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07.10 01:37 - Encyclopedia Largest prehistoric animals Vol.5 Molluscs part1Gastropods,Brachiopods and Bivalves
Автор: valentint Категория: Забавление   
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Gastropods (Gastropoda)

Snails and slugs
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The first gastropods were exclusively marine, with the earliest representatives of the group appearing in the Late Cambrian (Chippewaella, Strepsodiscus), though their only gastropod character is a coiled shell, so they could lie in the stem lineage, if they are gastropods at all. Early Cambrian organisms like Helcionella and Scenella are no longer considered gastropods, and the tiny coiled Aldanella of earliest Cambrian time is probably not even a mollusk.
As such, it"s not until the Ordovician that the first crown-group members arise.By the Ordovician period the gastropods were a varied group present in a range of aquatic habitats. Commonly, fossil gastropods from the rocks of the early Palaeozoic era are too poorly preserved for accurate identification. Still, the Silurian genus Poleumita contains fifteen identified species. Fossil gastropods were less common during the Palaeozoic era than bivalves.
Most of the gastropods of the Palaeozoic era belong to primitive groups, a few of which still survive. By the Carboniferous period many of the shapes seen in living gastropods can be matched in the fossil record, but despite these similarities in appearance the majority of these older forms are not directly related to living forms. It was during the Mesozoic era that the ancestors of many of the living gastropods evolved.
One of the earliest known terrestrial (land-dwelling) gastropods is Maturipupa, which is found in the Coal Measures of the Carboniferous period in Europe, but relatives of the modern land snails are rare before the Cretaceous period, when the familiar Helix first appeared.
In rocks of the Mesozoic era, gastropods are slightly more common as fossils; their shells are often well preserved. Their fossils occur in ancient beds deposited in both freshwater and marine environments. The "Purbeck Marble" of the Jurassic period and the "Sussex Marble" of the early Cretaceous period, which both occur in southern England, are limestones containing the tightly packed remains of the pond snail Viviparus.
Rocks of the Cenozoic era yield very large numbers of gastropod fossils, many of these fossils being closely related to modern living forms. The diversity of the gastropods increased markedly at the beginning of this era, along with that of the bivalves.
Certain trail-like markings preserved in ancient sedimentary rocks are thought to have been made by gastropods crawling over the soft mud and sand. Although these trace fossils are of debatable origin, some of them do resemble the trails made by living gastropods today.
Gastropod fossils may sometimes be confused with ammonites or other shelled cephalopods. An example of this is Bellerophon from the limestones of the Carboniferous period in Europe, the shell of which is planispirally coiled and can be mistaken for the shell of a cephalopod.
Gastropods are one of the groups that record the changes in fauna caused by the advance and retreat of the Ice Sheets during the Pleistocene epoch.


The largest-known of this group were in the genus Campanile, with the extinct Campanile giganteum having shell lengths up to 90 cm (35 in).
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The largest snail today is the giant African land snail, which can reach seven inches (18 cm) in length, and which has a  shell diameter of three-and-a-half inches (9 cm). Fairly large—for a snail. But now consider that the prehistoric C. giganteum, thought to be one of the largest (if not the largest) snails ever, could reach nearly two feet (60 cm) in length. The name was a giveaway, really. Paleontologists believe it lived in the oceans that covered France during the Eocene epoch 50 million years ago—and we can only imagine what sort of terror it might have inflicted upon the Spongebobs and Squidwards of that age.


Brachiopods (Brachiopoda)
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Brachiopods have a very long history of life on Earth (at least 550 million years). They first appear as fossils in rocks of earliest Cambrian age, and their descendants survive, albeit relatively rarely, in today"s oceans and seas. They were particularly abundant during the Palaeozoic Era (251 to 542 million years ago), and are often the most common fossils in rock of that age. Brachiopods are marine animals belonging to their own phylum (Brachiopoda) of the animal kingdom. Modern brachiopods occupy a variety of sea-bed habitats ranging from the tropics to the cold waters of the Arctic and Antarctic. Brachiopods are virtually defenceless and their shell, enclosing the animal"s organs, is their only protection. Most are permanently attached by a fleshy stalk (the pedicle) to a hard, sea-floor surface, such as a rock outcrop, boulder or some other shell. Braciopods are incapable of actively pursuing food. A few species of brachiopod can attach directly to soft sediment and others remain unattached. The pedicle is the only soft tissue that protrudes outside the shell which opens and closes to allow food-bearing currents of water to pass through it. Wide-hinged spiriferid brachiopods have been likened to birds. For example, in China, the common name of such forms translates as "stone swallows". There they are boiled in water with various herbs to produce medicinal potions and powders.

Gigantoproductus giganteus
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Gigantoproductus giganteus was a large brachiopod that superficially resembled a cockle. Fossils of this species have been found with widths of over 30 centimetres (12 in). It had a pair of thick dome-shaped valves joined together by a hinge. The valves had a small number of broad ribs that radiated from a thick umbo and there were large wing-shaped ears of calcareous material on either side. The valves were held together by a central strong adductor muscle which left a scar on the inside of the valves. The ventral valve, also known as the pedicle, was covered with spines on the outside. The inside of this valve was rough, being covered by numerous cone-shaped protrusions.These are visible in an internal mould of the brachiopod, a cast fossil which has been formed when a hole in sediment left by the soft tissues of the dead organism was later infiltrated by mineral matter.
The brachiopod genus Gigantoproductus is one of the most stratigraphically useful Early Carboniferous brachiopod clades. Species within this lineage are restricted to the late Visйan (Asbian-Brigantian-Middle Chesterian) strata of North America, Europe, Asia, and Africa. Some of the youngest members of this group display hingelines that measure up to 17 cm in length. With purported ancestry in the middle Visйan (Arundian-Holkerian), this lineage exhibits a distinct increase in both valve thickness and size through time. These macroevolutionary changes are consistent with Cope’s Rule of phyletic size increase. Unfortunately, the reasons for such changes remain unclear. Latitudinal size variations, deep water environmental settings, oxygen level of the atmosphere, and paleoclimatically induced changes have been postulated as factors creating increased individual growth. “Bergmann’s Rule” suggests that larger individuals with thinner shells characterize cooler waters at high latitudes and greater water depth. However, the widespread genus Gigantoproductus is characteristic of shallow water facies and does not appear to exhibit any distinguishable latitudinal clinal variations in size. Elevated atmospheric oxygen levels have been attributed as a cause of gigantism among Carboniferous terrestrial insects. This does not appear to be a proximate cause for size increases in Gigantoproductus, insofar as published trends in Carboniferous atmospheric oxygen levels show a significant increase beginning in the Late Serpukhovian, well after the Gigantoproductus lineage went extinct. While there is little evidence for geographic variations in size among species of Gigantoproductus, there do appear to be temporal trends. Early Visйan species assigned to this genus possess thinner and smaller shells, whereas, late Visйan to early Serpukhovian species exhibit thicker and larger shells. Such changes in shell character can be correlated with a late Visйan period of global climatic warming. The inferred warming in oceanic temperatures provided decreased levels of calcium carbonate solubility as well as increased oceanic mixing and concurrent increased nutrient levels which provided an opportunity for growth of thick-shelled individuals.

 

Bivalves (Bivalvia)
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The Cambrian explosion took place around 540 to 520 million years ago (Mya). In this geologically brief period, all the major animal phyla diverged and these included the first creatures with mineralized skeletons. Brachiopods and bivalves made their appearance at this time, and left their fossilized remains behind in the rocks.Possible early bivalves include Pojetaia and Fordilla; these probably lie in the stem rather than crown group. Watsonella and Anabarella are perceived to be (earlier) close relatives of these taxa.Only five genera of supposed Cambrian "bivalves" exist, the others being Tuarangia, Camya and Arhouriella and potentially Buluniella.Bivalves have also been proposed to have evolved from the rostroconchs.
Bivalve fossils can be formed when the sediment in which the shells are buried hardens into rock. Often, the impression made by the valves remains as the fossil rather than the valves. During the Early Ordovician, a great increase in the diversity of bivalve species occurred, and the dysodont, heterodont, and taxodont dentitions evolved. By the early Silurian, the gills were becoming adapted for filter feeding, and during the Devonian and Carboniferous periods, siphons first appeared, which, with the newly developed muscular foot, allowed the animals to bury themselves deep in the sediment.
By the middle of the Paleozoic, around 400 Mya, the brachiopods were among the most abundant filter feeders in the ocean, and over 12,000 fossil species are recognized.By the Permian–Triassic extinction event 250 Mya, bivalves were undergoing a huge radiation of diversity. The bivalves were hard hit by this event, but re-established themselves and thrived during the Triassic period that followed. In contrast, the brachiopods lost 95% of their species diversity.The ability of some bivalves to burrow and thus avoid predators may have been a major factor in their success. Other new adaptations within various families allowed species to occupy previously unused evolutionary niches. These included increasing relative buoyancy in soft sediments by developing spines on the shell, gaining the ability to swim, and in a few cases, adopting predatory habits.
For a long time, bivalves were thought to be better adapted to aquatic life than brachiopods were, outcompeting and relegating them to minor niches in later ages. These two taxa appeared in textbooks as an example of replacement by competition. Evidence given for this included the fact that bivalves needed less food to subsist because of their energetically efficient ligament-muscle system for opening and closing valves. All this has been broadly disproven, though; rather, the prominence of modern bivalves over brachiopods seems due to chance disparities in their response to extinction events

 

Platyceramus
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The largest and best known species is P. platinus. Individuals of this species typically reached 1 m (3 ft 3 in) or more in axial length, but fossil specimens 3 m (nearly 10 feet) long have been found, making it the
largest known bivalve. Its huge but very thin shell often provided shelter for schools of small fish, some of which became trapped and fossilised themselves. The outer shell often provided habitat for its own juveniles,also for oysters such as the epizootic oyster Pseudoperna congestaas shown in the image here, and barnacles.
Platyceramus
was a genus of Cretaceous bivalve molluscs belonging to the extinct inoceramid lineage. It is sometimes classified as a subgenus of Inoceramus.
Shells containing pearls have also been discovered.
Giant Middle Coniacian to Lower Campanian Platyceramus Seitz is among the largest Cretaceous bivalves, commonly reaching an axial length of over 1 m, and occasionally over 2–3 m in size. The genus is characterized by its large size, very low convexity, normal inflation limited mostly to the umbonal area, and flattened flanks. It is especially common in moderately deep calcareous shale facies, as well as in chalks and limestones of the Niobrara Formation and equivalents. Preferred facies contain abundant pyrite, elevated total organic carbon (TOC), and very low biotic diversity. The genus maintains its giant size in these facies, and becomes more abundant. It clearly prefers dysoxic facies. As such, it probably is chemosymbiotic; photosymbiosis is almost ruled out because of inferred water depths of 200–350 m. It is also found more sparsely, and of smaller size, in oxygenated facies, including shoreface sandstone.
Platyceramus was not really a bug, but its length of ten feet (3 m) more than makes up for the technicalities. P. platinus is one of the largest bivalves (clams, scallops) ever found. In comparison to the (itself enormous) modern giant clam, P. platinus would have been more than two-and-a-half times as wide, and probably much heavier.


Inoceramus
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Inoceramus is an
extinct genus of fossil marine pteriomorphian bivalves that superficially resembled the related winged pearly oysters of the extant genus Pteria. They lived from the Early Jurassic to latest Cretaceous.
Inoceramus is the largest known bivalve clam in the fossil record. It is thought that it grew so large so that it could have a larger gill area to cope with oxygen deficient waters. Like smaller versions, Inoceramus would have opened its shell to expose its soft tissue and filter food from the water. When threatened it would then close up to protect the fleshy parts within.
The taxonomy of the inoceramids is disputed, with genera such as Platyceramus sometimes classified as subgenus within Inoceramus. Also the number of valid species in this genus is disputed.
Inoceramids had a thick shell paved with "prisms" of calcite deposited perpendicular to the surface, which gave it a pearly luster in life.Most species have prominent growth lines which appear as raised semicircles concentric to the growing edge of the shell. Paleontologists suggest that the giant size of some species was an adaptation for life in the murky bottom waters, with a correspondingly large gill area that would have allowed the animal to survive in oxygen-deficient waters.




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Автор: valentint
Категория: Политика
Прочетен: 202599
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