Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Wang Shiqi, Li Chunxiao
Elephants are the largest mammals that live on land and belong to the order Proboscis. The proboscis order is represented by only three species of elephants, namely the African savannah elephant, the African forest elephant, and the Asian elephant, but the evolutionary history of the proboscis order is extremely prosperous, and at least more than 180 species of fossils have been found. In particular , proboscis is one of the few living mammals of the order I whose fossil representation can be found in the Paleocene. Although molecular biology presumes that the dominant order of living mammals, including primates, diverged 65 million years before the Great Cretaceous extinction, this is only a theory and does not have enough fossil evidence to support it, while the earliest representative of the proboscis order, Eritherium, was found in African Morocco, about 60 million years ago in the Paleocene.
Africa is a special region in the geological history of the Mesozoic Era, since the middle of the Jurassic 165 million years, the Pancontinent has been cracked, the African plate has been separated from other continents by the ocean, and early mammals began to evolve independently on the African continent, producing a taxon called the African Mammal, whose descendants have branched out from Africa and evolved to the present, including a number of species with very different morphological and ecological adaptations. Elephant shrews and golden shrews as small as rats, as well as rabbit-like hoofed rabbits, termite-eating aardvarks, and manatees that are fully adapted to marine life, as well as terrestrial behemoths, the elephants that are the protagonists of our article, may all derive from the ancestors of a common African beast. By 20 million years, the African continent and Eurasia had collided, and the mammals of the two continents had met again after hundreds of millions of years apart, and as the best of them, the proboscis went further than most African mammals, and eventually distributed on several continents except Australia and Antarctica. But we need to keep in mind the African origin attributes of proboscis, because the vast majority of later branches of proboscis, despite their widespread distribution around the globe, often originated from Africa, which seems to be somewhat similar to humans, but the ancestors of humans may have originated in Asia before the Oligocene (about 34 million years), and it was only after the Oligocene that the evolutionary centers of primates and humans shifted to Africa.
Returning to the evolution of proboscis, the closest living taxon to proboscis is the manatee order, and it is difficult to imagine that elephants have an aquatic distant brother, however, manatees also have fangs-like incisors close to elephants, as well as replacement growth buccal teeth, which indicate that the two are closely related. Another living group closely related to proboscis is the hoofed rabbit , the size of a rabbit , which lived in Africa and Arabia , but was also historically distributed in Places such as China. There are also two fossil taxa, one is a large group of heavy-legged mammals, of which the Egyptian heavy-legged beast has a large and a small pair of huge horns on its head; the other is the order Solodont, a strange large semi-aquatic mammal, distributed on both sides of the Pacific in Japan, the United States and other places. These strange animals, together with proboscis, form near-ungulates and represent a pet evolutionary lineage.
Protohoated rabbit
Heavy-footed beasts
Since the emergence of Akebonos in the Paleocene 60 million years ago, proboscisses have embarked on an independent evolutionary path. This is the first of three stages in the evolution of proboscis. At that time, the akebode was only 3 to 8 kilograms, about the size of a rabbit. However , its molars have evolved some of the features of later Eocene proboscis , namely molars that tend to have some bispincross. Note that this feature is far from the molar features of later elephants. By about 55 million years after the Eocene, the Phosphatherium had evolved, still found in Morocco, Africa. At this time, the weight of the phosphorus elephant increased to 10 to 15 kilograms, the size of a piglet, and the shape of a piglet. The molars of the apatite elephant develop into a true bisclinary, which is typical of the proboscis in the first stage; its upper and lower second incisors begin to enlarge, and in the future this enlarged incisor teeth will evolve into true fangs as one of the typical features of modern elephants; and the most important feature of the proboscis class, the proboscis, has not yet shown signs of development during this period. Proboscis of the same period or slightly later as the apatite elephant are the Daouitherium and the Numidotherium, which have reached the size of tapirs and have become some of the passers-by in the early evolution of the proboscis. By about 40 million years in the late Eocene, a true behemoth had emerged in the proboscis, the Barytherium, with a shoulder height of nearly 2 meters and a weight of about 2 tons. Since its discovery, the heavy beast has been regarded as a mysterious animal, until the skeleton behind its head was discovered, we know that the 40 million-year-old heavy beast has some modern elephant skull characteristics, but also shows that the larger body is a very early evolutionary trend of proboscis. A large body is of great significance for proboscis to protect itself and resist natural disasters; especially the later proboscis evolved a flexible proboscis, which can be grasped like a hand, solving the evolutionary problems of other large-sized animals, thus promoting the continuous prosperity of proboscis in the late Cenozoic generation for tens of millions of years.
Phosphorus
We have already said that the first stage of proboscis evolution is typically characterized by bispinnal molars, which reach the pinnacle of our familiar phobes and are the last remnants of the first stage of the evolution of proboscis. The fossil record of the Oligocene is missing, and by the Late Oligocene for about 25 million years, the first phobic elephant appeared, the Chilgatherium in Ethiopia, followed by prodeinotheirum and Deinotheirum, which spread to Europe and Asia at the same time as the indigodgets and mammoths described below. Dinosaurs are very large animals, weighing more than 10 tons, and have entered the largest proboscis class, it lacks upper incisors, and has a pair of lower front teeth that bend downward into a hook, and the nose should not be too long. It is particularly worth mentioning that before the beginning of this century, no fossils of dinosaurs have been found in China, and it seems to be a foregone conclusion that dinosaurs have not entered China. However, this speculation was shattered in 2007, when a research team led by Academician Qiu Zhanxiang and Researcher Deng Tao found fossils of Chinese proto-phobias in the early Late Miocene (about 10 million years) in Dongxiang County, Gansu Province, which became solid evidence that the dinosaurs entered China. The reason is likely that before the Late Miocene, there were dinosaurs in southern China, and there are reports of dinosaurs in Thailand and Myanmar, but the strata of this period are missing in southern China, so there is no fossil record of dinosaurs. At the turn of the Middle and Late Miocene, a mass extinction occurred in northern China, when the flourishing shovel-toothed elephant and the incarcerated tooth elephant were completely extinct, thus leaving ecological space for the southern elephant, so the elephant went north and left this invaluable record in China's Linxia region. As the climate of the Late Miocene cooled further, the elephant had to withdraw from northern China and gradually disappeared in Eurasia, and the last fear disappeared in Africa in the Middle Pleistocene (~100,000 years).
Fear
Returning to Late Eocene Africa, a proboscis that appeared at the same time as the heavy beast is more famous, the Moeritheirum, commonly known as the Archaeopteryx. It has long been considered the direct ancestor of elephants. Fossils such as later discoveries of apatite elephants gradually changed this understanding. The posterior skeletal morphology of the head of the Mohu beast suggests that it was suitable for amphibian life, such as today's hippopotamus, which also had enlarged incisors but did not have a long nose. The molars of the Mohu beast have begun to develop towards mound type, changing the paradigm of the bispinnal molars of the early proboscis class, and also foreshadowing some future evolutionary directions of the proboscis, although the evolution of the Mohu beast may have deviated from the main line of the evolution of the proboscis.
Archaeopteryx
After that, the evolution of proboscis entered the second stage. It was at this stage that proboscisses successfully spread across the globe, representing the beginning of the real prosperity of proboscis, and it was also at this stage that proboscis really evolved proboscis, although proboscis may not be as flexible as living elephants, so that proboscis in this period really became a proboscis taxa. At the beginning of the second stage, the real typical feature of the proboscis is the growth of the lower jaw, the upper and lower second incisors are particularly enlarged, the upper teeth become fangs, the lower teeth are shovel-shaped or stick-shaped, becoming a powerful feeding tool, and the proboscis is only used as an auxiliary tool to cooperate with the incisors and the lower jaw for feeding. From this period onwards , proboscisses have a more familiar name , elephants , that is , elephantoformes at the suborder level , while apatite elephants , Mohu beasts , and even dinosaurs , etc., cannot be taxonomically called elephants. The molars of elephants abandon the structure of the first stage of the biscingord, and the number of ridges gradually increases, changing to a mound or a complete mound, because the tip of their teeth is shaped like a papillary protrusion, so the elephants at this stage are collectively called mastodons. The teeth of the mastodon begin to evolve horizontally, secretly pointing to the third stage, which is not listed for the time being.
The earliest mastodons are represented by the Paleoeomastodon and Phiomia elephants of Africa in the early Oligocene (about 30 million years old). They were already quite large animals at the time, with their noses and jaws already extended, and their lower incisors in the shape of a shovel, apparently a powerful feeding tool. After the slow development of the entire Oligocene, mastodons began a large-scale differentiation at the time of entering the Miocene (about 25 million years), first differentiating into two major classes, mammoths and incarcerated elephants, represented by Losodokodon and Eritreum, respectively. Although some clues from South Asia and China suggest that it is possible that proboscisses reached southern and eastern Asia before 20 million years, proboscises entered Eurasia on a large scale in the middle of the Early Miocene (about 18 million years), forming the so-called intarsia land bridge event, which has also become an important symbol of the recombination of Africa and Eurasia in geological history. Also in the early Miocene, mastodons evolved into four family groups, the Mammothidae, the Mosaics, the Plesiosauridae, and the Shoveltooth. All of these mastodons had an elongated jaw at the beginning of their evolution, and the molars had three ridges or ridges, also known as three-sided long-jawed mastodons. Of these four families , the molars of the Mammothaceae are mound-shaped , and the other three families are mound-shaped , with different morphologies of the lower jaw and lower incisors. The leptophores inherit the lower incisors of the fayum elephant shovel, and the lower jaw is further widened; the lower jaw of the inlaid tooth is narrowed and the lower incisor becomes stick-like; while the larval tooth is the most special, the lower jaw is long grooved, but the lower incisor is degenerated, and only the blade structure protruding from the front end replaces the lower incisor. These differentiations show that mastodons have taken broad strides in different directions in adapting to different ecological environments. During this period , the Mammothidae were represented by the African Eozygodon and eurasian yokes ( Zygolophodon ) , both of which were mastodons with high ridged teeth ; the Family of Mammostomidae was represented by the African Afrochoerodon and the Asian yokehordon , and the Family of Aryntidae was represented by gomphotherium , which spread throughout Eurasia and Africa The family Ofobodondae was particularly prosperous, with archaeobelodon and proto-denatus found in Eurasia and Africa, while Platybelodon flourished in northern China. Shovel-toothed elephants have very exaggerated shovel-shaped lower jaws and lower incisors, and although it was early speculated that they lived at the water's edge and fed on aquatic plants, later evidence suggests that they most likely lived in semi-open areas between sparse forests, cutting off plants with their lower incisors and obtaining food. In many locations in the Miocene in China, such as Linxia in Gansu, Tongxin in Ningxia, and Tonguer in Inner Mongolia, shovel-toothed elephants are the most common molecules in fauna, far more numerous than other large mammals.
Fayoum elephant
Yoke tooth elephant (image from the Internet)
Incarcerated elephants
At the beginning of the Miocene (about 16 million years), the Miomastodon in the Mammoth family first crossed the Bering Strait and reached North America, and other taxa in the family Edophoridae followed, becoming an important member of the mammalian population of the North American continent. Later, it can be seen that when the evolution of mastodons in late Miosocene Eurasia suffered major setbacks, North America became the last refuge for many long-jawed mastodons, for example, the shoveling incisor elephant (Amebelodon) in the family Shoveltooth, which lived in North America to the late Miocene (~6 million years old) and evolved the largest jaw of land animals. The Gnatabelodon in the family Gnatabelodon also lived until the middle of the Late Miocene (~7 million years). The Mammut americanum of the Mammut americanum family survived in North America until the beginning of the Holocene (~10,000 years), and the descendants of the zosteria entered South America in the evening Cenozoic, creating the last boom of mastodons and surviving until the beginning of the Holocene, although both the American mastodon and the South American lacta have evolved into short-jawed elephants, which will be discussed further below.
American mastodon
From the second half of the Early Miocene to the end of the Miocene (18 million to 11 million years), seven or eight million years is the peak of the evolution of mastodons, and a variety of triangular long-jawed mastodons occupy the ecological space that large plant-eating animals can occupy, however, they are unaware that an evolutionary crisis of proboscis is quietly approaching. During this period of prosperity of the three-sided long-jawed mastodon, it corresponds to the "moderate climate suitable period" in geological history, which is also the last warm period in the world so far, and the mastodons that prosper in the warm period cannot predict the survival challenges brought to them by the global climate cooling after the end of the warm period, of course, the spearhead of this survival challenge is not only for the mastodons, but for the entire ecosystem. Since 14 million years, the climate has gradually cooled, forest ecosystems tend to decline in the middle and high latitudes of the world, and grassland ecosystems representing drought have become the mainstream ecological environment of middle and high latitudes. In particular, in most of Asia's mid- and high-latitude regions, drought continued in these regions with the rapid uplift of the Qinghai-Tibet Plateau, culminating in a major restructuring of eurasian ecosystems at the turn of the Mid-To-Miocene in the mid-to-late Miocene. According to research, in the northern region of East Asia, no large mammal has survived this boundary, and the three-sided long-jawed mastodon represented by the shovel-toothed elephant has suffered a catastrophe and has been extinct. Only a small number of triangular mastodons remain in the forested areas of the south, or survive in the Americas, where climate change is less intense, as described above.
Some mastodons have evolved new survival strategies when they have suffered a serious existential crisis. The first is the increase in the number of teeth ridges, from three to four edges, and even more edges to adapt to increasingly rough plants; the second change is that the molars have completely achieved horizontal replacement, one molar is worn out, and then another molar is used, thus greatly prolonging the use of molars; the most important change is that the mastodons have been trained for twenty million years, and the evolution of the proboscis has reached an unprecedented height in the history of evolution, and their noses are dexterous enough to be comparable to human hands to independently complete the function of feeding, so the most important change is that the mastodons have been trained for twenty million years, and the evolution of the proboscis has reached an unprecedented height in the history of evolution, and their noses are dexterous enough to be comparable to human hands to independently complete the function of feeding, so therefore, Mastodons began to abandon their long jaws, and their lower jaws began to evolve in a shortened direction. As mentioned earlier, a mound-type inlaid toothed elephant in the Americas, the jaw continued to shorten, and after the stage of the beaked elephant (Rhynchotherium), evolved the completely short-jawed southern mastodon (Notiomastodon) and Cuvieronius, which rapidly spread into the South American continent when the Isthmus of Panama was formed at the end of the Upper Miocene, becoming the dominant species in the local area. At the same time, mammoths also evolved in Eurasia and the Americas, producing the short-jawed Mammut borsoni and the American mastodon, of which the American mastodontis migrated from the Pliocene (~5 million years) or evolved from the native mammoths, which is a highly controversial topic. Perhaps similar to human evolution, the Mamm Elephant has migrated and merged more than once from Eurasia to the Americas. Although these mastodons of the Americas have a completely shortened lower jaw and the lower incisors eventually disappear, they still retain the original molar pattern of triangular edges.
The real evolutionary leap occurred in Eurasia and Africa in the Late Miocene, from the mound-type inlaid toothed elephant produced three branches, the more conservative one to its evolutionary end, only evolved into a tetragonal tooth, but the arrangement of the tooth ridge has changed, that is, the mutual prismatic tooth elephant (Anancus), the other two branches are more advanced, the number of tooth ridges can reach more than eight ridges, and in the Late Pleistocene it is more than twenty ridges. One of them, the molars that originated in southern Asia and still retain the low-crested molars, known as the saber-toothed elephant family, and the other branch originated in Africa, the home of proboscis evolution, and is the only proboscis that has survived to this day, the elephant family, or the true elephant family. When saber-toothed elephants and truths arose, proboscis also evolved to the third stage, representing the final prosperity of proboscis.
Reciprocal dentistry
Stegolophodon, the ancestor of the saber-toothed elephant ( Stegolophodon ) , exhibited quite progressive features at a very early age. In the early Miocene, other elephants were triangular molars, and when the form of the long jaw, the lower jaw of the ridged molar began to shorten, and the molars evolved into a tetragonal molar. Centered on Southeast Asia and spreading only to China, Japan, and South Asia, the ridged dentist is the dominant elephant in eastern China and Japan in the Miocene, which shows that it has been adapted to a relatively humid ecological environment. By the late Miocene, about 6 million years ago, the ridge-toothed elephant evolved into a saber-toothed elephant, which appeared in Yunnan, China, that is, the Zhaotong saber-toothed elephant, but confusingly, Some "saber-toothed elephant" buccal teeth were also reported in Africa about 7 million years ago. Since the full evolutionary sequence of the early saber-toothed elephants was found only in the eastern and southern parts of Asia, and the early "saber-toothed elephants" in Africa were produced at the same time as the true images, these African "saber-toothed elephants" are likely to be some of the earlier ones, and the morphology of the buccal teeth is very similar to that of the saber-toothed elephants. Those African "saber-toothed elephants" soon became extinct, while the Asian saber-toothed elephants flourished in the early Cenozoic (5 million years), spreading throughout the north and south of China, including the Yellow River elephants written in Chinese primary school textbooks and praised by literati in poems. The Yellow River elephant is also among the largest proboscis species, with a shoulder height of nearly 4 meters and an estimated weight of 12 tons. As the Arctic ice sheet of the Late Cenozoic (3 million years) decays year-round and the climate turns sharply colder, the saber-toothed elephant retreats into the south again. In China, it is represented by the pre-Eastern saber-toothed elephants of the Early Pleistocene, the Eastern saber-toothed elephants of the Middle Pleistocene and the Late Pleistocene. In Southeast Asia and South Asia, similar species also survive. Although the warm and humid climate in the south during the Pleistocene period became an excellent refuge for saber-toothed elephants compared to the harsh cold in the north, however, when the Holocene arrived in 10,000 years, the saber-toothed elephant eventually became extinct in southern China and on the planet. The Holocene was a period of global warming, theoretically more suitable for the survival of saber-toothed elephants, but why did saber-toothed elephants fall on the eve of dawn? The study found that saber-toothed elephants had a more limited range of food than their rivals, and a reduced food source may have been the main reason for their extinction during the Holocene Warm Period.
The fate of the saber-toothed elephant brother True Elephant class is even more ups and downs. The truth originated in Africa about 7 million years ago in the middle of the Late Miocene, and unlike the saber-toothed elephant, africa's in the Late Miocene was already a typical monsoon steppe climate, and the relatively arid climate prompted the truth to develop in the direction of increasing the crown of teeth at the beginning, and the huge molars were lined with dense tooth plates, like huge millstones, enough to crush any rough food. In the early true image, although the lower jaw has been shortened, a pair of lower incisors is unusually developed, which is the Stegotetrabelodon (Stegotetrabelodon) with four long teeth. I don't know if it was because there were too many four tusks and didn't have a big reason, but the saber-toothed elephant soon became extinct, producing a ridged double incisor elephant (Stegodibelodon) and primelephas with only one pair of upper incisors. Since then, three major true groups have emerged in Africa, namely the familiar African elephants (Loxodonta), Asian elephants (Elephas), and mammoths (Mammuthus). There is also a class of palaeoloxodons, which are more evolutionarily closer to Asian or African elephants. Morphology and molecular biology give different interpretations. The evolutionary history of African elephants is mainly limited to Africa, and Asian elephants and mammoths spread to Eurasia in the late Cenozoic, with mammoths becoming the most dominant elephant species in the northern eurasian region. In Eurasia, mammoths have undergone different stages of evolution in the Late Cenozoic Romanian elephant, the Early Pleistocene Southern elephant, the Middle Pleistocene steppe mammoth, and the Late Pleistocene true mammoth, the mammoth. Southern elephants entered North America during the Middle Pleistocene and evolved into columbus mammoths. Southern elephants, steppe mammoths, and Columbus mammoths became the leaders of individual sizes among proboscis, weighing nearly 20 tons. The ancient diamond tooth elephant can also reach this size. In the Pleistocene of the climate, due to the periodic changes in the Earth's orbit around the sun, glacial and interglacial periods alternated, and in Eurasia, the ancient rhombodons distributed at high latitudes and mammoths and middle and low latitudes disappeared one after another, and the distribution range also showed nodes with glacial periods and interglacial periods.
Saber-toothed elephant
Colombian mammoth
In the Pleistocene for more than two million years, although the rapid development of humans brought some trouble to elephants, they showed good adaptability in the harsh environment, spreading throughout Eurasia, Africa, and North America. However, the ultimate fate still falls on them. Similar to the fate of saber-toothed elephants, at the beginning of the Holocene Warm Period, the true elephant class also underwent a fundamental decline, and the ancient rhombodon and mammoth were completely extinct, although only around 3,000 years ago, there were still very small mammoth populations living on islands in the Arctic Ocean in northeastern Russia.
Proboscis is not the only victim of the arrival of the Holocene, along with the extinction of mammoths and ancient rhombus, there are also a large number of strange creatures such as woolly rhinoceros, earth sloths, sculptodonts, etc. Perhaps our entire ecosystem will once again face an ecological crisis of large animal extinction and even a global species extinction, but this crisis may be the death of the entire proboscis. Asian elephants and African elephants are still in danger on our planet; today, 11 million years after they faced their last crisis, elephants' unparalleled molars and earth-made proboscis have exhausted their full evolutionary potential — you can't imagine that they can evolve new organs like their ancestors, and thus have magical return power. The proboscis class will eventually withdraw from the curtain of history after 60 million years of difficult years and three great ups and downs. Perhaps we need to accept the fact that in the universe, there are rises and falls, there are births and deaths, species and life, have their own cycles, and in the process of species evolution, there is a force that we cannot fully understand and grasp, quietly determines their life and death, just like ourselves, we are also unable to grasp our own destiny. Although the past cannot be told, the comers may not be traceable, but the evolutionary story of the elephant class has composed a beautiful and colorful dream of life and death, which is enough to remain in the long river of history and the depths of our hearts that love nature.
Editor-in-Charge: Huang Xiaofeng
Proofreader: Ding Xiao