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24.08.2016 16:22 - A Complete Encyclopedia Of The Largest Prehistoric Animals Vol.2 Invertebrates part 6 Trilobites
Автор: valentint Категория: Забавление   
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Trilobites
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Trilobites made a sudden appearance in the fossil record. There appears to be a considerable evolutionary gap from possible earlier precursors such as Spriggina, which is found in the 550 million year old Ediacaran-age rocks of Australia, and thus predates trilobites by some 30 million years. The heat of the Cambrian sea may have contributed to trilobite emergence.However, evidence does suggest that significant diversification had already occurred before trilobites were preserved in the fossil record, allowing for the "sudden" appearance of diverse trilobite groups with complex derived characteristics (e.g. eyes).
Morphological similarities between trilobites and earlier arthropod-like creatures such as Spriggina, Parvancorina, and other "trilobitomorphs" of the Ediacaran period of the Precambrian are ambiguous enough to make a detailed analysis of their ancestry complex. Morphological similarities between early trilobites and other Cambrian arthropods (e.g. the Burgess shale fauna and the Maotianshan shales fauna) make analysis of ancestral relationships difficult as well.
Early trilobites show all the features of the trilobite group as a whole; transitional or ancestral forms showing or combining the features of trilobites with other groups (e.g. early arthropods) do not seem to exist.
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The earliest trilobites known from the fossil record are redlichiids and ptychopariid bigotinids dated to some 540 to 520 million years ago. Contenders for the earliest trilobites include Profallotaspis jakutensis (Siberia), Fritzaspis spp. (western USA), Hupetina antiqua (Morocco) and Serrania gordaensis (Spain).All trilobites are thought to have originated in present-day Siberia, with subsequent distribution and radiation from this location.
Trilobites saw incredible diversification over time.For such a long-lasting group of animals, it is no surprise that trilobite evolutionary history is marked by a number of extinction events where some groups perished and surviving groups diversified to fill ecological niches with comparable or unique adaptations. Generally, trilobites maintained high diversity levels throughout the Cambrian and Ordovician periods before entering a drawn-out decline in the Devonian, culminating in the final extinction of the last few survivors at the end of the Permian period. xactly why the trilobites became extinct is not clear; with repeated extinction events (often followed by apparent recovery) throughout the trilobite fossil record, a combination of causes is likely.
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After the extinction event at the end of the Devonian period, what trilobite diversity remained was bottlenecked into the order Proetida. Decreasing diversity of genera limited to shallow-water shelf habitats, coupled with a drastic lowering of sea level (regression) meant that the final decline of trilobites happened shortly before the end of the Permian mass extinction event.With so many marine species involved in the Permian extinction, the end of nearly 300 million successful years for the trilobites would not have been unexpected at the time.
Trilobites have no known direct descendants. Their closest living relatives would be the chelicerates. Though horseshoe crabs are often cited as their closest living relatives, they are no closer evolutionarily than other cheliceratans.


Hungioides bohemicus
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An isolated pygidium of Hungioides bohemicus implies that the full animal was 90 cm (35 in) long.



Isotelus rex
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Isotelus is a genus of asaphid trilobites from the middle and upper Ordovician period, fairly common in the Northeastern United States, northwest Manitoba, southwestern Quebec and southeastern Ontario. One species, Isotelus rex, is currently the world"s largest trilobite ever found as a complete fossil.
A specimen of Isotelus rex, from Churchill, Manitoba, is the largest complete trilobite ever found. Discovered by Dave Rudkin (Royal Ontario Museum), Robert Elias (University of Manitoba), Graham Young (Manitoba Museum) and Edward Dobrzanske (Manitoba Museum) in 1999, it measures 720 millimetres (28 in) in length, 400 millimetres (16 in) in maximum width (across the cephalon) and 70 millimetres (3 in) in maximum height (at the posterior midpoint of the cephalon)



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Автор: valentint
Категория: Политика
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