The Cambrian Period was the first geological period of the Paleozoic Era, of the Phanerozoic Eon. The Cambrian lasted 55.6 million years from the end of the preceding Ediacaran Period 541 million years ago to the beginning of the Ordovician Period 485.4 mya. Its subdivisions, its base, are somewhat in flux; the period was established by Adam Sedgwick, who named it after Cambria, the Latin name of Wales, where Britain's Cambrian rocks are best exposed. The Cambrian is unique in its unusually high proportion of lagerstätte sedimentary deposits, sites of exceptional preservation where "soft" parts of organisms are preserved as well as their more resistant shells; as a result, our understanding of the Cambrian biology surpasses that of some periods. The Cambrian marked a profound change in life on Earth. Complex, multicellular organisms became more common in the millions of years preceding the Cambrian, but it was not until this period that mineralized—hence fossilized—organisms became common; the rapid diversification of life forms in the Cambrian, known as the Cambrian explosion, produced the first representatives of all modern animal phyla.
Phylogenetic analysis has supported the view that during the Cambrian radiation, metazoa evolved monophyletically from a single common ancestor: flagellated colonial protists similar to modern choanoflagellates. Although diverse life forms prospered in the oceans, the land is thought to have been comparatively barren—with nothing more complex than a microbial soil crust and a few molluscs that emerged to browse on the microbial biofilm. Most of the continents were dry and rocky due to a lack of vegetation. Shallow seas flanked the margins of several continents created during the breakup of the supercontinent Pannotia; the seas were warm, polar ice was absent for much of the period. Despite the long recognition of its distinction from younger Ordovician rocks and older Precambrian rocks, it was not until 1994 that the Cambrian system/period was internationally ratified; the base of the Cambrian lies atop a complex assemblage of trace fossils known as the Treptichnus pedum assemblage. The use of Treptichnus pedum, a reference ichnofossil to mark the lower boundary of the Cambrian, is difficult since the occurrence of similar trace fossils belonging to the Treptichnids group are found well below the T. pedum in Namibia and Newfoundland, in the western USA.
The stratigraphic range of T. pedum overlaps the range of the Ediacaran fossils in Namibia, in Spain. The Cambrian Period was followed by the Ordovician Period; the Cambrian is divided into ten ages. Only three series and six stages are named and have a GSSP; because the international stratigraphic subdivision is not yet complete, many local subdivisions are still used. In some of these subdivisions the Cambrian is divided into three series with locally differing names – the Early Cambrian, Middle Cambrian and Furongian. Rocks of these epochs are referred to as belonging to Upper Cambrian. Trilobite zones allow biostratigraphic correlation in the Cambrian; each of the local series is divided into several stages. The Cambrian is divided into several regional faunal stages of which the Russian-Kazakhian system is most used in international parlance: *Most Russian paleontologists define the lower boundary of the Cambrian at the base of the Tommotian Stage, characterized by diversification and global distribution of organisms with mineral skeletons and the appearance of the first Archaeocyath bioherms.
The International Commission on Stratigraphy list the Cambrian period as beginning at 541 million years ago and ending at 485.4 million years ago. The lower boundary of the Cambrian was held to represent the first appearance of complex life, represented by trilobites; the recognition of small shelly fossils before the first trilobites, Ediacara biota earlier, led to calls for a more defined base to the Cambrian period. After decades of careful consideration, a continuous sedimentary sequence at Fortune Head, Newfoundland was settled upon as a formal base of the Cambrian period, to be correlated worldwide by the earliest appearance of Treptichnus pedum. Discovery of this fossil a few metres below the GSSP led to the refinement of this statement, it is the T. pedum ichnofossil assemblage, now formally used to correlate the base of the Cambrian. This formal designation allowed radiometric dates to be obtained from samples across the globe that corresponded to the base of the Cambrian. Early dates of 570 million years ago gained favour, though the methods used to obtain this number are now considered to be unsuitable and inaccurate.
A more precise date using modern radiometric dating yield a date of 541 ± 0.3 million years ago. The ash horizon in Oman from which this date was recovered corresponds to a marked fall in the abundance of carbon-13 that correlates to equivalent excursions elsewhere in the world, to the disappearance of distinctive Ediacaran fossils. There are arguments that the dated horizon in Oman does not correspond to the Ediacaran-Cambrian boundary, but represents a facies change from marine to evaporite-dominated strata — which w
National Museum of Natural History
The National Museum of Natural History is a natural history museum administered by the Smithsonian Institution, located on the National Mall in Washington, D. C. United States, it is open 364 days a year. In 2016, with 7.1 million visitors, it was the fourth most visited museum in the world and the most visited natural-history museum in the world. Opened in 1910, the museum on the National Mall was one of the first Smithsonian buildings constructed to hold the national collections and research facilities; the main building has an overall area of 1,500,000 square feet with 325,000 square feet of exhibition and public space and houses over 1,000 employees. The museum's collections contain over 126 million specimens of plants, fossils, rocks, human remains, human cultural artifacts, it is home to about 185 professional natural-history scientists—the largest group of scientists dedicated to the study of natural and cultural history in the world. The United States National Museum was founded in 1846 as part of the Smithsonian Institution.
The museum was housed in the Smithsonian Institution Building, better known today as the Smithsonian Castle. A formal exhibit hall opened in 1858; the growing collection led to the construction of the National Museum Building. Covering a then-enormous 2.25 acres, it was built in just 15 months at a cost of $310,000. It opened in March 1881. Congress authorized construction of a new building on June 28, 1902. On January 29, 1903, a special committee composed of members of Congress and representatives from the Smithsonian's board of regents published a report asking Congress to fund a much larger structure than planned; the regents began considering sites for the new building in March, by April 12 settled on a site on the north side of B Street NW between 9th and 12th Streets. The D. C. architectural firm of Hornblower & Marshall was chosen to design the structure. Testing of the soil for the foundations was set for July 1903, with construction expected to take three years; the Natural History Building opened its doors to the public on March 17, 1910, in order to provide the Smithsonian Institution with more space for collections and research.
The building was not completed until June 1911. The structure cost $3.5 million dollars. The Neoclassical style building was the first structure constructed on the north side of the National Mall as part of the 1901 McMillan Commission plan. In addition to the Smithsonian's natural history collection, it housed the American history and cultural collections. Between 1981 and 2003, the National Museum of Natural History had 11 acting directors. There were six directors alone between 1990 and 2002. Turnover was high as the museum's directors were disenchanted by low levels of funding and the Smithsonian's inability to define the museum's mission. Robert W. Fri was named the museum's director in 1996. One of the largest donations in Smithsonian history was made during Fri's tenure. Kenneth E. Behring donated $20 million in 1997 to modernize the museum. Fri resigned in 2001 after disagreeing with Smithsonian leadership over the reorganization of the museum's scientific research programs. J. Dennis O'Connor, Provost of the Smithsonian Institution was named acting director of the museum on July 25, 2001.
Eight months O'Conner resigned to become the vice president of research and dean of the graduate school at the University of Maryland. Douglas Erwin, a paleontologist at the National Museum of Natural History, was appointed interim director in June 2002. In January 2003, the Smithsonian announced that Cristián Samper, a Colombian with an M. Sc. and Ph. D. from Harvard University, would become the museum's permanent director on March 31, 2003. Samper founded the Alexander von Humboldt Biological Resources Research Institute and ran the Smithsonian Tropical Research Institute after 2001. Smithsonian officials said. Under Samper's direction, the museum opened the $100 million Behring Hall of Mammals in November 2003, received $60 million in 2004 for the Sant Hall of Oceans, received a $1 million gift from Tiffany & Co. for the purchase of precious gems for the National Gem Collection. On March 25, 2007, Lawrence M. Small, Secretary of the Smithsonian Institution and the organization's highest-ranking appointed official, resigned abruptly after public reports of lavish spending.
On March 27, 2007 Samper was appointed Acting Secretary of the Smithsonian. Paul G. Risser, former chancellor of the University of Oklahoma, was named Acting Director of the Museum of Natural History on March 29. Samper's tenure at the museum was not without controversy. In May 2007, Robert Sullivan, the former associate director in charge of exhibitions at the National Museum of Natural History, charged that Samper and Smithsonian Undersecretary for Science David Evans ordered "last minute" changes in the exhibit "Arctic: A Friend Acting Strangely" to tone down the role of human beings in the discussion of global warming, to make global warming seem more uncertain than depicted. Samper denied that he knew of any scientific objections to the changes, said that no political pressure had been applied to the Smithsonian to make the changes. In November 2007, The Washington Post reported that an interagency group of scientists from the Department of the Interior, NASA, Nati
Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres and have complex interactions with each other and their environments, forming intricate food webs. The category includes humans, but in colloquial use the term animal refers only to non-human animals; the study of non-human animals is known as zoology. Most living animal species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan; the Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes and molluscs—and the deuterostomes, containing the echinoderms and chordates.
Life forms interpreted. Many modern animal phyla became established in the fossil record as marine species during the Cambrian explosion which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified. Aristotle divided animals into those with those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between animal taxa. Humans make use of many other animal species for food, including meat and eggs. Dogs have been used in hunting, while many aquatic animals are hunted for sport.
Non-human animals have appeared in art from the earliest times and are featured in mythology and religion. The word "animal" comes from the Latin animalis, having soul or living being; the biological definition includes all members of the kingdom Animalia. In colloquial usage, as a consequence of anthropocentrism, the term animal is sometimes used nonscientifically to refer only to non-human animals. Animals have several characteristics. Animals are eukaryotic and multicellular, unlike bacteria, which are prokaryotic, unlike protists, which are eukaryotic but unicellular. Unlike plants and algae, which produce their own nutrients animals are heterotrophic, feeding on organic material and digesting it internally. With few exceptions, animals breathe oxygen and respire aerobically. All animals are motile during at least part of their life cycle, but some animals, such as sponges, corals and barnacles become sessile; the blastula is a stage in embryonic development, unique to most animals, allowing cells to be differentiated into specialised tissues and organs.
All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. During development, the animal extracellular matrix forms a flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible; this may be calcified, forming structures such as shells and spicules. In contrast, the cells of other multicellular organisms are held in place by cell walls, so develop by progressive growth. Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, desmosomes. With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues; these include muscles, which enable locomotion, nerve tissues, which transmit signals and coordinate the body. There is an internal digestive chamber with either one opening or two openings. Nearly all animals make use of some form of sexual reproduction, they produce haploid gametes by meiosis.
These fuse to form zygotes, which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement, it first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm. In most cases, a third germ layer, the mesoderm develops between them; these germ layers differentiate to form tissues and organs. Repeated instances of mating with a close relative during sexual reproduction leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits. Animals have evolved numerous mechanisms for avoiding close inbreeding. In some species, such as the splendid fairywren, females benefit by mating with multiple males, thus producing more offspring of higher genetic quality; some animals are capable of asexual reproduction, which results
Paranthropus boisei or Australopithecus boisei was an early hominin, described as the largest of the genus Paranthropus. It lived in Eastern Africa during the Pleistocene epoch from about 2.4 until about 1.4 million years ago. First discovered by anthropologist Mary Leakey on July 17, 1959, at Olduvai Gorge, the well-preserved cranium was dated to 1.75 million years ago and had characteristics distinctive of the robust australopithecines. Mary and her husband Louis Leakey classified the find as Zinjanthropus boisei: "Zinj" for the medieval East African region of Zanj, "anthropus" for "human being", "boisei" for Charles Watson Boise, the anthropologist team’s benefactor. Paranthropus boisei proved to be a treasure when the anthropologists' son Richard Leakey suggested it was the first hominin species to use stone tools. Bernard Wood of the University of Liverpool, posits that tools discovered in Ethiopia and dated to 2.5 million years ago could have been made by Paranthropus boisei. A well-preserved jaw, known as the Peninj Mandible, was found by Richard's colleague Kamoya Kimeu in 1964 in Peninj, Tanzania.
Another skull, specimen KNM ER 406, was unearthed in 1969 by Richard at Koobi Fora near Lake Turkana, in Kenya. The brain volume is quite small, about 500 to 550 cm3, not much larger than that of Australopithecus afarensis and Australopithecus africanus or modern-day chimpanzees. In P. boisei the foramen magnum is much shorter than in P. robustus. In addition, the cranial variation of P. boisei is remarkably high. The adult males were larger on average than females, as was the case in all australopithecine species. Males weighed some 49 kg and stood about 1.37 m tall, while females weighed about 34 kg and were 1.24 m tall. It had a skull specialized for heavy chewing and several traits seen in modern-day gorillas; the molar teeth were large, with an area over twice that of modern humans. The species is sometimes referred to as "Nutcracker Man" because it had the biggest, flattest cheek teeth and the thickest enamel of any known hominin. P. boisei had large chewing muscles attached to a pronounced sagittal crest.
To accommodate their enormous cheek teeth, P. boisei had a wide face, although the front teeth were much smaller than in similar species. Some argue that the craniodental morphology of this taxon is indicative of a diet of hard or tough foods such as ground tubers and seeds. However, research on the molar microwear of P. boisei has found a pattern different from that observed in P. robustus in South Africa, thought to have fed on hard foods as a fallback resource. This work suggests. Carbon isotope ratios suggest P. boisei had a diet dominated by C4 vegetation, unlike P. robustus in South Africa. In 1993, A. Amzaye found fossils of P. boisei at Ethiopia. The partial skull is dated to 1.4 million years ago. It is the biggest skull specimen found of P. boisei. The oldest specimen of P. boisei was found in Omo and dates to 2.3 million years ago, classified as, while the youngest specimen from Olduvai Gorge dates to 1.2 million years ago and is classified as OH 3 and OH 38. OH 5 Zinjanthropus, "Zinj" or "Nutcracker Man", was the first P. boisei specimen found by Mary Leakey at Olduvai Gorge, Tanzania.
The name "Nutcracker Man" may be a misnomer as it may only apply to Paranthropus robustus." KNM ER 406 is a small partial cranium discovered by Richard Leakey and H. Mutua in 1969, found at Koobi Fora, which displays large zygomatic arches and a cranial capacity of 510 cm³. KNM WT 17400 is a partial cranium, with similar characteristics to KNM WT 17000, "Black skull", belonging to Paranthropus aethiopicus, was found at West Turkana, Kenya. Peninj Mandible is a well-preserved jaw, found by Kamoya Kimeu in the Lake Natron region, near the Peninj River in Tanzania. All primates and hominids are or were dietary generalists, meaning that they ate a wide variety of foods and did not depend on a single, specific food source; the most accepted theory about what foods P. boisei ate suggests that it fed on hard objects as its primary source of nutrition. Strong jaw muscles are believed to be an evolutionary adaptation of P. boisei for a diet of nuts and hard fruit. Analysis of the wear on the molars of P. boisei showed fine striations indicating constant chewing of small, hard food items, like nuts and seeds.
The foods that P. boisei would have ingested can be determined by means of carbon isotope measurements. Along with fruits and nuts, carbon isotope data on P. boisei fossils suggest that their diet was based on C4 resources grasses or sedges. One study has concluded that the diet of P. boisei consisted of grasses and sedges such as tigernut and that it ate fruits and nuts. According to another, the addition of tubers to its diet helped P. boisei to meet its daily caloric intake requirement. In any case, it would appear that P. boisei consumed larger quantities of plant matter than any other hominin studied to date. The diet of fruits and nuts that P. boisei would have eaten is reflected by its classification as a cosmopolitan species, meaning that its wide geographic range
Mammals are vertebrate animals constituting the class Mammalia, characterized by the presence of mammary glands which in females produce milk for feeding their young, a neocortex, fur or hair, three middle ear bones. These characteristics distinguish them from reptiles and birds, from which they diverged in the late Triassic, 201–227 million years ago. There are around 5,450 species of mammals; the largest orders are the rodents and Soricomorpha. The next three are the Primates, the Cetartiodactyla, the Carnivora. In cladistics, which reflect evolution, mammals are classified as endothermic amniotes, they are the only living Synapsida. The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that produced the non-mammalian Dimetrodon. At the end of the Carboniferous period around 300 million years ago, this group diverged from the sauropsid line that led to today's reptiles and birds; the line following the stem group Sphenacodontia split off several diverse groups of non-mammalian synapsids—sometimes referred to as mammal-like reptiles—before giving rise to the proto-mammals in the early Mesozoic era.
The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of non-avian dinosaurs, have been among the dominant terrestrial animal groups from 66 million years ago to the present. The basic body type is quadruped, most mammals use their four extremities for terrestrial locomotion. Mammals range in size from the 30–40 mm bumblebee bat to the 30-meter blue whale—the largest animal on the planet. Maximum lifespan varies from two years for the shrew to 211 years for the bowhead whale. All modern mammals give birth to live young, except the five species of monotremes, which are egg-laying mammals; the most species-rich group of mammals, the cohort called placentals, have a placenta, which enables the feeding of the fetus during gestation. Most mammals are intelligent, with some possessing large brains, self-awareness, tool use. Mammals can communicate and vocalize in several different ways, including the production of ultrasound, scent-marking, alarm signals and echolocation.
Mammals can organize themselves into fission-fusion societies and hierarchies—but can be solitary and territorial. Most mammals are polygynous. Domestication of many types of mammals by humans played a major role in the Neolithic revolution, resulted in farming replacing hunting and gathering as the primary source of food for humans; this led to a major restructuring of human societies from nomadic to sedentary, with more co-operation among larger and larger groups, the development of the first civilizations. Domesticated mammals provided, continue to provide, power for transport and agriculture, as well as food and leather. Mammals are hunted and raced for sport, are used as model organisms in science. Mammals have been depicted in art since Palaeolithic times, appear in literature, film and religion. Decline in numbers and extinction of many mammals is driven by human poaching and habitat destruction deforestation. Mammal classification has been through several iterations since Carl Linnaeus defined the class.
No classification system is universally accepted. George Gaylord Simpson's "Principles of Classification and a Classification of Mammals" provides systematics of mammal origins and relationships that were universally taught until the end of the 20th century. Since Simpson's classification, the paleontological record has been recalibrated, the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself through the new concept of cladistics. Though field work made Simpson's classification outdated, it remains the closest thing to an official classification of mammals. Most mammals, including the six most species-rich orders, belong to the placental group; the three largest orders in numbers of species are Rodentia: mice, porcupines, beavers and other gnawing mammals. The next three biggest orders, depending on the biological classification scheme used, are the Primates including the apes and lemurs. According to Mammal Species of the World, 5,416 species were identified in 2006.
These were grouped into 153 families and 29 orders. In 2008, the International Union for Conservation of Nature completed a five-year Global Mammal Assessment for its IUCN Red List, which counted 5,488 species. According to a research published in the Journal of Mammalogy in 2018, the number of recognized mammal species is 6,495 species included 96 extinct; the word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma. In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes and therian m
The Ordovician is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.2 million years from the end of the Cambrian Period 485.4 million years ago to the start of the Silurian Period 443.8 Mya. The Ordovician, named after the Celtic tribe of the Ordovices, was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison, who were placing the same rock beds in northern Wales into the Cambrian and Silurian systems, respectively. Lapworth recognized that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian systems, placed them in a system of their own; the Ordovician received international approval in 1960, when it was adopted as an official period of the Paleozoic Era by the International Geological Congress. Life continued to flourish during the Ordovician as it did in the earlier Cambrian period, although the end of the period was marked by the Ordovician–Silurian extinction events.
Invertebrates, namely molluscs and arthropods, dominated the oceans. The Great Ordovician Biodiversification Event increased the diversity of life. Fish, the world's first true vertebrates, continued to evolve, those with jaws may have first appeared late in the period. Life had yet to diversify on land. About 100 times as many meteorites struck the Earth per year during the Ordovician compared with today; the Ordovician Period began with a major extinction called the Cambrian–Ordovician extinction event, about 485.4 Mya. It lasted for about 42 million years and ended with the Ordovician–Silurian extinction events, about 443.8 Mya which wiped out 60% of marine genera. The dates given are recent radiometric dates and vary from those found in other sources; this second period of the Paleozoic era created abundant fossils that became major petroleum and gas reservoirs. The boundary chosen for the beginning of both the Ordovician Period and the Tremadocian stage is significant, it correlates well with the occurrence of widespread graptolite and trilobite species.
The base of the Tremadocian allows scientists to relate these species not only to each other, but to species that occur with them in other areas. This makes it easier to place many more species in time relative to the beginning of the Ordovician Period. A number of regional terms have been used to subdivide the Ordovician Period. In 2008, the ICS erected a formal international system of subdivisions. There exist Baltoscandic, Siberian, North American, Chinese Mediterranean and North-Gondwanan regional stratigraphic schemes; the Ordovician Period in Britain was traditionally broken into Early and Late epochs. The corresponding rocks of the Ordovician System are referred to as coming from the Lower, Middle, or Upper part of the column; the faunal stages from youngest to oldest are: Late Ordovician Hirnantian/Gamach Rawtheyan/Richmond Cautleyan/Richmond Pusgillian/Maysville/Richmond Middle Ordovician Trenton Onnian/Maysville/Eden Actonian/Eden Marshbrookian/Sherman Longvillian/Sherman Soudleyan/Kirkfield Harnagian/Rockland Costonian/Black River Chazy Llandeilo Whiterock Llanvirn Early Ordovician Cassinian Arenig/Jefferson/Castleman Tremadoc/Deming/Gaconadian The Tremadoc corresponds to the Tremadocian.
The Floian corresponds to the lower Arenig. The Llanvirn occupies the rest of the Darriwilian, terminates with it at the base of the Late Ordovician; the Sandbian represents the first half of the Caradoc. During the Ordovician, the southern continents were collected into Gondwana. Gondwana started the period in equatorial latitudes and, as the period progressed, drifted toward the South Pole. Early in the Ordovician, the continents of Laurentia and Baltica were still independent continents, but Baltica began to move towards Laurentia in the period, causing the Iapetus Ocean between them to shrink; the small continent Avalonia separated from Gondwana and began to move north towards Baltica and Laurentia, opening the Rheic Ocean between Gondwana and Avalonia. The Taconic orogeny, a major mountain-building episode, was well under way in Cambrian times. In the early and middle Ordovician, temperatures were mild, but at the beginning of the Late Ordovician, from 460 to 450 Ma, volcanoes along the margin of the Iapetus Ocean spewed massive amounts of carbon dioxide, a greenhouse gas, into the atmosphere, turning the planet into a hothouse.
Sea levels were high, but as Gondwana moved south, ice accumulated into glaciers and sea levels dropped. At first, low-lying sea beds increased diversity, but glaciation led to mass extinctions as the seas drained and continental shelves became dry land. During the Ordovician, in fact during the Tremadocian, marine transgressions worldwide were the greatest for which evidence is preserved; these volcanic island arcs collided with proto North America to form the Appalachian mountains. By the end of the Late Ordovician the volcanic emissions had stopped. Gondwana had by that time neared the South Pole and was glaciated
Australopithecus afarensis is an extinct hominin that lived between 3.9 and 2.9 million years ago in Africa. A. afarensis was slenderly built, like the younger Australopithecus africanus. A. afarensis is thought to be more related to the genus Homo, whether as a direct ancestor or a close relative of an unknown ancestor, than any other known primate from the same time. Some researchers include A. afarensis in the genus Praeanthropus. The most famous fossil is the partial skeleton named Lucy found by Donald Johanson and colleagues, who, in celebration of their find played the Beatles song "Lucy in the Sky with Diamonds". Australopithecus afarensis. Despite Laetoli being the type locality for A. afarensis, the most extensive remains assigned to the species are found in Hadar, Afar Region of Ethiopia, including the above-mentioned "Lucy" partial skeleton and the "First Family" found at the AL 333 locality. Other localities bearing A. afarensis remains include Omo, Maka and Belohdelie in Ethiopia, Koobi Fora and Lothagam in Kenya.
Compared to the modern and extinct great apes, A. afarensis has reduced canines and molars, although they are still larger than in modern humans. A. afarensis has a small brain size and a prognathic face. Considerable debate surrounds the locomotor behaviour of A. afarensis. Some studies suggest that A. afarensis was exclusively bipedal, while others propose that the creatures were arboreal. The anatomy of the hands and shoulder joints in many ways favour the latter interpretation. In particular, the morphology of the scapula appears to be ape-like and different from modern humans; the curvature of the finger and toe bones approaches that of modern-day apes, is suggestive of their ability to efficiently grasp branches and climb. Alternatively, the loss of an abductable great toe and therefore the ability to grasp with the foot suggests A. afarensis was no longer adapted to climbing. A number of traits in the A. afarensis skeleton reflect bipedalism, to the extent some researchers have suggested bipedality evolved long before A. afarensis.
In overall anatomy, the pelvis is far more human-like than ape-like. The iliac blades are short and wide, the sacrum is wide and positioned directly behind the hip joint, evidence of a strong attachment for the knee extensors is clear. While the pelvis is not wholly human-like, these features point to a structure that can be considered radically remodeled to accommodate a significant degree of bipedalism in the animals' locomotor repertoire; the femur angles in toward the knee from the hip. This trait would have allowed the foot to have fallen closer to the midline of the body, is a strong indication of habitual bipedal locomotion; the feet feature adducted big toes, making it difficult if not impossible to grasp branches with the hindlimbs. The loss of a grasping hindlimb increases the risk of an infant being dropped or falling, as primates hold onto their mothers while the mother goes about her daily business. Without the second set of grasping limbs, the infant cannot maintain as strong a grip, had to be held with help from the mother.
The problem of holding the infant would be multiplied if the mother had to climb trees. Bones of the foot indicate bipedality. Computer simulations using dynamic modeling of the skeleton's inertial properties and kinematics suggest A. afarensis was able to walk in the same way modern humans walk, with a normal erect gait or with bent hips and knees, but could not walk in the same way as chimpanzees. The upright gait would have been much more efficient than the bent knee and hip walking, which would have taken twice as much energy. A. Afarensis was quite an efficient bipedal walker over short distances, the spacing of the footprints at Laetoli indicates they were walking at 1.0 m/s or above, which matches human small-town walking speeds. Yet, this can be questioned, as finds of Australopithecus foot bones indicate the Laetoli footprints may not have been made by Australopithecus. Many scientists doubt the suggestion of bipedalism, argue that if Australopithecus did walk on two legs, it did not walk in the same way as humans.
The presence of a wrist-locking mechanism, might suggest they engaged in knuckle-walking.. The shoulder joint is oriented much more cranially than that in modern humans, but similar to that in the present-day apes. Combined with the long arms Au. afarensis is thought to have had, this is thought by many to be reflective of a heightened ability to use the arm above the head in climbing behaviour. Furthermore, scans of the skulls reveal a canal and bony labyrinth morphology, not supportive to proper bipedal locomotion. Upright bipedal walking is thought to have evolved from knuckle-walking with bent legs, in the manner used by chimpanzees and gorillas to move around on the ground, but fossils such as Orrorin tugenensis indicate bipedalism around 5 to 8 million years ago, in the same general period when genetic studies suggest the lineage of chimpanzees and humans diverged. Modern apes and their fossil ancestors show skeletal adaptations to an upright posture used in tree-climbing, upright