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71-561: Palaeosciurus ( fossil ) Sciurotamias Spermophilinus (fossil) Ammospermophilus Spermophilus Notocitellus Otospermophilus Callospermophilus Xerospermophilus Cynomys Poliocitellus Ictidomys Arctomyoides (fossil) Miospermophilus (fossil) Paenemarmota (fossil) Palaearctomys (fossil) Protospermophilus (fossil) Marmota Eutamias Neotamias Nototamias (fossil) Tamias Urocitellus and see text Ground squirrels are rodents of

142-595: A bird that is thought to have reached its present location by rafting is the weak-flying South American hoatzin , whose ancestors apparently floated over from Africa. Colonization of groups of islands can occur by an iterative rafting process sometimes called island hopping. Such a process appears to have played a role, for example, in the colonization of the Caribbean by mammals of South American origin (i.e., caviomorphs, monkeys and sloths ). A remarkable example of iterative rafting has been proposed for spiders of

213-472: A constant rate. These " molecular clocks ", however, are fallible, and provide only approximate timing: for example, they are not sufficiently precise and reliable for estimating when the groups that feature in the Cambrian explosion first evolved, and estimates produced by different techniques may vary by a factor of two. Organisms are only rarely preserved as fossils in the best of circumstances, and only

284-426: A fraction of such fossils have been discovered. This is illustrated by the fact that the number of species known through the fossil record is less than 5% of the number of known living species, suggesting that the number of species known through fossils must be far less than 1% of all the species that have ever lived. Because of the specialized and rare circumstances required for a biological structure to fossilize, only

355-499: A hierarchical classification system still in use today. Darwin and his contemporaries first linked the hierarchical structure of the tree of life with the then very sparse fossil record. Darwin eloquently described a process of descent with modification, or evolution, whereby organisms either adapt to natural and changing environmental pressures, or they perish. When Darwin wrote On the Origin of Species by Means of Natural Selection, or

426-418: A mid-Ordovician age. Such index fossils must be distinctive, be globally distributed and occupy a short time range to be useful. Misleading results are produced if the index fossils are incorrectly dated. Stratigraphy and biostratigraphy can in general provide only relative dating ( A was before B ), which is often sufficient for studying evolution. However, this is difficult for some time periods, because of

497-536: A mutation first appeared. Phylogenetics and paleontology work together in the clarification of science's still dim view of the appearance of life and its evolution. Niles Eldredge 's study of the Phacops trilobite genus supported the hypothesis that modifications to the arrangement of the trilobite's eye lenses proceeded by fits and starts over millions of years during the Devonian . Eldredge's interpretation of

568-570: A portion of the deceased organism, usually that portion that was partially mineralized during life, such as the bones and teeth of vertebrates , or the chitinous or calcareous exoskeletons of invertebrates . Fossils may also consist of the marks left behind by the organism while it was alive, such as animal tracks or feces ( coprolites ). These types of fossil are called trace fossils or ichnofossils , as opposed to body fossils . Some fossils are biochemical and are called chemofossils or biosignatures . Gathering fossils dates at least to

639-611: A predator is in sight, they stop and watch 60% of the time. Ground squirrels are also known for their burrowing behavior. They have intricate tunnel systems with multiple entrances, which provide escape routes from predators. When a threat approaches, they quickly retreat underground, where they are safe from most predators. Their burrows are designed with multiple chambers and ranges between 5 and 30 feet (1.5 and 9.1 m), making it challenging for predators to reach them. This combination of vocal warnings and burrow construction makes ground squirrels highly adapted to evade danger and survive in

710-400: A richly diverse assembly of early multicellular eukaryotes . The fossil record and faunal succession form the basis of the science of biostratigraphy or determining the age of rocks based on embedded fossils. For the first 150 years of geology , biostratigraphy and superposition were the only means for determining the relative age of rocks. The geologic time scale was developed based on

781-448: A small percentage of life-forms can be expected to be represented in discoveries, and each discovery represents only a snapshot of the process of evolution. The transition itself can only be illustrated and corroborated by transitional fossils, which will never demonstrate an exact half-way point. The fossil record is strongly biased toward organisms with hard-parts, leaving most groups of soft-bodied organisms with little to no role. It

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852-449: Is a notable example of how knowledge encoded by the fossil record continues to contribute otherwise unattainable information on the emergence and development of life on Earth. For example, the research suggests Markuelia has closest affinity to priapulid worms, and is adjacent to the evolutionary branching of Priapulida , Nematoda and Arthropoda . Despite significant advances in uncovering and identifying paleontological specimens, it

923-666: Is also supported by the enigmatic Chinese genus Sciurotamias , which may be the most ancient living lineage of this group, or—if the chipmunks are not included here—close to the common ancestor of the Tamiini and the Marmotini sensu stricto . In any case, expansion of the Marmotini to Africa was probably prevented by competitive exclusion by their close relatives the Protoxerini and Xerini —the native terrestrial and palm squirrels of that continent, which must have evolved at

994-412: Is any preserved remains, impression, or trace of any once-living thing from a past geological age . Examples include bones , shells , exoskeletons , stone imprints of animals or microbes , objects preserved in amber , hair , petrified wood and DNA remnants. The totality of fossils is known as the fossil record . Though the fossil record is incomplete, numerous studies have demonstrated that there

1065-538: Is enough information available to give a good understanding of the pattern of diversification of life on Earth. In addition, the record can predict and fill gaps such as the discovery of Tiktaalik in the arctic of Canada . Paleontology includes the study of fossils: their age, method of formation, and evolutionary significance. Specimens are usually considered to be fossils if they are over 10,000 years old. The oldest fossils are around 3.48 billion years to 4.1 billion years old. The observation in

1136-479: Is generally accepted that the fossil record is vastly incomplete. Approaches for measuring the completeness of the fossil record have been developed for numerous subsets of species, including those grouped taxonomically, temporally, environmentally/geographically, or in sum. This encompasses the subfield of taphonomy and the study of biases in the paleontological record. Paleontology seeks to map out how life evolved across geologic time. A substantial hurdle

1207-438: Is replete with the mollusks , the vertebrates , the echinoderms , the brachiopods and some groups of arthropods . Fossil sites with exceptional preservation—sometimes including preserved soft tissues—are known as Lagerstätten —German for "storage places". These formations may have resulted from carcass burial in an anoxic environment with minimal bacteria, thus slowing decomposition. Lagerstätten span geological time from

1278-470: Is the difficulty of working out fossil ages. Beds that preserve fossils typically lack the radioactive elements needed for radiometric dating . This technique is our only means of giving rocks greater than about 50 million years old an absolute age, and can be accurate to within 0.5% or better. Although radiometric dating requires careful laboratory work, its basic principle is simple: the rates at which various radioactive elements decay are known, and so

1349-413: Is the science of deciphering the "layer-cake" that is the sedimentary record. Rocks normally form relatively horizontal layers, with each layer younger than the one underneath it. If a fossil is found between two layers whose ages are known, the fossil's age is claimed to lie between the two known ages. Because rock sequences are not continuous, but may be broken up by faults or periods of erosion , it

1420-702: Is the species Moggridgea rainbowi , the only Australian member of a genus otherwise endemic to Africa, with a divergence date of 2 to 16 Ma ago. However, oceanic dispersal of terrestrial species may not always take the form of rafting; in some cases, swimming or simply floating may suffice. Tortoises of the genus Chelonoidis arrived in South America from Africa in the Oligocene; they were probably aided by their ability to float with their heads up, and to survive up to six months without food or fresh water. South American tortoises then went on to colonize

1491-467: Is very difficult to match up rock beds that are not directly adjacent. However, fossils of species that survived for a relatively short time can be used to match isolated rocks: this technique is called biostratigraphy . For instance, the conodont Eoplacognathus pseudoplanus has a short range in the Middle Ordovician period. If rocks of unknown age have traces of E. pseudoplanus , they have

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1562-801: The Cambrian period to the present . Worldwide, some of the best examples of near-perfect fossilization are the Cambrian Maotianshan Shales and Burgess Shale , the Devonian Hunsrück Slates , the Jurassic Solnhofen Limestone , and the Carboniferous Mazon Creek localities. A fossil is said to be recrystallized when the original skeletal compounds are still present but in a different crystal form, such as from aragonite to calcite . Replacement occurs when

1633-557: The Early Oligocene , more than 30 million years ago (Mya), but the genus probably persisted at least until the mid- Miocene , some 15 Mya. Where the Marmotini originated is unclear. The subtribes probably diverged in the early to mid- Oligocene , as primitive marmots and chipmunks are known from the Late Oligocene of North America. The fossil record of the "true" ground squirrels is less well known, beginning only in

1704-957: The Mozambique Channel by rafting between 50 and 60 Ma ago. Likewise, the New World monkeys are thought to have originated in Africa and rafted to South America by the Oligocene , when the continents were much closer than they are today. Madagascar also appears to have received its tenrecs (25–42 Ma ago), nesomyid rodents (20–24 Ma ago) and euplerid carnivorans (19–26 Ma ago) by this route and South America its caviomorph rodents (over 30 Ma ago). Simian primates (ancestral to monkeys) and hystricognath rodents (ancestral to caviomorphs) are believed to have previously rafted from Asia to Africa about 40 Ma ago. Among reptiles, several iguanid species in

1775-586: The Phacops fossil record was that the aftermaths of the lens changes, but not the rapidly occurring evolutionary process, were fossilized. This and other data led Stephen Jay Gould and Niles Eldredge to publish their seminal paper on punctuated equilibrium in 1971. Synchrotron X-ray tomographic analysis of early Cambrian bilaterian embryonic microfossils yielded new insights of metazoan evolution at its earliest stages. The tomography technique provides previously unattainable three-dimensional resolution at

1846-569: The Renaissance . Leonardo da Vinci concurred with Aristotle's view that fossils were the remains of ancient life. For example, Leonardo noticed discrepancies with the biblical flood narrative as an explanation for fossil origins: If the Deluge had carried the shells for distances of three and four hundred miles from the sea it would have carried them mixed with various other natural objects all heaped up together; but even at such distances from

1917-602: The Song dynasty during the 11th century, who kept a specific seashell fossil with his own poem engraved on it. In his Dream Pool Essays published in 1088, Song dynasty Chinese scholar-official Shen Kuo hypothesized that marine fossils found in a geological stratum of mountains located hundreds of miles from the Pacific Ocean was evidence that a prehistoric seashore had once existed there and shifted over centuries of time . His observation of petrified bamboos in

1988-533: The West Indies and Galápagos Islands . The dispersal of semiaquatic species is likely to occur similarly. The dispersal of anthracotheres from Asia to Africa about 40 Ma ago, and the much more recent dispersal of hippos (relatives and possible descendants of anthracotheres) from Africa to Madagascar may have occurred by floating or swimming. Ancestors of the Nile crocodile are thought to have reached

2059-402: The law of superposition ) preserved different assemblages of fossils, and that these assemblages succeeded one another in a regular and determinable order. He observed that rocks from distant locations could be correlated based on the fossils they contained. He termed this the principle of faunal succession . This principle became one of Darwin's chief pieces of evidence that biological evolution

2130-426: The squirrel family ( Sciuridae ) that generally live on the ground or in burrows , rather than in trees like the tree squirrels . The term is most often used for the medium-sized ground squirrels, as the larger ones are more commonly known as marmots (genus Marmota ) or prairie dogs , while the smaller and less bushy-tailed ground squirrels tend to be known as chipmunks (genus Tamias ). Together, they make up

2201-403: The thunderbird . There is no such direct mythological connection known from prehistoric Africa, but there is considerable evidence of tribes there excavating and moving fossils to ceremonial sites, apparently treating them with some reverence. In Japan, fossil shark teeth were associated with the mythical tengu , thought to be the razor-sharp claws of the creature, documented some time after

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2272-829: The "marmot tribe " of squirrels, Marmotini , a clade within the large and mainly ground squirrel subfamily Xerinae , and containing six living genera . Well-known members of this largely Holarctic group are the marmots ( Marmota ), including the American groundhog , the chipmunks , the susliks ( Spermophilus ), and the prairie dogs ( Cynomys ). They are highly variable in size and habitus , but most are remarkably able to rise up on their hind legs and stand fully erect comfortably for prolonged periods. They also tend to be far more gregarious than other squirrels, and many live in colonies with complex social structures. Most Marmotini are rather short-tailed and large squirrels. At up to 8 kg (18 lb) or more, certain marmots are

2343-453: The 16th century. Roman naturalist Pliny the Elder wrote of " tongue stones ", which he called glossopetra . These were fossil shark teeth, thought by some classical cultures to look like the tongues of people or snakes. He also wrote about the horns of Ammon , which are fossil ammonites , whence the group of shelled octopus-cousins ultimately draws its modern name. Pliny also makes one of

2414-739: The 19th century that certain fossils were associated with certain rock strata led to the recognition of a geological timescale and the relative ages of different fossils. The development of radiometric dating techniques in the early 20th century allowed scientists to quantitatively measure the absolute ages of rocks and the fossils they host. There are many processes that lead to fossilization , including permineralization , casts and molds, authigenic mineralization , replacement and recrystallization, adpression, carbonization , and bioimmuration. Fossils vary in size from one- micrometre (1 μm) bacteria to dinosaurs and trees, many meters long and weighing many tons. A fossil normally preserves only

2485-413: The 8th century AD. In medieval China, the fossil bones of ancient mammals including Homo erectus were often mistaken for " dragon bones" and used as medicine and aphrodisiacs . In addition, some of these fossil bones are collected as "art" by scholars, who left scripts on various artifacts, indicating the time they were added to a collection. One good example is the famous scholar Huang Tingjian of

2556-493: The Americas from Africa 5 to 6 Ma ago. The first documented example of colonization of a land mass by rafting occurred in the aftermath of hurricanes Luis and Marilyn in the Caribbean in 1995. A raft of uprooted trees carrying fifteen or more green iguanas was observed by fishermen landing on the east side of Anguilla – an island where they had never before been recorded. The iguanas had apparently been caught on

2627-642: The Atlantic from Africa to South America and Fernando de Noronha , respectively, during the last 9 Ma. Skinks from the same group have also rafted from Africa to Cape Verde , Madagascar, the Seychelles , the Comoros and Socotra . (Among lizards, skinks and geckos seem especially capable of surviving long transoceanic journeys. ) Surprisingly, even burrowing amphisbaenians and blind snakes appear to have rafted from Africa to South America. An example of

2698-615: The Preservation of Favoured Races in the Struggle for Life , the oldest animal fossils were those from the Cambrian Period, now known to be about 540 million years old. He worried about the absence of older fossils because of the implications on the validity of his theories, but he expressed hope that such fossils would be found, noting that: "only a small portion of the world is known with accuracy." Darwin also pondered

2769-566: The Proterozoic and deeper still in the Archean is only "recounted by microscopic fossils and subtle chemical signals." Molecular biologists, using phylogenetics , can compare protein amino acid or nucleotide sequence homology (i.e., similarity) to evaluate taxonomy and evolutionary distances among organisms, with limited statistical confidence. The study of fossils, on the other hand, can more specifically pinpoint when and in what organism

2840-603: The South Pacific have been hypothesized to be descended from iguanas that rafted 10,000 kilometres (6,200 mi) from Central or South America (an alternative theory involves dispersal of a putative now-extinct iguana lineage from Australia or Asia ). Similarly, a number of clades of American geckos seem to have rafted over from Africa during both the Paleogene and Neogene. Skinks of the related genera Mabuya and Trachylepis also apparently both floated across

2911-788: The Tamiina subtribe and some basal genera) derived from maximum parsimony analysis. N. adocetus N. annulatus A. harrisii A. leucurus A. harrisii A. interpres O. atricapillus O. beecheyi O. variegatus C. saturatus C. lateralis C. madrensis M. monax M. marmota M. flaviventris M. caligata M. olympus M. vancouveriensis M. broweri M. menzbieri M. caudata M. baibacina M. bobak M. camtschatica M. himalayana M. sibirica S. musicus S. pygmaeus S. major S. pygmaeus S. dauricus S. xanthopyrmnus Fossil A fossil (from Classical Latin fossilis , lit.   ' obtained by digging ' )

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2982-413: The beach, indicating the fossils were once living animals. He had previously explained them in terms of vaporous exhalations , which Persian polymath Avicenna modified into the theory of petrifying fluids ( succus lapidificatus ). Recognition of fossil seashells as originating in the sea was built upon in the 14th century by Albert of Saxony , and accepted in some form by most naturalists by

3053-714: The beginning of recorded history. The fossils themselves are referred to as the fossil record. The fossil record was one of the early sources of data underlying the study of evolution and continues to be relevant to the history of life on Earth . Paleontologists examine the fossil record to understand the process of evolution and the way particular species have evolved. Fossils have been visible and common throughout most of natural history, and so documented human interaction with them goes back as far as recorded history, or earlier. There are many examples of paleolithic stone knives in Europe, with fossil echinoderms set precisely at

3124-467: The colonization of isolated land masses by mammals. Prominent examples include Madagascar , which has been isolated for ~120 million years ( Ma ), and South America , which was isolated for much of the Cenozoic . Both land masses, for example, appear to have received their primates by this mechanism. According to genetic evidence, the common ancestor of the lemurs of Madagascar appears to have crossed

3195-670: The deity Sopdu , the Morning Star, equivalent of Venus in Roman mythology. Fossils appear to have directly contributed to the mythology of many civilizations, including the ancient Greeks. Classical Greek historian Herodotos wrote of an area near Hyperborea where gryphons protected golden treasure. There was indeed gold mining in that approximate region , where beaked Protoceratops skulls were common as fossils. A later Greek scholar, Aristotle , eventually realized that fossil seashells from rocks were similar to those found on

3266-466: The dry northern climate zone of what is now Yan'an , Shaanxi province, China, led him to advance early ideas of gradual climate change due to bamboo naturally growing in wetter climate areas. In medieval Christendom , fossilized sea creatures on mountainsides were seen as proof of the biblical deluge of Noah's Ark . After observing the existence of seashells in mountains, the ancient Greek philosopher Xenophanes (c. 570 – 478 BC) speculated that

3337-415: The earlier known references to toadstones , thought until the 18th century to be a magical cure for poison originating in the heads of toads, but which are fossil teeth from Lepidotes , a Cretaceous ray-finned fish. The Plains tribes of North America are thought to have similarly associated fossils, such as the many intact pterosaur fossils naturally exposed in the region, with their own mythology of

3408-550: The earliest known stromatolites are over 3.4 billion years old. The fossil record is life's evolutionary epic that unfolded over four billion years as environmental conditions and genetic potential interacted in accordance with natural selection. The Virtual Fossil Museum Paleontology has joined with evolutionary biology to share the interdisciplinary task of outlining the tree of life, which inevitably leads backwards in time to Precambrian microscopic life when cell structure and functions evolved. Earth's deep time in

3479-464: The earth during earthquake and subsidences, and petrifies whatever comes into contact with it. As a matter of fact, the petrifaction of the bodies of plants and animals is not more extraordinary than the transformation of waters. From the 13th century to the present day, scholars pointed out that the fossil skulls of Deinotherium giganteum , found in Crete and Greece, might have been interpreted as being

3550-478: The existence of a world previous to ours, destroyed by some kind of catastrophe. Interest in fossils, and geology more generally, expanded during the early nineteenth century. In Britain, Mary Anning 's discoveries of fossils, including the first complete ichthyosaur and a complete plesiosaurus skeleton, sparked both public and scholarly interest. Early naturalists well understood the similarities and differences of living species leading Linnaeus to develop

3621-624: The genus Amaurobioides . Members of this genus inhabit coastal sites and build silken cells which they seal at high tide; however, they do not balloon . DNA sequence analysis suggests that ancestors of the genus dispersed from southern South America to South Africa about 10 Ma ago, where the most basal clade is found; subsequent rafting events then took the genus eastward with the Antarctic Circumpolar Current to Australia, then to New Zealand and finally to Chile by about 2 Ma ago. Another example among spiders

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3692-519: The hand grip, dating back to Homo heidelbergensis and Neanderthals . These ancient peoples also drilled holes through the center of those round fossil shells, apparently using them as beads for necklaces. The ancient Egyptians gathered fossils of species that resembled the bones of modern species they worshipped. The god Set was associated with the hippopotamus , therefore fossilized bones of hippo-like species were kept in that deity's temples. Five-rayed fossil sea urchin shells were associated with

3763-433: The heaviest squirrels. The chipmunks of the genus Tamias frequently spend time in trees. Also closer to typical squirrels in other aspects, they are occasionally considered a tribe of their own (Tamiini). Palaeosciurus from Europe is the oldest known ground squirrel species, and it does not seem to be particularly close to any of the two to three living lineages (subtribes) of Marmotini. The oldest fossils are from

3834-535: The limits of fossilization. Fossils of two enigmatic bilaterians, the worm-like Markuelia and a putative, primitive protostome , Pseudooides , provide a peek at germ layer embryonic development. These 543-million-year-old embryos support the emergence of some aspects of arthropod development earlier than previously thought in the late Proterozoic. The preserved embryos from China and Siberia underwent rapid diagenetic phosphatization resulting in exquisite preservation, including cell structures. This research

3905-556: The mid-Miocene, when modern susliks and prairie dogs are known to have inhabited their present-day range already. Whether the Marmotini dispersed between North America and Eurasia via " island-hopping " across the Bering Straits or the Greenland region—both of which were temperate habitat at that time—and from which continent they dispersed to which, or if both continents brought forth distinct subtribes which then spread to

3976-496: The other, is not known and would probably require more fossil material to be resolved. In any case, the fairly comprehensive fossil record of Europe—at the relevant time separated from Asia by the Turgai Sea —lacks ancient Marmotini except the indeterminate Palaeosciurus , which might be taken to indicate an East Asian or western North American origin with trans-Beringia dispersal being the slightly more satisfying hypothesis. This

4047-545: The problems involved in matching rocks of the same age across continents . Family-tree relationships also help to narrow down the date when lineages first appeared. For instance, if fossils of B or C date to X million years ago and the calculated "family tree" says A was an ancestor of B and C, then A must have evolved earlier. It is also possible to estimate how long ago two living clades diverged, in other words approximately how long ago their last common ancestor must have lived, by assuming that DNA mutations accumulate at

4118-443: The ratio of the radioactive element to its decay products shows how long ago the radioactive element was incorporated into the rock. Radioactive elements are common only in rocks with a volcanic origin, and so the only fossil-bearing rocks that can be dated radiometrically are volcanic ash layers, which may provide termini for the intervening sediments. Consequently, palaeontologists rely on stratigraphy to date fossils. Stratigraphy

4189-462: The relative ages of rock strata as determined by the early paleontologists and stratigraphers . Since the early years of the twentieth century, absolute dating methods, such as radiometric dating (including potassium/argon , argon/argon , uranium series , and, for very recent fossils, radiocarbon dating ) have been used to verify the relative ages obtained by fossils and to provide absolute ages for many fossils. Radiometric dating has shown that

4260-801: The same time as the Marmotini did. Ground squirrels can measure anywhere from about 7.2 inches (18 cm) in height up to nearly 30 inches (76 cm). They can weigh between 0.09 pounds (0.041 kg) and 24 pounds (11 kg). Open areas including rocky outcrops, fields, pastures, and sparsely wooded hillsides comprise their habitat. Ground squirrels also live in grassy areas such as pastures, golf courses, cemeteries, and parks. Ground squirrels have developed several defense mechanisms to protect themselves from predators. When threatened, they emit high-pitched warning calls to alert others in their colony. This alarm call serves as an early warning system, allowing nearby squirrels to seek cover. The squirrels spend about one-third of their time standing to watch and when

4331-429: The sea and that they were still living when the strait of Gibraltar was cut through. In the mountains of Parma and Piacenza multitudes of shells and corals with holes may be seen still sticking to the rocks.... In 1666, Nicholas Steno examined a shark, and made the association of its teeth with the "tongue stones" of ancient Greco-Roman mythology, concluding that those were not in fact the tongues of venomous snakes, but

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4402-423: The sea we see the oysters all together and also the shellfish and the cuttlefish and all the other shells which congregate together, found all together dead; and the solitary shells are found apart from one another as we see them every day on the sea-shores. And we find oysters together in very large families, among which some may be seen with their shells still joined together, indicating that they were left there by

4473-504: The shell, bone, or other tissue is replaced with another mineral. In some cases mineral replacement of the original shell occurs so gradually and at such fine scales that microstructural features are preserved despite the total loss of original material. Scientists can use such fossils when researching the anatomical structure of ancient species. Several species of saurids have been identified from mineralized dinosaur fossils. Oceanic dispersal Rafting has played an important role in

4544-586: The skulls of the Cyclopes of Greek mythology , and are possibly the origin of that Greek myth. Their skulls appear to have a single eye-hole in the front, just like their modern elephant cousins, though in fact it's actually the opening for their trunk. In Norse mythology , echinoderm shells (the round five-part button left over from a sea urchin) were associated with the god Thor , not only being incorporated in thunderstones , representations of Thor's hammer and subsequent hammer-shaped crosses as Christianity

4615-467: The sudden appearance of many groups (i.e. phyla ) in the oldest known Cambrian fossiliferous strata. Since Darwin's time, the fossil record has been extended to between 2.3 and 3.5 billion years. Most of these Precambrian fossils are microscopic bacteria or microfossils . However, macroscopic fossils are now known from the late Proterozoic. The Ediacara biota (also called Vendian biota) dating from 575 million years ago collectively constitutes

4686-451: The teeth of some long-extinct species of shark. Robert Hooke (1635–1703) included micrographs of fossils in his Micrographia and was among the first to observe fossil forams . His observations on fossils, which he stated to be the petrified remains of creatures some of which no longer existed, were published posthumously in 1705. William Smith (1769–1839) , an English canal engineer, observed that rocks of different ages (based on

4757-500: The trees and rafted 200 mi (320 km) across the ocean from Guadeloupe , where they are indigenous. Examination of the weather patterns and ocean currents indicated that they had probably spent three weeks at sea before landfall. This colony began breeding on the new island within two years of its arrival. The advent of human civilization has created opportunities for organisms to raft on floating artifacts, which may be more durable than natural floating objects. This phenomenon

4828-453: The wild. Ground squirrels are omnivorous, and not only eat a diet rich in fungi, nuts, fruits, and seeds, but also occasionally eat insects, eggs, and other small animals. Basal and incertae sedis genera Subtribe Tamiina : chipmunks (might be full tribe) Subtribe Marmotina : marmots and prairie dogs Subtribe Spermophilina : true ground squirrels Below is a partial cladogram of ground squirrels (tribe Marmotini , but excluding

4899-524: The world was once inundated in a great flood that buried living creatures in drying mud. In 1027, the Persian Avicenna explained fossils' stoniness in The Book of Healing : If what is said concerning the petrifaction of animals and plants is true, the cause of this (phenomenon) is a powerful mineralizing and petrifying virtue which arises in certain stony spots, or emanates suddenly from

4970-458: Was adopted, but also kept in houses to garner Thor's protection. These grew into the shepherd's crowns of English folklore, used for decoration and as good luck charms, placed by the doorway of homes and churches. In Suffolk , a different species was used as a good-luck charm by bakers, who referred to them as fairy loaves , associating them with the similarly shaped loaves of bread they baked. More scientific views of fossils emerged during

5041-408: Was real. Georges Cuvier came to believe that most if not all the animal fossils he examined were remains of extinct species. This led Cuvier to become an active proponent of the geological school of thought called catastrophism . Near the end of his 1796 paper on living and fossil elephants he said: All of these facts, consistent among themselves, and not opposed by any report, seem to me to prove

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