees have been criticised by other molecular phylogeneticists, sometimes quite harshly: [53]
* Mitochondrial DNA's mutation rate in mammals varies from region to region - some parts hardly ever change and some change extremely quickly and even show large variations between individuals within the same species.[54][55]
* Mammalian mitochondrial DNA mutates so fast that it causes a problem called "saturation", where random noise drowns out any information that may be present. If a particular piece of mitochondrial DNA mutates randomly every few million years, it will have changed several times in the 60 to 75M years since the major groups of placental mammals diverged.[56]
Evolution of mammalian features
Jaws and middle ears
Hadrocodium, whose fossils date from the early Jurassic, provides the first clear evidence of fully mammalian jaw joints and middle ears. Curiously it is usually classified as a member of the mammaliformes rather than a as a true mammal.
Milk production (lactation)
It has been suggested that lactation's original function was to keep eggs moist. Much of the argument is based on monotremes (egg-laying mammals):[59][60][61]
* Monotemes do not have nipples but secrete milk from a hairy patch on their bellies.
* During incubation, monotremes' eggs are covered in sticky substance whose origin is not known. Before the eggs are laid, their shells have only three layers. Afterwards a fourth layer appears, and its composition is different from that of the original three. The sticky substance and the fourth layer may be produced by the mammary glands.
* If so, that may explain why the patches from which monotremes secrete milk are hairy - it is easier to spread moisture and other substances over the egg from a broad, hairy area than from a small, bare nipple.
Hair and fur
The first clear evidence of hair or fur is in fossils of Castorocauda, from 164M years ago in the mid Jurassic.
From 1955 onwards some scientists have interpreted the foramina (passages) in the maxillae (upper jaws) and premaxillae (small bones in front of the maxillae) of cynodonts as channels which supplied blood vessels and nerves to vibrissae (whiskers), and suggested that this was evidence of hair or fur.[62][63] But foramina do not necessarily show that an animal had vibrissae - for example the modern lizard Tupinambis has foramina which are almost identical to those found in the non-mammalian cynodont Thrinaxodon.[64][65]
Erect limbs
The evolution of erect limbs in mammals is incomplete - living and fossil monotremes have sprawling limbs. In fact some scientists think that the parasagittal (non-sprawling) limb posture is a synapomorphy (distinguishing characteristic) of the Boreosphenida, a group which contains the Theria and therefore includes the last common ancestor of modern marsupial and placentals - and therefore that all earlier mammals had sprawling limbs.[66]
Sinodelphys (the earliest known marsupial) and Eomaia (the earliest known eutherian) lived about 125M years ago, so erect limbs must have evolved before then.
Warm-bloodedness
"Warm-bloodedness" is a complex and rather ambiguous term, because it includes some or all of:
* Endothermy, i.e. the ability to generate heat internally rather than via behaviors such as basking or muscular activity.
* Homeothermy, i.e. maintaining a fairly constant body temperature.
* Tachymetabolism, i.e. maintaining a high metabolic rate, particularly when at rest. This requires a fairly high and stable body temperature, since: biochemical processes run about half as fast if an animal's temperature drops by 10�C; most enzymes have an optimum operating temperature and their efficiency drops rapidly outside the preferred range.
Since we can't know much about the internal mechanisms of extinct creatures, most discussion focuses on homeothermy and tachymetabolism.
Modern monotremes have a lower body temperature and more variable metabolic rate than marsupials and placentals.[67] So the main question is when a monotreme-like metabolism evolved in mammals. The evidence found so far suggests Triassic cynodonts may have had fairly high metabolic rates, but is not conclusive.
[Respiratory turbinates
Modern mammals have respiratory turbinates, convoluted structures of thin bone in the nasal cavity. These are lined with mucous membranes which warm and moisten inhaled air and extract heat and moisture from exhaled air. An animal with respiratory turbinates can maintain a high rate of breathing without the danger of drying its lungs out, and therefore may have a fast metabolism. Unfortunately these bones are very delicate and therefore have not yet been found in fossils. But rudimentary ridges like those which support respiratory turbinates have been found in Triassic therapsids such as Thrinaxodon and Diademodon, which suggests that they may have had fairly high metabolic rates. [68] [69][70]
Bony secondary palate
Mammals have a secondary bony palate which separates the respiratory passage from the mouth, allowing them to eat and breathe at the same time. Secondary bony palates have been found in the more advanced cynodonts and have been used as evidence of high metabolic rates.[71][72] [73] But some cold-blooded vertebrates have secondary bony palates (crocodilians and some lizards), while birds, which are warm-blooded, do not have them.[74]
Diaphragm
A muscular diaphragm helps mammals to breathe, especially during strenuous activity. For a diaphragm to be work, the ribs must not restrict the abdomen, so that expansion of the chest can be compensated for by reduction in the volume of the abdomen and vice versa. The advanced cynodonts have very very mammal-like rib cages, with greatly reduced lumbar ribs. This suggests that these animals had diaphragms, were capable of strenuous activity for fairly long periods and therefore had high metabolic rates.[75][76] On the other hand these mammal-like rib cages may have evolved to increase agility.[77] But the movement of even advanced therapsids was "like a wheelbarrow", with the hindlimbs providing all the thrust while the forelimbs only steered the animal, in other words advanced therapsids were not as agile as either modern mammals or the early dinosaurs.[78] So the idea that the main function of these mammal-like rib cages was to increase agility is doubtful.
Limb posture
The therapsids had sprawling forelimbs and semi-erect hindlimbs.[79][80] This suggests that Carrier's constraint would have made it rather difficult for them to move and breathe at the same time, but not as difficult as it is for animals such as lizards which have completely sprawling limbs.[81] But cynodonts (advanced therapsids) had costal plates which stiffened the rib cage and therefore may have reduced sideways flexing of the trunk while moving, which would have made it a little easier for them to breathe while moving .[82] These facts suggest that advanced therapsids were significantly less active than modern mammals of similar size and therefore may have had slower metabolisms.
Insulation (hair and fur)
Insulation is the "cheapest" way to maintain a fairly constant body temperature. So possession of hair of fur would be good evidence of homeothermy, but would not be such strong evidence of a high metabolic rate.[83] [84]
We have already seen that: the first clear evidence of hair or fur is in fossils of Castorocauda, from 164M years ago in the mid Jurassic; arguments that advanced therapsids had hair are unconvincing
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