science

Placentalia

Placentalia is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Placentalia rather than just read about it. In short: Placentalia () is one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguished from monotremes and marsupials in that the fetus is carried in the uterus of its mother to a relatively late stage of development.

Key takeaways

Placentalia belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Placentalia to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Placentalia from memory before moving on to harder problems.

Reference excerpt

Placentalia () is one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguished from monotremes and marsupials in that the fetus is carried in the uterus of its mother to a relatively late stage of development. The name is something of a misnomer, considering that marsupials also nourish their fetuses via a placenta, though for a relatively briefer period, giving birth to less-developed young, which are then nurtured for a period inside the mother's pouch. Placentalia represents the only living group within Eutheria, which contains all mammals that are more closely related to placentals than they are to marsupials.

Anatomical features Placental mammals are anatomically distinguished from other mammals by:

a sufficiently wide opening at the bottom of the pelvis to allow the birth of a large baby relative to the size of the mother. the absence of epipubic bones extending forward from the pelvis, which are found in all other mammals. (Their function in non-placental mammals is to stiffen the body during locomotion, but in placentals they would inhibit the expansion of the abdomen during pregnancy.) the rearmost bones of the foot fit into a socket formed by the ends of the tibia and fibula, forming a complete mortise and tenon upper ankle joint. the presence of a malleolus at the bottom of the fibula. instead of a cloaca like monotremes, marsupials and most other vertebrates, the urogenital ducts exit through the vulva or penis and the rectum opens as the anus. the presence of a corpus callosum in between the cerebral hemispheres.

Subdivisions Analysis of molecular data led to rapid changes in assessments of the phylogeny of placental orders at the close of the 20th century. A novel phylogeny and classification of placental orders appeared with Waddell, Hasegawa and Okada in 1999. "Jumping genes"-type retroposon presence/absence patterns have provided corroboration of phylogenetic relationships inferred from molecular sequences. It is now widely accepted that there are three major subdivisions or lineages of placental mammals: Boreoeutheria, Xenarthra, and Afrotheria. All of these diverged from common ancestors. 2022 studies of Bertrand, O. C. and Sarah L. Shelley have identified leptictids, palaeoryctids and taeniodonts as basal placental mammal clades. Leptictids were also proposed to be close relatives of Boreoeutheria. The 19 living orders of Placentalia in the three groups are:

Magnorder Atlantogenata Superorder Xenarthra Order Cingulata (armadillos) Order Pilosa (sloths and anteaters) Superorder Afrotheria Grandorder Afroinsectiphilia Order Tubulidentata (aardvarks) Mirorder Afroinsectivora Order Afrosoricida (tenrecs, otter shrews, and golden moles) Order Macroscelidea (elephant shrews) Grandorder Paenungulata Order Hyracoidea (hyraxes) Mirorder Tethytheria Order Proboscidea (elephants) Order Sirenia (dugongs and manatees) Magnorder Boreoeutheria Superorder Euarchontoglires Grandorder Gliriformes Mirorder Glires Order Lagomorpha (rabbits, hares, and pikas) Order Rodentia (rodents: mice, rats, hamsters, guinea pigs, chinchillas, capybaras, porcupines, voles, squirrels, beavers, etc.) Grandorder Euarchonta Order Scandentia (treeshrews) Mirorder Primatomorpha Order Dermoptera (colugos) Order Primates (primates: monkeys, apes (including humans), lemurs, lorisids, galagos, and tarsiers) Superorder Laurasiatheria Order Eulipotyphla (hedgehogs, gymnures, shrews, moles, and solenodons) Order Chiroptera (bats) Grandorder Ferungulata Mirorder Euungulata Order Artiodactyla (even-toed ungulates: cattle, antelopes, sheep, deer, camelids, pigs, giraffes, cetaceans, hippopotamuses, goats, buffalo, etc.) Order Perissodactyla (odd-toed ungulates: horses, asses, zebras, rhinoceroses, and tapirs) Mirorder Ferae Order Pholidota (pangolins) Order Carnivora (carnivorans: dogs, cats, bears, pinnipeds, mongooses, raccoons, skunks, hyenas, weasels, etc.) The exact relationships among these three lineages is currently a subject of debate, and four different hypotheses have been proposed with respect to which group is basal or diverged first from other placentals. These hypotheses are Atlantogenata (basal Boreoeutheria), Epitheria (basal Xenarthra), Exafroplacentalia (basal Afrotheria) and a hypothesis supporting a near simultaneous divergence. Estimates for the divergence times among these three placental groups mostly range from 105 to 120 million years ago (MYA), depending on the type of DNA, whether it is translated, and the phylogenetic method (e.g. nuclear or mitochondrial), and varying interpretations of paleogeographic data. In addition, a strict molecular clock does not hold, so it is necessary to assume models of how evolutionary rates change along lineages. These assumptions alone can make substantial differences to the relative ages of different mammal groups estimated with genomic data.

Cladogram and classification based on Amrine-Madsen, H. et al.. (2003) and Asher, R. J. et al.. (2009) Compare with Waddell, Hasegawa and Okada (1999) and Waddell et al. (2001).

Genomics As of 2020, the genome has been sequenced for at least one species in each extant placental order and in 83% of families (105 of 127 extant placental families). See list of sequenced animal genomes.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Placentalia

Start with the simplest possible case. Write down what Placentalia claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Placentalia before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Placentalia ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Placentalia

In research: Placentalia appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Placentalia in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Placentalia is common in secondary-school and first-year university syllabi. It links to neighbouring topics Extant Paleocene first appearances, Mammal taxonomy, Placentalia, so understanding it makes those chapters shorter.
In everyday life: Look for Placentalia outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Placentalia in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Placentalia means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Placentalia out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Placentalia in simple terms?

Why does Placentalia matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Placentalia?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Placentalia.