Who are Placental Mammals?
Placental Mammals are the species of the kingdom Animalia. The only reason for placental mammals definition is the placenta that is attached to them. It is one of the placental mammals characteristics. The placenta is the vascular organ found in most mammals that attaches the uterus of the mother to the fetus. This organ develops during the gestation period and helps in the metabolic exchange of wastes and nutrients between the blood of the fetus and the mother. This also serves as the medium of respiration for the young.
Placental mammals are one of the three surviving subdivisions of the class of animals Mammalia. The other two non placental mammals are Marsupialia and Monotremata. Some authorities consider Marsupials to be placental mammals despite their under-developed placenta. The Marsupial placenta is also comparatively less efficient. These placentas even limit the gestation period. The placenta within the true placental animals requires a longer period of development within the womb along with its protection. This is one of the main reasons for the successful evolution of the group.
Types of Placental Mammals
The classification of placental mammals is analyzed based on the retroposon presence or absence patterns. This analysis provides a rapid and unmistakable means for revealing the history of the evolution of organisms. There are majorly 3 lineages or types of placental mammals: Xenarthra, Boreoeutheria and, Afrotheria. All of these diverge from common ancestors. The living orders in the 3 groups include:
1. Afrotheria (Golden Moles, Tenrecs, Elephants, Elephant Shrews, Manatees, and Hyraxes)
Superorder – Afroinsectiphilia
Order – Afrosoricida (golden moles and tenrecs)
Order – Macroscelidea (elephant shrews)
Order – Tubulidentata (aardvark)
Superorder – Paenungulata
Order – Hyracoidea (hyraxes)
Mirorder - Tethytheria (dugongs, manatees and, elephants)
Order – Sirenia (manatees and dugong)
Order – Proboscidea (elephants)
2. Boreoeutheria
Superorder – Euarchontoglires (rodents, primates, rabbits, colugos, hares and, treeshrews)
Grand Order – Gliriformes
Mirorder – Glires
Order – Rodentia (rodents: beavers, mice, rats, squirrels, voles, etc.)
Order – Lagomorpha (pikas, hares, and rabbits)
Grandorder – Euarchonta
Order – Scandentia (treeshrews)
Mirorder – Primatomorpha
Order – Primates (lorises, lemurs, apes, monkeys, and humans)
Order – Dermoptera (colugos)
Superorder – Laurasiatheria (hoofed mammals, seals, cats, dogs, bats, whales, moles, shrews, and hedgehogs)
Order – Chiroptera (bats)
Order – Eulipotyphla (solenodons, moles, shrews, gymnures, hedgehogs)
Order – Perissodactylia (odd-toed ungulates: tapirs, rhinoceroses, zebras, donkeys, and horses)
Order – Artiodactyla (even-toed ungulates: whales, pigs, camels, deer, antelope, cattle, etc.)
Mirorder - Firae (mongooses, seals, bears, cats, dogs, pangolins, etc.)
Order – Carnivora (carnivorans: mongooses, seals, bears, cats, dogs, etc.)
Order – Pholidota (pangolins)
3. Xenarthra (Sloths, Anteaters and, Armadillos)
Order – Pilosa (sloths and anteaters)
Order – Cingulata (armadillos)
The exact relationship between these three major types of placentals is still debatable. Four hypotheses have also been proposed which explains which group is first diverged from other placentals and which group is basal. The hypotheses are Epitheria (basal Xenarthra), Exafroplacentalia (basal Afrotheria), Atlantogenata (basal Boreoeutheria), and another hypothesis based on a near-simultaneous divergence. Depending upon the type of DNA, mitochondrial or nuclear, varying interpretations of the paleogeographic data, the divergence times among these three placental groups is estimated to be ranging from 105 to 120 million years ago.
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Features
The anatomical features of placental mammals are different from that of other mammals by:
The presence of a wide opening at the bottom of the pelvis is sufficient enough to allow a baby which is relatively large when compared to the mother.
Extending forward from the pelvis, the epipubic bones are absent, which are commonly found in other non placental mammals. These bones inhibit the expansion of the abdomen in mammals during pregnancy but on the contrary, helps the non placental mammals in locomotion by stiffening their body.
The foot forms a complete mortise and tenon upper ankle joint by fitting the rear-most bones of the foot into a socket which is formed by the ends of the fibula and tibia.
At the bottom of the fibula, malleolus is present.
Origin and Evolution
The placental mammals which include all the modern placentals along with the crown group belong to the stem-group members of the clade Eutheria. They have existed and evolved approximately between 163 million and 157 million years ago, that is, since the middle Jurassic period (201.3 million to around 145 million years ago). The early eutherians were small in size and were adapted for life in trees. They were nocturnal and fed themselves on insects. The true placentals are suspected to have an origin in the Late Cretaceous, around 90 million years ago. But the earliest unchallenged fossils are found to be from the early Paleocene, around 66 million years ago, following the Cretaceous-Paleogene event of extinction.
The species Protungulatum donnae was considered to be a stem-ungulate, known to be a meter above the Cretaceous-Paleogene boundary in the geological stratum which marks the extinction event of Cretaceous-Paleogene and, the species Purgatorius was thought to be a stem-primate previously, which appears for at most 300,000 years after the K-Pg boundary. However, both of these species are now considered to be non-placental.
According to the molecular clocks, after the mass extinction of placentals at the end of the Cretaceous and then their rapid appearance suggests that the group had already originated and gone through an initial diversification in the Late Cretaceous. Around 90 million years ago, the lineages leading to Afrotheria and Xenarthra was probably originated. The Boreoeutheria, on the other hand, underwent an initial diversification around 70 to 80 million years ago and produced lineages that led to carnivorans, insectivores, rodents, modern primates, and, artiodactyls.
The modern placental mammals meaning, the placentals with origins in the Paleogene about 66 to 23 million years ago. When most dinosaurs and other animals disappeared with the Chicxulub asteroid impact, a lot of ecological niches were left open which the mammals took advantage of and quickly evolved. The evolution of crown orders like rodents, carnivores and modern primates seem to be a part of such adaptive radiation. With the engagement of new niches, the size of mammal bodies increased significantly and the niches of large herbivores and large carnivores have been taken over which were left open by the destruction of the dinosaurs.
Placentals have also exploited many niches that the dinosaurs had not. For instance, whales firstly occupied the rivers and freshwater lakes and then moved into the oceans; and bats are highly effective aerial, nocturnal insectivores that evolved echolocation and flight. Meanwhile, the primates acquired hands and feet, specialized for grasping branches along with keener vision that helps them in foraging in the dark. Unlike smaller placentals like primates and rodents who left Laurasia, colonized Africa and then South America with the help of rafting, land placentals cannot cross large water bodies. So, their evolution followed different pathways in different countries.
In Africa, Afrotheria went through major adaptive radiation, which eventually led to golden moles, elephants, elephant shrews, manatees, tenrecs and aardvarks. Similarly in South America, the radiation of Xenarthra led to armadillos, modern sloths and, anteaters, along with the extinct ground sloths and glyptodonts.
Boreoeutheria dominated the expansion in Laurasia, which includes carnivores, insectivores, artiodactyls, perissodactyls, primates, and rodents. Due to the formation of land bridges that linked Africa to Eurasia and North America to South America, these groups grew beyond a single continent. According to a study on eutherian diversity, during the Paleocene, the placental diversity was constrained while on the other hand, the multituberculate mammals diversified. Later on, the multituberculates declined and the placentals exploded in diversity.
Conclusion
Due to the presence of fossil evidence of the extinct non-placental eutherians, who were the predecessors of the placentals of the modern-day, potential for confusion is always associated. To overcome this confusion, the clade Pacentalia is made a classification by scientists for placental mammals meaning, this clade comprises all the living placental mammals along with their most recent common ancestors.
FAQs on Placental Mammal
1. What Do you Know About the Early Development in the Eutherians?
Ans: Just like their closest relative, the marsupials, Eutherians also give birth to the young ones. The young are nurtured within the body of the mother with the help of the placenta. Marsupial young are born hairless. In the initial years, they are helpless and crawl into a special pouch that helps them in their further development. A similar trait is seen in some species of eutherian mammals, such as some carnivores and rodents where the young are born blind and hairless. Before they are eligible enough to live on their own, they are nurtured by their mother. The basic characteristic that differs them from the Marsupials is that the young are not raised in a pouch. The degree of development among Eutherian groups varies extensively. Where young ungulates can start walking within minutes of being born, a human child may take months or even years to accomplish the same. Along with this, the time of parental care also varies from about a month in some cases to years in others.
2. What is the Diet Followed By Eutherians?
Ans: Just like the vast variety of groups of Eutheria, the choices of food also differ greatly. The herbivores like elephants and ungulates feed on plants. They may also feed on the leaves of trees and shrubs (browsing), low herbs (grazing) or, on grasses. The Carnivora are carnivores who feed primarily on the flesh of other animals. Some of the carnivores also get their nourishment on a particular prey like fishes or insects, which they specialize in. Other omnivores include bears and raccoons who live on both plants and animal meat.
A vast study is conducted regarding the Eutherian mammals based on the correlation of their size and the amount of food required by them to maintain a constant body temperature. From studies, it is found that in general, with the decrease in size, the metabolic resting rate, (that is, the rate with which the body burns energy while being at rest) increases exponentially. For instance, a mouse eats more food concerning the size of its body than elephants eat, to maintain its body. This may become a serious issue for species that live in areas like the arctic region where the amount of food is scarce for several months throughout the year. This problem is tackled by collecting food before such months, by building up a fat layer and hibernating, or by seasonal migration to more favourable habitats.
3. What are the Details of Placental Reproduction?
Ans: The pros of placental reproduction are: The fetus can grow for a long time with the help of the placenta in the uterus. As a result, the fetus grows large and becomes mature before its birth. This helps in the survival of the fetus.
The cons of placental reproduction are: The process is very draining and most importantly, risky for the mother. The mother has to keep in check her diet and eat more food so that the fetus is nourished properly. With the increase in the size of the fetus, the body of the mother becomes heavier and reduces mobility in her as a result, her chances of escaping a predator reduces extensively. As the fetus is inside her body, she cannot abandon the fetus in case she is hunted or if food is scarce. Most importantly, giving birth to a large infant becomes risky and may lead to the death of the mother, in some cases.
4. What Do You Know About the First Placental Mammals?
Ans: Researches have been conducted for years to infer when the placental mammals originated and when the groups’ modern orders first emerged. Molecular sequences from living species and the analysis of several anatomical features of both extinct and modern mammals are used to find the same. The study hints at what the first placental mammals looked like. Previously, a lot of conflicts have been yielded in attempting to reconstruct the evolutionary history of mammals. Studies based on molecular data show that the group may have appeared around 100 million years ago when the dinosaurs were still thriving. On the other hand, fossil studies show that the placentals were seen shortly after an asteroid that slammed and destroyed the dinosaurs, around 65 million years ago. However, results show that the ancestors of all placental mammals are inferred to be tree-climbing and insect-eating mammals with weights ranging from 6 to 245 grams. It had a long tail, the fur-covered body which had a complex brain and gave birth to a single young.
