The first true vertebrates appeared. Vertebrates. Vertebrate skin
The ancestors of vertebrates, apparently, were close to primitive forms of benthic-pelagic non-cranial animals that had not yet acquired an atrial cavity. Presumably, these were small aquatic animals, with typical signs of chordates, but did not yet have a strong internal skeleton and powerful muscles. They probably separated from the original groups in the Ordovician, i.e., at least 500 million years ago. However, they are not known to us and are unlikely to ever be discovered, since their small size and the absence of solid skeletal formations make the preservation of fossil remains unlikely. Poorly preserved remains of primitive, but undoubtedly already fully formed vertebrates (scutellum agnathans) are known from the Ordovician-Lower Silurian deposits (about 450 million years old). They probably lived in fresh water bodies, and their remains were carried by the current to shallow sea bays, where they were deposited. Their existence in river deltas and desalinated areas of the sea is not ruled out. It is very interesting that the remains of vertebrates begin to occur in typical marine deposits only from the middle of the Devonian, i.e., in deposits of an age of about 350 ml. years ago and later. These paleontological data suggest that the formation of vertebrates took place not in the seas, but in fresh waters. What could be the reason for the transition of marine chordates - the ancestors of vertebrates from the seas to fresh water?
Judging by paleontological data, in the Ordovician and Silurian period, the bottom biocenoses of the seas and oceans were rich in various animals - worms, mollusks, crustaceans and echinoderms. At the bottom, there were probably numerous lower chordates, including tunicates; such powerful predators as large cephalopods and giant scorpion crustaceans (reaching 3-5 m in length) also lived here. The possibility of the emergence of a new large group (a subtype of vertebrates) under the conditions of such saturated biocenoses with intense rivalry seems unlikely. At the same time, in fresh water bodies there already existed a fairly rich flora (mainly algae) and a significant number of various invertebrates; large and strong predators were few in number. Therefore, the penetration of the ancestors of vertebrates here and their subsequent development seems possible. However, compared with the seas, fresh waters also had unfavorable features that prevented the introduction of marine organisms. An insignificant amount of salts dissolved in water threatened marine invaders with water-permeable covers by excessive watering of the body, a sharp violation of the salt composition and osmotic pressure in the tissues, which should have led to a general metabolic disorder. Fresh water bodies, in comparison with the sea, are characterized by more unstable chemistry (including sharp fluctuations in oxygen content) and a changeable temperature regime. Settlement in the rivers also required high mobility in order to stay in a certain area and resist drift by the current.
But the weak pressure of predators, the relative abundance of food and the less acute competition in freshwater biocenoses created opportunities for the penetration of some representatives of chordates and their consistent adaptation to new conditions. Apparently, it was this way that the formation of vertebrates took place, whose ancestors first penetrated into the estuaries, then moved to the lower reaches of the rivers and began to rise upstream, populating the lakes. In such places, kidneys characteristic of vertebrates could have arisen, capable of filtering out huge amounts of water from the body, eliminating the violation of the osmotic pressure of body juices and preventing the loss of salts that are deficient in new conditions. Seasonal fluctuations in the oxygen content in freshwater bodies required the improvement of the respiratory organs, to which, apparently, the appearance of gills was an evolutionary response. The need to overcome the force of currents contributed to the improvement of mobility: a stronger, but retaining flexibility, axial skeleton in the form of a spine developed - a support for the increasing mass of motor muscles. In turn, greater mobility required the complication of the nervous system and sensory organs, circulatory, digestive and excretory systems. Strengthening of the myochordial complex was facilitated by an increase in the size of the body, which, in turn, made it possible to increase the speed of movement, since rarer vibrations of a long body are mechanically beneficial than frequent ones with a short body. An increase in the speed of movement in water with an increase in the absolute dimensions of the body can be shown by the example of fish (Table 3).
Table 3. Changes in the swimming speed of fish with an increase in the absolute body size
| View | Body length, cm | Swimming speed, km/h |
| Trout (Salmo trutta) | 5,1 | 3,3 |
| lt; | 15,0 | 5,8 |
| | 30,5 | 13,2 |
| Salmon (Salmo salar) | 3,2 | 0,4 |
| | 75,0 | 21,6 |
| Carp (Carassius auratus) | 12-1,9 | 0,8 |
| | 8,0-10,0 | 4,5-5,4 |
| | 13,2 | 6,0 |
Already in the middle Devonian (i.e., about 320 million years ago), amphibians (Amphibia) separated from lobe-finned fish. In the Carboniferous period, they were represented by several groups of various sizes, structure and appearance, which settled in the coastal zones of fresh water bodies, but then many of them disappeared; in the Triassic period (about 170-180 million years ago), large amphibians - stegocephals - died out.
In the middle of the Carboniferous period (about 250-260 million years ago), reptiles (Reptilia) separated from amphibians. Throughout the Mesozoic era, for more than 120 ml. years, they dominated the Earth, having successfully mastered almost all spheres of life, including fresh and marine waters and air. Having adapted to live in a variety of habitats, reptiles broke up into 6-7 subclasses. The flowering of reptiles predetermined the extinction of the ancient amphibians. By the end of the Cretaceous period (about 60-80 million years ago), many groups of reptiles also died out relatively quickly. The species that have survived to this day represent the comparatively poor remnants of three subclasses. The extinction of reptiles was facilitated not only by adverse climate changes and a change in the nature of the vegetation cover that accompanied the Alpine mountain-building cycle, but also by the intensive speciation of birds and mammals, which by that time had become serious competitors of the Mesozoic reptiles. However, these two classes of higher vertebrates separated from their reptilian ancestors much earlier, for a long time (tens of millions of years) were few in number and, apparently, led a relatively secretive lifestyle.
Only at the end of the Cretaceous begins the rapid development of birds and mammals, and at the same time the extinction of reptiles. This was facilitated by global changes on Earth (mountain-building processes and increased volcanic activity, an increase in the continentality of the climate in many areas, etc.; even assumptions were made about the influence
cosmic factors). Birds (Aves) separated from highly organized reptiles - some archosaurs - apparently in the middle of the Triassic, although the most ancient and primitive birds are now known only from the deposits of the Jurassic period (about 135 million years old). Representatives of some modern orders have already been found in the deposits of the late Cretaceous period.
Mammals (Mammalia) separated themselves from one of the most ancient groups of reptiles in terms of time - animal-like reptiles (subclass Theromorpha, seu Synapsida); synapsids probably arose in the middle of the Carboniferous, experienced extensive adaptive radiation in the Permian, and disappeared by the end of the Triassic. Therefore, the formation of mammals, probably, should have occurred at the beginning of the Triassic (according to some paleontologists, at the end of the Permian). The history of Mesozoic mammals is poorly known. In the middle of the Mesozoic (Triassic-Jurassic), several groups, which died out relatively soon, apparently already separated (but among them were monotremes that survived to our time). Marsupials and placentals are known from the Jurassic, some from the Cretaceous. Extensive adaptive radiation of placental mammals and the formation of modern orders took place already in the Tertiary period of the Cenozoic era (about 60-40 million years ago).
A figurative idea of the successive evolution of chordates can be obtained from such a comparison. If we take a period of one Earth year as the scale of the entire history of the planet Earth, then the emergence of life will occur at the end of May - the beginning of June, the appearance of the lower types of invertebrates - at the end of June - the beginning of July, and other types of invertebrates and the most primitive chordates - at the end of September ( Cambrian period of the Paleozoic era). In mid-October, the first vertebrates appear - primitive jawless (the end of the Ordovician - the beginning of the Silurian), and at the end of October (Silur) the first jawless - primitive fish separate from the jawless. At the end of the first - the beginning of the second decade of November (Middle Devonian), the first amphibians separate from the lobe-finned fish; probably, at the beginning of the last five days of November (in the middle of the Carboniferous period), the first reptiles appear, and from the end of November - the first days of December (Permian period), the extinction of amphibians and the beginning of the flowering of reptiles begin, continuing until the end of the second decade of December (the entire Mesozoic era). At the beginning of the Triassic period (approximately December 3-4 on our scale), ancient mammals separated from primitive reptiles, and at the end of the same period (December 7-8), ancient birds separated from progressive reptiles - archosaurs.
However, only at the end of the second decade of December (the end of the Cretaceous period) does the rapid development of birds and mammals begin and the extinction of many groups of Mesozoic reptiles. The Cenozoic era - the period of formation of modern groups of higher vertebrates - begins only around December 23, and the formation of many modern families - from December 28 (the beginning of the Neogene). The beginning of the Pleistocene (Quaternary) period (at this scale) falls approximately on
6-8 p.m. December 31; this is the time of the appearance of primitive (ancient) species of people and modern or close to modern species of mammals and birds. Modern man (Homo sapiens - a reasonable man) appeared about 100 thousand years ago, that is, according to our time scale, only in the last 20-15 minutes of December 31, and the history of human culture from ancient Egypt to the present day takes only the last 3- 5 minutes of the year!
The chronology of the evolution of chordates draws attention to the unevenness of the evolutionary process, in which periods of vigorous morphogenesis were replaced by times of relatively slow and narrow adaptive transformations. A. N. Severtsov proposed to designate such a regular alternation as a change in periods of aromorphic evolutionary changes leading to morphophysiological transformations that raise the vital activity of the organism to a higher energy level (an increase in the energy of vital activity) - periods of idioadaptation or the realization of acquired advantages by increasing the number, widespread settlement, adaptation to local conditions and breakup into subordinate groups (families, genera, species). G. Osborne called the last process adaptive radiation. The formation and evolution of chordates is a classic example of an aromorphic path - the emergence of classes - with the subsequent idioadaptive flowering of each of them, which in turn occupied a dominant position in the fauna of individual periods.
Attention should be paid to one more feature of the evolution of chordates. A new class formed by aromorphoses, usually in the early stages of its development, separated branches that occupied a subordinate position, often pushed back into unfavorable biotopes. Forced to master new environments and adapt to alien living conditions, they evolved slowly, but could acquire adaptations of general significance and, after their formation and consolidation, forced out their predecessors. As a rule, this was preceded by a change in the earth's surface (cycles of mountain building), climate, and vegetation cover.
Thus, Agnatha and Gnathostomata apparently formed at a close time, but later the jawless ones, adapting to life in flowing waters, forced the jawless ones out of most ecological niches. The same thing happened in the evolution of fish: armored fish were replaced by cartilaginous, and the latter by bony fish. When terrestrial vertebrates (amphibians) appeared, a branch early separated from their ancient group (ichthyostegs), which later gave rise to reptiles, and at the early stages of the formation of the latter, a group of animal-toothed animals separated, giving rise to mammals. It should be emphasized that the formation of a new class of chordates has always been associated with the development of a new "adaptive zone", a new habitat (G. Simpson). So, chordates, who left the sea for fresh waters, became vertebrates; evolution and changes in fish classes were associated with successive penetration into the waters of the estuary and from the lower reaches to the upper reaches of the rivers. This is even more obvious for the superclass of tetrapods (terrestrial) vertebrates. Details
her conditions, factors and ways of evolution are considered in the description of individual classes of vertebrates.
During the evolution of vertebrates, not only the structure (bodily organization) developed progressively, but on the basis of the development of the nervous system and activity, the relations of individuals became more complicated and the importance of population organization increased. The complication of communication means that transmit complex and capacious information through optical, acoustic, chemical and other channels ensures efficient reproduction, streamlines the spatial distribution of animals, improves orientation in space, and enhances the impact on the environment. Mobile groupings (families, herds and flocks) expand the possibilities of using natural resources and increase the chances in the struggle for existence.
Vertebrates
Structural features compared to other types
Active movements provide vertebrates with the opportunity to change habitats depending on changes in living conditions and needs at different stages of their life cycle, for example, during development, puberty, reproduction, wintering, etc. The indicated general biological features of vertebrates are directly related to the features of their morphological organization and with physiology.
In the region of the anterior part of the intestinal tube, moving parts of the skeleton arise, from which the oral apparatus is formed, and in the vast majority - the jaw apparatus, which provides grasping, holding food, and in higher vertebrates, grinding it.
The structure of vertebrates
Vertebrates are united by a common morphophysiological organization. In all organ systems of these animals, one can trace the features of successive changes in connection with the evolutionary transformation of organs. Below is a general plan of the structure, functioning and laying in the ontogeny of individual organ systems.
Vertebrate skin
Muscular system
The layer of muscles located under the skin is the bulk of the muscles, called the musculature of the body, or somatic. It provides animals with the ability to move in the environment and consists of striated muscle tissue. In lower vertebrates, as in non-cranial ones, the musculature has a segmented character. In higher vertebrates, due to the general complication of body movements, with the development of limbs, segmentation is disturbed, and the trunk muscles are grouped, forming such parts of the body as the torso, head, and organs of movement.
In addition to the somatic muscles, vertebrates have the muscles of the intestines and some other internal organs (vessels, canals). This muscle is called visceral. It is composed of smooth muscle tissue and provides, in particular, the movement of food in the intestines, the contraction of the walls of blood vessels.
Trunk musculature embryonicly arises from the inner sheet of the myotome, that is, the dorsal mesoderm. The visceral musculature is a derivative of the lateral plate, that is, the abdominal mesoderm.
Skeleton
The internal skeleton is the supporting base of the vertebrate body. The skeleton is involved in the movement of the body, protects the internal organs. The musculature is attached to the skeleton. In the bones of the skeleton are hematopoietic tissues - in particular, red bone marrow. The skeleton also serves as a depot of substances - it stores reserves of calcium and other substances.
Topographically, the skeleton of vertebrates can be divided into axial, visceral, limb girdle and free limb skeletons.
The axial skeleton in its original form is represented by a chord surrounded by a thick connective tissue membrane. The latter covers not only the chord, but also the neural tube lying above it. The notochord develops from the rudiment of the dorsal side of the primary intestine, that is, it has an endodermal origin. In the axial skeleton, the vertebral column and the brain skull are distinguished.
In most vertebrates, the notochord is displaced and replaced by a cartilaginous or bony skeleton. The cartilaginous and bone skeletons develop as derivatives of the above connective tissue (mesoderm in origin) sheath. This shell is thus skeletal.
Limb belts are always located inside the body of the animal. The skeleton of the free limb in vertebrates is of two types: the fin of fish and the five-fingered limb of terrestrial vertebrates. In the first case, the skeleton is represented by several rows of cartilage or bones that move relative to the belt as a single lever. The skeleton of a five-fingered limb consists of a number of levers that can move both together relative to the limb belt, and separately - one relative to the other. The laying of the limb skeleton occurs in the connective tissue layer of the skin.
Digestive organs
The digestive system is represented by a tube that starts at the mouth and ends at the anus. The epithelium of the digestive tract is endodermal. Only in the region of the oral and anal openings does the endodermal epithelium imperceptibly pass into the ectodermal.
The digestive tract is divided into the following main sections:
- oral cavityserving for eating;
- pharynx - a department always associated with the respiratory organs: in fish, gill slits open into the pharynx, in terrestrial vertebrates, a laryngeal slit is located in the pharynx; the pharynx is rightly called the respiratory section of the digestive tube;
- stomach - an expansion of the intestinal tract, which in some cases has a very complex device;
- intestine, typically subdivided into the anterior, or small, middle, or large, and posterior, or rectum.
The morphological complication of the intestinal tract in a number of vertebrates follows the path of its elongation and differentiation into sections. The ducts of three types of digestive glands open into the digestive tube: salivary, liver, pancreas.
Respiratory system
The respiratory organs of vertebrates are of two types - gills and lungs, and in a significant part of vertebrates, skin is essential in breathing.
The gill apparatus is a system of paired, usually symmetrically located, slits that serve to communicate the pharynx with the external environment. The anterior and posterior walls of the gill slits are lined with a mucous membrane that forms lamellar outgrowths; outgrowths are divided into petals, called gill. Each gill plate above the petals is called a half-gill. Between the gill slits (in the gill septa) are the visceral gill arches. Thus, each gill arch is connected to two half-gills of two different gill slits.
The respiratory organs of terrestrial vertebrates - the lungs - in the diagram are a pair of sacs that open into the pharynx through the laryngeal fissure. Embryonally, the lungs arise in the form of a protrusion of the abdominal wall of the pharynx in the back of the gill apparatus, that is, they are of endodermal origin. In the early stages of embryonic development, the lung buds resemble a pair of internal (endodermal) gill slits. These circumstances, as well as the blood supply and innervation features common to the lungs and gills, force us to consider the lungs as homologues of the posterior pair of gill sacs.
The skin is involved in respiration in cases where there are no dense horny or bony scales in it, for example, in amphibians, naked-skinned fish.
Functionally, the respiratory system is involved in the enrichment of blood with oxygen and in the removal of carbon dioxide. Ammonia is released through the respiratory system in lower aquatic animals. In warm-blooded animals, it is involved in the processes of thermoregulation. The principle of operation of the respiratory system is the exchange of CO 2 and O 2 between the flows of gas and blood, directed countercurrent towards each other.
Circulatory organs
The circulatory system in vertebrates, like in non-cranial ones, is closed. It consists of interconnected blood vessels, which in a rough scheme can be reduced to two trunks: the dorsal, where blood flows from the head to the tail, and the abdominal, through which it moves in the opposite direction. Unlike non-cranial vertebrates, the movement of blood is associated with the activity of the heart.
Nervous system
The functions of the nervous system are the perception of external stimuli and the transmission of emerging excitations to cells, organs, tissues, as well as the unification and coordination of the activities of individual organ systems and the body as a whole into a single functioning living system. Embryonically, the nervous system of vertebrates arises, as well as in non-cranial ones, in the form of a hollow tube laid in the ectoderm on the dorsal side of the embryo. Subsequently, its differentiation occurs, leading to the formation of: a) central nervous system, represented by the brain and spinal cord, b) peripheral nervous system, consisting of nerves extending from the brain and spinal cord, and c) sympathetic nervous system, consisting mainly of nerve nodes located near the spinal column and connected by longitudinal strands.
An unpaired protrusion grows from the bottom of the diencephalon - a funnel, to which is adjacent a complex in structure and function formation - the pituitary gland. The anterior pituitary gland develops from the epithelium of the oral cavity, the posterior - from the medulla. The hypothalamus is also located there.
The roof of the midbrain forms paired swellings - visual lobes (tubercles). The third pair of head nerves (oculomotor) depart from the midbrain. The fourth pair of head nerves (trochlear) departs at the border between the middle and medulla oblongata, all other head nerves depart from the medulla oblongata.
sense organs
This group of organs arises as derivatives of different parts of the embryo and at different stages of its development. These are the organs of smell, vision, hearing, vestibular apparatus, lateral line organs, organs of taste, touch, specific organs that perceive the Earth's magnetic field, electric fields, thermal radiation, etc.
The data of comparative embryology suggest that the sense of smell is one of the most ancient functions of the brain. The organs of smell are laid in the embryo as a thickening of the ectoderm simultaneously with the neural plate. In parallel, the skeleton of the olfactory capsules, which are part of the brain skull, is formed. At first, the olfactory capsules communicate only with the external environment and have external nostrils. Subsequently, due to the terrestrial existence, the nostrils become through.
The organs of sight also belong to the ancient sense organs. Light-sensitive reception occurs at a very early stage in the evolution of chordates and is formed in early embryogenesis.
The organs of vision of vertebrates are divided into paired and unpaired. Both are outgrowths of the diencephalon. Paired eyes are laid as outgrowths of the lateral parts of the diencephalon, unpaired - as sequentially located in the roof of the diencephalon (pineal gland and parietal organ). The laying of paired eyes is accompanied by the formation of visual capsules around them, which are part of the brain skull.
The organs of hearing in vertebrates have a complex origin. The earliest in evolution is the formation of the inner ear, which is laid in the ectoderm of the embryo, deepens in the form of a fossa and takes shape as an auditory vesicle lying in the auditory capsule. The auditory vesicle is divided by constriction into two parts. The upper section turns into the vestibular apparatus. It is the organ of balance. It allows you to feel the position of the body in the three-dimensional space of the Earth. This organ is the 3 semicircular canals in the inner ear. The lower part of the auditory vesicle is the inner ear itself - the auditory sac. The middle and outer ear are formed in the late stages of the emergence of vertebrates in connection with landfall.
excretory organs
Sex organs
The sex glands of vertebrates - the ovaries in females and the testes in males - are usually paired. Embryonally, they develop from the mesoderm section at the site of division of this rudiment into the somite and lateral plate.
Initially (in jawless) the sex glands did not have excretory ducts and the reproductive products fell out through ruptures in the walls of the gonads into the body cavity, from where they were excreted into the external environment through special pores. Subsequently, the genital tract arose, which in males is associated with the excretory organs (Wolf's canal). And in females, the muller canal functions as an oviduct, which maintains the connection of the coelom with the external environment.
Metabolism
Ecology
The high vital organization led to the wide distribution of vertebrates and their penetration into all living environments. This circumstance, as well as the abundance and diversity of vertebrate species, make them the most important factor in the geographic environment.
Origin
Vertebrates appeared at the turn of the Ordovician - Silurian, and in the Jurassic there were already representatives of all their now known classes. The total number of modern species is about 40 thousand.
The ancestors of vertebrates are the lower chordates: tunicates and non-cranial.
Classification
Used sources
- Biological Encyclopedic Dictionary, edited by M. S. Gilyarov et al., M., ed. Soviet Encyclopedia, 1989.
- Zoology of vertebrates. V. M. Konstantinov, S. P. Naumov, S. P. Shatalova. M., 2000.
Vetulicolia, first found in Greenland in 1911, are found in the Cambrian deposits of different regions of the world - from China to Canada. Blind, but with a wide mouth, they could swim thanks to the movements of the tail. From an ecological point of view, vetulicolia resembled miniature whale sharks, filtering plankton and organic matter from the water column.
To date, 14 species of these animals are known to science, but due to their extremely poor preservation, their family ties, as well as details of morphology and appearance, have so far remained poorly understood. It is only known that the body of Vetulicolia was clearly divided into two sections - a hollow anterior and a segmented posterior. The structure of the anterior section suggests that the closest relatives of vetulicolia could be primitive chordates like salps and ascidians, and the tail evokes thoughts of arthropods. A new discovery helped to understand the family ties of the group.
Recently, a group of Australian paleontologists led by Diego Garcia-Bellido of the University of Adelaide discovered fossils of a new species of vetulicolia on Kangaroo Island. Scientists named it Nesonektris aldridgei, in honor of the famous researcher of this group, Dick Eldridge from the British University of Leicester. The generic name of the animal in Greek means "island swimmer".
Nesonectrises grew to about 13 cm in length. A curious feature of their preservation is that the tails and front sections are often found separately, that is, they were not firmly connected and were crushed shortly after the death of the animals. Paleontologists were even more interested in the tube that runs along the body like an intestine: it was divided by partitions into separate blocks.
“This is completely inconsistent with the gut (which is a hollow tube), but fits well with a cartilaginous notochord (or notochord), Garcia-Bellido said. “Thus, now we can draw conclusions about exactly where this group was located on the tree of life.”
The notochord is found in almost all vertebrates at the embryonic stages of development, usually giving way to the spine as they grow older. Some primitive chordates retain the notochord throughout their lives, while others, like sea squirts, have it only in the early stages of development. If vetulicolians did have a notochord, then this places them in the group of ancestors or at least "cousins" of all other chordates, including humans.
“They are close relatives of vertebrates. Vetulicolia has a long tail supported by a rigid shaft resembling a notochord, which is the precursor of the backbone and is a unique feature of vertebrates and their relatives,” said the Australian paleontologist. Garcia-Bellido and his team now intend to re-examine the known remains of other vetulicolians to try to find notochords in them as well. In addition, excavations will continue on Kangaroo Island, where, thanks to a rare set of circumstances, imprints of the soft tissues of animals of the Cambrian period have been preserved in a fossil state. 
Article A new vetulicolian from Australia and its bearing on the chordate affinities of an enigmatic Cambrian group published in BMC Evolutionary Biology
The jaws of vertebrates are thought to have evolved from the anterior pair of gill arches of their earliest fish-like ancestors. American scientists have finally found documentary evidence of this hypothesis in the fossil of the Cambrian organism Metaspriggina.
Metaspriggina walcotti. Reconstruction: Marianne Collins
According to modern scientific concepts, the jaws and several small bones of the human inner ear trace their ancestry to the gill arches - peculiar skeletal structures that supported the gills of primitive fish-like vertebrates in Cambrian or even Precambrian times. However, due to the poor preservation and incompleteness of the geological record, it was not possible until recently to find these arcs at their location in the body of an organism.
Professor Simon Morris of the University of Cambridge in the UK and his colleague Dr. Jean-Bernard Caron of the University of Toronto and the Royal Ontario Museum have just described the gill structures of the early chordate Metaspriggina walcotti, which lived 505 million years ago. Fossils of this uniquely preserved metaspriggin were found in 2012 at the famous site of the early Cambrian fauna of Burges Shale, along with four dozen other fossils.
“The detail in these fossils is stunning,” Morris said. “Even the eyes are perfectly preserved and perfectly visible.” The two large eyes of the metaspriggin turned out to be placed at the very end of the muzzle and therefore seem to be bulging. And not far from them, scientists saw slightly curved rods - the same gill arches that were destined to eventually give rise to the jaws and auditory bones of vertebrates.
“These arcs have long been known to have played a key role in the evolution of vertebrates, including the emergence of their jaws and tiny bones involved in the transmission of sound in mammals,” said the British professor. “Until now, however, the lack of quality fossils has meant that our understanding of the location of these arches in early vertebrates has been purely hypothetical.”
It turned out that the gill arches were located in the body of the animal in pairs, which is the best possible confirmation of the hypothesis of the origin of a pair of jaws from them. At the same time, the first pair of arches in Metaspriggina was also thicker than all the others, which, perhaps, is the first evolutionary step on the way to the emergence of gnats.
After additionally analyzing the structure and location of the metaspriggin muscles, paleontologists came to the conclusion that this animal was an active and mobile swimmer, not inferior to modern trout. And a pair of large eyes and an olfactory organ allowed them to perfectly navigate the sea of the Cambrian period.
Chordates are about 700 million years old, but a good geological history of their evolution only begins in the Cambrian. In the oceans of the Cambrian and Ordovician, only such chordates are known that did not have jaws.
In the Silure, it happened, it happened, it happened, amphibians with jaws were formed ... Sharks and rays, sturgeon fish, bony fish and amphibians come from them.
In the Carboniferous period, terrestrial animals appeared - anopsids ... Synopsids and diopsids come from anopsids. This classification, as it were, takes into account the number of temporal pits, but in reality this is not so. For example, anopsid ankylosaurs are classified as diopsids, and birds with open temporal depressions, hollows ... Synopsids include monotremes, similar in skull structure to dinosaurs and birds. Monotremes are classified as mammals because of their skin secretions and wool. But the secretions of monotremes are not like milk, but the secretion of the coccygeal gland of birds, and the wool is the same as the hair of pterosaurs ...
In general, several classes of animals developed from anopsids, combining and developing their features in different ways. These trunks are: lizards and snakes, crocodiles, dinosaurs (including monotremes), plesiosaurs, theromorphs (including ichthyosaurs).
Descendants of theromorphs, mammals, are known in the Triassic and Cretaceous. But mammals received the opportunity to spread widely only in the Paleogene and Neogene eras. This became possible after the mass extinction in the Upper Cretaceous of many species in all classes of vertebrates.
In the Paleogene, mammals with a folded cerebral cortex appeared: predatory, hoofed-hyrax, monkeys. The ancestor of all these orders of mammals were some kind of lemurs. All ancient mammals with a folded cerebral cortex are similar to lemurs.
Sharks have lost all their bones in the process of evolution
29.05.2015 14:22
A fossil shark found in western Australia has literally turned scientists' ideas about the evolution of this group of fish upside down. If until recently dangerous sea predators were considered quite primitive creatures, now biologists will have to treat them as very advanced creatures that have come a long evolutionary path.
Gogoselachus lynnbeazleyae. Reconstruction: John Long
The revolution in ichthyology was staged by paleontologist John Long of Flinders University. Long has spent three decades excavating the Devonian Gogo formation in the Kimberley region. In 2005, he found there the fossilized skeleton of a shark that lived in a warm tropical sea 380 million years ago. The study and description of the animal was delayed, and the article about Gogoselachus lynnbeazleyae was published only now.
“Sharks are considered to be primitive fish on the basis that their skeleton is made of cartilage and they never form bones,” says Professor Long. “However, we are now turning this idea on its head, arguing that the early fossil sharks actually had a real bone skeleton, and only subsequently lost it.”
Indeed, bone tissue in modern sharks can only be found in the roots of the teeth, and the skeleton and even the skull are composed of cartilage tissue, which is considered the precursor of bone tissue. These ideas were extended to all sharks of the past. But when Long looked at the gogoselachus cartilage under high magnification using microtomography, he saw in it real osteocytes - the cells that make up bones.
“Our fossil shark showed for the first time a true bony structure tying tiny cartilages together. So we're seeing a shark that's actually descended from someone with a lot more bones in their skeleton. And at the other end of this line are modern sharks that have completely lost their bones and become cartilaginous. Thus, our fossil allows us to observe the evolution of tissues, and explains the reasons why modern sharks have become so successful today - they simply gave up bones in order to become lighter, ”Long explained.
“This is a really interesting discovery,” said Per Ahlberg, professor of paleontology at Uppsala University. – The skeletons of modern sharks are made up of a peculiar tissue called prismatic calcified cartilage. This cartilage is mineralized and does not look like hard sheets, but like a mosaic of tiny mineral prisms. Such tissue is quite different from bone, and its origin is not yet well understood. The new shark from Gogo shows that the early version of prismatic calcified cartilage seems to have had gaps between the prisms filled with bone cells, unlike modern ones.”
“Studying sharks is very much like solving a grand puzzle,” added Professor Long. “They appeared 250 million years before the last dinosaurs and have not changed much since then, successfully hitting the winning formula. But although their appearance has remained almost the same, the structure of the tissues has undergone major changes.
Long managed to find out all these details thanks to the specific conditions of preservation of fossil material in the Gogo formation. Usually, Devonian fishes are preserved in fossils highly deformed, flattened by the pressure of sedimentary rocks. But here in the Kimberley, ancient fish have come down to us three-dimensional and voluminous thanks to carbonate nodules that formed on the site of a sponge-algae reef.
“At that time, life was in full swing here, many species of fish lived, for example, the long-extinct armored placoderms and early bony fish, the descendants of which dominate today. We were hoping to see a lot of sharks here, but for some reason they weren't common on this reef,” Long added.
For this reason, the discovery of the skeleton of a 75-centimeter shark attracted a lot of attention. Both branches of the lower jaw, fragments of the shoulder girdle that supported the pectoral fins, gill arches, about 80 teeth and several hundred scales fell into the hands of scientists. The specimen was subjected to many different studies, and one of them led to a sensational result, forcing us to reconsider both the early evolution of sharks and the attitude towards them as a primitive, delayed group in their development.
First Shark from the Late Devonian (Frasnian) Gogo Formation, Western Australia Sheds New Light on the Development of Tessellated Calcified Cartilage published by PLOS ONE
Doi: 10.1371/journal.pone.0126066
According to new data from American scientists, birds are not descendants of dinosaurs and are descended from a special group of archosaurs that separated from giant lizards in the distant past.
Scansoriopteryx. Reconstruction: Matt Martyniuk
A sensational discovery that could literally turn all modern paleontology on its head was made by Stephen Cherkas of the Blanding Dinosaur Museum and Alan Feduccia of the University of North Carolina. Using their new technique, they studied the remains of the tiny feathered pangolin Scansoriopteryx and came to the conclusion that there is no reason to consider it a dinosaur.
Scansoriopteryx, whose name is translated into Russian approximately as “wing-flyer” or “lap-wing”, was a small archosaur, approximately the size of a sparrow. The remains of a single, immature individual of Scansoriopteryx were found at the beginning of the 21st century in the Jurassic deposits of the Chinese province of Liaoning. Judging by the fossils that have survived to this day, Scansoriopteryx was arranged more primitive than the famous Archeopteryx and was well able to climb trees, gliding from them with the help of small wings.
Until recently, Scansoriopteryx was attributed to the coelurosaurs, a group of theropods from which, according to most scientists, modern birds originated. However, the study of Cherkas (who, by the way, discovered Scansoriopteryx) and Feduccia forces us to reconsider this approach. The duo of scientists used 3D microscopy and low-angle photography to elucidate structures not clearly seen before. Thanks to this, paleontologists were able to refine the natural outlines of the bones of the pelvis, tail and limbs, and at the same time find elongated tendons that stretched along the tail vertebrae, like in Velociraptor.
However, most of the evidence suggests that Scansoriopteryx lacked fundamental skeletal structural features to qualify it as a dinosaur. Rather, he is a descendant of early archosaurs who mastered tree climbing long before the appearance of terrible lizards. Accordingly, the birds, at the very roots of the family tree of which Scansoriopteryx is located, are not descendants of dinosaurs, but at best their cousins.
Meanwhile, typical avian adaptations are clearly visible in Scansoriopteryx itself, such as elongated forelimbs that have turned into feathered wings, a specialized lunate carpal bone and adapted to sitting on paw branches. Most likely, this animal was at the beginning of the development of the flight, in which it set off, gliding from the branches of trees.
Vertebrates are common in marine and freshwater reservoirs, on land - from the tropics to the high latitudes of the Arctic and Antarctic. About 42 thousand species belong to vertebrates.
Subtype characteristic
Vertebrates (Vertebrata), or cranial (Craniota), are the most highly organized group of the chordate type. Vertebrates have the most developed organ systems: integuments of the body, the apparatus of movement, external metabolism (digestive, respiratory and excretory systems), internal metabolism (circulatory and lymphatic systems), regulation (endocrine and nervous systems) and reproduction. Vertebrates differ from other subtypes of chordates in their active way of searching for and capturing food. The higher organization of their body is connected with this (Fig.). Vertebrates have perfect sensory organs necessary for finding food, developed organs of locomotion, movable mouthparts, and a complex brain.
A - longitudinal section; B - cross section of the head; B - transverse section of the body; D - cross section of the tail: 1 - chord; 2 - spinal cord; 3 - brain; 4 - gill slits; 5 - heart; 6 - lung; 7 - head kidney, or pronephros; 8 - trunk kidney, or mesonephros; 9 - pelvic kidney, or metanephros; 10 - gonad; 11 - stomach; 12 - gut; 13 - pancreas; 14 - liver; 15 - spleen; 16 - bladder; 17 - cloaca; 18 - postanal gut; 19 - medulla oblongata; 20 - muscles; 21 - skull; 22 - gill petals; 23 - oropharynx; 24 - abdominal aorta; 25 - internal gill opening; 26 - external gill opening; 27 - gill sac; 28 - right posterior cardinal vein; 29 - dorsal aorta; 30 - duct of the head kidney; 31 - secondary body cavity, or whole; 32 - right lateral vein; 33 - fin ray; 34 - upper arch of the vertebra; 35 - lower arch of the vertebra; 36 - tail artery; 37 - tail vein |
body integuments. There are two layers of skin - outer (multilayered epidermis) and inner (corium). Derivatives of the skin are scales, feathers, hair.
Skeleton. The development of the skull is associated with the evolution of the brain, sense organs and mouth parts. In addition to the brain skull, the visceral skeleton develops from the arches located between the gill slits. It consists of supporting gill arches and supports the respiratory apparatus of lower aquatic vertebrates (gills). In terrestrial vertebrates, the visceral skull is greatly reduced and transformed. In mammals, these transformations reach the highest degree. The elements of only the first 2 arches are preserved, from which 3 auditory ossicles are formed: the hammer, anvil and stirrup, and from the remnants of the second and third dut - laryngeal cartilages.
The segmented skeleton, consisting of vertebrae, is sufficiently rigid and moderately flexible, providing strength of support and a variety of movements. The skeleton of vertebrates is characterized by limbs articulated with the spine. They can be unpaired (dorsal and caudal fins) and paired. In the skeleton of paired limbs, belts and a free limb are distinguished. An older form of free limb is fish fins. In the process of evolution, in connection with the transition to a terrestrial way of life, five-fingered limbs of a terrestrial type developed. Phylogenetically they are related to the paired fins of lobe-finned fishes.
musculature It is subdivided into somatic (body muscles) and visceral (internal organs). In lower vertebrates, the somatic musculature retains a segmental structure. The somatic musculature is striated, is a derivative of the somites of the embryo. The visceral musculature consists of smooth and striated muscle fibers and is of mesodermal origin.
Digestive system. In the intestine, the anterior, middle and posterior sections are distinguished. In representatives of different classes, their structure differs in features. Apparatus for grinding food and digestive glands are characteristic; of these, the most important are the liver and pancreas.
Respiratory system. The respiratory organs are topographically and genetically related to the intestine. They are presented in the form of gills or lungs and develop from a protrusion of the anterior intestinal tube.
excretory system. The organs of excretion are paired kidneys, equipped with excretory channels - ureters. In representatives of different classes, the structure of the kidneys is not the same, but they always consist of numerous excretory tubules, the number of which increases as the organization becomes more complex. In the embryonic development of higher vertebrates, there is a change of three types of kidneys: pronephros, primary and secondary. The pronephros is similar to the metanephridia. In the primary kidney, the ciliated funnel is partially replaced by a capsule with filtration tubules. Finally, in the secondary kidney, such a replacement is carried out completely.
Vascular system. An active, very mobile lifestyle is ensured by a high level of metabolism and, consequently, rapid blood circulation, vigorous removal of unnecessary and harmful metabolic products from the body. Vertebrates have a special pulsating organ of the circulatory system - the heart. It is divided into several chambers, of which the main ones are the atrium and the ventricle. From the heart, blood moves through the vessels, called arteries, to the heart - through the veins. The circulatory system is always closed. In addition, vertebrates have an open lymphatic system. It consists of vessels that communicate with intercellular spaces, and together with the circulatory system performs the function of internal metabolism. The lymphatic system includes glands in which white blood cells are formed that perform protective functions.
Endocrine system. In the regulation of metabolism, an important role is played by the endocrine glands: the pituitary gland, adrenal glands, thyroid, parathyroid, pancreas, genital, etc.
Nervous system divided into central and peripheral. The central nervous system includes the brain and spinal cord. During embryonic development, the brain is laid down in the form of three primary cerebral vesicles. The anterior and posterior vesicles separate repeatedly, forming 5 main brain regions: anterior, intermediate, middle, posterior, and oblong. Behind the oblong is the spinal cord. The cranial nerves leave the brain. In lower vertebrates, there are 10 pairs of them, in higher ones - 12. Spinal nerves depart metamerically from the spinal cord. The sense organs - sight, hearing, smell, taste and touch - reach a high level of development.
Reproductive organs. All vertebrates (with the exception of a few species) are dioecious. Sex glands are paired. Insemination is external or internal. Sexual dimorphism is usually well expressed.
The subtype of vertebrates includes 6 classes: cyclostomes (Cyclostomata), fishes (Pisces), amphibians (Amphibia), reptiles (Reptilia), birds (Aves) and mammals (Mammalia).
On the basis of the absence or presence of the amniotic membrane, the subtype of vertebrates is divided into lower (Anamnia) and higher (Amniota). The lower ones include vertebrates, the development of which is associated with the aquatic environment and the embryos of which are devoid of amnion (cyclostomes, fish, amphibians). The higher ones are terrestrial inhabitants whose embryos develop inside the embryonic membranes. Amniotes include reptiles, birds and mammals.
The main classes of vertebrates studied under the program for entering universities are presented in tab. eighteen.
| Table 18. Comparative characteristics of classes of vertebrates | |||||
| Class | Skin covering | Respiratory system | A heart | hearing organ | Fertilization |