Transitional fossil

A transitional fossil is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group.[1] This is especially important where the descendant group is sharply differentiated by gross anatomy and mode of living from the ancestral group. These fossils serve as a reminder that taxonomic divisions are human constructs that have been imposed in hindsight on a continuum of variation. Because of the incompleteness of the fossil record, there is usually no way to know exactly how close a transitional fossil is to the point of divergence. Therefore, it cannot be assumed that transitional fossils are direct ancestors of more recent groups, though they are frequently used as models for such ancestors.[2]

In 1859, when Charles Darwin's On the Origin of Species was first published, the fossil record was poorly known. Darwin described the perceived lack of transitional fossils as, "... the most obvious and gravest objection which can be urged against my theory," but explained it by relating it to the extreme imperfection of the geological record.[3] He noted the limited collections available at that time, but described the available information as showing patterns that followed from his theory of descent with modification through natural selection.[4] Indeed, Archaeopteryx was discovered just two years later, in 1861, and represents a classic transitional form between earlier, non-avian dinosaurs and birds. Many more transitional fossils have been discovered since then, and there is now abundant evidence of how all classes of vertebrates are related, including many transitional fossils.[5] Specific examples of class-level transitions are: tetrapods and fish, birds and dinosaurs, and mammals and "mammal-like reptiles".

The term "missing link" has been used extensively in popular writings on human evolution to refer to a perceived gap in the hominid evolutionary record. It is most commonly used to refer to any new transitional fossil finds. Scientists, however, do not use the term, as it refers to a pre-evolutionary view of nature.

Evolutionary and phylogenetic taxonomy

Transitions in phylogenetic nomenclature

Spindle diagram
Traditional spindle diagram showing the vertebrates classes "budding" off from each other. Transitional fossils typically represent animals from near the branching points.

In evolutionary taxonomy, the prevailing form of taxonomy during much of the 20th century and still used in non-specialist textbooks, taxa based on morphological similarity are often drawn as "bubbles" or "spindles" branching off from each other, forming evolutionary trees.[6] Transitional forms are seen as falling between the various groups in terms of anatomy, having a mixture of characteristics from inside and outside the newly branched clade.[7]

With the establishment of cladistics in the 1990s, relationships commonly came to be expressed in cladograms that illustrate the branching of the evolutionary lineages in stick-like figures. The different so-called "natural" or "monophyletic" groups form nested units, and only these are given phylogenetic names. While in traditional classification tetrapods and fish are seen as two different groups, phylogenetically tetrapods are considered a branch of fish. Thus, with cladistics there is no longer a transition between established groups, and the term "transitional fossils" is a misnomer. Differentiation occurs within groups, represented as branches in the cladogram.[8]

In a cladistic context, transitional organisms can be seen as representing early examples of a branch, where not all of the traits typical of the previously known descendants on that branch have yet evolved.[9] Such early representatives of a group are usually termed "basal taxa" or "sister taxa,"[10] depending on whether the fossil organism belongs to the daughter clade or not.[8]

Transitional versus ancestral

A source of confusion is the notion that a transitional form between two different taxonomic groups must be a direct ancestor of one or both groups. The difficulty is exacerbated by the fact that one of the goals of evolutionary taxonomy is to identify taxa that were ancestors of other taxa. However, it is almost impossible to be sure that any form represented in the fossil record is a direct ancestor of any other. In fact, because evolution is a branching process that produces a complex bush pattern of related species rather than a linear process producing a ladder-like progression, and because of the incompleteness of the fossil record, it is unlikely that any particular form represented in the fossil record is a direct ancestor of any other. Cladistics deemphasizes the concept of one taxonomic group being an ancestor of another, and instead emphasizes the identification of sister taxa that share a more recent common ancestor with one another than they do with other groups. There are a few exceptional cases, such as some marine plankton microfossils, where the fossil record is complete enough to suggest with confidence that certain fossils represent a population that was actually ancestral to a later population of a different species.[11] But, in general, transitional fossils are considered to have features that illustrate the transitional anatomical features of actual common ancestors of different taxa, rather than to be actual ancestors.[2]

Prominent examples

Archaeopteryx

Archaeopteryx lithographica (Berlin specimen)
Archaeopteryx is one of the most famous transitional fossils and gives evidence for the evolution of birds from theropod dinosaurs.

Archaeopteryx is a genus of theropod dinosaur closely related to the birds. Since the late 19th century, it has been accepted by palaeontologists, and celebrated in lay reference works, as being the oldest known bird, though a study in 2011 has cast doubt on this assessment, suggesting instead that it is a non-avialan dinosaur closely related to the origin of birds.[12]

It lived in what is now southern Germany in the Late Jurassic period around 150 million years ago, when Europe was an archipelago in a shallow warm tropical sea, much closer to the equator than it is now. Similar in shape to a European magpie, with the largest individuals possibly attaining the size of a raven,[13] Archaeopteryx could grow to about 0.5 metres (1.6 ft) in length. Despite its small size, broad wings, and inferred ability to fly or glide, Archaeopteryx has more in common with other small Mesozoic dinosaurs than it does with modern birds. In particular, it shares the following features with the deinonychosaurs (dromaeosaurs and troodontids): jaws with sharp teeth, three fingers with claws, a long bony tail, hyperextensible second toes ("killing claw"), feathers (which suggest homeothermy), and various skeletal features.[14] These features make Archaeopteryx a clear candidate for a transitional fossil between dinosaurs and birds,[15] making it important in the study both of dinosaurs and of the origin of birds.

The first complete specimen was announced in 1861, and ten more Archaeopteryx fossils have been found since then. Most of the eleven known fossils include impressions of feathers—among the oldest direct evidence of such structures. Moreover, because these feathers take the advanced form of flight feathers, Archaeopteryx fossils are evidence that feathers began to evolve before the Late Jurassic.[16]

Australopithecus afarensis

Lucy Skeleton cropped
A. afarensis - walking posture.

The hominid Australopithecus afarensis represents an evolutionary transition between modern bipedal humans and their quadrupedal ape ancestors. A number of traits of the A. afarensis skeleton strongly reflect bipedalism, to the extent that some researchers have suggested that bipedality evolved long before A. afarensis.[17] 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, and there is clear evidence of a strong attachment for the knee extensors, implying an upright posture.[17]:122

While the pelvis is not entirely like that of a human (being markedly wide, or flared, with laterally orientated iliac blades), these features point to a structure radically remodelled to accommodate a significant degree of bipedalism. The femur angles in toward the knee from the hip. This trait allows the foot to fall closer to the midline of the body, and strongly indicates habitual bipedal locomotion. Present-day humans, orangutans and spider monkeys possess this same feature. The feet feature adducted big toes, making it difficult if not impossible to grasp branches with the hindlimbs. Besides locomotion, A. afarensis also had a slightly larger brain than a modern chimpanzee[18] (the closest living relative of humans) and had teeth that were more human than ape-like.[19]

Pakicetids, Ambulocetus

Pakicetus BW
Reconstruction of Pakicetus
Ambulocetus natans
Skeleton of Ambulocetus natans

The cetaceans (whales, dolphins and porpoises) are marine mammal descendants of land mammals. The pakicetids are an extinct family of hoofed mammals that are the earliest whales, whose closest sister group is Indohyus from family Raoellidae.[20][21] They lived in the Early Eocene, around 53 million years ago. Their fossils were first discovered in North Pakistan in 1979, at a river not far from the shores of the former Tethys Sea.[22] Pakicetids could hear under water, using enhanced bone conduction, rather than depending on tympanic membranes like most land mammals. This arrangement does not give directional hearing under water.[23]

Ambulocetus natans, which lived about 49 million years ago, was discovered in Pakistan in 1994. It was probably amphibious, and looked like a crocodile.[24] In the Eocene, ambulocetids inhabited the bays and estuaries of the Tethys Ocean in northern Pakistan.[25] The fossils of ambulocetids are always found in near-shore shallow marine deposits associated with abundant marine plant fossils and littoral molluscs.[25] Although they are found only in marine deposits, their oxygen isotope values indicate that they consumed water with a range of degrees of salinity, some specimens showing no evidence of sea water consumption and others none of fresh water consumption at the time when their teeth were fossilized. It is clear that ambulocetids tolerated a wide range of salt concentrations.[26] Their diet probably included land animals that approached water for drinking, or freshwater aquatic organisms that lived in the river.[25] Hence, ambulocetids represent the transition phase of cetacean ancestors between freshwater and marine habitat.

Tiktaalik

Tiktaalik Chicago
Tiktaalik roseae had spiracles (air holes) above the eyes.
Tiktaalik BW
Life restoration of Tiktaalik roseae

Tiktaalik is a genus of extinct sarcopterygian (lobe-finned fish) from the Late Devonian period, with many features akin to those of tetrapods (four-legged animals).[27] It is one of several lines of ancient sarcopterygians to develop adaptations to the oxygen-poor shallow water habitats of its time—adaptations that led to the evolution of tetrapods.[28] Well-preserved fossils were found in 2004 on Ellesmere Island in Nunavut, Canada.[29]

Tiktaalik lived approximately 375 million years ago. Paleontologists suggest that it is representative of the transition between non-tetrapod vertebrates 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 a "fishapod."[30][31] Unlike many previous, more fish-like transitional fossils, the "fins" of Tiktaalik have basic wrist bones and simple rays reminiscent of fingers. They may have been weight-bearing. Like all modern tetrapods, it had rib bones, a mobile neck with a separate pectoral girdle, and lungs, though it had the gills, scales, and fins of a fish.[27]

Tetrapod footprints found in Poland and reported in Nature in January 2010 were "securely dated" at 10 million years older than the oldest known elpistostegids[32] (of which Tiktaalik is an example), implying that animals like Tiktaalik, possessing features that evolved around 400 million years ago, were "late-surviving relics rather than direct transitional forms, and they highlight just how little we know of the earliest history of land vertebrates."[33]

Amphistium

Pseudopleuronectes americanus
Modern flatfish are asymmetrical, with both eyes on the same side of the head.
Amphistium
Fossil of Amphistium with one eye at the top-center of the head.

Pleuronectiformes (flatfish) are an order of ray-finned fish. The most obvious characteristic of the modern flatfish is their asymmetry, with both eyes on the same side of the head in the adult fish. In some families the eyes are always on the right side of the body (dextral or right-eyed flatfish) and in others they are always on the left (sinistral or left-eyed flatfish). The primitive spiny turbots include equal numbers of right- and left-eyed individuals, and are generally less asymmetrical than the other families. Other distinguishing features of the order are the presence of protrusible eyes, another adaptation to living on the seabed (benthos), and the extension of the dorsal fin onto the head.[34]

Amphistium is a 50-million-year-old fossil fish identified as an early relative of the flatfish, and as a transitional fossil[35] In Amphistium, the transition from the typical symmetric head of a vertebrate is incomplete, with one eye placed near the top-center of the head.[36] Paleontologists concluded that "the change happened gradually, in a way consistent with evolution via natural selection—not suddenly, as researchers once had little choice but to believe."[35]

Amphistium is among the many fossil fish species known from the Monte Bolca Lagerstätte of Lutetian Italy. Heteronectes is a related, and very similar fossil from slightly earlier strata of France.[36]

Runcaria

Runcaria megasporangium and cupule drawing
The Devonian fossil plant Runcaria resembles a seed but lacks a solid seed coat and means to guide pollen.

A Middle Devonian precursor to seed plants has been identified from Belgium, predating the earliest seed plants by about 20 million years. Runcaria, small and radially symmetrical, is an integumented megasporangium surrounded by a cupule. The megasporangium bears an unopened distal extension protruding above the multilobed integument. It is suspected that the extension was involved in anemophilous pollination. Runcaria sheds new light on the sequence of character acquisition leading to the seed, having all the qualities of seed plants except for a solid seed coat and a system to guide the pollen to the seed.[37]

Fossil record

Not every transitional form appears in the fossil record, because the fossil record is not complete. Organisms are only rarely preserved as fossils in the best of circumstances, and only a fraction of such fossils have been discovered. Paleontologist Donald Prothero noted that this is illustrated by the fact that the number of species known through the fossil record was 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.[38]

Because of the specialized and rare circumstances required for a biological structure to fossilize, logic dictates that known fossils represent only a small percentage of all life-forms that ever existed—and that each discovery represents only a snapshot of evolution. The transition itself can only be illustrated and corroborated by transitional fossils, which never demonstrate an exact half-way point between clearly divergent forms.[39]

The fossil record is very uneven and, with few exceptions, is heavily slanted toward organisms with hard parts, leaving most groups of soft-bodied organisms with little to no fossil record.[38] The groups considered to have a good fossil record, including a number of transitional fossils between traditional groups, are the vertebrates, the echinoderms, the brachiopods and some groups of arthropods.[40]

History

Post-Darwin

Archæopteryx, fig 1, Nordisk familjebok
A historic 1904 reconstruction of Archæopteryx
Rhynia reconstruction
Reconstruction of Rhynia

The idea that animal and plant species were not constant, but changed over time, was suggested as far back as the 18th century.[41] Darwin's On the Origin of Species, published in 1859, gave it a firm scientific basis. A weakness of Darwin's work, however, was the lack of palaeontological evidence, as pointed out by Darwin himself. While it is easy to imagine natural selection producing the variation seen within genera and families, the transmutation between the higher categories was harder to imagine. The dramatic find of the London specimen of Archaeopteryx in 1861, only two years after the publication of Darwin's work, offered for the first time a link between the class of the highly derived birds, and that of the more primitive reptiles.[42] In a letter to Darwin, the palaeontologist Hugh Falconer wrote:

Had the Solnhofen quarries been commissioned—by august command—to turn out a strange being à la Darwin—it could not have executed the behest more handsomely—than in the Archaeopteryx.[43]

Thus, transitional fossils like Archaeopteryx came to be seen as not only corroborating Darwin's theory, but as icons of evolution in their own right.[44] For example, the Swedish encyclopedic dictionary Nordisk familjebok of 1904 showed an inaccurate Archaeopteryx reconstruction (see illustration) of the fossil, "ett af de betydelsefullaste paleontologiska fynd, som någonsin gjorts" ("one of the most significant paleontological discoveries ever made").[45]

The rise of plants

Transitional fossils are not only those of animals. With the increasing mapping of the divisions of plants at the beginning of the 20th century, the search began for the ancestor of the vascular plants. In 1917, Robert Kidston and William Henry Lang found the remains of an extremely primitive plant in the Rhynie chert in Aberdeenshire, Scotland, and named it Rhynia.[46]

The Rhynia plant was small and stick-like, with simple dichotomously branching stems without leaves, each tipped by a sporangium. The simple form echoes that of the sporophyte of mosses, and it has been shown that Rhynia had an alternation of generations, with a corresponding gametophyte in the form of crowded tufts of diminutive stems only a few millimetres in height.[47] Rhynia thus falls midway between mosses and early vascular plants like ferns and clubmosses. From a carpet of moss-like gametophytes, the larger Rhynia sporophytes grew much like simple clubmosses, spreading by means of horizontal growing stems growing rhizoids that anchored the plant to the substrate. The unusual mix of moss-like and vascular traits and the extreme structural simplicity of the plant had huge implications for botanical understanding.[48]

Misunderstandings

Missing links

Human pidegree
The human pedigree back to amoeba shown as a reinterpreted chain of being with living and fossil animals. From G. Avery's critique of Ernst Haeckel, 1873.
Pithecanthropus-erectus
"Java Man" or Pithecanthropus erectus (now Homo erectus), the original "missing link" found in Java in 1891–92.

The term "missing link" refers back to the originally static pre-evolutionary concept of the great chain of being, a deist idea that all existence is linked, from the lowest dirt, through the living kingdoms to angels and finally to God.[49] The idea of all living things being linked through some sort of transmutation process predates Darwin's theory of evolution. Jean-Baptiste Lamarck envisioned that life is generated in the form of the simplest creatures constantly, and then strive towards complexity and perfection (i.e. humans) through a series of lower forms.[50] In his view, lower animals were simply newcomers on the evolutionary scene.[51]

After On the Origin of Species, the idea of "lower animals" representing earlier stages in evolution lingered, as demonstrated in Ernst Haeckel's figure of the human pedigree.[52] While the vertebrates were then seen as forming a sort of evolutionary sequence, the various classes were distinct, the undiscovered intermediate forms being called "missing links."

The term was first used in a scientific context by Charles Lyell in the third edition (1851) of his book Elements of Geology in relation to missing parts of the geological column, but it was popularized in its present meaning by its appearance on page xi of his book Geological Evidences of the Antiquity of Man of 1863. By that time it was generally thought that the end of the last glacial period marked the first appearance of humanity, but Lyell drew on new findings in his Antiquity of Man to put the origin of human beings much further back in the deep geological past. Lyell wrote that it remained a profound mystery how the huge gulf between man and beast could be bridged.[53] Lyell's vivid writing fired the public imagination, inspiring Jules Verne's Journey to the Center of the Earth (1864) and Louis Figuier's 1867 second edition of La Terre avant le déluge ("Earth before the Flood"), which included dramatic illustrations of savage men and women wearing animal skins and wielding stone axes, in place of the Garden of Eden shown in the 1863 edition.[54]

The idea of a "missing link" between humans and so-called "lower" animals remains lodged in the public imagination.[55] The search for a fossil showing transitional traits between apes and humans, however, was fruitless until the young Dutch geologist Eugène Dubois found a skullcap, a molar and a femur on the banks of Solo River, Java in 1891. The find combined a low, ape-like skull roof with a brain estimated at around 1000 cc, midway between that of a chimpanzee and an adult human. The single molar was larger than any modern human tooth, but the femur was long and straight, with a knee angle showing that "Java Man" had walked upright.[56] Given the name Pithecanthropus erectus ("erect ape-man"), it became the first in what is now a long list of human evolution fossils. At the time it was hailed by many as the "missing link," helping set the term as primarily used for human fossils, though it is sometimes used for other intermediates, like the dinosaur-bird intermediary Archaeopteryx.[57][58]

Punctuated Equilibrium
Sudden jumps with apparent gaps in the fossil record have been used as evidence for punctuated equilibrium. Such jumps can be explained either by macromutation or simply by relatively rapid episodes of gradual evolution by natural selection, since a period of say 10,000 years barely registers in the fossil record.

"Missing link" is still a popular term, well recognized by the public and often used in the popular media.[59] It is, however, avoided in the scientific press, as it relates to the concept of the great chain of being and to the notion of simple organisms being primitive versions of complex ones, both of which have been discarded in biology.[5] In any case, the term itself is misleading, as any known transitional fossil, like Java Man, is no longer missing. While each find will give rise to new gaps in the evolutionary story on each side, the discovery of more and more transitional fossils continues to add to our knowledge of evolutionary transitions.[5][60]

Punctuated equilibrium

The theory of punctuated equilibrium developed by Stephen Jay Gould and Niles Eldredge and first presented in 1972[61] is often mistakenly drawn into the discussion of transitional fossils.[62] This theory, however, pertains only to well-documented transitions within taxa or between closely related taxa over a geologically short period of time. These transitions, usually traceable in the same geological outcrop, often show small jumps in morphology between extended periods of morphological stability. To explain these jumps, Gould and Eldredge envisaged comparatively long periods of genetic stability separated by periods of rapid evolution. Gould made the following observation concerning creationist misuse of his work to deny the existence of transitional fossils:

Since we proposed punctuated equilibria to explain trends, it is infuriating to be quoted again and again by creationists—whether through design or stupidity, I do not know—as admitting that the fossil record includes no transitional forms. The punctuations occur at the level of species; directional trends (on the staircase model) are rife at the higher level of transitions within major groups.

See also

References

  1. ^ Freeman & Herron 2004, p. 816
  2. ^ a b Prothero 2007, pp. 133–135
  3. ^ Darwin 1859, pp. 279–280
  4. ^ Darwin 1859, pp. 341–343
  5. ^ a b c Prothero, Donald R. (1 March 2008). "Evolution: What missing link?". New Scientist. 197 (2645): 35–41. doi:10.1016/s0262-4079(08)60548-5. ISSN 0262-4079. Retrieved 13 May 2015.
  6. ^ For example, see Benton 1997
  7. ^ Prothero 2007, p. 84.
  8. ^ a b Kazlev, M. Alan. "Amphibians, Systematics, and Cladistics". Palaeos. Retrieved 9 May 2012.
  9. ^ Prothero 2007, p. 127.
  10. ^ Prothero 2007, p. 263.
  11. ^ Prothero, Donald R.; Lazarus, David B. (June 1980). "Planktonic Microfossils and the Recognition of Ancestors". Systematic Biology. 29 (2): 119–129. doi:10.1093/sysbio/29.2.119. ISSN 1063-5157.
  12. ^ Xing Xu; Hailu You; Kai Du; Fenglu Han (28 July 2011). "An Archaeopteryx-like theropod from China and the origin of Avialae". Nature. 475 (7357): 465–470. doi:10.1038/nature10288. ISSN 0028-0836. PMID 21796204.
  13. ^ Erickson, Gregory M.; Rauhut, Oliver W. M.; Zhonghe Zhou; et al. (9 October 2009). "Was Dinosaurian Physiology Inherited by Birds? Reconciling Slow Growth in Archaeopteryx". PLOS ONE. 4 (10): e7390. Bibcode:2009PLoSO...4.7390E. doi:10.1371/journal.pone.0007390. ISSN 1545-7885. PMC 2756958. PMID 19816582.
  14. ^ Yalden, Derek W. (September 1984). "What size was Archaeopteryx?". Zoological Journal of the Linnean Society. 82 (1–2): 177–188. doi:10.1111/j.1096-3642.1984.tb00541.x. ISSN 0024-4082.
  15. ^ "Archaeopteryx: An Early Bird". University of California Museum of Paleontology. Berkeley, CA: University of California, Berkeley. Retrieved 18 October 2006.
  16. ^ Wellnhofer 2004, pp. 282–300
  17. ^ a b Lovejoy, C. Owen (November 1988). "Evolution of Human walking" (PDF). Scientific American. 259 (5): 82–89. Bibcode:1988SciAm.259e.118L. doi:10.1038/scientificamerican1188-118. ISSN 0036-8733. PMID 3212438.
  18. ^ "Australopithecus afarensis". Human Evolution. Washington, D.C.: Smithsonian Institution's Human Origins Program. Retrieved 15 May 2015.
  19. ^ White, Tim D.; Suwa, Gen; Simpson, Scott; Asfaw, Berhane (January 2000). "Jaws and teeth of Australopithecus afarensis from Maka, Middle Awash, Ethiopia". American Journal of Physical Anthropology. 111 (1): 45–68. doi:10.1002/(SICI)1096-8644(200001)111:1<45::AID-AJPA4>3.0.CO;2-I. ISSN 0002-9483. PMID 10618588.
  20. ^ Northeastern Ohio Universities Colleges of Medicine and Pharmacy (21 December 2007). "Whales Descended From Tiny Deer-like Ancestors". Science Daily. Rockville, MD: ScienceDaily, LLC. Retrieved 15 May 2015.
  21. ^ Gingerich & Russell 1981
  22. ^ Castro & Huber 2003
  23. ^ Nummela, Sirpa; Thewissen, J. G. M.; Bajpai, Sunil; et al. (12 August 2004). "Eocene evolution of whale hearing". Nature. 430 (7001): 776–778. Bibcode:2004Natur.430..776N. doi:10.1038/nature02720. ISSN 0028-0836. PMID 15306808.
  24. ^ Thewissen, J. G. M.; Williams, Ellen M.; Roe, Lois J.; et al. (20 September 2001). "Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls". Nature. 413 (6853): 277–281. Bibcode:2001Natur.413..277T. doi:10.1038/35095005. ISSN 0028-0836. PMID 11565023.
  25. ^ a b c Thewissen, J. G. M.; Williams, Ellen M. (November 2002). "The Early Radiations of Cetacea (Mammalia): Evolutionary Pattern and Developmental Correlations". Annual Review of Ecology and Systematics. 33: 73–90. doi:10.1146/annurev.ecolsys.33.020602.095426. ISSN 1545-2069.
  26. ^ Thewissen, J. G. M.; Bajpai, Sunil (December 2001). "Whale Origins as a Poster Child for Macroevolution" (PDF). BioScience. 51 (12): 1037–1049. doi:10.1641/0006-3568(2001)051[1037:WOAAPC]2.0.CO;2. ISSN 0006-3568. Retrieved 16 May 2015.
  27. ^ a b Daeschler, Edward B.; Shubin, Neil H.; Jenkins, Farish A., Jr. (6 April 2006). "A Devonian tetrapod-like fish and the evolution of the tetrapod body plan". Nature. 440 (7085): 757–763. Bibcode:2006Natur.440..757D. doi:10.1038/nature04639. ISSN 0028-0836. PMID 16598249.
  28. ^ Clack, Jennifer A. (December 2005). "Getting a Leg Up on Land". Scientific American. 293 (6): 100–107. Bibcode:2005SciAm.293f.100C. doi:10.1038/scientificamerican1205-100. ISSN 0036-8733. PMID 16323697.
  29. ^ Easton, John (23 October 2008). "Tiktaalik's internal anatomy explains evolutionary shift from water to land". University of Chicago Chronicle. 28 (3). ISSN 1095-1237. Retrieved 19 April 2012.
  30. ^ Wilford, John Noble (5 April 2006). "Scientists Call Fish Fossil the 'Missing Link'". The New York Times. Retrieved 17 May 2015.
  31. ^ Shubin 2008
  32. ^ Niedźwiedzki, Grzegorz; Szrek, Piotr; Narkiewicz, Katarzyna; et al. (7 January 2010). "Tetrapod trackways from the early Middle Devonian period of Poland". Nature. 463 (7227): 43–48. Bibcode:2010Natur.463...43N. doi:10.1038/nature08623. ISSN 0028-0836. PMID 20054388.
  33. ^ "Four feet in the past: trackways pre-date earliest body fossils". Nature (Editor's summary). 463 (7227). 7 January 2010. ISSN 0028-0836.
  34. ^ Chapleau & Amaoka 1998, pp. 223–226
  35. ^ a b Minard, Anne (9 July 2008). "Odd Fish Find Contradicts Intelligent-Design Argument". National Geographic News. Washington, D.C.: National Geographic Society. Archived from the original on 4 August 2008. Retrieved 17 July 2008.
  36. ^ a b Friedman, Matt (10 July 2008). "The evolutionary origin of flatfish asymmetry". Nature. 454 (7201): 209–212. Bibcode:2008Natur.454..209F. doi:10.1038/nature07108. ISSN 0028-0836. PMID 18615083.
  37. ^ Gerrienne, Philippe; Meyer-Berthaud, Brigitte; Fairon-Demaret, Muriel; et al. (29 October 2004). "Runcaria, a Middle Devonian Seed Plant Precursor". Science. 306 (5697): 856–858. Bibcode:2004Sci...306..856G. doi:10.1126/science.1102491. ISSN 0036-8075. PMID 15514154.
  38. ^ a b Prothero 2007, pp. 50–53
  39. ^ Isaak, Mark, ed. (5 November 2006). "Claim CC200: Transitional fossils". TalkOrigins Archive. Houston, TX: The TalkOrigins Foundation, Inc. Retrieved 30 April 2009.
  40. ^ Donovan & Paul 1998
  41. ^ Archibald, J. David (August 2009). "Edward Hitchcock's Pre-Darwinian (1840) 'Tree of Life'" (PDF). Journal of the History of Biology. 42 (3): 561–592. CiteSeerX 10.1.1.688.7842. doi:10.1007/s10739-008-9163-y. ISSN 0022-5010. PMID 20027787.
  42. ^ Darwin 1859, Chapter 10.
  43. ^ Williams, David B. (September 2011). "Benchmarks: September 30, 1861: Archaeopteryx is discovered and described". EARTH. ISSN 1943-345X. Retrieved 24 February 2012.
  44. ^ Wellnhofer 2009
  45. ^ Leche 1904, pp. 1379–1380
  46. ^ Kidston, Robert; Lang, William Henry (27 February 1917). "XXIV.—On Old Red Sandstone Plants showing Structure, from the Rhynie Chert Bed, Aberdeenshire. Part I. Rhynia Gwynne-Vaughanii, Kidston and Lang". Transactions of the Royal Society of Edinburgh. 51 (3): 761–784. doi:10.1017/S0263593300006805. ISSN 0080-4568. OCLC 704166643. Retrieved 18 May 2015.
  47. ^ Kerp, Hans; Trewin, Nigel H.; Hass, Hagen (2003). "New gametophytes from the Early Devonian Rhynie chert". Transactions of the Royal Society of Edinburgh: Earth Sciences. 94 (4): 411–428. doi:10.1017/S026359330000078X. ISSN 0080-4568.
  48. ^ Andrews 1967, p. 32
  49. ^ Lovejoy 1936.
  50. ^ Lamarck 1815–1822
  51. ^ Appel, Toby A. (Fall 1980). "Henri De Blainville and the Animal Series: A Nineteenth-Century Chain of Being". Journal of the History of Biology. 13 (2): 291–319. doi:10.1007/BF00125745. ISSN 0022-5010. JSTOR 4330767.
  52. ^ Haeckel 2011, p. 216.
  53. ^ Bynum, William F. (Summer 1984). "Charles Lyell's Antiquity of Man and its critics". Journal of the History of Biology. 17 (2): 153–187. doi:10.1007/BF00143731. ISSN 0022-5010. JSTOR 4330890.
  54. ^ Browne 2003, pp. 130, 218, 515.
  55. ^ Sambrani, Nagraj (10 June 2009). "Why the term 'missing links' is inappropriate". Biology Times (Blog). Retrieved 19 May 2015.
  56. ^ Swisher, Curtis & Lewin 2001
  57. ^ Reader 2011
  58. ^ Benton, Michael J. (March 2001). "Evidence of Evolutionary Transitions". actionbioscience. Washington, D.C.: American Institute of Biological Sciences. Retrieved 29 March 2012.
  59. ^ Zimmer, Carl (19 May 2009). "Darwinius: It delivers a pizza, and it lengthens, and it strengthens, and it finds that slipper that's been at large under the chaise lounge [sic] for several weeks..." The Loom (Blog). Waukesha, WI: Kalmbach Publishing. Retrieved 10 September 2011.
  60. ^ Eldredge & Gould 1972, pp. 82–115
  61. ^ Bates, Gary (December 2006). "That quote!—about the missing transitional fossils". Creation. 29 (1): 12–15. ISSN 0819-1530. Retrieved 6 July 2014.
  62. ^ Gould 1980, p. 189.

Sources

External links

Ambulocetus

Ambulocetus (meaning "walking whale") is an early cetacean with short limbs and large feet used for swimming. Along with other members of Ambulocetidae, it is a transitional fossil that shows how whales evolved from land-living mammals. While its name stems from the historical hypothesis that it was capable of walking on land, more recent research suggests that it was fully aquatic like modern cetaceans.

Amphistium

Amphistium paradoxum, the only species classified under the genus Amphistium, is a fossil fish which has been identified as a Paleogene relative of the flatfish, and as a transitional fossil. In a typical modern flatfish, the head is asymmetric with both eyes on one side of the head. In Amphistium, the transition from the typical symmetric head of a vertebrate is incomplete, with one eye placed near the top of the head.Amphistium is among the many fossil fish species known from the Monte Bolca Lagerstätte of Lutetian Italy. Heteronectes is a related, and very similar fossil from a slightly earlier strata of France.

Casineria

Casineria is an extinct genus of tetrapod which lived about 334 million years ago in the Mississippian epoch of the Carboniferous period. Its generic name, Casineria, is a latinization of Cheese Bay, the site near Edinburgh, Scotland where the holotype fossil was found. When originally described in 1999, it was identified as a transitional fossil noted for its mix of basal (amphibian-like) and advanced (reptile-like) characteristics, putting it at or very near the origin of the amniotes, the group containing all mammals, birds, modern reptiles, and other descendants of their reptile-like common ancestor. However, the sole known fossil is lacking key elements such as a skull, making exact analysis difficult. As a result, the classification of Casineria has been more controversial in analyses conducted since 1999. Other proposed affinities include a placement among the lepospondyls, seymouriamorphs, "gephyrostegids", or as a synonym of Caerorhachis, another controversial tetrapod which may have been an early temnospondyl.

Darwinopterus

Darwinopterus (meaning "Darwin's wing") is a genus of pterosaur, discovered in China and named after biologist Charles Darwin. Between 30 and 40 fossil specimens have been identified, all collected from the Tiaojishan Formation, which dates to the middle Jurassic period, 160.89–160.25 Ma ago. The type species, D. modularis, was described in February 2010. D. modularis was the first known pterosaur to display features of both long-tailed ('rhamphorhynchoid') and short-tailed (pterodactyloid) pterosaurs, and was described as a transitional fossil between the two groups. Two additional species, D. linglongtaensis and D. robustodens, were described from the same fossil beds in December 2010 and June 2011, respectively.

Doleserpeton

Doleserpeton is an extinct, monospecific genus of dissorophoidean temnospondyl within the family Amphibamidae that lived during the Upper Permian, 285 million years ago. Doleserpeton is represented by a single species, Doleserpeton annectens, which was first described by John R. Bolt in 1969. Fossil evidence of Doleserpeton was recovered from the Dolese Brothers Limestone Quarry in Fort Sill, Oklahoma. The genus name Doleserpeton is derived from the initial discovery site in Dolese quarry of Oklahoma and the Greek root "serp-", meaning "low or close to the ground". This transitional fossil displays primitive traits of amphibians that allowed for successful adaptation from aquatic to terrestrial environments. In many phylogenies, frogs appear as the sister group of Doleserpeton.

Fram Formation

The Fram Formation is an Upper Devonian sequence of rock strata on Ellesmere Island that came into prominence in 2006 with the discovery in its rocks of examples of the transitional fossil, Tiktaalik, a sarcopterygian or lobe-finned fish showing many tetrapod characteristics. The Fram Formation is a Middle to Upper Devonian clastic wedge forming an extensive continental facies consisting of sediments derived from deposits laid down in braided stream systems that formed some 375 million years ago, at a time when the North American craton ("Laurentia") was straddling the equator.

Heteronectes

Heteronectes chaneti is a fossil fish which has been identified as an early relative of the flatfish, and as a transitional fossil. In a typical modern flatfish, the head is asymmetric with both eyes on one side of the head. In Heteronectes, the transition from the typical symmetric head of a vertebrate is incomplete, with one eye positioned near the top of the head, very similar, but less so than its Italian relative, Amphistium.The evolutionary transition from a symmetric position of eyes in many fish to the position of both eyes on the same side of the head in flatfish was cited as a transition difficult to imagine by St. George Jackson Mivart. This was presented as a difficulty for gradual evolution. The discovery, in 2008, of Heteronectes and Amphistium was considered a vindication of the viability of such a transition.Heteronectes is found in the early to middle Eocene of France.

Ichthyostega

Ichthyostega (Greek: "fish roof") is an early genus of tetrapodomorphs that lived at the end of the Late Devonian Period. It was one of the first four-limbed vertebrates in the fossil record. Ichthyostega possessed lungs and limbs that helped it navigate through shallow water in swamps. Although Ichthyostega is often labelled a "tetrapod" due to the possession of limbs and fingers, it was more basal ("primitive") than true crown-tetrapods, and could more accurately be referred to as a stegocephalian or stem tetrapod. Likewise, while undoubtedly of amphibian build and habit, it is not considered a true member of the group in the narrow sense, as the first modern amphibians (members of the group Lissamphibia) appeared in the Triassic Period. Until finds of other early stegocephalians and closely related fishes in the late 20th century, Ichthyostega stood alone as a transitional fossil between fish and tetrapods, combining fish- and tetrapod-like features. Newer research has shown that it had an unusual anatomy, functioning more akin to a seal than a salamander, as previously assumed.

Kulindroplax

Kulindroplax perissokomos is a Silurian mollusk, known from a single fossil from the Wenlock Series Lagerstätte fauna of England. It lived during the Homerian Age (about 425 million years ago). It is considered a basal aplacophoran. Unlike all modern aplacophorans, which are shell-less, Kulindroplax has a chiton-like shell, and it is considered a transitional fossil in the evolution of molluscs.The only known specimen, described in 2012, was conserved at the Oxford University Museum of Natural History.

Le Père de nos pères

Le Père de nos pères is a novel, by Bernard Werber, released in 1998. It is the first volume in the trilogy of Lucrèce et Isidore, named for the two main characters.

In this book, Werber deals with the origins of humankind through two new characters, Isidore Katzenberg and Lucrèce Nemrod. He puts forward an alternative and surprising idea for the transitional fossil in the evolution of humanity, also called the Missing Link theory.

List of index fossils

Index fossils (also known as guide fossils or indicator fossils) are fossils used to define and identify geologic periods (or faunal stages). Index fossils must have a short vertical range, wide geographic distribution and rapid evolutionary trends. Another term, Zone fossil is used when the fossil have all the characters stated above except wide geographical distribution, they are limited to a zone and can't be used for correlations of stratas.

Macroevolution

Macroevolution is evolution on a scale at or above the level of species, in contrast with microevolution, which refers to smaller evolutionary changes of allele frequencies within a species or population. Macroevolution and microevolution describe fundamentally identical processes on different scales.The process of speciation may fall within the purview of either, depending on the forces thought to drive it. Paleontology, evolutionary developmental biology, comparative genomics and genomic phylostratigraphy contribute most of the evidence for macroevolution's patterns and processes.

Macrofossil

Macrofossils, also known as megafossils, are preserved organic remains large enough to be visible without a microscope. The term macrofossil stands in opposition to the term microfossil. Microfossils, by contrast, require substantial magnification for evaluation by fossil-hunters or professional paleontologists. As a result, most fossils observed in the field and most "museum-quality" specimens are macrofossils.

Panderichthys

Panderichthys is a genus of extinct sarcopterygian (lobe-finned fish) from the late Devonian period, about 380 Mya. Panderichthys, which was recovered from Frasnian (early Late Devonian) deposits in Latvia, is represented by two species. P. stolbovi is known only from some snout fragments and an incomplete lower jaw. P. rhombolepis is known from several more complete specimens. Although it probably belongs to a sister group of the earliest tetrapods, Panderichthys exhibits a range of features transitional between tristichopterid lobe-fin fishes (e.g., Eusthenopteron) and early tetrapods. It is named after the German-Baltic paleontologist Christian Heinrich Pander. A recent study uncovered tetrapod tracks dating back to before the appearance of Panderichthys in the fossil record, which suggests that Panderichthys is not a transitional fossil, but nonetheless shows the traits that evolved during the fish-tetrapod evolution

Protoclaviger

Protoclaviger is an extinct Early Eocene transitional fossil myrmecophile of the rove beetle subfamily Pselaphinae, and a stem group of the modern supertribe Clavigeritae, of which Claviger is a representative. All modern Clavigeritae are morphologically specialized obligate colony parasites of ant nests, soliciting food via trophallaxis from worker ants, and preying on the nest brood.

The amber-embedded holotype specimen of the single species, Protoclaviger trichodens, was recovered from a piece of 52 million-year-old Cambay amber from Gujarat, India. P. trichodens differs to modern Clavigeritae in its possession of a segmented dorsal abdomen; in extant species, the abdominal tergites are fused into single large segment. P. trichodens also possesses 8 antennal segments and mouthparts that extend outside the oral cavity, whereas modern species have between 3 and 6 antennal segments, and mouthparts that are contained within the oral cavity.

Protoclaviger is believed to represent an intermediate stage in the evolution of morphological specialization that adapted modern Clavigeritae to life inside ant colonies. Its discovery in Cambay amber, which holds one of the earliest diverse assemblages of modern ant subfamilies, indicates that the ant-beetle symbiosis is evolutionarily ancient.

Puijila

Puijila darwini is an extinct species of basal pinniped which lived during the Miocene epoch about 21 to 24 million years ago. Approximately a metre (three feet) in length, the animal possessed only minimal physical adaptations for swimming. Unlike modern pinnipeds, it did not have flippers and its overall form was otter-like, albeit more specialized; its skull and teeth are the features that most clearly indicate that it is a seal.It is considered to be the most primitive pinniped yet found. The genus name is an Inuktitut word for a young seal; the species name honours the English naturalist Charles Darwin. The holotype and only known specimen is a nearly complete fossilised skeleton. It is being housed at the Canadian Museum of Nature in Ottawa, Ontario.

Science, Evolution, and Creationism

Science, Evolution, and Creationism is a publication by the United States National Academy of Sciences. The book's authors intended to provide a current and comprehensive explanation of evolution and "its importance in the science classroom". It was "intended for use by scientists, teachers, parents, and school board members who wanted to engage in more constructive conversations with others who remain uncertain about evolution and its place in the public school curriculum." The book, published on January 3, 2008, is available as a free PDF file on the National Academies Press website.Science, Evolution, and Creationism differs from prior National Academy of Sciences publications regarding creation and evolution in public education and the creation-evolution controversy; it is intended "specifically for the lay public", devoting much of its space to "explaining the differences between science and religion, and asserting that acceptance of evolution does not require abandoning belief in God." The book provides statements from notable biologists and clergy members to support the claim that "attempts to pit science and religion against each other create controversy where none needs to exist."The authors of Science, Evolution, and Creationism, who include Francisco J. Ayala and Bruce Alberts, highlight developments in evolutionary biology and its relevance to the study of emerging infectious diseases, the 2006 discovery of the transitional fossil Tiktaalik, and the application of evolutionary theory in many areas of science and engineering beyond biology.The book was released as several states, particularly Texas and Florida, considered revisions in state science standards.A study at Arizona State University used the book as part of a two-week module, within an introductory biology course, focusing on science, evolution, and religion. The percentage of students who held the view that there was a conflict between religion and evolution was reduced by about half.

Ted Daeschler

Edward B. 'Ted' Daeschler is an American vertebrate paleontologist and Associate Curator and Chair of Vertebrate Biology at the Academy of Natural Sciences in Philadelphia. He is a specialist in fish paleontology, especially in the Late Devonian, and in the development of the first limbed vertebrates.

He is the discoverer of the transitional fossil tetrapod Hynerpeton bassetti, and a Devonian fish-like specimen of Sauripterus taylorii with fingerlike appendages, and was also part of a team of researchers that discovered the transitional fossil Tiktaalik.

He received a Ph.D. from the University of Pennsylvania in 1998. He has held recent research grants from the National Science Foundation, the National Geographic Society, and other donors. He is also known for his work on the preservation of natural history collections.

Triadobatrachus

Triadobatrachus is an extinct genus of salientian frog-like amphibians, including only one known species, Triadobatrachus massinoti. It is the oldest member of the frog lineage known, and an excellent example of a transitional fossil. It lived during the Early Triassic about 250 million years ago, in what is now Madagascar.

Triadobatrachus was 10 cm (3.9 in) long, and still retained many primitive characteristics, such as possessing at least 26 vertebrae, where modern frogs have only four to nine. At least 10 of these vertebrae formed a short tail, which the animal may have retained as an adult. It probably swam by kicking its hind legs, although it could not jump, as most modern frogs can. Its skull resembled that of modern frogs, consisting of a latticework of thin bones separated by large openings.This creature, or a relative, evolved eventually into modern frogs, the earliest example of which is Prosalirus, millions of years later in the Early Jurassic.It was first discovered in the 1930s, when Adrien Massinot, near the village of Betsiaka in northern Madagascar, found an almost complete skeleton. The animal must have fossilized soon after its death, because all bones lay in their natural anatomical position. Only the anterior part of the skull and the ends of the limbs were missing. This fossil was initially described under the name Protobatrachus massinoti by the French paleontologist Jean Piveteau in 1936. Much more detailed description were published more recently.Although it was found in marine deposits, the general structure of Triadobatrachus shows that it probably lived for part of the time on land and breathed air. Its proximity to the mainland is further borne out by the remains of terrestrial plants found with it, and because most extant amphibians do not tolerate saltwater, and that this saltwater intolerance was probably present in the earliest lissamphibians.

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