When was the origin of mammals

Then came the therapsids from c. Then mya a mass extinction event wiped out over 90 per cent of animals and plants. Miraculously, a few therapsids survived including the burrowing dicynodont called Lystrosaurus and cynodonts that evolved throughout the Triassic and gave rise to mammals. The earliest mammal was a mouse-sized animal called Morganucodon from mya. Access to the complete content on Very Short Introductions online requires a subscription or purchase.

Public users are able to search the site and view the abstracts and keywords for each book and chapter without a subscription. Please subscribe or login to access full text content. If you have purchased a print title that contains an access token, please see the token for information about how to register your code.

For questions on access or troubleshooting, please check our FAQs , and if you can't find the answer there, please contact us. Very Short Introductions online.

Publications Pages Publications Pages. Recently viewed 0 Save Search. Mammals: A Very Short Introduction. Kemp T. Google Preview. The discoveries are also starting to reveal the evolutionary origins of many of the key traits of mammals — such as lactation, large brains and superbly keen senses.

This avalanche of discovery is also stirring up debate: some researchers disagree over which fossil groups are true mammals and the shape of the mammal family tree. In , at the Geological Society of London, naturalist William Buckland presented bones from one of the first known dinosaurs, Megalosaurus.

At the same talk, he revealed tiny mammalian jaws that had been found in the same fossil deposit. Their presence suggested that mammals had a very deep history, but as would happen repeatedly, the dinosaur discoveries completely overshadowed the mammal ones. The slow trickle of mammal finds from around the world continued for years.

Then in , researchers described the first ancient mammal from the fossil-rich rocks of Liaoning in northeastern China 2 , and the floodgates opened. Like the dinosaur fossils, they are dug up by local farmers and sold on to museums. But the dinosaurs continued to get the vast majority of the attention, says palaeontologist Steve Brusatte at the University of Edinburgh, UK. Typical mammal fossils from the Mesozoic era million to 66 million years ago are little more than teeth and jaw fragments, but Chinese specimens often have entire skeletons, with fur, skin and internal organs.

The finds overturned previous dogma. Mammals first appeared at least million years ago, and scampered amid the dinosaurs until the majority of those beasts, with the exception of the birds, were wiped out 66 million years ago. Among the first innovations that researchers began to find in fossil form were those to do with locomotion.

These animals swooped between the trees alongside some of the first flying dinosaurs, taking advantage of previously unexploitable food resources. Researchers found other specializations that they assumed had evolved only later: Agilodocodon could climb trees and gnawed into bark to feast on sap 6 ; the platypus-sized river-dweller Castorocauda had webbed feet and a beaver-like tail for swimming 7 ; and Docofossor had paws and claws for digging, and looked like a modern mole 8.

These mammals had also adapted to a multitude of diets, much more diverse than previously assumed. In , Krause described the groundhog-like Vintana from Madagascar 9 , a herbivore that perhaps fed on roots and seeds.

Many of these new-found fossil mammals belong to long-extinct subgroups, says Meng. In contrast to the panoply that existed in the Mesozoic, mammals today come in just three varieties: placentals, which make up the majority of species and include humans; marsupials, such as kangaroos and koalas, in which gestation in the womb is brief and development continues in a pouch; and the egg-laying monotremes, represented only by the platypus and several echidnas.

And a few dinosaur-era mammals were much bigger than suspected, too. The finds are not solely from China. Some of the most intriguing and oldest fossils — as well as the biggest gaps in our knowledge — relate to the southern continents, where only five genera of Mesozoic mammals and their relatives are known, compared with more than 70 genera from northern latitudes.

In the past two decades, Brazil has yielded several Triassic fossils that are more than million years old. The latest finds are also offering clues to the evolution of key mammal features.

For instance, the keen hearing of mammals is partly down to tiny bones in the middle ear — the malleus, incus and ectotympanic. But in the reptilian ancestors of mammals, these bones were part of the jaw, and were used for chewing instead of hearing. Mammal forerunners, such as shrew-like Morganucodon from million years ago, sported a prototype of the mammal arrangement that allowed for both functions The rat-sized fossil revealed three middle-ear bones, but they were still attached to the jaw by cartilage.

This was hard evidence of the evolutionary transition from jaw to ear. This exquisitely preserved million-year-old specimen of Maiopatagium furculiferum shows how early gliding evolved. Another unique trait of mammals is the sophisticated way they chew and ingest food in small parcels, rather than swallowing things whole as snakes and alligators do.

To make that possible, mammals evolved a wide variety of complex teeth for biting and grinding food. Drinking milk is made possible by the ability to suck and swallow, aided by the hyoid bones in the throat and muscles that support them. This apparatus also forms the voice box. In July, Luo published a paper revealing a million-year-old vole-sized docodont — a close relative of true mammals — that had the hyoid bones of its throat preserved Microdocodon gracilis is the earliest animal known to have been able to suckle like a modern mammal.

This level of detail is rare, and — similar to the study of the Kayentatherium hatchlings — the work on both the ear and throat bones has been made possible only through advances in microCT scanning techniques, says Krause.

The technique has also revealed details about the olfactory abilities and brains of early mammals. Early placental mammals evolved about million years ago and fossils of Eomaia suggest that it may have had several traits of placental mammals. It may have been only about 10 centimeters 4 inches long and was a tree climber that ate insects and worms.

The gradual and phased-out evolution of mammals led to the development of several features that differentiated between modern mammals and modern reptiles. Several mammalian traits can be attributed to their high levels of activity. A good example of this would be the system of double circulation with a four-chambered heart, the diaphragm, anucleate and biconcave erythrocytes, and the secondary palate that separates the food and air passages and facilitates breathing during suckling or mastication.

Hair on their body provides insulation which is known as endothermy or warm-bloodedness. It helps in maintaining the body temperature of an individual independent of the temperature of the external environment. Endothermy also allows mammals to have high levels of sustained activity. The unique characteristics of mammals are a result of the evolution of their complex and interrelated system. The mammals and reptiles evolved at a different pace and this led to distinct characteristics that separate them.

This evolution of mammals occurred as a response to a variety of interrelated conditions. At certain junctures in the period of transition from reptiles to mammals, some forms combined the characteristics of both groups.

This pattern of evolution is termed mosaic and is common in transitions that mark the origin of a new and major adaptive species. Some authors suggest that to have simplified definitions and strict delimitation of the Mammalia, only one trait like the teeth must be the base for creating a boundary. By using a single osteological character, the placement of numerous fossil species can be done in a logical classification. Other mammalian characteristics such as the nursing of the young, degree of endothermy, and the condition of the internal organs may not be evaluated.

If the advanced therapsids were alive today, it would have been difficult for taxonomists to classify which ones were to be placed in the Mammalia and which in the Reptilia category. Scientists have done the higher classification of Mammalia based on a broad array of characteristics. Evidence from comparative anatomy was considered extremely important earlier, but recent information from other disciplines like serology study of immune reactions in body fluids , physiology, and genetics has provided insights that can be used while considering relationships.

Paleontological findings have also supplemented the comparative study of living organisms. The fossil records provide a historical dimension to the existing information on mammalian relationships. In some mammals like horses, the fossil record has been used in detail to trace the lineages. The fossil record of mammals is better than that of other major vertebrate groups. Several factors affect fossilization, the most important ones being the habitat of the organism, its structure, and conditions at the time of death.

The teeth and the related jaw bones and the skull are the most common remains of mammals. The enamel that covers the mammalian tooth consists of prismatic rods of crystalline apatite. It is the hardest tissue in the body of mammals and is highly resistant to physical and chemical weathering. A large number of teeth were found in deposits of fossil mammals.

They were the reason for using dental characteristics in the interpretation of mammalian relationships and phylogeny. Dental features exhibit the broad radiation of mammalian feeding specializations from when they were predaceous.

It is the reason why they are ideal for this key role in classification. Based on their reproduction process, mammals can be classified into three classes: Eutheria, Prototheria, Metatheria. Mammals under this category produce young ones that develop inside the body of the mother in the form of babies. They receive nutrition through the placenta of the mother. This subclass can be classified further into 19 orders.

The primary ones include:. Kangaroos are a good example of Metatheria. This classification is divided into seven different orders:. Marsupial modes.