The Central American Seaway, also known as the Panamanic Inter-American and Proto-Caribbean Seaway, was a body of water that once separated North America from South America. It formed in the Mesozoic (200–154 Ma) during the breakup of the supercontinent Pangaea, and closed when the Isthmus of Panama was formed by volcanic activity in the late Pliocene (2.76–2.54 Ma).
The closure of the Central American Seaway had tremendous effects on oceanic circulation and the biogeography of the adjacent seas, isolating many species and triggering speciation and diversification of tropical and sub-tropical marine fauna. The inflow of nutrient-rich water of deep Pacific origin into the Caribbean was blocked, so local species had to adapt to an environment of lower productivity. It had an even larger impact on terrestrial life. The seaway had isolated South America for much of the Cenozoic, allowing the evolution of a wholly unique diverse mammalian fauna there; when it closed, a faunal exchange with North America ensued, leading to the extinction of many of the native South American forms.
The evidence for when the Central American landmass emerged and the closing of the Central American Seaway can be divided into three categories. The first is the direct geologic observation of crustal thickening and submarine deposits in Central America. The second is the Great American Interchange of vertebrates between North and South America which required a continuous land bridge across the two areas for the organisms to travel along with a climate that was very different than the climate today. Lastly is the development of differences in marine assemblages and their isotopic signatures in the Caribbean from those in the Pacific. The Central American Seaway was closed by the elevation of the Central American Isthmus which is proposed to have occurred three and a half to five million years ago. The closing of the Central American Seaway is also supported by the evolution of taxa on different sides of the Central American Isthmus along with the different histories of the oceans on either side of the isthmus.
The closing of the seaway allowed a major migration of land mammals between North and South America or The Great American Interchange. This allowed the entrance of species of mammals such as cats, horses, elephants and camels to migrate from North America to South America. There is much controversy about glacial and interglacial climates in South America. Research shows that vegetation in most of the Amazon basin has changed a very small amount since glacial times. It is also believed that the area was more of a savannah during glacial times, but it is believed that the area is quite the same. Predicted connections between a closed seaway and glaciation are causation for a very different North Atlantic Ocean circulation having an effect on the surrounding atmospheric temperatures. The emergence of the isthmus caused a reflection of the westward-flowing North Equatorial Current northward and enhanced the northward-flowing Gulf Stream.
Allopatric speciation (from Ancient Greek ἄλλος, allos, meaning "other", and πατρίς, patris, "fatherland"), also referred to as geographic speciation, vicariant speciation, or its earlier name, the dumbbell model, is a mode of speciation that occurs when biological populations of the same species become isolated from each other to an extent that prevents or interferes with gene flow.
Various geographic changes can arise such as the movement of continents, and the formation of mountains, islands, bodies of water, or glaciers. Human activity such as agriculture or developments can also change the distribution of species populations. These factors can substantially alter a region's geography, resulting in the separation of a species population into isolated subpopulations. The vicariant populations then undergo genetic changes as they become subjected to different selective pressures, experience genetic drift, and accumulate different mutations in the separated populations gene pools. The barriers prevent the exchange of genetic information between the two populations leading to reproductive isolation. If the two populations come into contact they will be unable to reproduce—effectively speciating. Other isolating factors such as population dispersal leading to emigration can cause speciation (for instance, the dispersal and isolation of a species on an oceanic island) and is considered a special case of allopatric speciation called peripatric speciation.
Allopatric speciation is typically subdivided into two major models: vicariance and peripatric. Both models differ from one another by virtue of their population sizes and geographic isolating mechanisms. The terms allopatry and vicariance are often used in biogeography to describe the relationship between organisms whose ranges do not significantly overlap but are immediately adjacent to each other—they do not occur together or only occur within in a narrow zone of contact. Historically, the language used to refer to modes of speciation directly reflected biogeographical distributions. As such, allopatry is a geographical distribution opposed to sympatry (speciation within the same area). Furthermore, the terms allopatric, vicariant, and geographical speciation are often used interchangeably in the scientific literature. This article will follow a similar theme, with the exception of special cases such as peripatric, centrifugal, among others.
Observation of nature creates difficulties in witnessing allopatric speciation from "start-to-finish" as it operates as a dynamic process. From this arises a host of various issues in defining species, defining isolating barriers, measuring reproductive isolation, among others. Nevertheless, verbal and mathematical models, laboratory experiments, and empirical evidence overwhelmingly supports the occurrence of allopatric speciation in nature. Mathematical modeling of the genetic basis of reproductive isolation supports the plausibility of allopatric speciation; whereas laboratory experiments of Drosophila and other animal and plant species have confirmed that reproductive isolation evolves as a byproduct of natural selection.Atlantic Ocean
The Atlantic Ocean is the second largest of the world's oceans, with an area of about 106,460,000 square kilometers (41,100,000 square miles). It covers approximately 20 percent of the Earth's surface and about 29 percent of its water surface area. It separates the "Old World" from the "New World".
The Atlantic Ocean occupies an elongated, S-shaped basin extending longitudinally between Europe and Africa to the east, and the Americas to the west. As one component of the interconnected global ocean, it is connected in the north to the Arctic Ocean, to the Pacific Ocean in the southwest, the Indian Ocean in the southeast, and the Southern Ocean in the south (other definitions describe the Atlantic as extending southward to Antarctica). The Equatorial Counter Current subdivides it into the North Atlantic Ocean and the South Atlantic Ocean at about 8°N.Scientific explorations of the Atlantic include the Challenger expedition, the German Meteor expedition, Columbia University's Lamont-Doherty Earth Observatory and the United States Navy Hydrographic Office.Atlantic meridional overturning circulation
The Atlantic meridional overturning circulation (AMOC) is the zonally-integrated component of surface and deep currents in the Atlantic Ocean. It is characterized by a northward flow of warm, salty water in the upper layers of the Atlantic, and a southward flow of colder, deep waters that are part of the thermohaline circulation. These "limbs" are linked by regions of overturning in the Nordic and Labrador Seas and the Southern Ocean. The AMOC is an important component of the Earth’s climate system, and is a result of both atmospheric and thermohaline drivers.Central America
Central America (Spanish: América Central, pronounced [aˌmeɾika senˈtɾal], Centroamérica [sentɾoaˈmeɾika]) is located on the southern tip of North America, or is sometimes defined as a subcontinent of the Americas, bordered by Mexico to the north, Colombia to the southeast, the Caribbean Sea to the east, and the Pacific Ocean to the west and south. Central America consists of seven countries: Belize, Costa Rica, El Salvador, Guatemala, Honduras, Nicaragua, and Panama. The combined population of Central America has been estimated to be 41,739,000 (2009 estimate) and 42,688,190 (2012 estimate).Central America is a part of the Mesoamerican biodiversity hotspot, which extends from northern Guatemala through to central Panama. Due to the presence of several active geologic faults and the Central America Volcanic Arc, there is a great deal of seismic activity in the region. Volcanic eruptions and earthquakes occur frequently; these natural disasters have resulted in the loss of many lives and much property.
In the Pre-Columbian era, Central America was inhabited by the indigenous peoples of Mesoamerica to the north and west and the Isthmo-Colombian peoples to the south and east. Soon after the Spanish expedition of Christopher Columbus's voyages to the Americas, the Spanish began to colonize the Americas. From 1609 until 1821, most of the territory within Central America—except for the lands that would become Belize and Panama—was governed by the Viceroyalty of New Spain from Mexico City as the Captaincy General of Guatemala. After New Spain achieved independence from Spain in 1821, some of its provinces were annexed to the First Mexican Empire, but soon seceded from Mexico to form the Federal Republic of Central America, which lasted from 1823 to 1838. The seven states finally became independent autonomous states: beginning with Nicaragua, Honduras, Costa Rica, and Guatemala (1838); followed by El Salvador (1841); then Panama (1903); and finally Belize (1981). Even today, people in Central America sometimes refer to their nations as if they were provinces of a Central American state. For example, it is not unusual to write "C.A." after the country names in formal and informal contexts and the automobile licence plates of many of the countries in the region show the legend "Centroamerica" in addition to the country name.Culebra Formation
The Culebra Formation (Tcb) is a geologic formation in Panama. It preserves fossils dating back to the Miocene period; Early Miocene epoch, Aquitanian to Burdigalian stages (Late Arikareean to Hemingfordian in the NALMA classification). Fossils of Culebrasuchus have been found in and named after the formation. The thickness of the formation is at least 250 metres (820 ft) thick, and the age has been estimated as from 23 to 19 Ma.Evidence of common descent
Evidence of common descent of living organisms has been discovered by scientists researching in a variety of disciplines over many decades, demonstrating that all life on Earth comes from a single ancestor. This forms an important part of the evidence on which evolutionary theory rests, demonstrates that evolution does occur, and illustrates the processes that created Earth's biodiversity. It supports the modern evolutionary synthesis—the current scientific theory that explains how and why life changes over time. Evolutionary biologists document evidence of common descent, all the way back to the last universal common ancestor, by developing testable predictions, testing hypotheses, and constructing theories that illustrate and describe its causes.
Comparison of the DNA genetic sequences of organisms has revealed that organisms that are phylogenetically close have a higher degree of DNA sequence similarity than organisms that are phylogenetically distant. Genetic fragments such as pseudogenes, regions of DNA that are orthologous to a gene in a related organism, but are no longer active and appear to be undergoing a steady process of degeneration from cumulative mutations support common descent alongside the universal biochemical organization and molecular variance patterns found in all organisms. Additional genetic information conclusively supports the relatedness of life and has allowed scientists (since the discovery of DNA) to develop phylogenetic trees: a construction of organisms evolutionary relatedness. It has also led to the development of molecular clock techniques to date taxon divergence times and to calibrate these with the fossil record.
Fossils are important for estimating when various lineages developed in geologic time. As fossilization is an uncommon occurrence, usually requiring hard body parts and death near a site where sediments are being deposited, the fossil record only provides sparse and intermittent information about the evolution of life. Evidence of organisms prior to the development of hard body parts such as shells, bones and teeth is especially scarce, but exists in the form of ancient microfossils, as well as impressions of various soft-bodied organisms. The comparative study of the anatomy of groups of animals shows structural features that are fundamentally similar (homologous), demonstrating phylogenetic and ancestral relationships with other organisms, most especially when compared with fossils of ancient extinct organisms. Vestigial structures and comparisons in embryonic development are largely a contributing factor in anatomical resemblance in concordance with common descent. Since metabolic processes do not leave fossils, research into the evolution of the basic cellular processes is done largely by comparison of existing organisms' physiology and biochemistry. Many lineages diverged at different stages of development, so it is possible to determine when certain metabolic processes appeared by comparing the traits of the descendants of a common ancestor.
Evidence from animal coloration was gathered by some of Darwin's contemporaries; camouflage, mimicry, and warning coloration are all readily explained by natural selection. Special cases like the seasonal changes in the plumage of the ptarmigan, camouflaging it against snow in winter and against brown moorland in summer provide compelling evidence that selection is at work. Further evidence comes from the field of biogeography because evolution with common descent provides the best and most thorough explanation for a variety of facts concerning the geographical distribution of plants and animals across the world. This is especially obvious in the field of insular biogeography. Combined with the well-established geological theory of plate tectonics, common descent provides a way to combine facts about the current distribution of species with evidence from the fossil record to provide a logically consistent explanation of how the distribution of living organisms has changed over time.
The development and spread of antibiotic resistant bacteria provides evidence that evolution due to natural selection is an ongoing process in the natural world. Natural selection is ubiquitous in all research pertaining to evolution, taking note of the fact that all of the following examples in each section of the article document the process. Alongside this are observed instances of the separation of populations of species into sets of new species (speciation). Speciation has been observed in the lab and in nature. Multiple forms of such have been described and documented as examples for individual modes of speciation. Furthermore, evidence of common descent extends from direct laboratory experimentation with the selective breeding of organisms—historically and currently—and other controlled experiments involving many of the topics in the article. This article summarizes the varying disciplines that provide the evidence for evolution and the common descent of all life on Earth, accompanied by numerous and specialized examples, indicating a compelling consilience of evidence.Geography of North America
North America is the third largest continent, and is also a portion of the second largest supercontinent if North and South America are combined into the Americas and Africa, Europe, and Asia are considered to be part of one supercontinent called Afro-Eurasia.
With an estimated population of 380 million and an area of 21,346,000 km² (824,714 mi²), the northernmost of the two continents of the Western Hemisphere is bounded by the Pacific Ocean on the west; the Atlantic Ocean on the east; the Caribbean Sea, Atlantic and Pacific oceans, and South America on the south; and the Arctic Ocean on the north.
The northern half of North America is sparsely populated and covered mostly by Canada, except for the northeastern portion, which is occupied by Greenland, and the northwestern portion, which is occupied by Alaska, the largest state of the U.S. The central and southern portions of the continent are represented by the United States, Mexico, and numerous smaller states primarily in Central America and in the Caribbean.
The continent is delimited on the southeast by most geographers at the Darién watershed along the Colombia-Panama border, placing all of Panama within North America. Alternatively, a less common view would end North America at the man-made Panama Canal. Islands generally associated with North America include Greenland, the world's largest island, and archipelagos and islands in the Caribbean. The terminology of the Americas is complex, but "Anglo-America" can describe Canada and the U.S., while "Latin America" comprises Mexico and the countries of Central America and the Caribbean, as well as the entire continent of South America.
Natural features of North America include the northern portion of the American Cordillera, represented by the geologically new Rocky Mountains in the west; and the considerably older Appalachian Mountains to the east. The north hosts an abundance of glacial lakes formed during the last glacial period, including the Great Lakes. North America's major continental divide is the Great Divide, which runs north and south down through Rocky Mountains. The major watersheds all drain to the east: The Mississippi/Missouri and Rio Grande into the Gulf of Mexico, and St. Lawrence into the Atlantic.
Climate is determined to a large extent by the latitude, ranging from Arctic cold in the north to tropical heat in the south. The western half of North America tends to have milder and wetter climate than other areas with equivalent latitude, although there are steppes (known as "prairies") in the central and western portions, and deserts in the Southwestern United States of Arizona, Colorado, California, Nevada, New Mexico, Utah, Oklahoma, and Texas; along with the Mexican states of Baja California, Baja California Sur, Sonora, Chihuahua, Coahuila, Nuevo Leon and Tamaulipas.Great American Interchange
The Great American Interchange was an important late Cenozoic paleozoogeographic event in which land and freshwater fauna migrated from North America via Central America to South America and vice versa, as the volcanic Isthmus of Panama rose up from the sea floor and bridged the formerly separated continents. Although there were earlier dispersals, probably over water, the migration accelerated dramatically about 2.7 million years (Ma) ago during the Piacenzian age. It resulted in the joining of the Neotropic (roughly South America) and Nearctic (roughly North America) ecozones definitively to form the Americas. The interchange is visible from observation of both biostratigraphy and nature (neontology). Its most dramatic effect is on the zoogeography of mammals but it also gave an opportunity for reptiles, amphibians, arthropods, weak-flying or flightless birds, and even freshwater fish to migrate.
The occurrence of the interchange was first discussed in 1876 by the "father of biogeography", Alfred Russel Wallace. Wallace had spent 1848–1852 exploring and collecting specimens in the Amazon basin. Others who made significant contributions to understanding the event in the century that followed include Florentino Ameghino, W. D. Matthew, W. B. Scott, Bryan Patterson, George Gaylord Simpson and S. David Webb. The Pliocene timing of the formation of the connection between North and South America was discussed in 1910 by Henry Fairfield Osborn.Analogous interchanges occurred earlier in the Cenozoic, when the formerly isolated land masses of India and Africa made contact with Eurasia c. 50 and 30 Ma ago, respectively.Hawaiian monk seal
The Hawaiian monk seal, Neomonachus schauinslandi (formerly Monachus schauinslandi), is an endangered species of earless seal in the family Phocidae that is endemic to the Hawaiian Islands.The Hawaiian monk seal is one of two remaining monk seal species; the other is the Mediterranean monk seal. A third species, the Caribbean monk seal, is extinct.The Hawaiian monk seal is the only seal native to Hawaii, and, along with the Hawaiian hoary bat, is one of only two mammals endemic to the islands.These monk seals are a conservation reliant endangered species. The small population of about 1,400 individuals is threatened by human encroachment, very low levels of genetic variation, entanglement in fishing nets, marine debris, disease, and past commercial hunting for skins. There are many methods of conservation biology when it comes to endangered species; translocation, captive care, habitat cleanup, and educating the public about the Hawaiian monk seal are some of the methods that can be employed.Index of Central America-related articles
This is an Index of Central America-related articles. This index defines Central America as the seven nations of Belize, Costa Rica, El Salvador, Guatemala, Honduras, Nicaragua, and Panama.Isthmus of Panama
The Isthmus of Panama (Spanish: Istmo de Panamá), also historically known as the Isthmus of Darien (Istmo de Darién), is the narrow strip of land that lies between the Caribbean Sea and the Pacific Ocean, linking North and South America. It contains the country of Panama and the Panama Canal. Like many isthmuses, it is a location of great strategic value.
The isthmus formed around 2.8 million years ago. This major geological event separated the Atlantic and Pacific Oceans and caused the creation of the Gulf Stream. This was first suggested in 1910 by North American paleontologist Henry Fairfield Osborn. He based the proposal on the fossil record of mammals in Central America. This conclusion provided a foundation for Alfred Wegener when he proposed the theory of continental drift in 1912.Megalodon
Megalodon (Carcharocles megalodon), meaning "big tooth", is an extinct species of shark that lived approximately 23 to 2.6 million years ago (mya), during the Early Miocene to the end of the Pliocene. It was formerly thought to be a member of the Lamnidae family, making it closely related to the great white shark (Carcharodon carcharias). However presently there is near unanimous consensus that it belongs to the extinct family Otodontidae, which diverged from the ancestry of the great white shark during the Early Cretaceous. Its genus placement is still debated, authors placing it in either Carcharocles, Megaselachus, Otodus, or Procarcharodon.
Scientists suggest that megalodon looked like a stockier version of the great white shark, though it may have looked similar to the basking shark (Cetorhinus maximus) or the sand tiger shark (Carcharias taurus). Regarded as one of the largest and most powerful predators to have ever lived, fossil remains of megalodon suggest that this giant shark reached a maximum length of 18 meters (59 ft) with the average size being 10.5 meters (34 ft). Their large jaws could exert a bite force of up to 110,000 to 180,000 newtons (25,000 to 40,000 lbf). Their teeth were thick and robust, built for grabbing prey and breaking bone.
Megalodon probably had a major impact on the structure of marine communities. The fossil record indicates that it had a cosmopolitan distribution. It probably targeted large prey, such as whales, seals, and sea turtles. Juveniles inhabited warm coastal waters and fed on fish and small whales. Unlike the great white, which attacks prey from the soft underside, megalodon probably used its strong jaws to break through the chest cavity and puncture the heart and lungs of its prey.
The animal faced competition from whale-eating cetaceans, such as Livyatan and other macroraptorial sperm whales, and smaller ancestral killer whales such as Orcinus citoniensis. As the shark preferred warmer waters, it is thought that oceanic cooling associated with the onset of the ice ages, coupled with the lowering of sea levels and resulting loss of suitable nursery areas, may have also contributed to its decline. A reduction in the diversity of baleen whales and a shift in their distribution toward polar regions may have reduced megalodon's primary food source. The extinction of the shark appeared to affect other animals; for example, the size of baleen whales increased significantly after the shark had disappeared.Megalonyx
Megalonyx (Greek, "large claw") is an extinct genus of ground sloths of the family Megalonychidae, which was endemic to North America from the Hemphillian of the Late Miocene through to the Rancholabrean of the Pleistocene, living from ~10.3 Mya—11,000 years ago, existing for approximately 10.289 million years. The type species, M. jeffersonii, measured about 3 meters (9.8 ft) and weighed up to 1,000 kilograms (2,200 lb).Monk seal
Monk seals are earless seals of the tribe Monachini. They are the only earless seals found in tropical climates. The two genera of monk seals, Monachus and Neomonachus, comprise three species: the Mediterranean monk seal, Monachus monachus; the Hawaiian monk seal, Neomonachus schauinslandi; and the Caribbean monk seal, Neomonachus tropicalis, which became extinct in the 20th century. The two surviving species are now rare and in imminent danger of extinction. All three monk seal species were classified in genus Monachus until 2014, when the Caribbean and Hawaiian species were placed into a new genus, Neomonachus.
Monk seals have a slender body and are agile. They have a broad, flat snout with nostrils on the top. Monk seals are polygynous, and group together in harems. They feed mainly on bony fish and cephalopods, but they are opportunistic. The skin is covered in small hairs, which are generally black in males and brown or dark gray in females. Monk seals are found in the Hawaiian archipelago, certain areas in the Mediterranean Sea (such as Cabo Blanco and Gyaros island), and formerly in the tropical areas of the west Atlantic Ocean.
All species experienced overhunting by sealers. The Hawaiian monk seal experienced population drops in the 19th century and during World War II, and the Caribbean monk seal was exploited since the 1500s until the 1850s, when populations were too low to hunt commercially. The Mediterranean monk seal has experienced commercial hunting since the Middle Ages and eradication by fishermen. Monk seals have developed a fear of humans, and may even abandon beaches due to human presence. Currently, around 1,700 monk seals remain.Pinniped
Pinnipeds, commonly known as seals, are a widely distributed and diverse clade of carnivorous, fin-footed, semiaquatic marine mammals. They comprise the extant families Odobenidae (whose only living member is the walrus), Otariidae (the eared seals: sea lions and fur seals), and Phocidae (the earless seals, or true seals). There are 33 extant species of pinnipeds, and more than 50 extinct species have been described from fossils. While seals were historically thought to have descended from two ancestral lines, molecular evidence supports them as a monophyletic lineage (descended from one ancestral line). Pinnipeds belong to the order Carnivora and their closest living relatives are bears and musteloids (weasels, raccoons, skunks, and red pandas), having diverged about 50 million years ago.
Seals range in size from the 1 m (3 ft 3 in) and 45 kg (99 lb) Baikal seal to the 5 m (16 ft) and 3,200 kg (7,100 lb) southern elephant seal, which is also the largest member of the order Carnivora. Several species exhibit sexual dimorphism. They have streamlined bodies and four limbs that are modified into flippers. Though not as fast in the water as dolphins, seals are more flexible and agile. Otariids use their front limbs primarily to propel themselves through the water, while phocids and walruses use their hind limbs. Otariids and walruses have hind limbs that can be pulled under the body and used as legs on land. By comparison, terrestrial locomotion by phocids is more cumbersome. Otariids have visible external ears, while phocids and walruses lack these. Pinnipeds have well-developed senses—their eyesight and hearing are adapted for both air and water, and they have an advanced tactile system in their whiskers or vibrissae. Some species are well adapted for diving to great depths. They have a layer of fat, or blubber, under the skin to keep warm in the cold water, and, other than the walrus, all species are covered in fur.
Although pinnipeds are widespread, most species prefer the colder waters of the Northern and Southern Hemispheres. They spend most of their lives in the water, but come ashore to mate, give birth, molt or escape from predators, such as sharks and killer whales. They feed largely on fish and marine invertebrates; but a few, like the leopard seal, feed on large vertebrates, such as penguins and other seals. Walruses are specialized for feeding on bottom-dwelling mollusks. Male pinnipeds typically mate with more than one female (polygyny), although the degree of polygyny varies with the species. The males of land-breeding species tend to mate with a greater number of females than those of ice breeding species. Male pinniped strategies for reproductive success vary between defending females, defending territories that attract females and performing ritual displays or lek mating. Pups are typically born in the spring and summer months and females bear almost all the responsibility for raising them. Mothers of some species fast and nurse their young for a relatively short period of time while others take foraging trips at sea between nursing bouts. Walruses are known to nurse their young while at sea. Seals produce a number of vocalizations, notably the barks of California sea lions, the gong-like calls of walruses and the complex songs of Weddell seals.
The meat, blubber and fur coats of pinnipeds have traditionally been used by indigenous peoples of the Arctic. Seals have been depicted in various cultures worldwide. They are commonly kept in captivity and are even sometimes trained to perform tricks and tasks. Once relentlessly hunted by commercial industries for their products, seals and walruses are now protected by international law. The Japanese sea lion and the Caribbean monk seal have become extinct in the past century, while the Mediterranean monk seal and Hawaiian monk seal are ranked endangered by the International Union for Conservation of Nature. Besides hunting, pinnipeds also face threats from accidental trapping, marine pollution, and conflicts with local people.Pliometanastes
Pliometanastes is an extinct genus of giant ground sloths of the family Megalonychidae endemic to North America during the Late Miocene epoch through very early Pliocene epoch (Hemphillian in the NALMA classification). Its fossils have been found in Costa Rica and across the southern United States from California to Florida.Thinobadistes
Thinobadistes is an extinct genus of ground sloth of the family Mylodontidae, endemic to North America during the Miocene-Pliocene epochs (Hemphillian). It lived from 10.3—4.9 mya, existing for approximately 5.4 million years.Thinobadistes and Pliometanastes were the first of the giant sloths to appear in N. America. Both Pliometanastes and Thinobadistes were in N. America before the Panamanian Land Bridge formed around 2.5 million years ago. It is then reasonable to presume that the ancestors of Thinobadistes island-hopped across the Central American Seaway from South America, where sloths in general first evolved.Walrus
The walrus (Odobenus rosmarus) is a large flippered marine mammal with a discontinuous distribution about the North Pole in the Arctic Ocean and subarctic seas of the Northern Hemisphere. The walrus is the only living species in the family Odobenidae and genus Odobenus. This species is subdivided into three subspecies: the Atlantic walrus (O. r. rosmarus) which lives in the Atlantic Ocean, the Pacific walrus (O. r. divergens) which lives in the Pacific Ocean, and O. r. laptevi, which lives in the Laptev Sea of the Arctic Ocean.
Adult walrus are easily recognized by their prominent tusks, whiskers, and bulk. Adult males in the Pacific can weigh more than 2,000 kg (4,400 lb) and, among pinnipeds, are exceeded in size only by the two species of elephant seals. Walruses live mostly in shallow waters above the continental shelves, spending significant amounts of their lives on the sea ice looking for benthic bivalve mollusks to eat. Walruses are relatively long-lived, social animals, and they are considered to be a "keystone species" in the Arctic marine regions.
The walrus has played a prominent role in the cultures of many indigenous Arctic peoples, who have hunted the walrus for its meat, fat, skin, tusks, and bone. During the 19th century and the early 20th century, walruses were widely hunted and killed for their blubber, walrus ivory, and meat. The population of walruses dropped rapidly all around the Arctic region. Their population has rebounded somewhat since then, though the populations of Atlantic and Laptev walruses remain fragmented and at low levels compared with the time before human interference.