Transitional forms. Presentation on the topic "transitional forms of evolution"
No fossil remains cause as much controversy as those classified as “transitional forms”: Ichthyostega, Archeopteryx, rhinophytes, etc. For some, such finds are clear evidence of the process of evolution, connecting bridges between different groups. For others, it is a reason to question the possibility of transitions between large taxa.
The concept of “transitional form” can have two various interpretations: phylogenetic and comparative anatomical. From a phylogenetic point of view, transitional forms are the descendants of one group, which are the ancestors of another. From a comparative anatomical point of view, transitional forms are organisms that combine the characteristics of different groups. Such organisms can be not only extinct, but also modern. Thus, by comparing existing species, we can see a reflection of the stages through which the evolution of a particular trait could occur. Let's look at one example. It seemed incredible to C. Darwin that such a complex organ as the eye could arise gradually, because its different parts are meaningless without each other. Research on modern coelenterates and worms has shown the possibility of multiple transitional stages from pigment spots through lensless pits to true eyes.
Stages of eye complexity observed in modern animals. 1. Single photosensitive cell. 2. Palpebral socket. 3. A goblet eye without a lens. 4. Eye with lens.
Alas, carriers of transitional states of certain characteristics are not in all cases preserved in the modern fauna. The first terrestrial vertebrates would now not have been able to withstand competition with highly developed tetrapods, and the first birds would not have been able to withstand competition with those who had reached high degrees of perfection modern types. In these cases, the fossil record provides invaluable data. This is precisely the significance of such finds as Ichthyostega, Archeopteryx, and rhinophytes.
The fact that this or that organism is a transitional form in the phylogenetic sense can be stated only in exceptional cases, when the paleontological record preserves the complete sequences of ancestors and descendants. This is possible when, in the habitat inhabited by evolving populations of certain species, there is a continuous deposition of sediment containing the remains of organisms. Why are phylogenetic transitional forms preserved so rarely?
The transition from one large group to another is also a decisive change in lifestyle. Each large group occupies a characteristic complex of ecological niches (adaptive zone). Sometimes, during evolution, species appear that change their way of life. Having passed through an unstable state, such species are able to move to another adaptive zone and give rise to a new taxon. Only groups that occupy sufficiently wide adaptive zones can be numerous and have a high chance of being preserved in the fossil record. The surprise is not that we find few intermediate forms, but that we sometimes manage to find them! This is usually due to the fact that transitional forms have occupied some specific ecological niche and have become quite widespread. This means that the transitional forms known to us are most likely not the common ancestors of the emerging groups.
“Transitional forms” were often associated with short-lived adaptive zones. This feature made them few in number and short-lived.
So, is Ichthyostega not the ancestor of all tetrapods, and Archeopteryx not the ancestor of all birds? Of course not! Perhaps these transitional forms are quite closely related to the common ancestors of the new groups, or perhaps they are not. That's not the point. They show what paths evolution could take, how the characteristics of one group could be combined with the characteristics of another.
Classic reconstruction of Ichthyostega. Here she is depicted as a clumsy land animal
It is now clear that the first four-legged animals were predators that lived in shallow water. Both limbs and lungs were formed as adaptations to life in water, but later turned out to be successful acquisitions for life on land.
Tulerpeton is another representative of the Upper Devonian tetrapods, found in the Tula region. Tulerpeton and Ichthyostega belong to different evolutionary branches of tetrapods.
Pandericht is an Upper Devonian lobe-finned fish, which in some respects is adapted to crawling on land even better than the first tetrapods.
How to confuse the problem of “transitional forms”?
The difficulty in understanding the problem of transitional forms is intensively used by opponents of evolutionism. The main technique is to convince non-specialists that the presence of a full spectrum of transitional forms is an obligatory consequence of evolution. To achieve this, the properties of both the evolutionary process and the fossil record are deliberately distorted.
"According to generally accepted evolutionary theory, from the fossil record one would expect: 1. the gradual appearance of the simplest forms of life; 2. gradual transformation of simple forms into more complex ones; 3. many intermediate “links” between different types; 4. the beginnings of new features of the body, such as limbs, bones and organs. According to the creation model, one would expect from the fossil record: 1. the sudden appearance of complex life forms; 2. reproduction of complex life forms “according to their kind” (biological families), not excluding variations”; 3. absence of intermediate “links” between different biological families; 4. absence of partially developed characters; complete completeness of all parts of the body."
All theses attributed to evolutionists are based on the idea that evolution proceeds in small steps at a constant speed, and the fossil record diligently records all the emerging forms, both widespread and rare. Incomplete fulfillment of the statements attributed to evolutionists does not refute the fact of evolution, but only corrects our ideas about its mechanisms. Nevertheless, in general terms the above conditions are met. In the fossil record, remains of unicellular, primitive multicellular animals and then highly developed invertebrates, successive groups of vertebrates (jawless, fish, the first terrestrial tetrapods, reptiles, etc.) successively appear. Both in the fossil record and among modern forms one can find a significant number of intermediate links in terms of their structure or lifestyle. Looking at well-documented phylogenetic lineages, one can see the development of what the book's authors call "the beginnings of new characters." Shallow folds on the teeth of the first horses develop into a powerful system of food-grinding ridges. The rays of the fins of lobe-finned fish are transformed into the bones of the limbs of vertebrates. Small areas The neocortex in reptiles became a stage in the process that led to the development of the huge human hemispheres.
Were there any transitional forms?
“If evolution were based on facts, one would expect that the fossil record would reveal the beginnings of new structures in living organisms. At least some fossils would show developing arms, legs, wings, eyes, and other bones and organs. For example, there should be fins of fish that turn into the legs of amphibians, and gills that gradually turn into lungs. There would have to be reptiles whose forelimbs would turn into bird wings, their hind limbs into clawed paws, their scales into feathers, and their mouth into a horny beak.”
The above quotation (like many similar statements scattered throughout the anti-evolution literature) indicates the lack of competence of its authors. It is unlikely that creationists who make claims like these are so naive that they do not think to consult publicly available reference books and textbooks to see if their opinion is wrong. Most likely, their only goal is to mislead naive readers.
The paw-shaped fins of lobe-finned fish are well known. During studies of modern coelacanths, films were shot from aboard a submarine showing how successfully these fish walk along the rocky bottom on their fins. The transformation of gills into lungs was not envisioned by any sane specialist. On the contrary, a number of fish (including modern lungfish) have both gills and lungs. The lungs developed as a protrusion of the wall of the esophagus. The classic "transitional form" Archeopteryx (like Protoavis) fits well with the last description in the quoted passage. The wings of these animals retain many common features with the forelimbs of typical reptiles. As embryological data show, bird feathers are transformed scales of reptiles. It is difficult to understand what the transformation of the hind limbs of reptiles into the clawed paws of birds consists of: the hind limbs of birds did not experience any significant restructuring. It is interesting that the evolution of the hind limbs towards the formation of the tarsus (an additional section of the limbs) began in typical reptiles. Both toothed and toothless birds are known. There is nothing supernatural in the beak of birds, contrary to the following statement: “...Birds are distinguished from reptiles by their beaks. There are beaks with which they crack nuts or filter food from muddy water, hollow out food in trees, as well as crossed beaks that open Pine cones, - the variety seems endless. And yet, about the beak, which has such purposefulness, it is said that it was formed by chance from the nose of a reptile! Do you think this explanation is plausible? .
The beak is the horny cover located on the jaws. Beaks have appeared repeatedly in different groups of reptiles. The well-known tuateria (belonging to the Beak-headed order) has both a small beak and teeth. All turtles have lost teeth and have remarkable beaks, adapted in shape to the type of nutrition characteristic of each species. Many extinct reptiles such as mammals (for example, anomodonts), dinosaurs (psittacosaurs) and flying lizards (pteranodons) had beaks. Adaptation to the flight of birds required lightening the body, and especially the head. The jaws bearing teeth turned out to be heavier than those covered with a horny sheath. In this respect, the birds followed the path trodden by many groups of their relatives. And the various modifications of the beak described in the above passage are the result of later adaptation to different lifestyles.
One way to discredit an opponent in a dispute is to distort his views, and then brilliantly refute his own fabrications. Polemics with a caricatured image of an opponent often indicate that the actual arguments of the opposite side turned out to be irrefutable.
“How is it that flight could have developed in four various groups: insects, birds, reptiles and mammals? Did everyone have transitional forms? Did all flying animals evolve from a single intermediate and then continue to evolve into mammals (such as bats) and/or insects?” . The reader who believes that evolutionists express such views will inevitably be outraged by their nonsense. The only catch is that such ideas are expressed precisely by opponents of evolution. Of course, in all the named evolutionary lines there were transitional forms; Naturally, they were different. However, there were some similar features between these forms (and especially between different vertebrates), and they are explained by the similarity of the problems that were solved in each of these evolutionary branches.
By the way, it is very likely that the flight occurred not four, but more times. It is very likely that both birds and flying dinosaurs arose polyphyletically (in several branches). Gliding flight has been mastered by marsupials and common flying squirrels, woolly wings, copepods, several groups of modern (flying dragons from agamas and lobe-tailed geckos) and extinct lizards, decorated tree snakes, flying fish and squids, and even spiders using long webs for this!
In a short article it is impossible to examine in detail the origins of all the groups whose emergence is considered miraculous by creationists. We have already looked at some examples, we will look at some later. In all cases, an unbiased examination of the facts turns miracles requiring divine intervention into normal problems amenable to scientific study.
Literature Cited
13. Ichas M. On the nature of living things: mechanisms and meaning. M.: Mir, 1994.
14. Harrub B., Thompson B. Archaeopteryx, archaeoraptor, and the“dinosaurs-to-birds” theory. Part I
Transitional form
Transitional form- an organism with an intermediate state that necessarily exists during a gradual transition from one biological type of structure to another. Transitional forms are characterized by the presence of more ancient and primitive (in the sense of primary) traits than their later relatives, but, at the same time, the presence of more progressive (in the sense of later) traits than their ancestors. As a rule, when speaking about intermediate forms, they mean fossil species, although intermediate species do not necessarily have to die out. Many transitional forms are known, illustrating the origin of tetrapods from fish, reptiles from amphibians, birds from dinosaurs, mammals from theriodonts, cetaceans from land mammals, horses from a five-toed ancestor, and humans from ancient hominids.
Evolution theory
Examples
Examples of transitional forms include Ambulocetus - the “walking whale” (a transitional form between cetaceans and land mammals), Tiktaalik and Ichthyostega (a transitional form from fish to amphibians), Mesohippus (between early equids and the modern horse).
Transitional forms in the evolution of Homo Sapiens
Currently, many transitional forms are known between Homo sapiens and his ape-like ancestors. For example, the remains of Sahelanthropus, Ardipithecus, Australopithecus (African, Afar and others), Homo habilis, Homo working, Homo erectus, Homo precursor, Heidelberg man, Neanderthal and early Homo sapiens have been discovered.
Dispute about the gradualness of evolutionary transformations
Transitional forms are the object of study in the debate between gradualists and punctualists. Gradualists believe that evolution is a gradual, continuous process. Punctualists, or proponents, believe that species remain the same for a long time, but changes occur in a very short period of time. Transitional forms are an argument on both sides. Punctualists argue that most species appear suddenly and change little throughout their history. Gradualists note that among vertebrates there is not a single indisputable example of a very rapid transition from one species to another; most transitions are generally poorly documented by fossils. There are also many well-described intraspecific variations. For example, changes in Middle Cenozoic rabbits, Miocene rodents, hipparions, Eocene artiodactyls, and the transition between genera of Eocene primates are described.
Missing Links
| 1850 | |
| 1900 |  |
| 1950 |  |
| 2002 |  |
| Progress in the study of human evolution. Increase in the number of hominin species known to science over time. Each species is depicted as a rectangle showing the boundaries within which skull volume varied and the species' place in the fossil record. One can see the gradual filling of the gaps between types. | |
Many transitional forms between living organisms have not yet been found. These are commonly referred to as missing links. The rarity of transitional forms is explained by the incompleteness of the fossil record. Proponents of the theory of punctuated equilibrium also point to the rapidity of evolutionary transformations. However, paleontologists are constantly finding new fossils, filling in the gaps (for example, in 2004, Tiktaalik was found - another transitional link between fish and amphibians)
Incomplete fossil record
Not every transitional form exists as a fossil due to the fundamental incompleteness of the fossil record. The incompleteness is caused by the peculiarities of the process of fossilization, that is, the transition to a fossilized state. For a fossil to form, the dead organism must be buried under a large layer of sediment. Due to the extremely slow rate of sedimentation on land, purely terrestrial species rarely become fossilized and persist. In addition, species living in the deep ocean are rarely discovered due to the rarity of their rise to the surface. large areas bottom. Thus, most known fossils (and, accordingly, transitional forms) are either species living in shallow waters, seas and rivers, or terrestrial species leading a semi-aquatic lifestyle, or living close to the coastline. The patterns of burial processes of the remains of living beings are studied by a special branch of paleontology - taphonomy.
Phylogenetic series
Phylogenetic series are series of species that successively replaced each other in the process of evolution of various groups of animals and plants.
They were first discovered by V. O. Kovalevsky, who showed that modern one-toed ungulates descend from ancient five-toed small omnivores.
Creationism
Proponents of creationism claim that no transitional forms have been found. The scientific community considers such statements to be false and deliberately misleading.
see also
Notes
Literature
- Darwin, Charles. Origin of species by means of natural selection: In 2 books. - M.: TERRA - Book Club, 2009. - ISBN 978-5-275-02114-1
- Carroll R. Paleontology and evolution of vertebrates: In 3 volumes - Mir, 1992. - 280 p. - ISBN 5-03-001819-0
Links
- The myth that “the missing link has not been found...” on the portal Anthropogenesis.ru
Wikimedia Foundation. 2010.
Transitional form
Transitional form- an organism with an intermediate state that necessarily exists during a gradual transition from one biological type of structure to another. Transitional forms are characterized by the presence of more ancient and primitive (in the sense of primary) traits than their later relatives, but, at the same time, the presence of more progressive (in the sense of later) traits than their ancestors. As a rule, when speaking about intermediate forms, they mean fossil species, although intermediate species do not necessarily have to die out. Many transitional forms are known, illustrating the origin of tetrapods from fish, reptiles from amphibians, birds from dinosaurs, mammals from theriodonts, cetaceans from land mammals, horses from a five-toed ancestor, and humans from ancient hominids.
Evolution theory
Examples
Examples of transitional forms include Ambulocetus - the “walking whale” (a transitional form between cetaceans and land mammals), Tiktaalik and Ichthyostega (a transitional form from fish to amphibians), Mesohippus (between early equids and the modern horse).
Transitional forms in the evolution of Homo Sapiens
Currently, many transitional forms are known between Homo sapiens and his ape-like ancestors. For example, the remains of Sahelanthropus, Ardipithecus, Australopithecus (African, Afar and others), Homo habilis, Homo working, Homo erectus, Homo precursor, Heidelberg man, Neanderthal and early Homo sapiens have been discovered.
Dispute about the gradualness of evolutionary transformations
Transitional forms are the object of study in the debate between gradualists and punctualists. Gradualists believe that evolution is a gradual, continuous process. Punctualists, or proponents, believe that species remain the same for a long time, but changes occur in a very short period of time. Transitional forms are an argument on both sides. Punctualists argue that most species appear suddenly and change little throughout their history. Gradualists note that among vertebrates there is not a single indisputable example of a very rapid transition from one species to another; most transitions are generally poorly documented by fossils. There are also many well-described intraspecific variations. For example, changes in Middle Cenozoic rabbits, Miocene rodents, hipparions, Eocene artiodactyls, and the transition between genera of Eocene primates are described.
Missing Links
| 1850 | |
| 1900 | |
| 1950 | |
| 2002 | |
| Progress in the study of human evolution. Increase in the number of hominin species known to science over time. Each species is depicted as a rectangle showing the boundaries within which skull volume varied and the species' place in the fossil record. One can see the gradual filling of the gaps between types. | |
Many transitional forms between living organisms have not yet been found. These are commonly referred to as missing links. The rarity of transitional forms is explained by the incompleteness of the fossil record. Proponents of the theory of punctuated equilibrium also point to the rapidity of evolutionary transformations. However, paleontologists are constantly finding new fossils, filling in the gaps (for example, in 2004, Tiktaalik was found - another transitional link between fish and amphibians)
Incomplete fossil record
Not every transitional form exists as a fossil due to the fundamental incompleteness of the fossil record. The incompleteness is caused by the peculiarities of the process of fossilization, that is, the transition to a fossilized state. For a fossil to form, the dead organism must be buried under a large layer of sediment. Due to the extremely slow rate of sedimentation on land, purely terrestrial species rarely become fossilized and persist. In addition, it is rarely possible to discover species living in the depths of the ocean, due to the rarity of large areas of the bottom rising to the surface. Thus, most known fossils (and, accordingly, transitional forms) are either species living in shallow waters, seas and rivers, or terrestrial species leading a semi-aquatic lifestyle, or living close to the coastline. The patterns of burial processes of the remains of living beings are studied by a special branch of paleontology - taphonomy.
Phylogenetic series
Phylogenetic series are series of species that successively replaced each other in the process of evolution of various groups of animals and plants.
They were first discovered by V. O. Kovalevsky, who showed that modern one-toed ungulates descend from ancient five-toed small omnivores.
Creationism
Proponents of creationism claim that no transitional forms have been found. The scientific community considers such statements to be false and deliberately misleading.
see also
Notes
Literature
- Darwin, Charles. Origin of species by means of natural selection: In 2 books. - M.: TERRA - Book Club, 2009. - ISBN 978-5-275-02114-1
- Carroll R. Paleontology and evolution of vertebrates: In 3 volumes - Mir, 1992. - 280 p. - ISBN 5-03-001819-0
Links
- The myth that “the missing link has not been found...” on the portal Anthropogenesis.ru
Wikimedia Foundation. 2010.
Well-known evidence of evolution is the presence of so-called intermediate forms (transitional forms), that is, organisms that combine characteristic features different types(or different taxa of higher rank - genera, families, etc.). As a rule, when speaking about intermediate (or transitional) forms, they mean fossil species, although intermediate species do not necessarily have to die out. Based on the phylogenetic tree, evolutionary theory predicts which intermediate forms actually existed (and therefore can be found) and which did not. According to the scientific method, predictions that come true confirm the theory. For example, knowing the structure of the organisms of reptiles and birds, one can predict some features of the transitional form between them. We predict the possibility of finding the remains of animals like reptiles but with feathers, or the remains of animals like birds but with teeth or long tails. However, we predict that transitional forms between birds and mammals will not be found, such as mammal fossils with feathers or bird-like fossils with mammal-like middle ear bones.
Shortly after the publication of The Origin of Species, the first skeleton of Archeopteryx, an intermediate form between reptiles and birds, was discovered. Archeopteryx had developed plumage (a typical bird feature), and its skeletal structure differed little from dinosaurs. It had claws on the forelimbs, teeth and a long bony tail, and there were few characteristic “bird” features of the skeleton (hooked processes on the ribs, a fork). Later, other transitional forms between reptiles and birds were found.
Anti-evolutionists, as a rule, have extremely misconceptions about what the transitional forms found in the fossil record should be. For example, they “demand” that the transitional form between A and B must be intermediate between A and B in all respects without exception. If some features of a transitional form are closer to A, and others to B, anti-evolutionists declare this form “not transitional, but simply mosaic.” Thus, anti-evolutionists impose testable consequences on evolutionary theory that do not follow from it at all. Evolutionary theory does not claim that all traits of organisms evolve at exactly the same rate. The uneven pace of evolution is one of the common places modern evolutionary theory. Naturally, during the evolutionary transition from A to B, some characteristics of A become “like those of B” earlier, others later. There is no reason why this should not be the case. In other words, evolutionary theory precisely predicts that most transitional forms should be “mosaic.”
Another frequently encountered type of unlawful “demands” of anti-evolutionists is based on the scheme: “In your opinion, ducks descended from crocodiles, so show us a crocodile.” But modern ducks did not evolve from modern crocodiles. Both of them descended from a common ancestor who lived a long time ago (apparently in the Permian period, more than 250 million years ago) and was neither a crocodile nor a duck. He was, for that matter, an ancient, primitive archosaur. And from him came not only ducks and crocodiles, but also all dinosaurs, and only then birds came from dinosaurs. Any two living organisms have a common ancestor, but this ancestor is, of course, neither of them. Therefore, we cannot and should not present crocodiles to anyone. However, since birds actually evolved from dinosaurs, we are able to present whole line the most real bird-dinosaurs (see below).
Another way anti-evolutionists fight transitional forms is a “word game.” "Is this shape closer to A? So it's just A. Closer to B? Well then it's just B." Such essentialism is supported by the formal rules of biological classification. Biologists, according to these rules, cannot write on the label “a form transitional between species A and species B.” The maximum that they can do is to isolate the transitional form into a separate species, call it, say, A1, and sign below in small letters “a species transitional between A and B.” But anti-evolutionists do not notice these additions and say: “What kind of transitional form is this? It’s just species A1.”
Another type of anti-evolutionist distortion is the demand to prove that a given form transitional between A and B is indeed the direct ancestor of B. This is the same as demanding strict evidence that the Omo I skull found in Africa (see: 195,000 years ago in Ethiopia lived “ anatomically modern people ) belongs to Mitochondrial Eve herself, and not to her sister or more distant relative. The chances of finding someone's direct ancestor in the very incomplete fossil record are very low, and even when such ancestors are found, there is no way to prove that it is the direct ancestor and not his. close relative. The transitional forms that are actually found are forms that are closely related to the sought-after direct ancestor (or its little changed descendants); to demand more means, again, to attribute to the evolutionary theory “consequences” that do not follow from it.
Transformation of dinosaurs into birds ("ornithization of theropod dinosaurs")
A series of brilliant paleontological discoveries made in last years, shed light on many details of the evolutionary transformation of dinosaurs into birds. As it turned out, many predatory theropod dinosaurs evolved “in the avian direction” during the Jurassic and Cretaceous periods. Only one of these evolutionary lines gave rise to modern birds and did not go extinct 65.5 million years ago along with the rest of the dinosaurs.
Below are short reviews Several recent studies (see hyperlinks for details):
1) Feathers were first used for beauty, and later came in handy for flight. In 2008, a furry, flightless "bird-dinosaur" was found with four very long tail feathers, like modern birds of paradise. Researchers believe that the long tail feathers served solely “for beauty,” that is, to attract a mate. The males of many modern birds show off long tail feathers in front of the females, which are no longer good for anything and only interfere with flight, and the females give preference to those grooms who have these feathers longer. Paleontologists have previously assumed that some feathered dinosaurs had large feathers that were unsuitable for other purposes and could have been used for mating displays, but such an “obvious” case had not yet been found. This study, along with many others, showed that the origin of bird feathers is a typical example change function organ in evolution. Feathers first evolved for thermal insulation, and perhaps to improve the dinosaur's aerodynamic performance when running quickly; then they began to be used for mating displays, and, finally, they were useful for gliding, and then for flapping flight.
2) The small size of bird genomes is a legacy of the dinosaur era. American and British paleontologists showed in 2007 that the microstructure of fossil bones can be used to judge the genome size of extinct tetrapods. It turned out that one of distinctive features modern birds - small genome size - was characteristic of the ancestors of birds (theropod dinosaurs) from the very beginning of their history. This is another “bird-like” feature that, along with feathers and some skeletal features, formed in dinosaurs long before they learned to fly.
3) The ancestors of birds may have been four-winged. In 2009, a well-preserved specimen of a feathered “bird-dinosaur” was found in the Jurassic sediments of Liaoning Province (Northeast China). Anchiornis huxleyi, which lived several million years earlier than Archeopteryx. The find showed that the ancestors of birds, apparently, initially had large contour feathers suitable for flight, not only on the forelimbs, but also on the hind limbs. This find is another shining example evolutionary prediction come true. Paleontologists understood that there must have been half-bird-half-dinosaurs, more ancient and at the same time closer to reptiles than Archeopteryx. However, until now, all finds of this kind have been younger (experiments in “ornithization” among dinosaurs continued long after the appearance of real birds). Now, finally, the sought-after feathered “pre-Archaeopteryx” dinosaur has been found.
Anchiornis huxleyi. Imprints of large contour feathers are visible, located on all four limbs and on the tail. The length of the scale bar is 5 cm.
4) How the Velociraptor forgot how to fly. On the bone of a velociraptor from Mongolia in 2007, special tubercles were discovered to which feathers were attached. It became clear that not only small representatives of theropods had feathers and flew, but large, apparently flightless dinosaurs also had feathers.
The diagram shows the main branches of dinosaur evolution that led to birds. Red rectangles note the found (and not supposed!) signs of feathers in these reptiles. And the numbers show 4 gigantism events. It can be seen that the reptiles located at the base of the tree are small. This diagram does not take into account the aforementioned four-winged dinosaur Anchiornis huxleyi, which was found only in 2009, and the bird-dinosaur Epidexipteryx with four long tail feathers, found in 2008.
5) Bones of a giant bird-like dinosaur were found in China. In 2007, Chinese paleontologists reported the discovery of another unusual dinosaur, close to the supposed ancestors of birds and called Gigantoraptor erlianensis. Unlike other bird-like dinosaurs, which rarely weighed more than 40 kg, Gigantoraptor was a true giant and weighed about one and a half tons.
6) A furry dinosaur was found in China. In 2009, a small (about 70 cm) herbivorous dinosaur from the group of heterodontosaurs, who lived about 125 million years ago, was found. Along with the bones, bundles of long filament-like formations similar to hair, and even more like “proto-feathers”, characteristic of some lizard-hipped maniraptor dinosaurs, the ancestors of birds, were preserved. The presence of proto-feathers in a non-avian dinosaur shows that furry (or feathered) forms were more common among dinosaurs than was generally believed.
Transitional (intermediate) forms- organisms that combine in their structure the characteristics of two large systematic groups.
Transitional forms are characterized by the presence of more ancient and primitive (in the sense of primary) features than later forms, but, at the same time, the presence of more progressive (in the sense of later) features than their ancestors. As a rule, the term “transitional forms” is used in relation to fossil forms, although intermediate species do not necessarily have to die.
Transitional forms are used as one of the proofs of the existence of biological evolution.
History of the concept
In 1859, when Charles Darwin’s work “The Origin of Species” was published, the number of fossil remains was extremely small, and transitional forms were not known to science. Darwin described the absence of intermediate forms "as the most obvious and severe objection that can be raised against the theory", but attributed this to the extreme incompleteness of the geological record. He noted the limited number of collections available at the time, while at the same time describing the available information about available fossil specimens in terms of evolution and the action of natural selection. Only two years later, in 1961. Archeopteryx was found, which represented the classic transitional form between reptiles and birds. His findings became not only a confirmation of Darwin's theory, but also a landmark fact confirming the reality of the existence of biological evolution. Since then, a large number of fossil forms have been found that show that all classes of vertebrate animals are related to each other, most of them through transitional forms.
With increasing knowledge of the taxonomic diversity of vascular plants at the beginning of the twentieth century, research began to search for their possible ancestor. In 1917, Robert Kidston and William Henry Land discovered the remains of a very primitive plant near the village Rhynia in Scotland. This plant was named Rhynia. It combines the characteristics of green algae and vascular plants.
Interpretation of the concept
Transitional forms between two groups of organisms are not necessarily descendants of one group and the ancestor of the other. From fossils, it is usually impossible to accurately determine whether a certain organism is the ancestor of another. Moreover, the likelihood of finding a direct ancestor of a particular form in the fossil record is extremely low. It is much more likely to find relatively close relatives of this ancestor who are similar in structure to it. Therefore, any transitional form is automatically interpreted as lateral branch evolution, and not “a section of the phylogenetic trunk”.
Transitional forms and taxonomy
Evolutionary taxonomy remained the dominant form of taxonomy throughout the twentieth century. The identification of taxa is based on various signs, as a result of which taxa are depicted as branches of a branched evolutionary tree. Transitional forms are seen as "falling" between different groups in terms of anatomy, they are a mixture of characteristics from the internal and external clade that has recently split.
With the development of cladistics in the 1990s. Relationships are usually depicted in the form of a cladogram, illustrating the dichotomous branching of evolutionary lines. Therefore, in cladistics, transitional forms are considered as earlier branches of the tree, where not all the traits characteristic of previously known descendants on this branch have yet developed. Such early representatives of the group are usually called the main taxon. Basal taxa) or sister taxon Sister taxa), depending on whether the fossil organism belongs to a given clade or not.
Problems of identification and interpretation
The lack of transitional forms between many groups of organisms is a subject of criticism from creationists. However, not every transitional form exists in the form of fossils due to the fundamental incompleteness of the fossil record. The incompleteness is caused by the peculiarities of the process of fossilization, that is, the transition to a petrified state. For a fossil to form, the organism that died must be buried under a large layer of sediment. Due to the very slow rate of sedimentation on land, land species rarely become fossilized and persist. In addition, it is rarely possible to identify species that live in the depths of the ocean through rare cases of large areas of the bottom rising to the surface. Thus, most known fossils (and, accordingly, transitional forms) are either species that live in shallow waters, seas and rivers, or terrestrial species that lead a semi-aquatic lifestyle or live near the coastline. To the problems mentioned above should be added the extremely small (on a planetary scale) number of paleontologists who carry out excavations.
Transitional forms, as a rule, do not live over large territories and do not exist for a long time, otherwise they would be persistent. This fact also reduces the likelihood of fossilization and subsequent discovery of transitional forms.
Therefore, the probability of detecting intermediate forms is extremely low.
Examples among animals
Ichthyostegas are considered the oldest representatives of amphibians. They are considered a transitional link between lobe-finned fish and amphibians. Despite the fact that Ichthyostega had a five-fingered tail, adapted to life on land, they spent a significant part of their life as fish, had a caudal fin, a lateral line and some other signs of fish.
Batrachosaurs, which existed in the Carboniferous and Permian periods, are considered a transitional form between amphibians and reptiles. Batrachosaurs, although they spent their adult lives on land (like reptiles), were closely associated with bodies of water and retained a number of features inherent in amphibians, in particular, the laying of eggs and the development of larvae in water, the presence of gills, and the like.
A large number of reptiles have been discovered that have developed the ability to fly, some of them had feathers, so they are considered as transitional forms between reptiles and birds. The most famous is Archeopteryx. It was about the size of a modern crow. The body shape, structure of the limbs and the presence of plumage similar to modern birds may have flown. Common to reptiles was the special structure of the pelvis and ribs, the presence of a beak with conical teeth, three free fingers on the wings, flexed vertebrae, a long tail with 20-21 vertebrae, bones that could not be pneumatized, and a sternum without a keel. Other well-known transitional forms between reptiles and birds are Protoavis and Confuciusornis.
A large number of fossil forms of animal-like reptiles (synapsids, therapsids, pelycosaurs, various dinosaurs, etc.) found in many areas globe, existed in the Jurassic and Cretaceous periods, combining the characteristics of reptiles and mammals, reveal possible directions and methods of formation of various groups of tetrapods, in particular mammals. For example, an animal-like reptile from the therapsid group - Lycenops (Lycaenops) on bone development oral cavity, differentiation of teeth into canines, incisors, incisor teeth and a number of other signs of body structure resembles predatory mammals, although according to other characteristics and way of life they were real reptiles.
One of the forms preserved in fossil form is ambulocetus Ambulocetus natans(“walking whale”) is a transitional form between land mammals and cetaceans, which are aquatic forms. Outwardly, the animal resembled a cross between a crocodile and a dolphin. The skin should be partially reduced hair. The animal had webbed paws; the tail and limbs are adapted as auxiliary organs of movement in water.
Examples among plants
The first terrestrial plants from the rhyniopsid class, the rhinium and psilophyte families, living in the Silurian - Devonian, combined the characteristics of green algae and primitive forms of higher plants. Their body was leafless, a cylindrical axial organ - the body in the upper part was dichotomously branched at the apices with sporangia. The function of mineral nutrition for rhiniopsids was performed by rhizoids.
Fossil forms of seed ferns that flourished in the late Devonian combine features of ferns and gymnosperms. They formed not only spores (like ferns), but also seeds (like plant seeds). The structure of the conductive tissue of their stems resembles the wood of gymnosperms (cycads).
Another seed plant precursor has been identified from Middle Devonian deposits. Runkaria (Runcaria heinzelinii) existed about 20 million years ago. It was a small plant with radial symmetry; had a sporangium surrounded by an integument and a plus. Runkaria demonstrates the path of plant evolution from spore to seed.
Transitional forms in human evolution
In our time, a large number of fossil remains have been found that reveal the evolutionary path of Homo sapiens from its anthropoid ancestors. Forms that, to a greater or lesser extent, can be classified as transitional include: Sahelanthropus, Ardipithecus, Australopithecus (African, Afar and others), Homo habilis, Homo working, Homo erectus, Homo precursor, Heidelberg man and Cro-Magnons.
Among the forms mentioned, Australopithecus deserves significant attention. From an evolutionary point of view, Australopithecus afarensis lies between modern bipedal humans and their four-legged ancient ancestors. A large number of the skeletons of this Australopithecus clearly reflect bipedality, to such an extent that some researchers believe that this property arose long before the appearance of Australopithecus afarensis. Among its common anatomical features, its pelvis is much more similar to those of humans than to those of apes. The edges of the ilium are shorter and wider, the sacrum is wide and located directly behind hip joint. There is clear evidence of the existence of attachment sites for the knee extensor muscles, which provides for the vertical position of this organism. While the Australopithecus pelvis is not exactly human-like (noticeably wider, with the edge of the iliac bones oriented outward), these features indicate a fundamental restructuring associated with walking on two legs. The femur forms an angle towards the knee. This feature allows the leg to be placed closer to the midline of the body and is a clear indication of the habitual nature of locomotion on two legs. Nowadays, homo sapiens, orangutans and koats have the same features. Australopithecus's feet had big toes, which makes it almost impossible for the foot to grasp tree branches. In addition to the characteristics of locomotion, Australopithecus was also significantly bigger brain than modern chimpanzees and the teeth were significantly more similar modern man than to monkeys.
Phylogenetic series
Phylogenetic series are series of fossil forms that are interconnected in the process of evolution and reflect gradual changes in their historical development.
They were studied by the Russian scientist A. Kovalevsky and the English scientist J. Simpson. They showed that modern single-toed ungulates descend from ancient small omnivores. Analysis of fossil horses helped to establish the gradual process of evolution within this group of animals, in particular, how, changing over time, fossil forms became more and more similar to modern horses. Comparing the Eocene Eohypus with the modern horse, it is difficult to prove their phylogenetic relationship. However, the presence of a number of transitional forms that successively replaced each other over large areas of Eurasia and North America, made it possible to restore the phylogenetic series of horses and establish the direction of their evolutionary changes. It consists of a number of the following forms (in a simplified form): Phenacodus — Eohippus — Miohippus — Parahippus — Pliohippus — Equus.
Gilgendorf (1866) described a paleontological series of gastropods from Miocene sediments that accumulated over two million years in lacustrine sediments of the Steinheim basin (Württemberg, Germany). 29 different forms belonging to the planorbis series were discovered in successive layers (Planorbis). Ancient mollusks had a shell in the form of a spiral, and later ones had a shell in the form of a turbospiral. The row had two branches. It is believed that the change in the turtle's shape was caused by rising temperatures and increased calcium carbonate content as a result of hot volcanic springs.
Thus, phylogenetic series represent a historical sequence of transitional forms.
Currently, phylogenetic series are known for ammonites (Waagen, 1869), gastropods of the genus Viviparidae (Viviparus)(Neymayrom, 1875), rhinoceroses, elephants, camels, artiodactyls and other animals.
