Evolution of nervous systems
The evolution of nervous systems dates back to the first development of nervous systems in animals. Neurons developed as specialized electrical signaling cells in multicellular animals, adapting the mechanism of action potentials present in motile single-celled and colonial eukaryotes. Simple nerve nets seen in animals like Cnidaria evolved first, consisted of polymodal neurons which serve a dual purpose in motor and sensory functions. Cnidarians can be compared to Ctenophores, which although are both jellyfish, have different nervous systems. Unlike Cnidarians, Ctenophores have neurons; this was perplexing because the phylum Ctenophora was considered to be more ancient than that of Porifera, which have no nervous system at all. This led to the rise of two theories. One theory stated that the nervous system came about in an ancestor basal to all of these phylum, however was lost in Porifera; the other theory states that the nervous system arose independently twice, one basal to Cnidarians and one basal to Ctenophores.
Bilateral animals – ventral nerve cords in invertebrates and dorsal nerve cords supported by a notochord in chordates-- evolved with a central nervous system, around a central region, a process known as cephalization. Action potentials, which are necessary for neural activity, evolved in single-celled eukaryotes; these use calcium rather than sodium action potentials, but the mechanism was adapted into neural electrical signaling in multicellular animals. In some colonial eukaryotes such as Obelia electrical signals do propagate not only through neural nets, but through epithelial cells in the shared digestive system of the colony. Several non-metazoan phyla, including choanoflagellates and mesomycetozoea, have been found to have synaptic protein homologs, including secretory SNAREs, Homer. In choanoflagellates and mesomycetozoea, these proteins are upregulated during colonial phases, suggesting the importance of these proto-synaptic proteins for cell to cell communication; the history of ideas on how neurons and the first nervous systems emerged in evolution has been discussed in a recent book.
Sponges have no cells connected to each other by synaptic junctions, that is, no neurons, therefore no nervous system. They do, have homologs of many genes that play key roles in synaptic function. Recent studies have shown that sponge cells express a group of proteins that cluster together to form a structure resembling a postsynaptic density. However, the function of this structure is unclear. Although sponge cells do not show synaptic transmission, they do communicate with each other via calcium waves and other impulses, which mediate some simple actions such as whole-body contraction. Jellyfish, comb jellies, related animals have diffuse nerve nets rather than a central nervous system. In most jellyfish the nerve net is spread less evenly across the body; the nerve nets consist of sensory neurons that pick up chemical and visual signals, motor neurons that can activate contractions of the body wall, intermediate neurons that detect patterns of activity in the sensory neurons and send signals to groups of motor neurons as a result.
In some cases groups of intermediate neurons are clustered into discrete ganglia. The development of the nervous system in radiata is unstructured. Unlike bilaterians, radiata only have two primordial cell layers and ectoderm. Neurons are generated from a special set of ectodermal precursor cells, which serve as precursors for every other ectodermal cell type. Neural induction represents the initial step in the generation of the nervous system and begins with the segregation of neural and glial cells from other types of tissues. Experiments and research pertaining to neural induction are focused on invertebrates C. Elegans and Drosophila as well as vertebrates frogs. Invertebrates are much more powerful genetic systems however due to how researchers can screen for they are looking for; this process is called forward genetics. Frogs on the other hand are not as good as the aforementioned invertebrates because of their slower life cycle and their tetraploid genes, which are much more difficult to manipulate.
One benefit that studying vertebrates such as frogs brings however are their big eggs, in which cellular changes can be observed. The neurogenic region of invertebrates begins at the ventrolateral regions of the embryo. In Drosophila Melanogaster, development begins once the ventral furrow folds into the embryo interior; the invaginated cells become the mesoderm. The closure of the furrow creates a midline; the neuroblasts of the ectoderm enlarge and squeeze away the epithelium layer through a process called delamination. Delamination occurs in 5 total waves, called niches, each creating about 60 neuroblasts; these neuroblasts undergo cell division to produce the "ganglion mother cell". The GMC divides only once to produce neurons or glia. After fertilization, the egg is polarized into an animal hemisphere; the animal hemisphere, at the top of the egg has smaller cells than the rest of the egg. Following this polarization, a Blastula is formed after the egg undergoes multiple division, with a blactocoel being the outcome.
The blastocoel differs from the blastula because of the tiny pocket or space, created. Following this, the gastrula is formed via the process of gastrulation which leads to th
Micropaleontology
Micropaleontology is the branch of paleontology that studies microfossils, or fossils that require the use of a microscope to see the organism, its morphology and its characteristic details. Microfossils are fossils that are between 0.001mm and 1 mm in size, the study of which requires the use of light or electron microscopy. Fossils which can be studied with the naked eye or low-powered magnification, such as a hand lens, are referred to as macrofossils. For example, some colonial organisms, such as Bryozoa have large colonies, but are classified by fine skeletal details of the small individuals of the colony. In another example, many fossil genera of Foraminifera, which are protists are known from shells that were as big as coins, such as the genus Nummulites. Microfossils are a common feature of the geological record, from the Precambrian to the Holocene, they are most common in deposits of marine environments, but occur in brackish water, fresh water and terrestrial sedimentary deposits.
While every kingdom of life is represented in the microfossil record, the most abundant forms are protist skeletons or cysts from the Chrysophyta, Sarcodina and chitinozoans, together with pollen and spores from the vascular plants. In 2017, fossilized microorganisms, or microfossils, were announced to have been discovered in hydrothermal vent precipitates in the Nuvvuagittuq Belt of Quebec, Canada that may be as old as 4.28 billion years old, the oldest record of life on Earth, suggesting "an instantaneous emergence of life", after ocean formation 4.41 billion years ago, not long after the formation of the Earth 4.54 billion years ago. Nonetheless, life may have started earlier, at nearly 4.5 billion years ago, as claimed by some researchers. Micropaleontology can be divided into four areas of study on the basis of microfossil composition: calcareous, as in coccoliths and foraminifera, phosphatic, as in the study of some vertebrates, siliceous, as in diatoms and radiolaria, or organic, as in the pollen and spores studied in palynology.
This division reflects differences in the mineralogical and chemical composition of microfossil remains rather than any strict taxonomic or ecological distinctions. Most researchers in this field, known as micropaleontologists, are specialists in one or more taxonomic groups. Calcareous microfossils include coccoliths, calcareous dinoflagellate cysts, ostracods. Phosphatic microfossils include conodonts, some scolecodonts, Shark spines and teeth, other Fish remains. Siliceous microfossils include diatoms, silicoflagellates, phytoliths, some scolecodonts, sponge spicules; the study of organic microfossils is called palynology. Organic microfossils include pollen, chitinozoans, acritarchs, dinoflagellate cysts, fungal remains. Sediment or rock samples are collected from either cores or outcrops, the microfossils they contain are extracted by a variety of physical and chemical laboratory techniques, including sieving, density separation by centrifuge or in heavy liquids, chemical digestion of the unwanted fraction.
The resulting concentrated sample of microfossils is mounted on a slide for analysis by light microscope. Taxa are identified and counted; the enormous numbers of microfossils that a small sediment sample can yield allows the collection of statistically robust datasets which can be subjected to multivariate analysis. A typical microfossil study will involve identification of a few hundred specimens from each sample. Microfossils are specially noteworthy for their importance in biostratigraphy. Since microfossils are extremely abundant and quick to appear and disappear from the stratigraphic record, they constitute ideal index fossils from a biostratigraphic perspective; the planktonic and nektonic habits of some microfossils give them the bonus of appearing across a wide range of facies or paleoenvironments, as well as having near-global distribution, making biostratigraphic correlation more powerful and effective. Microfossils from deep-sea sediments provide some of the most important records of global environmental change on long, medium or short timescales.
Across vast areas of the ocean floor, the shells of planktonic micro-organisms sinking from surface waters provide the dominant source of sediment, they continuously accumulate. Study of changes in assemblages of microfossils and changes in their shell chemistry are fundamental to research on climate change in the geological past. In addition to providing an excellent tool for sedimentary rock dating and for paleoenvironmental reconstruction – used in both petroleum geology and paleoceanography – micropaleontology has found a number of less orthodox applications, such as its growing role in forensic police investigation or in determining the provenance of archaeological artefacts. Micropaleontology is a tool of geoarchaeology used in the archaeological reconstruction of human habitation sites and environments. Changes in the microfossil population abundance in the stratigraphy of current and former water bodies reflect changes in environmental conditions. Occurring ostracods in freshwater bodies are impacted b
Paleopedology
Paleopedology is the discipline that studies soils of past geological eras, from quite recent to the earliest periods of the Earth's history. Paleopedology can be seen either as a branch of soil science or of paleontology, since the methods it uses are in many ways a well-defined combination of the two disciplines. Paleopedology's earliest developments arose from observations in Scotland circa 1795 whereby it was found that some soils in cliffs appeared to be remains of a former exposed land surface. During the nineteenth century there were many other finds of former soils throughout Europe and North America. However, most of this was only found in the search for animal and/or plant fossils and it was not until soil science first developed that buried soils of past geological ages were considered of any value, it was only when the first relationships between soils and climate were observed in the steppes of Russia and Kazakhstan that there was any interest in applying the finds of former soils to past ecosystems.
This occurred because, by the 1920s, some soils in Russia had been found by K. D. Glinka that did not fit with present climates and were seen as relics of warmer climates in the past. Eugene W. Hilgard, in 1892, had related soil and climate in the United States in the same manner, by the 1950s analysis of Quaternary stratigraphy to monitor recent environmental changes in the northern hemisphere had become established; these developments have allowed soil fossils to be classified according to USDA soil taxonomy quite with all recent soils. Interest in earlier soil fossils was much slower to grow, but has developed since the 1960s owing to the development of such techniques as X-ray diffraction which permit their classification; this has allowed many developments in paleoecology and paleogeography to take place because the soils' chemistry can provide a good deal of evidence as to how life moved onto land during the Paleozoic era. Remains of former soils can either be found under deposited sediment in unglaciated areas or in steep cliffs where the old soil can be seen below the young present-day soil.
In cases where volcanoes have been active, some soil fossils occur under the volcanic ash. If there is continued deposition of sediment, a sequence of soil fossils will form after the retreat of glaciers during the Holocene. Soil fossils can exist where a younger soil has been eroded, as in the Badlands of South Dakota. Soil fossils, whether buried or exposed, suffer from alteration; this occurs because all past soils have lost their former vegetative covering and the organic matter they once supported has been used up by plants since the soil was buried. However, if remains of plants can be found, the nature of the soil fossil can be made a great deal clearer than if no flora can be found because roots can nowadays be identified with respect to the plant group from which they come. Patterns of root traces including their shape and size, is good evidence for the vegetation type the former soil supported. Bluish colours in the soil tend to indicate; the horizons of fossil soils are defined only in the top layers, unless some of the parent material has not been obliterated by soil formation.
The kinds of horizons in fossil soils are, though the same as those found in present-day soils, allowing easy classification in modern taxonomy of all but the oldest soils. Chemical analysis of soil fossils focuses on their lime content, which determines both their pH and how reactive they will be to dilute acids. Chemical analysis is useful through solvent extraction to determine key minerals; this analysis can be of some use in determining the structure of a soil fossil, but today X-ray diffraction is preferred because it permits the exact crystal structure of the former soil to be determined. With the aid of X-ray diffraction, paleosols can now be classified into one of the 12 orders of Soil Taxonomy. Many Precambrian soils, when examined do not fit the characteristics for any of these soil orders and have been placed in a new order called green clays; the green colour is due to the presence of certain unoxidised minerals found in the primitive earth because O2 was not present. There are some forest soils of more recent times that cannot be classified as Alfisols or as Spodosols because, despite their sandy horizons, they are not nearly acidic enough to have the typical features of a Spodosol.
Paleopedology is an important scientific discipline for the understanding of the ecology and evolution of ancient ecosystems, both on Earth and the emerging field of exoplanet research. Models The different definitions applied to soils is indicative of the different approaches taken to them. Where farmers and engineers experience different soil challenges, soil scientists have a different view again; these differing views of the definition of soil are different theoretical bases for their study. Soils can be thought of as open systems in that they represent a boundary between the earth and the atmosphere where materials are transported and are changed. There are four basic types of flux: additions, subtractions and transformations. Examples of addition can include mineral grains and le
Evolution of reptiles
Reptiles arose about 310–320 million years ago during the Carboniferous period. Reptiles, in the traditional sense of the term, are defined as animals that have scales or scutes, lay land-based hard-shelled eggs, possess ectothermic metabolisms. So defined, the group is paraphyletic, excluding endothermic animals like birds and mammals that are descended from early reptiles. A definition in accordance with phylogenetic nomenclature, which rejects paraphyletic groups, includes birds while excluding mammals and their synapsid ancestors. So defined, Reptilia is identical to Sauropsida. Though lots of reptiles today are apex predators, many examples of apex reptiles have existed in the past. Reptiles have an diverse evolutionary history that has led to biological successes, such as dinosaurs, plesiosaurs and ichthyosaurs. Reptiles first arose from amphibians in the swamps of the late Carboniferous. Increasing evolutionary pressure and the vast untouched niches of the land powered the evolutionary changes in amphibians to become more and more land-based.
Environmental selection propelled the development of certain traits, such as a stronger skeletal structure and more protective coating became more favorable. The evolution of lungs and legs are the main transitional steps towards reptiles, but the development of hard-shelled external eggs replacing the amphibious water bound eggs is the defining feature of the class Reptilia and is what allowed these amphibians to leave water. Another major difference from amphibians is the increased brain size, more the enlarged cerebrum and cerebellum. Although their brain size is small when compared to birds and mammals, these enhancements prove vital in hunting strategies of reptiles; the increased size of these two regions of the brain allowed for improved motor skills and an increase in sensory development. The origin of the reptiles lies about 320–310 million years ago, in the swamps of the late Carboniferous period, when the first reptiles evolved from advanced reptiliomorph labyrinthodonts; the oldest known animal that may have been an amniote, a reptile rather than an amphibian, is Casineria.
A series of footprints from the fossil strata of Nova Scotia, dated to 315 million years, show typical reptilian toes and imprints of scales. The tracks are attributed to the oldest unquestionable reptile known, it was a small, lizard-like animal, about 20 to 30 cm long, with numer ous sharp teeth indicating an insectivorous diet. Other examples include Westlothiana and Paleothyris, both of similar build and similar habit. One of the best known early reptiles is Mesosaurus, a genus from the early Permian that had returned to water, feeding on fish; the earliest reptiles were overshadowed by bigger labyrinthodont amphibians, such as Cochleosaurus, remained a small, inconspicuous part of the fauna until after the small ice age at the end of the Carboniferous. It was traditionally assumed that first reptiles were anapsids, having a solid skull with holes only for the nose, spinal cord, etc.. Soon after the first reptiles appeared, they split into two branches. One branch, had one opening in the skull roof behind each eye.
The other branch, Sauropsida, is itself divided into two main groups. One of them, the aforementioned Parareptilia, contained taxa with anapsid-like skull, as well as taxa with one opening behind each eye. Members of the other group, possessed a hole in their skulls behind each eye, along with a second hole located higher on the skull; the function of the holes in both synapsids and diapsids was to lighten the skull and give room for the jaw muscles to move, allowing for a more powerful bite. Turtles have been traditionally believed to be surviving anapsids, on the basis of their skull structure; the rationale for this classification was disputed, with some arguing that turtles are diapsids that reverted to this primitive state in order to improve their armor. Morphological phylogenetic studies with this in mind placed turtles within Diapsida. All molecular studies have upheld the placement of turtles within diapsids, most as a sister group to extant archosaurs. A basic cladogram of the origin of mammals.
Important developments in the transition from reptile to mammal were the evolution of warm-bloodedness, of molar occlusion, of the three-ossicle middle ear, of hair, of mammary glands. By the end of the Triassic, there were many species that looked like modern mammals and, by the Middle Jurassic, the lineages leading to the three extant mammal groups — the monotremes, the marsupials, the placentals — had diverged. Near the end of the Carboniferous, while the terrestrial reptiliomorph labyrinthodonts were still present, the synapsids evolved the first terrestrial large vertebrates, the pelycosaurs such as Edaphosaurus. In the mid-Permian period, the climate turned drier, resulting in a change of fauna: The primitive pelycosaurs were replaced by the more advanced therapsids; the anapsid reptiles, whose massive skull roofs had no postorbital holes and flourished throughout the Permian. The pareiasaurs reached giant proportions in the late Pe
Human evolution
Human evolution is the evolutionary process that led to the emergence of anatomically modern humans, beginning with the evolutionary history of primates—in particular genus Homo—and leading to the emergence of Homo sapiens as a distinct species of the hominid family, the great apes. This process involved the gradual development of traits such as human bipedalism and language, as well as interbreeding with other hominins, which indicate that human evolution was not linear but a web; the study of human evolution involves several scientific disciplines, including physical anthropology, archaeology, neurobiology, linguistics, evolutionary psychology and genetics. Genetic studies show that primates diverged from other mammals about 85 million years ago, in the Late Cretaceous period, the earliest fossils appear in the Paleocene, around 55 million years ago. Within the Hominoidea superfamily, the Hominidae family diverged from the Hylobatidae family some 15–20 million years ago. Human evolution from its first separation from the last common ancestor of humans and chimpanzees is characterized by a number of morphological, developmental and behavioral changes.
The most significant of these adaptations are bipedalism, increased brain size, lengthened ontogeny, decreased sexual dimorphism. The relationship between these changes is the subject of ongoing debate. Other significant morphological changes included the evolution of a power and precision grip, a change first occurring in H. erectus. Bipedalism is the basic adaptation of the hominid and is considered the main cause behind a suite of skeletal changes shared by all bipedal hominids; the earliest hominin, of primitive bipedalism, is considered to be either Sahelanthropus or Orrorin, both of which arose some 6 to 7 million years ago. The non-bipedal knuckle-walkers, the gorilla and chimpanzee, diverged from the hominin line over a period covering the same time, so either of Sahelanthropus or Orrorin may be our last shared ancestor. Ardipithecus, a full biped, arose 5.6 million years ago. The early bipeds evolved into the australopithecines and still into the genus Homo. There are several theories of the adaptation value of bipedalism.
It is possible that bipedalism was favored because it freed the hands for reaching and carrying food, saved energy during locomotion, enabled long distance running and hunting, provided an enhanced field of vision, helped avoid hyperthermia by reducing the surface area exposed to direct sun. A new study provides support for the hypothesis that walking on two legs, or bipedalism, evolved because it used less energy than quadrupedal knuckle-walking. However, recent studies suggest that bipedality without the ability to use fire would not have allowed global dispersal; this change in gait saw a lengthening of the legs proportionately when compared to the length of the arms, which were shortened through the removal of the need for brachiation. Another change is the shape of the big toe. Recent studies suggest that Australopithecines still lived part of the time in trees as a result of maintaining a grasping big toe; this was progressively lost in Habilines. Anatomically, the evolution of bipedalism has been accompanied by a large number of skeletal changes, not just to the legs and pelvis, but to the vertebral column and ankles, skull.
The femur evolved into a more angular position to move the center of gravity toward the geometric center of the body. The knee and ankle joints became robust to better support increased weight. To support the increased weight on each vertebra in the upright position, the human vertebral column became S-shaped and the lumbar vertebrae became shorter and wider. In the feet the big toe moved into alignment with the other toes to help in forward locomotion; the arms and forearms shortened relative to the legs making it easier to run. The foramen magnum migrated under more anterior; the most significant changes occurred in the pelvic region, where the long downward facing iliac blade was shortened and widened as a requirement for keeping the center of gravity stable while walking. A drawback is that the birth canal of bipedal apes is smaller than in knuckle-walking apes, though there has been a widening of it in comparison to that of australopithecine and modern humans, permitting the passage of newborns due to the increase in cranial size but this is limited to the upper portion, since further increase can hinder normal bipedal movement.
The shortening of the pelvis and smaller birth canal evolved as a requirement for bipedalism and had significant effects on the process of human birth, much more difficult in modern humans than in other primates. During human birth, because of the variation in size of the pelvic region, the fetal head must be in a transverse position during entry into the birth canal and rotate about 90 degrees upon exit; the smaller birth canal became a limiting factor to brain size increases in early humans and prompted a shorter gestation period leading to the relative immaturity of human
Evolution of the eye
Many researchers have found the evolution of the eye attractive to study, because the eye distinctively exemplifies an analogous organ found in many animal forms. Simple light detection is found in bacteria, single-celled organisms and animals. Complex, image-forming eyes have evolved independently several times. Complex eyes appeared first within the few million years of the Cambrian explosion. Prior to the Cambrian, no evidence of eyes has survived, but diverse eyes are known from the Burgess shale of the Middle Cambrian, from the older Emu Bay Shale. Eyes are adapted to the various requirements of their owners, they vary in their visual acuity, the range of wavelengths they can detect, their sensitivity in low light, their ability to detect motion or to resolve objects, whether they can discriminate colours. In 1802, philosopher William Paley called it a miracle of "design". Charles Darwin himself wrote in his Origin of Species, that the evolution of the eye by natural selection seemed at first glance "absurd in the highest possible degree".
However, he went on that despite the difficulty in imagining it, its evolution was feasible:...if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is the case. He suggested a stepwise evolution from "an optic nerve coated with pigment, without any other mechanism" to "a moderately high stage of perfection", gave examples of existing intermediate steps. Current research is investigating evolution. Biologist D. E. Nilsson has independently theorized about four general stages in the evolution of a vertebrate eye from a patch of photoreceptors. Nilsson and S. Pelger estimated in a classic paper that only a few hundred thousand generations are needed to evolve a complex eye in vertebrates. Another researcher, G. C. Young, has used the fossil record to infer evolutionary conclusions, based on the structure of eye orbits and openings in fossilized skulls for blood vessels and nerves to go through. All this adds to the growing amount of evidence.
The first fossils of eyes found to date are from the lower Cambrian period. The lower Cambrian had a burst of rapid evolution, called the "Cambrian explosion". One of the many hypotheses for "causes" of the Cambrian explosion is the "Light Switch" theory of Andrew Parker: It holds that the evolution of eyes started an arms race that accelerated evolution. Before the Cambrian explosion, animals may have sensed light, but did not use it for fast locomotion or navigation by vision; the rate of eye evolution is difficult to estimate, because the fossil record of the lower Cambrian, is poor. How fast a circular patch of photoreceptor cells evolve into a functional vertebrate eye has been estimated based on rates of mutation, relative advantage to the organism, natural selection. However, the time needed for each state was overestimated and the generation time was set to one year, common in small animals. With these pessimistic values, the vertebrate eye would still evolve from a patch of photoreceptor cells in less than 364,000 years.
Whether the eye evolved once or many times depends on the definition of an eye. All eyed animals share much of the genetic machinery for eye development; this suggests that the ancestor of eyed animals had some form of light-sensitive machinery – if it was not a dedicated optical organ. However photoreceptor cells may have evolved more than once from molecularly similar chemoreceptor cells. Photoreceptor cells existed long before the Cambrian explosion. Higher-level similarities – such as the use of the protein crystallin in the independently derived cephalopod and vertebrate lenses – reflect the co-option of a more fundamental protein to a new function within the eye. A shared trait common to all light-sensitive organs are opsins. Opsins belong to a family of photo-sensitive proteins and fall into nine groups, which existed in the urbilaterian, the last common ancestor of all bilaterally symmetrical animals. Additionally, the genetic toolkit for positioning eyes is shared by all animals: The PAX6 gene controls where eyes develop in animals ranging from octopuses to mice and fruit flies.
Such high-level genes are, by implication, much older than many of the structures that they control today. Eyes and other sensory organs evolved before the brain: There is no need for an information-processing organ before there is information to process; the earliest predecessors of the eye were photoreceptor proteins that sense light, found in unicellular organisms, called "eyespots". Eyespots can only sense ambient brightness: they can distinguish light from dark, sufficient for photoperiodism and daily synchronization of circadian rhythms, they are insufficient for vision, as they cannot distinguish shapes or determine the direction light is coming from. Eyespots are found in nearly all major animal groups, are common among unicellular organisms, including euglena; the euglena's eyespot, called a stigma, is located at its anterior end. It is a small splotch of red pigment. Together with th
Paleobotany
Paleobotany spelled as palaeobotany, is the branch of paleontology or paleobiology dealing with the recovery and identification of plant remains from geological contexts, their use for the biological reconstruction of past environments, both the evolutionary history of plants, with a bearing upon the evolution of life in general. A synonym is paleophytology. Paleobotany includes the study of terrestrial plant fossils, as well as the study of prehistoric marine photoautotrophs, such as photosynthetic algae, seaweeds or kelp. A related field is palynology, the study of fossilized and extant spores and pollen. Paleobotany is important in the reconstruction of ancient ecological systems and climate, known as paleoecology and paleoclimatology respectively. Paleobotany has become important to the field of archaeology for the use of phytoliths in relative dating and in paleoethnobotany; the emergence of paleobotany as a scientific discipline can be seen in the early 19th century in the works of the German palaeontologist Ernst Friedrich von Schlotheim, the Czech nobleman and scholar Kaspar Maria von Sternberg, the French botanist Adolphe-Théodore Brongniart.
Macroscopic remains of true vascular plants are first found in the fossil record during the Silurian Period of the Paleozoic era. Some dispersed, fragmentary fossils of disputed affinity spores and cuticles, have been found in rocks from the Ordovician Period in Oman, are thought to derive from liverwort- or moss-grade fossil plants. An important early land plant fossil locality is the Rhynie Chert, found outside the village of Rhynie in Scotland; the Rhynie chert is an Early Devonian sinter deposit composed of silica. It is exceptional due to its preservation of several different clades of plants, from mosses and lycopods to more unusual, problematic forms. Many fossil animals, including arthropods and arachnids, are found in the Rhynie Chert, it offers a unique window on the history of early terrestrial life. Plant-derived macrofossils become abundant in the Late Devonian and include tree trunks and roots; the earliest tree was thought to be Archaeopteris, which bears simple, fern-like leaves spirally arranged on branches atop a conifer-like trunk, though it is now known to be the discovered Wattieza.
Widespread coal swamp deposits across North America and Europe during the Carboniferous Period contain a wealth of fossils containing arborescent lycopods up to 30 meters tall, abundant seed plants, such as conifers and seed ferns, countless smaller, herbaceous plants. Angiosperms evolved during the Mesozoic, flowering plant pollen and leaves first appear during the Early Cretaceous 130 million years ago. A plant fossil is any preserved part of a plant; such fossils may be prehistoric impressions that are many millions of years old, or bits of charcoal that are only a few hundred years old. Prehistoric plants are various groups of plants. Plant fossils can be preserved in a variety of ways, each of which can give different types of information about the original parent plant; these modes of preservation are discussed in the general pages on fossils but may be summarised in a palaeobotanical context as follows. Adpressions; these are the most found type of plant fossil. They provide good morphological detail of dorsiventral plant parts such as leaves.
If the cuticle is preserved, they can yield fine anatomical detail of the epidermis. Little other detail of cellular anatomy is preserved. Petrifactions; these provide fine detail of the cell anatomy of the plant tissue. Morphological detail can be determined by serial sectioning, but this is both time consuming and difficult. Moulds and casts; these only tend to preserve the more robust plant parts such as seeds or woody stems. They can provide information about the three-dimensional form of the plant, in the case of casts of tree stumps can provide evidence of the density of the original vegetation. However, they preserve any fine morphological detail or cell anatomy. A subset of such fossils are pith casts, where the centre of a stem is either hollow or has delicate pith. After death, sediment forms a cast of the central cavity of the stem; the best known examples of pith casts are in cordaites. Authigenic mineralisations; these can provide fine, three-dimensional morphological detail, have proved important in the study of reproductive structures that can be distorted in adpressions.
However, as they are formed in mineral nodules, such fossils can be of large size. Fusain. Fire destroys plant tissue but sometimes charcoalified remains can preserve fine morphological detail, lost in other modes of preservation. Fusain fossils are delicate and small, but because of their buoyancy can drift for long distances and can thus provide evidence of vegetation away from areas of sedimentation. Plant fossils always represent disarticulated parts of plants; those few examples of plant fossils that appear to be the remains of whole plants in fact are incomplete as the internal cellular tis
