Misplaced Pages

#77922

99-742: Although fossils are generally lacking in the Whitemud Formation, in southern Saskatchewan the Whitemud contains coprolites (fossilized feces ) and fossilized intestines of fish . In most areas the formation can be subdivided into three units: In southwestern Alberta around the Red Deer and Oldman Rivers , the middle carbonaceous shale unit is absent and the formation consists of white-weathering, cross-bedded kaolinitic sandstones interbedded with white- to cream-weathering silty and sandy clay. Sedimentology and fossils indicate that

198-445: A Cambridge University expert on tetrapod evolution, said of Tiktaalik , "It's one of those things you can point to and say, 'I told you this would exist,' and there it is." After five years of digging on Ellesmere Island, in the far north of Nunavut, they hit pay dirt: a collection of several fish so beautifully preserved that their skeletons were still intact. As Shubin's team studied the species they saw to their excitement that it

297-398: A crocodile than most fish. The rear edge of the skull was excavated by a pair of indentations known as otic notches . These notches may have housed spiracles on the top of the head, which suggest the creature had primitive lungs as well as gills. Tiktaalik also lacked a characteristic most fishes have—bony plates in the gill area that restrict lateral head movement. This makes Tiktaalik

396-475: A crocodile , including a shoulder , elbow and wrist . The fossil discovered in 2004 did not include the rear fins and tail, which were found in other specimens. It had rows of sharp teeth indicative of a predator fish, and its neck could move independently of its body, which is not common in other fish ( Tarrasius , Mandageria , placoderms and extant seahorses being some exceptions; see also Lepidogalaxias and Channallabes apus ). The animal had

495-410: A paraphyletic grade of fish incrementally closer to tetrapods. Elpistostegalian fish have few unique traits which are not retained from earlier fish or inherited by later tetrapods. In response, Daescler et al. (2006) redefined Elpisostegalia as a clade , including all vertebrates descended from the common ancestor of Panderichthys , Elpistostege and tetrapods. Nevertheless, they still retained

594-476: A sister taxon to Elpistostege and directly above Panderichthys , which was preceded by Eusthenopteron . Tiktaalik was thus inserted below Acanthostega and Ichthyostega , acting as a transitional form between limbless fish and limbed vertebrates ("tetrapods"). Some press coverage also used the term " missing link ", implying that Tiktaalik filled an evolutionary gap between fish and tetrapods. Nevertheless, Tiktaalik has never been claimed to be

693-547: A " fishapod ". Tiktaalik is a transitional fossil ; it is to tetrapods what Archaeopteryx is to birds , troodonts and dromaeosaurids . While it may be that neither is ancestor to any living animal, they serve as evidence that intermediates between very different types of vertebrates did once exist. The mixture of both fish and tetrapod characteristics found in Tiktaalik include these traits: The phylogenetic analysis of Daeschler et al. (2006) placed Tiktaalik as

792-482: A close relative. Tiktaalik was covered by rhombic (diamond-shaped) bony scales , most similar to Panderichthys among lobe-finned fish. The scales are roughly textured, slightly broader than long, and overlap from front-to-back. Strong lungs (as supported by the plausible presence of a spiracle) may have led to the evolution of a more robust ribcage , a key evolutionary trait of land-living creatures. The more robust ribcage of Tiktaalik would have helped support

891-472: 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

990-491: A detailed look at the internal head skeleton of Tiktaalik roseae , in the October 16, 2008, issue of Nature , researchers show how Tiktaalik was gaining structures that could allow it to support itself on solid ground and breathe air, a key intermediate step in the transformation of the skull that accompanied the shift to life on land by our distant ancestors. More than 60 specimens of Tiktaalik have been discovered, though

1089-477: A direct ancestor to tetrapods. Rather, its fossils help to illuminate evolutionary trends and approximate the hypothetical true ancestor to the tetrapod lineage, which would have been similar in form and ecology. In its original description, Tiktaalik was described as a member of Elpistostegalia , a name previously used to refer to particularly tetrapod-like fish such as Elpistostege and Panderichthys . Daeschler et al. (2006) recognized that this term referred to

SECTION 10

#1732787703078

1188-537: A flat skull resembling a crocodile's; eyes on top of its head; a neck and ribs similar to those of tetrapods, with the ribs being used to support its body and aid in breathing via lungs ; well developed jaws suitable for catching prey; and a small gill slit called a spiracle that, in more derived animals, became an ear . Spiracles would have been useful in shallow water, where higher water temperature would lower oxygen content. The discoverers said that in all likelihood, Tiktaalik flexed its proto-limbs primarily on

1287-426: 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

1386-499: 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

1485-412: A low semi-cartilaginous pubic process in front of the acetabulum (hip socket). This contrasts with the more complex pelvis of tetrapods, which have three separate bones (the ilium, pubis and ischium ) making up the hip. In addition, in tetrapods the left and right pelvises often connect to each other or the spinal column, while in Tiktaalik each side of the pelvis is fully separate. The orientation of

1584-418: 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

1683-536: 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

1782-570: 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

1881-700: A quadrupedal gait. Tiktaalik ' s discoverers were skeptical about the Zachelmie trackways. Daeschler said that trace evidence was not enough for him to modify the theory of tetrapod evolution, while Shubin argued that Tiktaalik could have produced very similar footprints. In a later study, Shubin expressed a significantly modified opinion that some of the Zachelmie footprints, those which lacked digits, may have been made by walking fish. However, Ahlberg insisted that those tracks could not have possibly been formed either by natural processes or by transitional species such as Tiktaalik or Panderichthys . Instead,

1980-472: A reanalysis in 2015 of the Zachelmie trackways find that it fails the criteria for it being identified as Devonian tetrapod trackways and were instead reinterpreted as fish nests or feeding traces, with the trackways at Easter Ross, Valentia Island and Genoa River being interpreted to have been produced by tetrapods. Tiktaalik generally had the characteristics of a lobe-finned fish, but with front fins featuring arm-like skeletal structures more akin to those of

2079-400: 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

SECTION 20

#1732787703078

2178-448: 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

2277-492: A triangular, flattened head and unusual, cleaver-shaped fins. Its fins have thin ray bones for paddling like most fish, but they also have sturdy interior bones that would have allowed Tiktaalik to prop itself up in shallow water and use its limbs for support as most four-legged animals do. Those fins and other mixed characteristics mark Tiktaalik as a crucial transition fossil , a link in evolution from swimming fish to four-legged vertebrates. This and similar animals might be

2376-613: Is a monospecific genus of extinct sarcopterygian (lobe-finned fish) from the Late Devonian Period, about 375 Mya (million years ago), having many features akin to those of tetrapods (four-legged animals). Tiktaalik is estimated to have had a total length of 1.25–2.75 metres (4.1–9.0 ft) based on various specimens. Unearthed in Arctic Canada , Tiktaalik is a non-tetrapod member of Osteichthyes (bony fish), complete with scales and gills—but it has

2475-449: 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

2574-530: Is conformably overlain by the Battle Formation . Fossil A fossil (from Classical Latin fossilis , lit.   ' obtained by digging ' ) 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

2673-434: Is difficult to homologize with the more splayed lower limbs of tetrapods. Tiktaalik retains a metapterygial axis with distinctly enlarged axial bones, a very fish-like condition. Even Panderichthys , which is otherwise more fish-like, seems to be more advanced towards a tetrapod-like limb. Nevertheless, the internal skeleton of the pectoral fin can still be equated to the forelimb bones of tetrapods. The first axial, at

2772-479: 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

2871-657: Is known as the fossil record . Though the fossil record is incomplete, numerous studies have demonstrated that there 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

2970-438: 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

3069-470: 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

Whitemud Formation - Misplaced Pages Continue

3168-413: 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

3267-467: 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

3366-462: The Australian lungfish , the fin skeleton is based around a straight string of midline bones, making up the metapterygial axis. The component bones of the axis are known as axials or mesomeres. The axis is flanked by one or two series of rod-like bones known as radials. Radials can be characterized as preaxial (in front of the axials) or postaxial (behind the axials). This semi-symmetrical structure

3465-801: 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

3564-586: 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

3663-569: 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

3762-602: 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

3861-402: 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

3960-403: 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

4059-453: 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

Whitemud Formation - Misplaced Pages Continue

4158-739: 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

4257-413: 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

4356-580: The April 6, 2006 issue of Nature and quickly recognized as a transitional form. In 2004, three fossilized Tiktaalik skeletons were discovered in the Late Devonian fluvial Fram Formation on Ellesmere Island , Nunavut , in northern Canada . Estimated ages were reported at 375 Ma, 379 Ma and 383 Ma. At the time of the species' existence, Ellesmere Island was part of the continent Laurentia (modern eastern North America and Greenland ), which

4455-581: The Devonian equator, suggesting a benthic animal that lived on the bottom of shallow waters and perhaps even out of the water for short periods, with a skeleton indicating that it could support its body under the force of gravity whether in very shallow water or on land. At that period, for the first time, deciduous plants were flourishing and annually shedding leaves into the water, attracting small prey into warm oxygen-poor shallows that were difficult for larger fish to swim in. Tiktaalik has been used as

4554-615: 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

4653-566: 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

4752-517: The Whitemud sediments accumulated in river channels , low-gradient streams , and shallow lakes and ponds on a low-lying coastal plain . Its kaolinite was produced by intense chemical weathering that altered the original feldspar content to kaolinite during and after deposition. Kaolinite-rich clays have been quarried from the Whitemud Formation since before 1920 and are used in the manufacture of stoneware products ranging from pottery to sewer pipes. Plant fossils and palynomorphs found in

4851-732: The animal's body any time it ventured outside a fully aquatic habitat. Tiktaalik is sometimes compared to gars (especially the alligator gar ), with whom it shares a number of characteristics: Tiktaalik roseae is the only species classified under the genus. Tiktaalik lived approximately 375 million years ago. It is representative of the transition between non-tetrapod vertebrates (fish) such as Panderichthys , known from fossils 380 million years old, and early tetrapods such as Acanthostega and Ichthyostega , known from fossils about 365 million years old. Its mixture of primitive fish and derived tetrapod characteristics led one of its discoverers, Neil Shubin, to characterize Tiktaalik as

4950-673: The apparent delay in fossil appearances. Another approach is that elpistostegalian and tetrapod similarities are a case of convergent evolution . In this interpretation, tetrapods would originate in the Middle Devonian while elpisostegalians originate independently in the Late Devonian, before going extinct near the end of the period. Estimates published after the discovery of Zachelmie tracks suggested that digited tetrapods may have appeared as early as 427.4 Mya and questioned attempts to read absolute timing of evolutionary events in early tetrapod evolution from stratigraphy. However,

5049-469: The authors of the publication suggested that " ichthyostegalian "-grade tetrapods were the responsible trackmakers, based on available pes morphology of those animals. Narkiewicz, co-author of the article on the Zachelmie trackways, claimed that the Polish "discovery has disproved the theory that elpistostegids were the ancestors of tetrapods", a notion partially shared by Philippe Janvier . To resolve

SECTION 50

#1732787703078

5148-401: The base of the fin, has developed into the humerus , the single large bone making up the stylopodium (upper arm). This is followed by the two bones of the zeugopodium (forearm): the radius (i.e., the first preaxial radial) and ulna (i.e., the second axial). The radius is much larger than the ulna, and its front edge thins into a sharp blade like that of Panderichthys . Further down,

5247-413: 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

5346-714: 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

5445-408: The body, the pelvis (hip) was intermediate in form between earlier lobe-finned fish (like Gooloogongia and Eusthenopteron ) and tetrapods (like Acanthostega ). The pelvis was much larger than in other fish, nearly the same size as the shoulder girdle, like tetrapods. In terms of shape, the pelvis is a single bone, much more similar to fish. There is a broad upper iliac blade continuous with

5544-409: The carbonaceous shales indicate the presence of aquatic plants such as water lilies , as well as trees and shrubs related to figs , hickories and redwoods . Coprolites provide the only evidence of vertebrates . In southern Saskatchewan the formation contains scattered coprolites that represent feces of fish, most probably sturgeons . The feces have been preserved due to the replacement of

5643-575: The common ancestors of all vertebrate terrestrial fauna : amphibians, reptiles, birds and mammals. The first Tiktaalik fossils were found in 2004 on Ellesmere Island in Nunavut , Canada. The discovery, made by Edward B. Daeschler of the Academy of Natural Sciences , Neil H. Shubin from the University of Chicago , and Harvard University Professor Farish A. Jenkins Jr. , was published in

5742-670: 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

5841-442: The digital arch model, is supported by numerous developmental studies. A consistent set of Hox genes are responsible for moderating both the rear edge of the fin (in several modern fish) and the digits of modern tetrapods as their embryos develop. The digital arch model posits that the metapterygial axis was bent forwards at a sharp angle near the origin of tetrapods. This allowed the axials to transform into wrist bones, while

5940-445: The digits to develop symmetrically from the distal fin radials. Another school of thought, popularized in the 1940s, is that the hand was neomorphic. This means that it was an entirely new structure that spontaneously evolved once the distal axials and radials were reduced. A third hypothesis, emphasized by Shubin and Alberch (1986), is that digits are homologous to postaxial radials in particular. This interpretation, better known as

6039-466: 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

SECTION 60

#1732787703078

6138-415: 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

6237-550: 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

6336-486: The earliest-known fish to have a neck, with the pectoral (shoulder) girdle separate from the skull. This would give the creature more freedom in hunting prey on land or in the shallows. The "fins" of Tiktaalik have helped to contextualize the origin of weight-bearing limbs and digits . The fin has both a robust internal skeleton, like tetrapods, surrounded by a web of simple bony fin rays ( lepidotrichia ), like fish. The lepidotrichia are thickest and most extensive on

6435-464: 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

6534-478: 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

6633-425: The fin was both muscular and had the ability to flex like a wrist joint. These wrist-like features would have helped anchor the creature to the bottom in a fast current. One of the persistent questions facing paleontologists is the evolution of the tetrapod limb: specifically, how the internal bones of lobed fins evolved into the feet and toes of tetrapods. In many lobe-finned fish, including living coelacanths and

6732-984: The fishes died. The Whitemud Formation is part of the Edmonton Group . It rests conformably on the Eastend Formation in Saskatchewan and in the Cypress Hills area; on the Horseshoe Canyon Formation in the Drumheller area; and on the St. Mary River Formation in the Oldman River area. In southeastern Saskatchewan it is unconformably overlain by the Frenchman Formation ; and in all other areas it

6831-430: The floor of streams and may have pulled itself onto the shore for brief periods. In 2014, the discovery of the animal's pelvic girdle was announced; it was strongly built, indicating the animal could have used them for moving in shallow water and across mudflats. Neil Shubin and Daeschler, the leaders of the team, have been searching Ellesmere Island for fossils since 2000: We're making the hypothesis that this animal

6930-436: The front edge and upper side of the fin, leaving more room for muscle and skin on the underside of the fin. The pectoral fin was clearly weight bearing, being attached to a massive shoulder girdle with expanded scapular and coracoid elements attached to the body armor. Moreover, there are large muscle scars on the underside of the forefin bones, and the distal joints of the wrist are highly mobile. Together, these suggest that

7029-519: 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

7128-461: The hip socket is halfway between the rear-facing socket of other fish and the sideways-facing socket of tetrapods. The hindlimbs, also known as pelvic fins , appear to be almost as long as the forelimbs. This is yet another trait more similar to tetrapods than to other fish. Though not all bones are preserved in the fossil, it is clear that the hindlimbs of Tiktaalik had lepidotrichia and at least three large rod-like ankle bones. If fully preserved,

7227-434: The holotype remains the most complete and well-described fossil. Tiktaalik provides insights on the features of the extinct closest relatives of the tetrapods. Tiktaalik was a large fish: the largest known fossils have an estimated length of 2.75 m (9.02 feet), with the longest lower jaws reaching a length of 31 centimetres (1.0 ft). The skull of Tiktaalik was low and flat, more similar in shape to that of

7326-449: The internal skeleton transitions into the mesopodium , which in tetrapods contains the bones of the wrist. Tiktaalik has two large wrist bones: the narrow intermedium (i.e., the second preaxial radial) and the blocky ulnare (i.e., the third axial). In tetrapods, the wrist is followed by the hand and finger bones. The origin of these bones has long been a topic of contention. In the early 20th century, most paleontologists considered

7425-535: 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

7524-430: The middle of the fin. Only three of the finger-like radials are postaxial, while the model predicts that most or all of the radials should be postaxial. It remains to be seen whether any of the distal radials of Tiktaalik are homologous to fingers. Finger-like distal radials are also known in other elpistostegalians: Panderichthys (which has at least four) and Elpistostege (which has 19). As with other regions of

7623-410: The narrower postaxial radials splay out and evolve into fingers. Tiktaalik presents a contradictory set of traits. As predicted by the digital arch model, there are multiple (at least eight) rectangular distal radials arranged in a dispersed pattern, similar to fingers. Some of the radials are even arranged sequentially, akin to finger joints. However, the metapterygial axis is straight and runs down

7722-450: The original fecal material by goethite , siderite and other iron minerals . They typically range in length from about 2 to 10 cm (1 to 4 in), although specimens as long as 23 cm (9 in) have been reported. A variety of coiled and uncoiled morphologies have been described, and some spiral forms with intricate systems of internal folds are thought to represent fossilized intestines containing feces that were unexpelled when

7821-529: The pelvic fins would probably have been internally and externally very similar to the pectoral fins. The torso of Tiktaalik is elongated by the standards of most Devonian tetrapodomorphs. Although the vertebrae are not ossified, there are about 45 pairs of ribs between the skull and the hip region. The ribs are larger than in earlier fish, imbricating (overlapping) via blade-like flanges. Imbricating ribs are also known in Ichthyostega, though in that taxon

7920-459: The phrase "elpistostegalian fish" to refer to the grade of early elpisostegalians which had not acquired limbs, digits, or other specializations which define tetrapods. In this sense, Tiktaalik is an elpistostegalian fish. Later papers also use the term "elpisostegid" for the same category of Devonian fish. This order of the phylogenetic tree was initially adopted by other experts, most notably by Per Ahlberg and Jennifer Clack . However, it

8019-545: 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

8118-443: The questions posed by the Zachelmie trackways, several hypotheses have been suggested. One approach maintains that the first pulse of elpistostegalian and tetrapod evolution occurring in the Middle Devonian, a time when body fossils showing this trend are too rare to be preserved. This maintains the elpistostegalian–tetrapod ancestor–descendant relationship apparent in fossils, but also introduces long ghost lineages required to explain

8217-443: 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

8316-462: 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

8415-489: The ribs are more diverse in shape. Tiktaalik most likely lacked dorsal fins , like other elpistostegalians as well as tetrapods. The shape of the tail and caudal fin are unknown, since that portion of the skeleton has not been preserved. Many lobe-finned fish have a single anal fin on the underside of the tail, behind the pelvic fins. While not reported in Tiktaalik , an anal fin can be observed in Elpistostege ,

8514-429: 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

8613-423: 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

8712-564: 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. Tiktaalik Tiktaalik ( / t ɪ k ˈ t ɑː l ɪ k / ; Inuktitut ᑎᒃᑖᓕᒃ [tiktaːlik] )

8811-586: 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

8910-467: 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

9009-451: 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

9108-524: 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

9207-458: 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

9306-581: Was called into question by a discovery made in the Holy Cross Mountains of Poland . In January 2010, a group of paleontologists (including Ahlberg) published on a series of trackways from the Eifelian stage of the Middle Devonian, about 12 million years older than Tiktaalik . These trackways, discovered at the Zachełmie quarry, appear to have been created by fully terrestrial tetrapods with

9405-408: Was centered on the equator and had a warm climate. When discovered, one of the skulls was found sticking out of a cliff. Upon further inspection, the fossil was found to be in excellent condition for a 375-million-year-old specimen. The discovery by Daeschler, Shubin and Jenkins was published in the April 6, 2006 issue of Nature and quickly recognized as a transitional form. Jennifer A. Clack ,

9504-431: Was exactly the missing intermediate they were looking for. 'We found something that really split the difference right down the middle,' says Daeschler. Tiktaalik is an Inuktitut word meaning "large freshwater fish". The "fishapod" genus received this name after a suggestion by Inuit elders of Canada 's Nunavut Territory , where the fossil was discovered. The specific name roseae honours an anonymous donor. Taking

9603-447: Was questioned in a 2008 paper by Boisvert et al., who noted that Panderichthys , due to its more derived distal forelimb structure, might be closer to tetrapods than Tiktaalik or even that it was convergent with tetrapods. Ahlberg, co-author of the study, considered the possibility of Tiktaalik ' s fin having been "an evolutionary return to a more primitive form." The proposed origin of tetrapods among elpistostegalian fish

9702-408: 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

9801-500: Was specialized for living in shallow stream systems, perhaps swampy habitats, perhaps even to some of the ponds. And maybe occasionally, using its very specialized fins, for moving up overland. And that's what is particularly important here. The animal is developing features which will eventually allow animals to exploit land. The fossils of Tiktaalik were found in the Fram Formation , deposits of meandering stream systems near

#77922