title: Origin of birds

text: A close relationship between birds and dinosaurs was first proposed in the nineteenth century after the discovery of the primitive bird "Archaeopteryx" in Germany. Birds and extinct non-avian dinosaurs share many unique skeletal traits. Moreover, fossils of more than thirty species of non-avian dinosaur with preserved feathers have been collected. There are even very small dinosaurs, such as "Microraptor" and "Anchiornis", which have long, vaned arm and leg feathers forming wings. The Jurassic basal avialan "Pedopenna" also shows these long foot feathers. Paleontologist Lawrence Witmer concluded in 2009 that this evidence is sufficient to demonstrate that avian evolution went through a four-winged stage. Fossil evidence also demonstrates that birds and dinosaurs shared features such as hollow, pneumatized bones, gastroliths in the digestive system, nest-building and brooding behaviors.


title: Origin of birds

text: Although the origin of birds has historically been a contentious topic within evolutionary biology, only a few scientists still dispute the dinosaurian origin of birds, suggesting descent from other types of archosaurian reptiles. Within the consensus that supports dinosaurian ancestry, the exact sequence of evolutionary events that gave rise to the early birds within maniraptoran theropods is disputed. The origin of bird flight is a separate but related question for which there are also several proposed answers.


title: Origin of birds

text: Scientific investigation into the origin of birds began shortly after the 1859 publication of Charles Darwin's "On the Origin of Species". In 1860, a fossilized feather was discovered in Germany's Late Jurassic Solnhofen limestone. Christian Erich Hermann von Meyer described this feather as "Archaeopteryx lithographica" the next year. Richard Owen described a nearly complete skeleton in 1863, recognizing it as a bird despite many features reminiscent of reptiles, including clawed forelimbs and a long, bony tail.


title: Origin of birds

text: Biologist Thomas Henry Huxley, known as "Darwin's Bulldog" for his tenacious support of the new theory of evolution by means of natural selection, almost immediately seized upon "Archaeopteryx" as a transitional fossil between birds and reptiles. Starting in 1868, and following earlier suggestions by Karl Gegenbaur, and Edward Drinker Cope, Huxley made detailed comparisons of "Archaeopteryx" with various prehistoric reptiles and found that it was most similar to dinosaurs like "Hypsilophodon" and "Compsognathus". The discovery in the late 1870s of the iconic "Berlin specimen" of "Archaeopteryx", complete with a set of reptilian teeth, provided further evidence. Like Cope, Huxley proposed an evolutionary relationship between birds and dinosaurs. Although Huxley was opposed by the very influential Owen, his conclusions were accepted by many biologists, including Baron Franz Nopcsa, while others, notably Harry Seeley, argued that the similarities were due to convergent evolution.


title: Origin of birds

text: A turning point came in the early twentieth century with the writings of Gerhard Heilmann of Denmark. An artist by trade, Heilmann had a scholarly interest in birds and from 1913 to 1916, expanding on earlier work by Othenio Abel, published the results of his research in several parts, dealing with the anatomy, embryology, behavior, paleontology, and evolution of birds. His work, originally written in Danish as "Vor Nuvaerende Viden om Fuglenes Afstamning", was compiled, translated into English, and published in 1926 as "The Origin of Birds".


title: Origin of birds

text: Like Huxley, Heilmann compared "Archaeopteryx" and other birds to an exhaustive list of prehistoric reptiles, and also came to the conclusion that theropod dinosaurs like "Compsognathus" were the most similar. However, Heilmann noted that birds had clavicles (collar bones) fused to form a bone called the furcula ("wishbone"), and while clavicles were known in more primitive reptiles, they had not yet been recognized in dinosaurs. Since he was a firm believer in an interpretation of Dollo's law that stated that evolution was not "reversible", Heilmann could not accept that clavicles were lost in dinosaurs and re-evolved in birds. He was therefore forced to rule out dinosaurs as bird ancestors and ascribe all of their similarities to convergent evolution. Heilmann stated that bird ancestors would instead be found among the more primitive "thecodont" grade of reptiles. Heilmann's extremely thorough approach ensured that his book became a classic in the field, and its conclusions on bird origins, as with most other topics, were accepted by nearly all evolutionary biologists for the next four decades.


title: Origin of birds

text: Clavicles are relatively delicate bones and therefore in danger of being destroyed or at least damaged beyond recognition. Nevertheless, some fossil theropod clavicles had actually been excavated before Heilmann wrote his book, but these had been misidentified.


title: Origin of birds

text: The absence of clavicles in dinosaurs became the orthodox view despite the discovery of clavicles in the primitive theropod "Segisaurus" in 1936. The next report of clavicles in a dinosaur was in a Russian article in 1983.


title: Origin of birds

text: Contrary to what Heilmann believed, paleontologists now accept that clavicles and in most cases furculae are a standard feature not just of theropods but of saurischian dinosaurs. Up to late 2007 ossified furculae (i.e. made of bone rather than cartilage) have been found in all types of theropods except the most basal ones, "Eoraptor" and "Herrerasaurus". The original report of a furcula in the primitive theropod "Segisaurus" (1936) was confirmed by a re-examination in 2005. Joined, furcula-like clavicles have also been found in "Massospondylus", an Early Jurassic sauropodomorph.


title: Origin of birds

text: The tide began to turn against the 'thecodont' hypothesis after the 1964 discovery of a new theropod dinosaur in Montana. In 1969, this dinosaur was described and named "Deinonychus" by John Ostrom of Yale University. The next year, Ostrom redescribed a specimen of "Pterodactylus" in the Dutch Teyler Museum as another skeleton of "Archaeopteryx". The specimen consisted mainly of a single wing and its description made Ostrom aware of the similarities between the wrists of "Archaeopteryx" and "Deinonychus".


title: Origin of birds

text: In 1972, British paleontologist Alick Walker hypothesized that birds arose not from 'thecodonts' but from crocodile ancestors like "Sphenosuchus". Ostrom's work with both theropods and early birds led him to respond with a series of publications in the mid-1970s in which he laid out the many similarities between birds and theropod dinosaurs, resurrecting the ideas first put forth by Huxley over a century before. Ostrom's recognition of the dinosaurian ancestry of birds, along with other new ideas about dinosaur metabolism, activity levels, and parental care, began what is known as the dinosaur renaissance, which began in the 1970s and continues to this day.


title: Origin of birds

text: Ostrom's revelations also coincided with the increasing adoption of phylogenetic systematics (cladistics), which began in the 1960s with the work of Willi Hennig. Cladistics is an exact method of arranging species based strictly on their evolutionary relationships, which are calculated by determining the evolutionary tree implying the least number of changes in their anatomical characteristics. In the 1980s, cladistic methodology was applied to dinosaur phylogeny for the first time by Jacques Gauthier and others, showing unequivocally that birds were a derived group of theropod dinosaurs. Early analyses suggested that dromaeosaurid theropods like "Deinonychus" were particularly closely related to birds, a result that has been corroborated many times since.


title: Origin of birds

text: The early 1990s saw the discovery of spectacularly preserved bird fossils in several Early Cretaceous geological formations in the northeastern Chinese province of Liaoning. In 1996, Chinese paleontologists described "Sinosauropteryx" as a new genus of bird from the Yixian Formation, but this animal was quickly recognized as a more basal theropod dinosaur closely related to "Compsognathus". Surprisingly, its body was covered by long filamentous structures. These were dubbed 'protofeathers' and considered homologous with the more advanced feathers of birds, although some scientists disagree with this assessment. Chinese and North American scientists described "Caudipteryx" and "Protarchaeopteryx" soon after. Based on skeletal features, these animals were non-avian dinosaurs, but their remains bore fully formed feathers closely resembling those of birds. "Archaeoraptor", described without peer review in a 1999 issue of "National Geographic", turned out to be a smuggled forgery, but legitimate remains continue to pour out of the Yixian, both legally and illegally. Feathers or "protofeathers" have been found on a wide variety of theropods in the Yixian, and the discoveries of extremely bird-like non-avian dinosaurs, as well as non-avian dinosaur-like primitive birds, have almost entirely closed the morphological gap between non-avian theropods and birds.


title: Origin of birds

text: There is a debate between embryologists and paleontologists whether the hands of theropod dinosaurs and birds are essentially different, based on phalangeal counts—a count of the number of phalanges (finger bones) in the hand.


title: Origin of birds

text: Embryologists and some paleontologists who oppose the bird-dinosaur link have long numbered the digits of birds II-III-IV on the basis of multiple studies of the development in the egg. This is based on the fact that in most amniotes, the first digit to form in a 5-fingered hand is digit IV, which develops a primary axis. Therefore, embryologists have identified the primary axis in birds as digit IV, and the surviving digits as II-III-IV. The fossils of advanced theropod (Tetanurae) hands appear to have the digits I-II-III (some genera within Avetheropoda also have a reduced digit IV).


title: Origin of birds

text: If this is true, then the II-III-IV development of digits in birds is an indication against theropod (dinosaur) ancestry. However, with no ontogenical (developmental) basis to definitively state which digits are which on a theropod hand (because no non-avian theropods can be observed growing and developing today), the labelling of the theropod hand is not absolutely conclusive.


title: Origin of birds

text: Paleontologists have traditionally identified avian digits as I-II-III. They argue that the digits of birds number I-II-III, just as those of theropod dinosaurs do, by the conserved phalangeal formula. The phalangeal count for archosaurs is 2-3-4-5-3; many archosaur lineages have a reduced number of digits, but have the same phalangeal formula in the digits that remain. In other words, paleontologists assert that archosaurs of different lineages tend to lose the same digits when digit loss occurs, from the outside to the inside. The three digits of dromaeosaurs, and "Archaeopteryx" have the same phalangeal formula of I-II-III as digits I-II-III of basal archosaurs. Therefore, the lost digits would be V and IV. If this is true, then modern birds would also possess digits I-II-III.


title: Origin of birds

text: Also, one 1999 publication proposed a frame-shift in the digits of the theropod line leading to birds (thus making digit I into digit II, II to III, and so forth).


title: Origin of birds

text: However, such frame shifts are rare in amniotes and—to be consistent with the theropod origin of birds—would have had to occur solely in the bird-theropod lineage forelimbs and not the hindlimbs (a condition unknown in any animal).


title: Origin of birds

text: This is called "Lateral Digit Reduction" (LDR) versus "Bilateral Digit Reduction" (BDR) (see also "Limusaurus").


title: Origin of birds

text: A small minority, known by the acronym BAND (Birds Are Not Dinosaurs), including ornithologists Alan Feduccia and Larry Martin, continues to assert that birds are more closely related to earlier reptiles, such as "Longisquama" or "Euparkeria", than to dinosaurs. Embryological studies of bird developmental biology have raised questions about digit homology in bird and dinosaur forelimbs. However, due to the cogent evidence provided by comparative anatomy and phylogenetics, as well as the dramatic feathered dinosaur fossils from China, the idea that birds are derived dinosaurs, first championed by Huxley and later by Nopcsa and Ostrom, enjoys near-unanimous support among today's paleontologists.


title: Origin of birds

text: A 2011 publication suggested that selection for the expansion of skeletal muscle, rather than the evolution of flight, was the driving force for the emergence of this clade. Muscles became larger in prospectively endothermic saurians, according to this hypothesis, as a response to the loss of the vertebrate mitochondrial uncoupling protein, UCP1, which is thermogenic. In mammals, UCP1 functions within brown adipose tissue to protect newborns against hypothermia.


title: Origin of birds

text: In modern birds, skeletal muscle serves a similar function and is presumed to have done so in their ancestors. In this view, bipedality and other avian skeletal alterations were side effects of muscle hyperplasia, with further evolutionary modifications of the forelimbs, including adaptations for flight or swimming, and vestigiality, being secondary consequences of two-leggedness.


title: Origin of birds

text: "Archaeopteryx" has historically been considered the first bird, or "Urvogel". Although newer fossil discoveries filled the gap between theropods and "Archaeopteryx", as well as the gap between "Archaeopteryx" and modern birds, phylogenetic taxonomists, in keeping with tradition, almost always use "Archaeopteryx" as a specifier to help define Aves. Aves has more rarely been defined as a crown group consisting only of modern birds. Nearly all palaeontologists regard birds as coelurosaurian theropod dinosaurs. Within Coelurosauria, multiple cladistic analyses have found support for a clade named Maniraptora, consisting of therizinosauroids, oviraptorosaurs, troodontids, dromaeosaurids, and birds. Of these, dromaeosaurids and troodontids are usually united in the clade Deinonychosauria, which is a sister group to birds (together forming the node-clade Eumaniraptora) within the stem-clade Paraves.


title: Origin of birds

text: Other studies have proposed alternative phylogenies, in which certain groups of dinosaurs usually considered non-avian may have evolved from avian ancestors. For example, a 2002 analysis found that oviraptorosaurs were basal avians. Alvarezsaurids, known from Asia and the Americas, have been variously classified as basal maniraptorans, paravians, the sister taxon of ornithomimosaurs, as well as specialized early birds. The genus "Rahonavis", originally described as an early bird, has been identified as a non-avian dromaeosaurid in several studies. Dromaeosaurids and troodontids themselves have also been suggested to lie within Aves rather than just outside it.


title: Origin of birds

text: "Archaeopteryx", the first good example of a "feathered dinosaur", was discovered in 1861. The first specimen was found in the Solnhofen limestone in southern Germany, which is a "lagerstätte", a rare and remarkable geological formation known for its superbly detailed fossils. "Archaeopteryx" is a transitional fossil, with features clearly intermediate between those of non-avian theropod dinosaurs and birds. Discovered just two years after Darwin's seminal "Origin of Species", its discovery spurred the nascent debate between proponents of evolutionary biology and creationism. This early bird is so dinosaur-like that, without a clear impression of feathers in the surrounding rock, at least one specimen was mistaken for "Compsognathus".


title: Origin of birds

text: Since the 1990s, a number of additional feathered dinosaurs have been found, providing even stronger evidence of the close relationship between dinosaurs and modern birds. The first of these were initially described as simple filamentous "protofeathers", which were reported in dinosaur lineages as primitive as compsognathids and tyrannosauroids. However, feathers indistinguishable from those of modern birds were soon after found in non-avialan dinosaurs as well.


title: Origin of birds

text: A small minority of researchers have claimed that the simple filamentous "protofeather" structures are simply the result of the decomposition of collagen fiber under the dinosaurs' skin or in fins along their backs, and that species with unquestionable feathers, such as oviraptorosaurs and dromaeosaurs are not dinosaurs, but true birds unrelated to dinosaurs. However, a majority of studies have concluded that feathered dinosaurs are in fact dinosaurs, and that the simpler filaments of unquestionable theropods represent simple feathers. Some researchers have demonstrated the presence of color-bearing melanin in the structures—which would be expected in feathers but not collagen fibers. Others have demonstrated, using studies of modern bird decomposition, that even advanced feathers appear filamentous when subjected to the crushing forces experienced during fossilization, and that the supposed "protofeathers" may have been more complex than previously thought. Detailed examination of the "protofeathers" of "Sinosauropteryx prima" showed that individual feathers consisted of a central quill ("rachis") with thinner "barbs" branching off from it, similar to but more primitive in structure than modern bird feathers.


title: Origin of birds

text: Because feathers are often associated with birds, feathered dinosaurs are often touted as the "missing link" between birds and other dinosaurs. However, the multiple skeletal features also shared by the two groups represent the more important proof for paleontologists.


title: Origin of birds

text: Comparisons of bird and dinosaur skeletons, as well as cladistic analysis, strengthens the case for the link, particularly for a branch of theropods called Maniraptora. Skeletal similarities include the skull, tooth build, neck, uncinate processes on the ribs, an open hip socket, a retroverted long pubis, flexible wrist (semi-lunate carpal), long arms, three-fingered hand, general pectoral girdle, shoulder blade, furcula, and breast bones. Almost all skeletal traits of "Archaeopteryx" can be found in non-avian maniraptorans.


title: Origin of birds

text: A study comparing embryonic, juvenile and adult archosaur skulls concluded that bird skulls are derived from those of theropod dinosaurs by progenesis, a type of paedomorphic heterochrony, which resulted in retention of juvenile characteristics of their ancestors.


title: Origin of birds

text: Large meat-eating dinosaurs had a complex system of air sacs similar to those found in modern birds, according to an investigation led by Patrick M. O'Connor of Ohio University. In theropod dinosaurs (carnivores that walked on two legs and had birdlike feet) flexible soft tissue air sacs likely pumped air through the stiff lungs, as is the case in birds. "What was once formally considered unique to birds was present in some form in the ancestors of birds", O'Connor said.


title: Origin of birds

text: Computed tomography (CT) scans conducted in 2000 of the chest cavity of a specimen of the ornithopod "Thescelosaurus" found the apparent remnants of a complex four-chambered heart, much like those found in today's mammals and birds. The idea is controversial within the scientific community, criticised for being bad anatomical science or simply wishful thinking.


title: Origin of birds

text: A study published in 2011 applied multiple lines of inquiry to the question of the object's identity, including more advanced CT scanning, histology, X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. From these methods, the authors found that: the object's internal structure does not include chambers but is made up of three unconnected areas of lower density material, and is not comparable to the structure of an ostrich's heart; the "walls" are composed of sedimentary minerals not known to be produced in biological systems, such as goethite, feldspar minerals, quartz, and gypsum, as well as some plant fragments; carbon, nitrogen, and phosphorus, chemical elements important to life, were lacking in their samples; and cardiac cellular structures were absent. There was one possible patch with animal cellular structures. The authors found their data supported identification as a concretion of sand from the burial environment, not the heart, with the possibility that isolated areas of tissues were preserved.


title: Origin of birds

text: The question of how this find reflects metabolic rate and dinosaur internal anatomy is moot, though, regardless of the object's identity. Both modern crocodilians and birds, the closest living relatives of dinosaurs, have four-chambered hearts (albeit modified in crocodilians), so dinosaurs probably had them as well; the structure is not necessarily tied to metabolic rate.


title: Origin of birds

text: Fossils of the troodonts "Mei" and "Sinornithoides" demonstrate that the dinosaurs slept like certain modern birds, with their heads tucked under their arms. This behavior, which may have helped to keep the head warm, is also characteristic of modern birds.


title: Origin of birds

text: When laying eggs, female birds grow a special type of bone in their limbs. This medullary bone forms as a calcium-rich layer inside the hard outer bone, and is used as a calcium source to make eggshells. The presence of endosteally derived bone tissues lining the interior marrow cavities of portions of a "Tyrannosaurus rex" specimen's hind limb suggested that "T. rex" used similar reproductive strategies, and revealed that the specimen is female. Further research has found medullary bone in the theropod "Allosaurus" and ornithopod "Tenontosaurus". Because the line of dinosaurs that includes "Allosaurus" and "Tyrannosaurus" diverged from the line that led to "Tenontosaurus" very early in the evolution of dinosaurs, this suggests that dinosaurs in general produced medullary tissue.


title: Origin of birds

text: Several "Citipati" specimens have been found resting over the eggs in its nest in a position most reminiscent of brooding.


title: Origin of birds

text: Numerous dinosaur species, for example "Maiasaura", have been found in herds mixing both very young and adult individuals, suggesting rich interactions between them.


title: Origin of birds

text: A dinosaur embryo was found without teeth, which suggests some parental care was required to feed the young dinosaur, possibly the adult dinosaur regurgitated food into the young dinosaur's mouth ("see" altricial). This behaviour is seen in numerous bird species; parent birds regurgitate food into the hatchling's mouth.


title: Origin of birds

text: Both birds and dinosaurs use gizzard stones. These stones are swallowed by animals to aid digestion and break down food and hard fibres once they enter the stomach. When found in association with fossils, gizzard stones are called gastroliths.


title: Origin of birds

text: Gizzard stones are also found in some fish (mullets, mud shad, and the gillaroo, a type of trout) and in crocodiles.


title: Origin of birds

text: On several occasions, the extraction of DNA and proteins from Mesozoic dinosaurs fossils has been claimed, allowing for a comparison with birds. Several proteins have putatively been detected in dinosaur fossils, including hemoglobin.


title: Origin of birds

text: In the March 2005 issue of "Science", Dr. Mary Higby Schweitzer and her team announced the discovery of flexible material resembling actual soft tissue inside a 68-million-year-old "Tyrannosaurus rex" leg bone of specimen MOR 1125 from the Hell Creek Formation in Montana. The seven collagen types obtained from the bone fragments, compared to collagen data from living birds (specifically, a chicken), suggest that older theropods and birds are closely related. The soft tissue allowed a molecular comparison of cellular anatomy and protein sequencing of collagen tissue published in 2007, both of which indicated that "T. rex" and birds are more closely related to each other than either is to "Alligator". A second molecular study robustly supported the relationship of birds to dinosaurs, though it did not place birds within Theropoda, as expected. This study utilized eight additional collagen sequences extracted from a femur of the "mummified" "Brachylophosaurus canadensis" specimen MOR 2598, a hadrosaur. However, these results have been very controversial. No other peptides of a Mesozoic age have been reported. In 2008, it was suggested that the presumed soft tissue was in fact a bacterial microfilm. In response, it was argued that these very microfilms protected the soft tissue. Another objection was that the results could have been caused by contamination. In 2015, under more controlled conditions safeguarding against contamination, the peptides were still identified. In 2017, a study found that a peptide was present in the bone of the modern ostrich that was identical to that found in the "Tyrannosaurus" and "Brachylophosaurus" specimens, highlighting the danger of a cross-contamination.


title: Origin of birds

text: The successful extraction of ancient DNA from dinosaur fossils has been reported on two separate occasions, but upon further inspection and peer review, neither of these reports could be confirmed.


title: Origin of birds

text: Debates about the origin of bird flight are almost as old as the idea that birds evolved from dinosaurs, which arose soon after the discovery of "Archaeopteryx" in 1862. Two theories have dominated most of the discussion since then: the cursorial ("from the ground up") theory proposes that birds evolved from small, fast predators that ran on the ground; the arboreal ("from the trees down") theory proposes that powered flight evolved from unpowered gliding by arboreal (tree-climbing) animals. A more recent theory, "wing-assisted incline running" (WAIR), is a variant of the cursorial theory and proposes that wings developed their aerodynamic functions as a result of the need to run quickly up very steep slopes such as trees, which would help small feathered dinosaurs escape from predators.


title: Origin of birds

text: In March 2018, scientists reported that "Archaeopteryx" was likely capable of flight, but in a manner substantially different from that of modern birds.


title: Origin of birds

text: The cursorial theory of the origin of flight was first proposed by Samuel Wendell Williston, and elaborated upon by Baron Nopcsa. This hypothesis proposes that some fast-running animals with long tails used their arms to keep their balance while running. Modern versions of this theory differ in many details from the Williston-Nopcsa version, mainly as a result of discoveries since Nopcsa's time.


title: Origin of birds

text: Nopcsa theorized that increasing the surface area of the outstretched arms could have helped small cursorial predators keep their balance, and that the scales of the forearms elongated, evolving into feathers. The feathers could also have been used to trap insects or other prey. Progressively, the animals leapt for longer distances, helped by their evolving wings. Nopcsa also proposed three stages in the evolution of flight. First, animals developed passive flight, in which developing wing structures served as a sort of parachute. Second, they achieved active flight by flapping the wings. He used "Archaeopteryx" as an example of this second stage. Finally, birds gained the ability to soar.


title: Origin of birds

text: Current thought is that feathers did not evolve from scales, as feathers are made of different proteins. More seriously, Nopcsa's theory assumes that feathers evolved as part of the evolution of flight, and recent discoveries prove that assumption is false.


title: Origin of birds

text: Feathers are very common in coelurosaurian dinosaurs (including the early tyrannosauroid "Dilong"). Modern birds are classified as coelurosaurs by nearly all palaeontologists, though not by a few ornithologists. The modern version of the "from the ground up" hypothesis argues that birds' ancestors were small, feathered, ground-running predatory dinosaurs (rather like roadrunners in their hunting style) that used their forelimbs for balance while pursuing prey, and that the forelimbs and feathers later evolved in ways that provided gliding and then powered flight. The most widely suggested original functions of feathers include thermal insulation and competitive displays, as in modern birds.


title: Origin of birds

text: All of the "Archaeopteryx" fossils come from marine sediments, and it has been suggested that wings may have helped the birds run over water in the manner of the "Jesus Christ Lizard" (common basilisk).


title: Origin of birds

text: Most recent refutations of the "from the ground up" hypothesis attempt to refute the modern version's assumption that birds are modified coelurosaurian dinosaurs. The strongest attacks are based on embryological analyses that conclude that birds' wings are formed from digits 2, 3, and 4, (corresponding to the index, middle, and ring fingers in humans. The first of a bird's three digits forms the alula, which they use to avoid stalling in low-speed flight—for example, when landing). The hands of coelurosaurs, however, are formed by digits 1, 2, and 3 (thumb and first two fingers in humans). However, these embryological analyses were immediately challenged on the embryological grounds that the "hand" often develops differently in clades that have lost some digits in the course of their evolution, and that birds' "hands" do develop from digits 1, 2, and 3. This debate is complex and not yet resolved - see "Digit homology".


title: Origin of birds

text: The wing-assisted incline running (WAIR) hypothesis was prompted by observation of young chukar chicks, and proposes that wings developed their aerodynamic functions as a result of the need to run quickly up very steep slopes such as tree trunks, for example to escape from predators. This makes it a specialized type of cursorial ("from the ground up") theory. Note that in this scenario birds need "downforce" to give their feet increased grip. But early birds, including "Archaeopteryx", lacked the shoulder mechanism by which modern birds' wings produce swift, powerful upstrokes. Since the downforce WAIR depends on is generated by upstrokes, it seems that early birds were incapable of WAIR. Because WAIR is a behavioural trait without osteological specializations, the phylogenetic placement of the flight stroke before the divergence of the Neornithes, the group which contains all extant birds, makes it impossible to determine if WAIR is ancestral to the avian flight stroke or derived from it.


title: Origin of birds

text: Most versions of the arboreal hypothesis state that the ancestors of birds were very small dinosaurs that lived in trees, springing from branch to branch. This small dinosaur already had feathers, which were co-opted by evolution to produce longer, stiffer forms that were useful in aerodynamics, eventually producing wings. Wings would have then evolved and become increasingly refined as devices to give the leaper more control, to parachute, to glide, and to fly in stepwise fashion. The arboreal hypothesis also notes that, for arboreal animals, aerodynamics are far more energy efficient, since such animals simply fall to achieve minimum gliding speeds.


title: Origin of birds

text: Several small dinosaurs from the Jurassic or Early Cretaceous, all with feathers, have been interpreted as possibly having arboreal and/or aerodynamic adaptations. These include "Scansoriopteryx", "Epidexipteryx", "Microraptor", "Pedopenna", and "Anchiornis". "Anchiornis" is particularly important to this subject, as it lived at the beginning of the Late Jurassic, long before "Archaeopteryx".


title: Origin of birds

text: Analysis of the proportions of the toe bones of the most primitive birds "Archaeopteryx" and "Confuciusornis", compared to those of living species, suggest that the early species may have lived both on the ground and in trees.


title: Origin of birds

text: One study suggested that the earliest birds and their immediate ancestors did not climb trees. This study determined that the amount of toe claw curvature of early birds was more like that seen in modern ground-foraging birds than in perching birds.


title: Origin of birds

text: "Archaeopteryx" was the first and for a long time the only known feathered Mesozoic animal. As a result, discussion of the evolution of birds and of bird flight centered on "Archaeopteryx" at least until the mid-1990s.


title: Origin of birds

text: There has been debate about whether "Archaeopteryx" could really fly. It appears that "Archaeopteryx" had the brain structures and inner-ear balance sensors that birds use to control their flight. "Archaeopteryx" also had a wing feather arrangement like that of modern birds and similarly asymmetrical flight feathers on its wings and tail. But "Archaeopteryx" lacked the shoulder mechanism by which modern birds' wings produce swift, powerful upstrokes (see diagram above of supracoracoideus pulley); this may mean that it and other early birds were incapable of flapping flight and could only glide.


title: Origin of birds

text: But the discovery since the early 1990s of many feathered dinosaurs means that "Archaeopteryx" is no longer the key figure in the evolution of bird flight. Other small feathered coelurosaurs from the Cretaceous and Late Jurassic show possible precursors of avian flight. These include "Rahonavis", a ground-runner with a "Velociraptor"-like raised sickle claw on the second toe, that some paleontologists assume to have been better adapted for flight than "Archaeopteryx", "Scansoriopteryx", an arboreal dinosaur that may support the "from the trees down" theory, and "Microraptor", an arboreal dinosaur possibly capable of powered flight but, if so, more like a biplane, as it had well-developed feathers on its legs. As early as 1915, some scientists argued that the evolution of bird flight may have gone through a four-winged (or "tetrapteryx") stage.


title: Origin of birds

text: Hartman "et al". (2019) found that, because of how basal flying paravians are phylogenetically distributed, flight most likely evolved five times among paravians instead of only once. "Yi", "Archaeopteryx", "Rahonavis" and "Microraptor" were thus considered examples of convergent evolution instead of precursors of bird flight.


title: Origin of birds

text: A minority hypothesis, credited to the books "Predatory Dinosaurs of the World" (1988) and "Dinosaurs of the Air" (2002) by scientific illustrator Gregory Paul, suggests that some groups of non-flying carnivorous dinosaurs — especially deinonychosaurs, but perhaps others such as oviraptorosaurs, therizinosaurs, alvarezsaurids and ornithomimosaurs — actually descend from birds or other flighted maniraptorans. Paul also proposed that the ancestors of these groups were more advanced in their flight adaptations than "Archaeopteryx". The hypothesis would mean that "Archaeopteryx" is less closely related to extant birds than these dinosaurs are. In 2016, Paul suggested that omnivoropterygid avialans were closely related to oviraptorosaurs and that jeholornithid avialans were closely related to therizinosaurs; he considered them to not be avians but suggested that they shared a flighted ancestor.


title: Origin of birds

text: Mayr "et al." (2005) analyzed a new, tenth specimen of "Archaeopteryx", and concluded that "Archaeopteryx" was the sister clade to the Deinonychosauria, but that the more advanced bird "Confuciusornis" was within the Dromaeosauridae. This paper, however, excluded all other birds and thus did not sample their character distributions. The paper was criticized by Corfe and Butler (2006) who found the authors could not support their conclusions statistically. Mayr "et al." agreed that the statistical support for the authors' earlier paper was weak but stated that it is also weak for the alternative scenarios.


title: Origin of birds

text: Most subsequent cladistic analyses, an exception being that of Hartman and colleagues (2019), do not support Paul's hypothesis about the position of "Archaeopteryx". Instead, they indicate that "Archaeopteryx" is closer to birds, within the clade Avialae, than it is to deinonychosaurs or oviraptorosaurs. "Microraptor", "Pedopenna", and "Anchiornis" all have winged feet, share many features and lie close to the base of the clade Paraves. This suggests that the ancestral paravian may have been a four-winged glider. "Deinonychus" may also display partial volancy, with the young being capable of flight or gliding and the adults being flightless. In 2018, a study concluded that the last common ancestor of the Pennaraptora had joint surfaces on the fingers, and between the metatarsus and the wrist, that were optimised to stabilise the hand in flight. This was seen as an indication for secondary flightlessness in heavy basal members of that group.


title: Dioxins and dioxin-like compounds

text: Dioxins and dioxin-like compounds (DLCs) are a group of chemical compounds that are persistent organic pollutants (POPs) in the environment. They are mostly by-products of burning or various industrial processes - or, in case of dioxin-like PCBs and PBBs, unwanted minor components of intentionally produced mixtures.


title: Dioxins and dioxin-like compounds

text: Some of them are highly toxic, but the toxicity among them varies 30,000-fold. They are grouped together because their mechanism of action is the same. They activate the aryl hydrocarbon receptor (AH receptor), albeit with very different binding affinities, leading to high differences in toxicity and other effects. They include:


title: Dioxins and dioxin-like compounds

text: Dioxins have different toxicity depending on the number and position of the chlorine atoms. Because dioxins refer to such a broad class of compounds that vary widely in toxicity, the concept of toxic equivalency factor (TEF) has been developed to facilitate risk assessment and regulatory control. TEFs exist for seven congeners of dioxins, ten furans and twelve PCBs. The reference congener is the most toxic dioxin TCDD which per definition has a TEF of one. In essence, multiplying the amount of a particular congener with its TEF produces the amount toxicologically equivalent to TCDD, and after this conversion all dioxin-like congeners can be summed up, and the resulting toxicity equivalent quantity (TEQ) gives an approximation of toxicity of the mixture measured as TCDD.


title: Dioxins and dioxin-like compounds

text: Dioxins are virtually insoluble in water but have a relatively high solubility in lipids. Therefore, they tend to associate with organic matter such as plankton, plant leaves, and animal fat. In addition, they tend to be adsorbed to inorganic particles, such as ash and soil.


title: Dioxins and dioxin-like compounds

text: Dioxins are extremely stable and consequently tend to accumulate in the food chain. They are eliminated very slowly in animals, e.g. TCDD has a half-life of 7 to 9 years in humans. Incidents of contamination with PCBs are often reported as dioxin contamination incidents since these are of most public and regulatory concern.


title: Dioxins and dioxin-like compounds

text: Dioxins are a group of chemically related compounds that are persistent environmental pollutants. Dioxins are found throughout the world in the environment, and they accumulate in the food chain, mainly in the fatty tissue of animals. Dioxins are highly toxic and can cause reproductive and developmental problems, damage the nervous and immune systems, interfere with hormones, and also cause cancer. Owing to the highly toxic potential of dioxins, efforts need to be undertaken to reduce current background exposure. In these regards, prevention or reduction of human exposure and a strict control of industrial processes to reduce formation of dioxins are warranted.


title: Dioxins and dioxin-like compounds

text: There are 75 possible congeners of polychlorinated dibenzo"-p-"dioxins, but only 7 of them have affinity for the AH receptor and are toxic via this mechanism. The crucial structures are so called lateral chlorines in positions 2,3,7, and 8. These 4 chlorines also make the congeners persistent, because they prevent microbial degradation. Additional chlorines make the compounds less potent, but basically the effects remain the same although at higher doses. There are 135 possible dibenzofurans, and 10 in which the lateral chlorines are dioxin-like.


title: Dioxins and dioxin-like compounds

text: There are 209 PCB compounds. Analogously to PCDDs at least two lateral chlorines in each ring in positions 3,4, and/or 5 are needed for dioxin-like activity. Because the AH receptor requires a planar (flat) structure, only PCB congeners that can rotate freely along the C—C axis between the rings can attach the receptor. Substituents in ortho-positions 2 and 6 prevent rotation and thus hinder the molecule from assuming a planar position. Mono-ortho congeners (one Cl in 2, 2', 6, or 6') have minimal activity. No significant dioxin-like activities have been noticed, if there are two or more o-chlorines. Brominated dioxins and biphenyls have similar properties, but they have been studied much less.


title: Dioxins and dioxin-like compounds

text: Many natural compounds have very high affinity to AH receptors. These include indoles, flavones, benzoflavones, imidazoles and pyridines. These compounds are metabolized rapidly, but continuous intake from food may cause similar receptor activation as the background levels of dioxins. They do not reach concentrations causing typical dioxin-like toxicity, however.


title: Dioxins and dioxin-like compounds

text: The AH receptor is an ancient receptor, and its many functions have been revealed only recently. It is an over 600 million year old protein occurring in all vertebrates, and its homologs have been discovered in invertebrates and insects. It belongs to basic Helix–Loop–Helix-PAS proteins and acts as a transcription factor modifying transcription of a number of genes (see figure). AH receptor activity is necessary for normal development and many physiological functions. Mice lacking the AH receptor (knockouts) are sick with cardiac hypertrophy, liver fibrosis, reproductive problems, and impaired immunology.


title: Dioxins and dioxin-like compounds

text: The AH receptor is relevant in toxicology for two very different reasons. First, it induces several enzymes important in the metabolism of foreign substances, so called xenobiotics. These include both oxidative phase I enzymes and conjugative phase II enzymes, e.g. CYP1A2, CYP1B1, CYP2S1, CYP2A5, ALDH3, GSTA1, UGT1A1, UGT1A6, UGT1A7 and NQO1. This is in essence a protective function preventing toxic or carcinogenic effects of xenobiotics, but in some conditions it may also result in the production of reactive metabolites that are mutagenic and carcinogenic. This enzyme induction can be initiated by many natural or synthetic compounds, e.g., carcinogenic polycyclic hydrocarbons such as benzo"(a)"pyrene, several natural compounds, and dioxins. Secondly, AH receptors are involved in the activation or silencing of genes that lead to the toxic effects of high doses of dioxins. Because TCDD at high doses can influence the transcription of perhaps hundreds of genes, the genes crucial for the multitude of toxic effects of dioxins are still not known very well.


title: Dioxins and dioxin-like compounds

text: Binding of dioxin-like compounds to the AH receptor has made it possible to measure total dioxin-like activity of a sample using CALUX (Chemical Activated LUciferase gene eXpression) bioassay. The results have been comparable to TEQ levels measured by much more expensive gas chromatography-high resolution mass spectrometry in environmental samples.


title: Dioxins and dioxin-like compounds

text: Dioxin toxicity is based on inappropriate activation of a physiologically important receptor, and therefore dose-response must be carefully considered. Inappropriate stimulation of many receptors leads to toxic outcomes, e.g. overdose of vitamin A leads to inappropriate activation of retinoid receptors resulting in e.g. malformations, and overdoses of corticosteroids or sex hormones lead to a multitude of adverse effects. Therefore it is important to separate the effects of low doses causing activation of the receptor around the physiological range from the effects of high toxic doses. This is all the more important because of large differences in exposures even among human beings. Western populations today are exposed to dioxins at doses leading to concentrations of 5 to 100 picograms/g (as TEQ in body fat), and the highest concentrations in accidental or deliberate poisonings have been 10,000 to 144,000 pg/g leading to dramatic but not lethal outcomes.


title: Dioxins and dioxin-like compounds

text: The most relevant toxic outcomes of dioxins both in humans and animals are cancer and the developmental effects of offspring. Both have been documented at high doses, most accurately in animal experiments. As to developmental effects there is an agreement that the present dioxin levels in many populations are not very far from those causing some effects, but there is not yet consensus on the safe level. As to cancer, there is a disagreement on how to extrapolate the risk from high toxic doses to the present low exposures.


title: Dioxins and dioxin-like compounds

text: While the affinity of dioxins and related industrial toxicants to the Ah receptor may not fully explain all their toxic effects including immunotoxicity, endocrine effects and tumor promotion, toxic responses appear to be typically dose-dependent within certain concentration ranges. A multiphasic dose–response relationship has also been reported, leading to uncertainty and debate about the true role of dioxins in cancer rates. The endocrine disrupting activity of dioxins is thought to occur as a down-stream function of AH receptor activation, with thyroid status in particular being a sensitive marker of exposure. TCDD, along with the other PCDDs, PCDFs and dioxin-like coplanar PCBs are not direct agonists or antagonists of hormones, and are not active in assays which directly screen for these activities such as ER-CALUX and AR-CALUX. These compounds have also not been shown to have any direct mutagenic or genotoxic activity. Their main action in causing cancer is cancer promotion. A mixture of PCBs such as Aroclor may contain PCB compounds which are known estrogen agonists but are not classified as dioxin-like in terms of toxicity. Mutagenic effects have been established for some lower chlorinated chemicals such as 3-chlorodibenzofuran, which is neither persistent nor an AH receptor agonist.


title: Dioxins and dioxin-like compounds

text: High doses. The symptoms reported to be associated with dioxin toxicity in animal studies are incredibly wide-ranging, both in the scope of the biological systems affected and in the range of dosage needed to bring these about. Acute effects of single high dose dioxin exposure include reduced feed intake and wasting syndrome, and typically a delayed death of the animal in 1 to 6 weeks. By far most toxicity studies have been performed using 2,3,7,8-tetrachlorodibenzo-"p"-dioxin.


title: Dioxins and dioxin-like compounds

text: The of TCDD varies wildly between species and even strains of the same species, with the most notable disparity being between the seemingly similar species of hamster and guinea pig. The oral for guinea pigs is as low as 0.5 to 2 μg/kg body weight, whereas the oral for hamsters can be as high as 1 to 5 mg/kg body weight. Even between different mouse or rat strains there may be tenfold to thousandfold differences in acute toxicity. Many pathological findings are seen in the liver, thymus, and other organs. Some effects such as thymic atrophy are common in many species, but e.g. liver toxicity is typical in rabbits.


title: Dioxins and dioxin-like compounds

text: Low doses. Very few signs of toxicity are seen in adult animals after low doses, but developmental effects may occur at low dioxin levels, including foetal, neonatal, and possibly pubescent stages. Well established developmental effects are cleft palate, hydronephrosis, disturbances in tooth development and sexual development, and endocrine effects. Surprisingly, enzyme induction, several developmental effects and aversion to novel foods occur at similar dose levels in animals that respond differently to acute high-dose toxicity. Therefore it has been suggested that dioxin effects be divided to type I effects (enzyme induction etc.) and type II effects (lethality, liver damage, anorexia, and tumour promotion). The reason may be different requirements of the transactivation domain structure of the AH receptor for different genes. Some of these low-dose effects can in fact be interpreted as protective rather than toxic (enzyme induction, aversion to novel foods).


title: Dioxins and dioxin-like compounds

text: High doses. Toxicity of dioxins at high doses has been well documented after accidents, deliberate poisonings, food contamination episodes, and high industrial exposures. Three women in Vienna, Austria, were poisoned with large doses of TCDD in 1998. The highest concentration of TCDD in fat tissue was 144,000 pg/g, the highest ever reported in human beings. The main feature was chloracne, a serious skin disease. The victim survived, and other symptoms were modest after initial gastrointestinal symptoms and amenorrhea. Another acute incident was the deliberate poisoning of Victor Yushchenko, then presidential candidate of Ukraine, in 2004. TCDD concentration in fat was 108,000 pg/g. Also in this case the most prominent symptom was chloracne after initial stomach pain indicating hepatitis and pancreatitis. These episodes show that a human being is not as sensitive as the most sensitive animals, since the doses must have been up to 25 μg/kg.


title: Dioxins and dioxin-like compounds

text: Two serious food contamination accidents were caused by PCB oils used in heat exchangers. The PCB oil leaked to rice bran oil consumed by thousands of people in Japan (Yusho disease 1968) and Taiwan (Yu-cheng disease 1979). The toxic effects have been attributed to dioxin-like PCBs and PCDFs. Their daily intake was up to 100,000 times higher than average intake presently. There were many skin problems, chloracne, swelling of eyelids, and hypersecretion of Meibomian glands in the eyes. Babies born to Yusho and Yu-cheng mothers were smaller than normal, they had dark pigmentation and sometimes teeth at birth and tooth deformities. Foetal deaths and miscarriages were common.


title: Dioxins and dioxin-like compounds

text: Perhaps the best known dioxin accident occurred in Seveso, Italy, in 1976. A tank of chlorophenols released its contents to air including many kilograms of TCDD, and contaminated much of the city. The highest TCDD levels were found in children, up to 56,000 pg/g fat. Acute effects were limited to chloracne, although many animals such as rabbits died after eating contaminated grass. Dental aberrations were found after 25 years in persons exposed as children, and a slightly increased cancer risk was confirmed 35 years later.


title: Dioxins and dioxin-like compounds

text: In line with animal studies, developmental effects may be much more important than effects in adults. These include disturbances of tooth development, and of sexual development.


title: Dioxins and dioxin-like compounds

text: An example of the variation in responses is clearly seen in a study following the Seveso disaster indicating that sperm count and motility were affected in different ways in exposed males, depending on whether they were exposed before, during or after puberty.


title: Dioxins and dioxin-like compounds

text: In occupational settings many symptoms have been seen, but exposures have always been to a multitude of chemicals including chlorophenols, chlorophenoxy acid herbicides, and solvents. Therefore, definitive proof of dioxins as causative factors has been difficult to obtain. By far the best proven effect is chloracne. The suspected effects in adults are liver damage, and alterations in heme metabolism, serum lipid levels, thyroid functions, as well as diabetes and immunological effects.


title: Dioxins and dioxin-like compounds

text: Low exposures. Effects after low exposures such as from food have been difficult to prove. Levels of dioxins in contemporary population are 5 to 20 pg/g (TEQ in fat) and 50 to 100 pg in older people or at least 1000 times lower than those in poisonings (see above). Tooth deformities have been considered plausible after long breast-feeding, when the dioxin concentrations were high in 1970s and 1980s. When the concentrations decreased during 1990s and 2000s, the effects were no longer seen. According to a study in Russia, sperm counts in 18-19 year old young men were lower when dioxin levels were higher at the age of 8 to 9 years. This was in industrial environments causing relatively high exposures to boys as well as their mothers. The contamination panel of the European Food Safety Agency (EFSA) recommended decreasing tolerable weekly intake (TWI) levels based on the Russian children study. This recommendation can be challenged, because it does not properly consider competing risks following from lost benefits of important and healthy food items such as certain fish. TWI levels are not applied for breast feeding, because benefits of breast milk are judged to be far more important than the remote risks of dioxins. A general conclusion may be that safety margins are not very great concerning developmental effects, but toxic effects are not likely at the present population levels of dioxins.


title: Dioxins and dioxin-like compounds

text: A number of cross-sectional studies have shown associations between type 2 diabetes and several POP compounds including dioxins. Such observational studies cannot prove causality, i.e. there may be an association which does not prove that one is the cause of the other. The main problem is that similar associations can be found with many quite different POPs, which have only long half-lives and tendency to accumulate in lipids in common. This suggests that they may all be related to diet and obesity which are by far the most common causes of type 2 diabetes.


title: Dioxins and dioxin-like compounds

text: Over the years there have been speculations on various effects of dioxins on endometriosis, sexual development, liver function, thyroid hormone levels, white blood cell levels, immune functions, and even learning and intelligence. While some of these effects might be possible after heavy exposures (like in the Seveso disaster), these claims are only based on potential exposures of population, not supported by actual measurements of dioxin concentrations. E.g. absorption from bleached tampons claimed to be associated with endometriosis is insignificant compared with daily dioxin intake from food.


title: Dioxins and dioxin-like compounds

text: Dioxins are well established carcinogens in animal studies, although the precise mechanism is not clear. Dioxins are not mutagenic or genotoxic. The United States Environmental Protection Agency has categorised dioxin, and the mixture of substances associated with sources of dioxin toxicity as a "likely human carcinogen". The International Agency for Research on Cancer has classified TCDD as a human carcinogen (class 1) on the basis of clear animal carcinogenicity and limited human data, and subsequently also 2,3,4,7,8-PCDF and PCB 126 as class 1 carcinogens. The mechanism is thought to be mainly promotion, i.e. dioxins can accelerate the formation of tumours caused by other factors, and adversely affect the normal mechanisms for inhibiting tumour growth. Some researchers have also proposed that dioxin induces cancer progression through a very different mitochondrial pathway.


title: Dioxins and dioxin-like compounds

text: As with many toxic endpoints of dioxin, a clear dose–response relationship is difficult to establish. After accidental or high occupational exposures there is evidence on human carcinogenicity. Increases in cancer have been modest, in fact reaching statistical significance has been difficult even after high accidental or occupational exposures like in Yusho and Yucheng poisonings, Seveso accident, and combined occupational cohorts. Therefore controversies on cancer risk at low population levels of dioxins are understandable. The problem with IARC evaluations is that they only assess hazard, i.e. carcinogenicity at any dose. It is likely that there is a practical safe threshold for the non-genotoxic dioxins, and the present population levels do not possess any risk of cancer. There is thus some agreement on that cancer risk is taken care of as well, if daily intake limits are set to protect from developmental effects. Among fishermen with high dioxin concentrations in their bodies, cancer deaths were decreased rather than increased. All this means that in case of important beneficial food items and breast feeding a thorough benefit/risk analysis is needed before setting limits, in order to avoid increased other risks or lost benefits.


title: Dioxins and dioxin-like compounds

text: The uncertainty and variability in the dose–response relationship of dioxins in terms of their toxicity, as well as the ability of dioxins to bioaccumulate, have led WHO experts to recommending very low tolerable daily intake (TDI) of dioxin, 1-4 pg/kg body weight per day, i.e. 7x10 to 2.8x10g per 70-kg person per day, to allow for this uncertainty and ensure public safety in all instances. Authorities have then set weekly or monthly intake levels that equal to TDIs around 2 pg/kg. Because dioxins are eliminated very slowly, the body burden accumulated during the whole lifetime is high compared with daily doses, and occasional modest exceedances of limit values do not change it much. Therefore long-term intake is much more important than daily intake. Specifically, the TDI has been assessed to guarantee the safety of children born to mothers exposed to such a daily intake of dioxins all their lifetime prior to pregnancy. It is likely that the TDI for other population groups could be higher.


title: Dioxins and dioxin-like compounds

text: One important cause for differences in different assessments has been carcinogenicity. If the dose-response of TCDD in causing cancer is linear, it might be a true risk. If the dose-response is of a threshold-type or J-shape, there is little or no risk at the present concentrations. Understanding the mechanisms of toxicity better is hoped to increase the reliability of risk assessment. Recently also developmental effects have been reassessed by the Contamination Panel of the European Food Safety Agency (EFSA). They propose decreasing the tolerable weekly intake (TWI) from 14 pg/kg to 2 pg/kg. This is likely to cause another controversy before being accepted by European countries. Dioxin intake and levels in breast milk in 1970s and 1980s were 5 to 10 times higher than presently, and very few effects have been found, possibly mild developmental effects on teeth.


title: Dioxins and dioxin-like compounds

text: Greenpeace and some other environmental groups have called for the chlorine industry to be phased out. However, chlorine industry supporters say that "banning chlorine would mean that millions of people in the third world would die from want of disinfected water". Sharon Beder and others have argued that the dioxin controversy has been very political and that large companies have tried to play down the seriousness of the problems of dioxin. The companies involved have often said that the campaign against dioxin is based on "fear and emotion" and not on science.


title: Dioxins and dioxin-like compounds

text: Most intake of dioxin-like chemicals is from food of animal origin: meat, dairy products, or fish predominate, depending on the country. The daily intake of dioxins and dioxin-like PCBs as TEQ is of the order of 100 pg/day, i.e. 1-2 pg/kg/day. In many countries both the absolute and relative significance of dairy products and meat have decreased due to strict emission controls, and brought about the decrease of total intake. E.g. in the United Kingdom the total intake of PCDD/F in 1982 was 239 pg/day and in 2001 only 21 pg/day (WHO-TEQ). Since the half-lives are very long (for e.g. TCDD 7–8 years), the body burden will increase almost over the whole lifetime. Therefore, the concentrations may increase five to tenfold from age 20 to age 60. For the same reason, short term higher intake such as after food contamination incidents, is not crucial unless it is extremely high or lasts for several months or years.


title: Dioxins and dioxin-like compounds

text: The highest body burdens were found in Western Europe in the 1970s and early 1980s, and the trends have been similar in the U.S. The most useful measure of time trends is concentration in breast milk measured over decades. In many countries the concentrations have decreased to about one tenth of those in the 1970s, and the total TEQ concentrations are now of the order of 5-30 pg/g fat (please note the units, pg/g is the same as ng/kg, or the non-standard expression ppt used sometimes in the United States). The decrease is due to strict emission controls and also to the control of concentrations in food. In the U.S. young adult female population (age group 20-39), the concentration was 9.7 pg/g lipid in 2001-2002 (geometric mean).


title: Dioxins and dioxin-like compounds

text: Certain professions such as subsistence fishermen in some areas are exposed to exceptionally high amounts of dioxins and related substances. This along with high industrial exposures may be the most valuable source of information on the health risks of dioxins.


title: Dioxins and dioxin-like compounds

text: Dioxins are absorbed well from the digestive tract if they are dissolved in fats or oils (e.g. in fish or meat). On the other hand, dioxins tend to adsorb tightly to soil particles, and absorption may be quite low: 13.8% of the given dose of TEQs in contaminated soil was absorbed.


title: Dioxins and dioxin-like compounds

text: The same features causing persistence of dioxins in the environment also cause very slow elimination in humans and animals. Because of low water solubility, kidneys cannot excrete them in urine as such. They must first be metabolised to more-water-soluble metabolites, but that metabolism, especially in humans, is extremely slow. This results in biological half-lives of several years for all dioxins. That of TCDD is estimated to be 7 to 8 years, and for other PCDD/Fs from 1.4 to 13 years, PCDFs on average slightly shorter than PCDDs.


title: Dioxins and dioxin-like compounds

text: In mammals, dioxins are found mostly in fat. Concentrations in fat seem to be relatively similar, be it serum fat, adipose tissue fat, or milk fat. This permits measuring dioxin burden by analysing breast milk. Initially, however, at least in laboratory animals, after a single dose, high concentrations are found in the liver, but in a few days, adipose tissue will predominate. In rat liver, however, high doses cause induction of CYP1A2 enzyme, and this binds dioxins. Thus, depending on the dose, the ratio of fat and liver tissue concentrations may vary considerably in rodents.


title: Dioxins and dioxin-like compounds

text: Dioxins have no common uses. They are manufactured on a small scale for chemical and toxicological research, but mostly exist as by-products of industrial processes such as chlorine bleaching of paper pulp, pesticide manufacture, and combustion processes such as incineration. The defoliant Agent Orange contained dioxins. The wood preservative pentachlorophenol contained dioxins and dibenzofurans. The Stockholm Convention banned the production and use of dioxins in 2001.


title: Dioxins and dioxin-like compounds

text: PCDD/F-compounds were never synthesized for any purpose, except for small quantities for scientific research. Small amounts of PCDD/Fs are formed whenever organics, oxygen and chlorine are available at suitable temperatures. This is augmented by metal catalysts such as copper. The optimal temperature range is to . This means that formation is highest when organic material is burned in less-than-optimal conditions such as open fires, building fires, domestic fireplaces, and poorly operated and/or designed solid waste incinerators. Historically, municipal and medical waste incineration was the most important source of PCDD/Fs.


title: Dioxins and dioxin-like compounds

text: PCB-compounds, always containing low concentrations of dioxin-like PCBs and PCDFs, were synthesized for various technical purposes (see Polychlorinated biphenyls). They have entered the environment through accidents such as fires or leaks from transformers or heat exchangers, or from PCB-containing products in landfills or during incineration. Because PCBs are somewhat volatile, they have also been transported long distances by air leading to global distribution including the Arctic. Only a minor portion of PCBs in mixtures are dioxin-like.


title: Dioxins and dioxin-like compounds

text: Improvements and changes have been made to nearly all industrial sources to reduce PCDD/F production. In waste incineration, large amounts of publicity and concern surrounded dioxin-like compounds during the 1980s-1990s continues to pervade the public consciousness, especially when new incineration and waste-to-energy facilities are proposed. As a result of these concerns, incineration processes have been improved with increased combustion temperatures (over ), better furnace control, and sufficient residence time allotted to ensure complete oxidation of organic compounds. Ideally, an incineration process oxidizes all carbon to CO and converts all chlorine to HCl or inorganic chlorides prior to the gases passing through the temperature window of 400-700 °C where PCDD/F formation is possible. These substances cannot easily form organic compounds, and HCl is easily and safely neutralized in the scrubber while CO is vented to the atmosphere. Inorganic chlorides are incorporated into the ash.


title: Dioxins and dioxin-like compounds

text: Scrubber and particulate removal systems manage to capture some of the PCDD/F which forms even in sophisticated incineration plants. These PCDD/Fs are generally not destroyed but moved into the fly ash. Catalytic systems have been designed which destroy vapor-phase PCDD/Fs at relatively low temperatures. This technology is often combined with the baghouse or SCR system at the tail end of an incineration plant.


title: Dioxins and dioxin-like compounds

text: European Union limits for concentration of dioxin-like compounds in the discharged flue gas is 0.1 ng/Nm³ TEQ.


title: Dioxins and dioxin-like compounds

text: Both in Europe and in U.S.A., the emissions have decreased dramatically since the 1980s, by even 90% (see Figure). This has also led to decreases in human body burdens, which is neatly demonstrated by the decrease of dioxin concentrations in breast milk. With the substantial decrease of emissions from municipal waste incinerators, other potentially large sources of dioxin-like compounds, for example from forest and wild fires, have increased relative to industrial sources. They are however not included in the total inventory due to uncertainties in available data. A more recent study on the environmental effects of accidental fires, including forest fires, estimated the emissions from dioxins (PCDD/Fs) to be about equivalent to those from traffic and municipal waste combustion.


title: Dioxins and dioxin-like compounds

text: Open burning of waste (backyard barrel burning) has not decreased effectively, and in the U.S. it is now the most important source of dioxins. Total U.S. annual emissions decreased from in 1987 to in 2000. However, backyard barrel burning decreased only modestly from to , resulting in over one third of all dioxins in the year 2000 from backyard burning alone.


title: Dioxins and dioxin-like compounds

text: Low concentrations of dioxins have been found in some soils without any anthropogenic contamination. A puzzling case of milk contamination was detected in Germany. The source was found to be kaolin added to animal feed. Dioxins have been repeatedly detected in clays from Europe and USA since 1996, with contamination of clay assumed to be the result of ancient forest fires or similar natural events with concentration of the PCDD/F during clay sedimentation.


title: Dioxins and dioxin-like compounds

text: In sugarcane cultivation, the remaining bagasse after extraction of sugar is used in large amounts for energy production (both process heat and electric energy for use both in the sugar factory itself and for other consumers), and locally it has been thought to be a remarkable source of dioxins. This basically indicates that burning biomass produces dioxins and it should be done at high enough temperatures and there should be proper filtering of flue gases. For the treatment of gases and pollutants, sugarcane industries often use wet gas scrubbers, such as the Venturi type. In addition, other treatment systems also used are electrostatic precipitators and bag filters. These methods may be insufficient Improperly burned biomass, which undergoes only low temperature incomplete combustion, is responsible for much of the negative health effects of indoor air pollution and is a particular problem in the Global South where biomass like wood or cowdung are often the only commonly available fuels for cooking and home heating. In addition to dioxins, other harmful products of incomplete combustion - such as carbon monoxide - are also released when biomass is burned in low oxygen conditions. Using plastic, particularly chlorine-containing plastics such as polyvinyl chloride, as a fuel or firestarter further increases dioxin emissions.


title: Dioxins and dioxin-like compounds

text: All groups of dioxin-like compounds are persistent in the environment. Very few soil microbes nor animals can break down the PCDD/Fs with lateral chlorines (positions 2,3,7, and 8). This causes very slow elimination. However scientists at Martin Luther University recently found that a type of bacteria "Dehalococcoides" CBDB1 can extract the chlorine from dioxin compounds in the absence of oxygen. Ultraviolet light slowly breaks down these compounds. Lipophilicity (tendency to seek for fat-like environments) and very poor water solubility make these compounds move from water environment to living organisms having lipid cell structures. This is called bioaccumulation. Increase in chlorination increases both stability and lipophilicity. The compounds with the very highest chlorine numbers (e.g. octachlorodibenzo-p-dioxin) are, however, so poorly soluble that this hinders their bioaccumulation. Bioaccumulation is followed by biomagnification. Lipid-soluble compounds are first accumulated to microscopic organisms such as phytoplankton (plankton of plant character, e.g. algae). Phytoplankton is consumed by animal plankton, this by invertebrates such as insects, these by small fish, and further by large fish and seals. At every stage or trophic level, the concentration is higher, because the persistent chemicals are not "burned off" when the higher organism uses the fat of the prey organism to produce energy.


title: Dioxins and dioxin-like compounds

text: Due to bioaccumulation and biomagnification, the species at the top of the trophic pyramid are most vulnerable to dioxin-like compounds. In Europe, the white-tailed eagle and some species of seals have approached extinction due to poisoning by persistent organic pollutants. Likewise, in America, the population of bald eagles declined because of POPs causing thinning of eggshells and other reproductive problems. Usually, the failure has been attributed mostly to DDT, but dioxins are also a possible cause of reproductive effects. Both in America and in Europe, many waterfowl have high concentrations of dioxins, but usually not high enough to disturb their reproductive success. Due to supplementary winter feeding and other measures also, the white-tailed eagle is recovering (see White-tailed eagle). Also, ringed seals in the Baltic Sea are recovering.


title: Dioxins and dioxin-like compounds

text: Humans are also at the top of the trophic pyramid, particularly newborns. Exclusively breastfed newborns were estimated to be exposed to a total of 800 pg TEQ/day, leading to an estimated body weight-based dose of 242 pg TEQ/kg/day. Due to a multitude of food sources of adult humans exposure is much less averaging at 1 pg TEQ/kg-day, and dioxin concentrations in adults are much less at 10-100 pg/g, compared with 9000 to 340,000 pg/g (TEQ in lipid) in eagles or seals feeding almost exclusively on fish.


title: Dioxins and dioxin-like compounds

text: Because of different physicochemical properties, not all congeners of dioxin-like compounds find their routes to human beings equally well. Measured as TEQs, the dominant congeners in human tissues are 2,3,7,8-TCDD, 1,2,3,7,8-PeCDD, 1,2,3,6,7,8-HxCDD and 2,3,4,7,8-PeCDF. This is very different from most sources where hepta- and octa-congeners may predominate. The WHO panel re-evaluating the TEF values in 2005 expressed their concern that emissions should not be uncritically measured as TEQs, because all congeners are not equally important. They stated that "when a human risk assessment is to be done from abiotic matrices, factors such as fate, transport, and bioavailability from each matrix be specifically considered".


title: Dioxins and dioxin-like compounds

text: All POPs are poorly water-soluble, especially dioxins. Therefore, ground water contamination has not been a problem, even in cases of severe contamination due to the main chemicals such as chlorophenols. In surface waters, dioxins are bound to organic and inorganic particles.


title: Dioxins and dioxin-like compounds

text: The most important source of human exposure is fatty food of animal origin (see Human intake, above), and breast milk. There is much variation between different countries as to the most important items. In U.S. and Central Europe, milk, dairy products and meat have been by far the most important sources. In some countries, notably in Finland and to some extent in Sweden, fish is important due to contaminated Baltic fish and very low intake from any other sources. In most countries, a significant decrease of dioxin intake has occurred due to stricter controls during the last 20 years.


title: Dioxins and dioxin-like compounds

text: Historically, occupational exposure to dioxins has been a major problem. Dioxins are formed as important toxic side products in the production of PCBs, chlorophenols, chlorophenoxy acid herbicides, and other chlorinated organic chemicals. This caused very high exposures to workers in poorly controlled hygienic conditions. Many workers had chloracne. In a NIOSH study in the U.S., the average concentration of TCDD in exposed persons was 233 ng/kg (in serum lipid) while it was 7 ng/kg in unexposed workers, even though the exposure had been 15–37 years earlier. This indicates a huge previous exposure. In fact the exact back-calculation is debated, and the concentrations may have been even several times higher than originally estimated.


title: Dioxins and dioxin-like compounds

text: Handling and spraying of chlorophenoxy acid herbicides may also cause quite high exposures, as clearly demonstrated by the users of Agent Orange in the Malayan Emergency and in the Vietnam War. The highest concentrations were detected in nonflying enlisted personnel (e.g. filling the tanks of planes), although the variation was huge, 0 to 618 ng/kg TCDD (mean 23.6 ng/kg). Other occupational exposures (working at paper and pulp mills, steel mills and incinerators) have been remarkably lower.


title: Dioxins and dioxin-like compounds

text: Accidental exposures have been huge in some cases. The highest concentrations in people after the Seveso accident were 56,000 ng/kg, and the highest exposure ever recorded was found in Austria in 1998, 144,000 ng/kg (see TCDD). This is equivalent to a dose of 20 to 30 μg/kg TCDD, a dose that would be lethal to guinea pigs and some rat strains.


title: Dioxins and dioxin-like compounds

text: Exposure from contaminated soil is possible when dioxins are blown up in dust, or children eat dirt. Inhalation was clearly demonstrated in Missouri in the 1970s, when waste oils were used as dust suppressant in horse arenas. Many horses and other animals were killed due to poisoning. Dioxins are neither volatile nor water-soluble, and therefore exposure of human beings depends on direct eating of soil or production of dust which carries the chemical. Contamination of ground water or breathing vapour of the chemical are not likely to cause a significant exposure. Currently, in the US, there are 126 Superfund sites with a completed exposure pathway contaminated with dioxins.


title: Dioxins and dioxin-like compounds

text: Further, PCBs are known to pass through treatment plants and accumulate in sludge which is used on farm fields in certain countries. In 2011 in South Carolina, SCDHEC enacted emergency sludge regulations after PCBs were found to have been discharged to a waste treatment plant.


title: Dioxins and dioxin-like compounds

text: PCBs are also known to flush from industry and land (aka sludge fields) to contaminate fish, as they have up and down the Catawba River in North and South Carolina. State authorities have posted fish consumption advisories due to accumulation of PCBs in fish tissue.


title: Dioxins and dioxin-like compounds

text: There have been several food contamination episodes, one of the best known occurred in Belgium in 1999. A tank of recycled fats collected for animal feed production was contaminated by PCB oil containing about 1 g of dioxins and 2 g of DL-PCBs. This caused a major alarm in the European Union, but due to relatively fast response and slow accumulation of dioxins in humans there were no health impacts. There was a similar incidence in Ireland in 2008. In 2008, Chile experienced a pork crisis caused by high dioxin concentrations in their pork exports. The contamination was found to be due to zinc oxide used in pork feed, and caused reputational and financial losses for the country, as well as leading to the introduction of new food safety regulations. These episodes emphasize the importance of food control, and early detection guarantees that very slowly accumulating dioxins do not increase in humans to levels causing toxic effects.


title: Dioxins and dioxin-like compounds

text: All dioxin-like compounds share a common mechanism of action via the aryl hydrocarbon receptor (AHR), but their potencies are very different. This means that similar effects are caused by all of them, but much larger doses of some of them are needed than of TCDD. Binding to the AHR as well as persistence in the environment and in the organism depends on the presence of so-called "lateral chlorines", in case of dioxins and furans, chlorine substitutes in positions 2,3,7, and 8. Each additional non-lateral chlorine decreases the potency, but qualitatively the effects remain similar. Therefore, a simple sum of different dioxin congeners is not a meaningful measure of toxicity. To compare the toxicities of various congeners and to render it possible to make a toxicologically meaningful sum of a mixture, a toxicity equivalency (TEQ) concept was created.


title: Dioxins and dioxin-like compounds

text: Each congener has been given a toxicity equivalence factor (TEF). This indicates its relative toxicity as compared with TCDD. Most TEFs have been extracted from "in vivo" toxicity data on animals, but if these are missing (e.g. in case of some PCBs), less reliable "in vitro" data have been used. After multiplying the actual amount or concentration of a congener by its TEF, the product is the virtual amount or concentration of TCDD having effects of the same magnitude as the compound in question. This multiplication is done for all compounds in a mixture, and these "equivalents of TCDD" can then simply be added, resulting in TEQ, the amount or concentration of TCDD toxicologically equivalent to the mixture.


title: Dioxins and dioxin-like compounds

text: The TEQ conversion makes it possible to use all studies on the best studied TCDD to assess the toxicity of a mixture. This is most useful in regulatory work, but it can also be used in scientific studies. This resembles the common measure of all alcoholic drinks: beer, wine and whiskey can be added together as absolute alcohol, and this sum gives the toxicologically meaningful measure of the total impact.


title: Dioxins and dioxin-like compounds

text: The TEQ only applies to dioxin-like effects mediated by the AHR. Some toxic effects (especially of PCBs) may be independent of the AHR, and those are not taken into account by using TEQs.


title: Dioxins and dioxin-like compounds

text: TEFs are also approximations with certain amount of scientific judgement rather than scientific facts. Therefore, they may be re-evaluated from time to time. There have been several TEF versions since the 1980s. The most recent re-assessment was by an expert group of the World Health organization in 2005.


title: Malacostraca

text: Malacostraca (from New Latin; ) is the largest of the six classes of crustaceans, containing about 40,000 living species, divided among 16 orders. Its members, the malacostracans, display a great diversity of body forms and include crabs, lobsters, crayfish, shrimp, krill, prawns, woodlice, amphipods, mantis shrimp, tongue-eating lice and many other less familiar animals. They are abundant in all marine environments and have colonised freshwater and terrestrial habitats. They are segmented animals, united by a common body plan comprising 20 body segments (rarely 21), and divided into a head, thorax, and abdomen.


title: Malacostraca

text: The name Malacostraca was coined by a French zoologist Pierre André Latreille in 1802. He was curator of the arthropod collection at the National Museum of Natural History in Paris. The name comes from the Greek roots (', meaning "soft") and (', meaning "shell"). The name is misleading, since the shell is soft only immediately after moulting, and is usually hard. The word was used by Aristotle, who contrasted them with oysters, in comparison with which their shells are pliable. Malacostracans are sometimes contrasted with entomostracans, a name applied to all crustaceans outside the Malacostraca, and named after the obsolete taxon Entomostraca.


title: Malacostraca

text: The class Malacostraca includes about 40,000 species, and "arguably ... contains a greater diversity of body forms than any other class in the animal kingdom". Its members are characterised by the presence of three tagmata (specialized groupings of multiple segments) – a five-segmented head, an eight-segmented thorax and an abdomen with six segments and a telson, except in the Leptostraca, which retain the ancestral condition of seven abdominal segments. Malacostracans have abdominal appendages, a fact that differentiates them from all other major crustacean taxa except Remipedia. Each body segment bears a pair of jointed appendages, although these may be lost secondarily.


title: Malacostraca

text: The head bears two pairs of antennae, the first of which is often biramous (branching into two parts) and the second pair bear exopods (outer branches) which are often flattened into antennal scales known as scaphocerites. The mouthparts consist of pairs each of mandibles, maxillules (second pair of mouthparts) and maxillae. Usually a pair of stalked compound eyes is present, although in some taxa the eyes are unstalked, reduced or lost.


title: Malacostraca

text: Up to three thoracic segments may be fused with the head to form a cephalothorax; the associated appendages turn forward and are modified as maxillipeds (accessory mouthparts). A carapace may be absent, present or secondarily lost, and may cover the head, part or all of the thorax and some of the abdomen. It is variable in form and may be fused dorsally with some of the thoracic segments or occasionally be in two parts, hinged dorsally. Typically, each of the thoracic appendages is biramous and the endopods are the better developed of the branches, being used for crawling or grasping. Each endopod consist of seven articulating segments; the coxa, basis, ischium, merus, carpus, propodus and dactylus. In decapods, the claw is formed by the articulation of the dactylus against an outgrowth of the propodus. In some taxa, the exopods are lost and the appendages are uniramous.


title: Malacostraca

text: There is a clear demarcation between the thorax and the six or seven-segmented abdomen. In most taxa, each abdominal segment except the last carries a pair of biramous pleopods used for swimming, burrowing, gas exchange, creating a current or brooding eggs. The first and second abdominal pleopods may be modified in the male to form gonopods (accessory copulatory appendages). The appendages of the last segment are typically flattened into uropods, which together with the terminal telson, make up the "tail fan". It is the sudden flexion of this tail fan that provides the thrust for the rapid escape response of these crustaceans and the tail fan is also used in steering. In Leptostraca, the appendages on the telson instead form caudal rami (spine-like protrusions).


title: Malacostraca

text: The digestive tract is straight and the foregut consists of a short oesophagus and a two-chambered stomach, the first part of which contains a gizzard-like "gastric mill" for grinding food. The walls of this have chitinous ridges, teeth and calcareous ossicles. The fine particles and soluble material are then moved into the midgut where chemical processing and absorption takes place in one or more pairs of large digestive caeca. The hindgut is concerned with water reclamation and the formation of faeces and the anus is situated at the base of the telson.


title: Malacostraca

text: Like other crustaceans, malacostracans have an open circulatory system in which the heart pumps blood into the hemocoel (body cavity) where it supplies the needs of the organs for oxygen and nutrients before diffusing back to the heart. The typical respiratory pigment in malacostracans is haemocyanin. Structures that function as kidneys are located near the base of the antennae. A brain exists in the form of ganglia close to the antennae, there are ganglia in each segment and a collection of major ganglia below the oesophagus. Sensory organs include compound eyes (often stalked), ocelli (simple eyes), statocysts and sensory bristles. The naupliar eye is a characteristic of the nauplius larva and consists of four cup-shaped ocelli facing in different directions and able to distinguish between light and darkness.


title: Malacostraca

text: Malacostracans live in a wide range of marine and freshwater habitats, and three orders have terrestrial members: Amphipoda (Talitridae), Isopoda (Oniscidea, the woodlice) and Decapoda (terrestrial hermit crabs, crabs of the families Ocypodidae, Gecarcinidae, and Grapsidae, and terrestrial crayfish). They are abundant in all marine ecosystems, and most species are scavengers, although some, such as the porcelain crabs, are filter feeders, and some, such as mantis shrimps, are carnivores.


title: Malacostraca

text: Most species of malacostracans have distinct sexes (a phenomenon known as gonochorism), although a few species exhibit hermaphroditism. The female genital openings or gonopores are located on the sixth thoracic segment or its appendages, while the male gonopores are on the eighth segment or its appendages, or in a small number of species, on the seventh. The naupliar larval stages are often reduced and take place before hatching, but where they occur, a metamorphosis usually occurs between the larval and the adult forms. Primitive malacostracans have a free-swimming naupliar larval stage. Research suggests the common ancestor of Malacostraca had lost the nauplius larval stage, but re-evolved it again in Dendrobranchiata and Euphausiacea through heterochrony.


title: Malacostraca

text: Mating behavior has been studied in the freshwater shrimp "Caridina ensifera". Multiple paternity, common in the "Malacostrica", also occurs in "C. ensifera". Reproductive success of sires was found to correlate inversely with their genetic relatedness to the mother. This finding suggests that sperm competition and/or pre- and post-copulatory female choice occurs. Female choice may increase the fitness of progeny by avoiding inbreeding that can lead to expression of homozygous deleterious recessive mutations.


title: Malacostraca

text: The monophyly of Malacostraca is widely accepted. This is supported by several common morphological traits which are present throughout the group and is confirmed by molecular studies. However, a number of problems make it difficult to determine the relationships between the orders of Malacostraca. These include differences in mutation rates in different lineages, different patterns of evolution being apparent in different sources of data, including convergent evolution, and long branch attraction.


title: Malacostraca

text: There is less agreement on the status of the subclass Phyllocarida with its single extant order, Leptostraca, depending on whether foliaceous (leaf-like) limbs have a single or multiple origin. Some authors advocate placing Phyllocarida in Phyllopoda, a group used in former classification systems, which would then include branchiopods, cephalocarids and leptostracans. A molecular study by American biologists Trisha Spears and Lawrence Abele concluded that phylogenetic evidence did not support the monophyly of this grouping, and that Phyllocarida should be regarded as a subclass of Malacostraca that had diverged from the main lineage at an early date.


title: Malacostraca

text: Leptostraca is the only extant order of Phyllocarida, the other two orders, Archaeostraca and Hoplostraca being extinct. Leptostracans are thought to be the most primitive of the malacostracans and date back to the Cambrian period. They range in length from , most being suspension feeders though some are carnivores or scavengers. They have a two part carapace which encloses the head, the whole thorax and part of the abdomen and are the only malacostracans with seven abdominal segments. Three families are known with several genera and about twenty species. They are found worldwide from the intertidal zone to the deep ocean, all but one species being benthic (living on the seabed).


title: Malacostraca

text: Stomatopoda is the only extant order of Hoplocarida, the other two orders, Aeschronectida and Archaeostomatopoda being extinct. Stomatopodans, commonly known as mantis shrimps, range in length from and are predators. They have a dorso-ventrally flattened body and a shield-like carapace and are armed with powerful, raptorial claws normally carried in a folded position. There are about 300 species, most living in tropical and subtropical seas although some live in temperate areas. They are benthic, mostly hiding in cracks and crevices or living in burrows, some emerging to forage while others are ambush predators.


title: Malacostraca

text: The Eumalocostraca contains the vast majority of the approximately 40,000 living species of malacostracans and consists of three superorders, Syncarida, Peracarida and Eucarida. Syncaridans are mostly small and found in freshwater and subterranean habitats. Peracaridans are characterised by having a marsupium in which they brood their young. They are found in marine, freshwater and terrestrial habitats and include Amphipoda, Cumacea, Isopoda and Mysida. Eucarida includes lobsters, crabs, shrimps, prawns and krill.


title: Malacostraca

text: The first malacostracans appeared sometime in the Cambrian, when animals belonging to the Phyllocarida appeared.


title: Malacostraca

text: The following classification of living malacostracans is based on "An Updated Classification of the Recent Crustacea" (2001) by the American marine biologists Joel W. Martin, curator of crustaceans at the Natural History Museum of Los Angeles County, and George E. Davies. Extinct orders have been added to this and are indicated by an obelisk (†).


title: Aberdeenshire

text: It takes its name from the County of Aberdeen which has substantially different boundaries. The Aberdeenshire Council area includes all of the area of the historic counties of Aberdeenshire and Kincardineshire (except the area making up the City of Aberdeen), as well as part of Banffshire. The county boundaries are officially used for a few purposes, namely land registration and lieutenancy.


title: Aberdeenshire

text: Aberdeenshire Council is headquartered at Woodhill House, in Aberdeen, making it the only Scottish council whose headquarters are located outside its jurisdiction. Aberdeen itself forms a different council area (Aberdeen City). Aberdeenshire borders onto Angus and Perth and Kinross to the south, Highland and Moray to the west and Aberdeen City to the east.


title: Aberdeenshire

text: Traditionally, it has been economically dependent upon the primary sector (agriculture, fishing, and forestry) and related processing industries. Over the last 40 years, the development of the oil and gas industry and associated service sector has broadened Aberdeenshire's economic base, and contributed to a rapid population growth of some 50% since 1975. Its land represents 8% of Scotland's overall territory. It covers an area of .


title: Aberdeenshire

text: Aberdeenshire has a rich prehistoric and historic heritage. It is the locus of a large number of Neolithic and Bronze Age archaeological sites, including Longman Hill, Kempstone Hill, Catto Long Barrow and Cairn Lee. The area was settled in the Bronze Age by the Beaker culture, who arrived from the south around 2000–1800 BC. Stone circles and cairns were constructed predominantly in this era. In the Iron Age, hill forts were built. Around the 1st century AD, the Taexali people, who have left little history, were believed to have resided along the coast. The Picts were the next documented inhabitants of the area, and were no later than 800–900 AD. The Romans also were in the area during this period, as they left signs at Kintore. Christianity influenced the inhabitants early on, and there were Celtic monasteries at Old Deer and Monymusk.


title: Aberdeenshire

text: Since medieval times there have been a number of traditional paths that crossed the Mounth (a spur of mountainous land that extends from the higher inland range to the North Sea slightly north of Stonehaven) through present-day Aberdeenshire from the Scottish Lowlands to the Highlands. Some of the most well known and historically important trackways are the Causey Mounth and Elsick Mounth.


title: Aberdeenshire

text: Aberdeenshire played an important role in the fighting between the Scottish clans. Clan MacBeth and the Clan Canmore were two of the larger clans. Macbeth fell at Lumphanan in 1057. During the Anglo-Norman penetration, other families arrive such as House of Balliol, Clan Bruce, and Clan Cumming (Comyn). When the fighting amongst these newcomers resulted in the Scottish Wars of Independence, the English king Edward I travelled across the area twice, in 1296 and 1303. In 1307, Robert the Bruce was victorious near Inverurie. Along with his victory came new families, namely the Forbeses and the Gordons.


title: Aberdeenshire

text: These new families set the stage for the upcoming rivalries during the 14th and 15th centuries. This rivalry grew worse during and after the Protestant Reformation, when religion was another reason for conflict between the clans. The Gordon family adhered to Catholicism and the Forbeses to Protestantism. Aberdeenshire was the historic seat of the clan Dempster. Three universities were founded in the area prior to the 17th century, King's College in Old Aberdeen (1494), Marischal College in Aberdeen (1593), and the University of Fraserburgh (1597).


title: Aberdeenshire

text: After the end of the Revolution of 1688, an extended peaceful period was interrupted only by such fleeting events such as the Rising of 1715 and the Rising of 1745. The latter resulted in the end of the ascendancy of Episcopalianism and the feudal power of landowners. An era began of increased agricultural and industrial progress. During the 17th century, Aberdeenshire was the location of more fighting, centred on the Marquess of Montrose and the English Civil Wars. This period also saw increased wealth due to the increase in trade with Germany, Poland, and the Low Countries.


title: Aberdeenshire

text: The present council area is named after the historic county of Aberdeenshire, which has different boundaries and was abandoned as an administrative area in 1975 under the Local Government (Scotland) Act 1973. It was replaced by Grampian Regional Council and five district councils: Banff and Buchan, Gordon, Kincardine and Deeside, Moray and the City of Aberdeen. Local government functions were shared between the two levels. In 1996, under the Local Government, etc. (Scotland) Act 1994, the Banff and Buchan District, Gordon District and Kincardine and Deeside District were merged to form the present Aberdeenshire Council area. Moray and the City of Aberdeen were made their own council areas. The present Aberdeenshire Council area consists of all of the historic counties of Aberdeenshire and Kincardineshire (except the area of those two counties making up the City of Aberdeen), as well as north-east portions of Banffshire.


title: Aberdeenshire

text: The population of the council area has risen over 50% since 1971 to approximately , representing 4.7% of Scotland's total. Aberdeenshire's population has increased by 9.1% since 2001, while Scotland's total population grew by 3.8%.


title: Aberdeenshire

text: The census lists a relatively high proportion of under 16s and slightly fewer people of working age compared with the Scottish average.