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03.12.2016 00:32 - Largest prehistoric animals Vol.2 Arthropods part1 Chelicerata ch.1Eurypterids – enormous sea scorpions
Автор: valentint Категория: Забавление   
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Последна промяна: 03.12.2016 02:59


You find yourself sitting at the edge of a lake. It's a beautiful day: The sky is a clearer blue than you've ever seen it. Even the air feels cleaner, easier to breathe. You decid­e to wade into the lake; the cool water feels good against your skin. As you wade deeper, you stub your toe on something that feels like a long rock or log. You find a branch at one end, tapering off into a crescent shape and lined with spikes.
Suddenly the "log" moves, and it becomes painfully clear that the crescent-shaped part is an enormous claw. It grabs your leg, pulling you under, as another claw grabs your torso. You've just encountered a giant sea scorpion, and your chances of surviving are pretty
Fortunately, you would have to travel back about 400 million years to visit during the Devonian period of the Paleozoic Era for this scenario to play out. This period was a time of gigantism among living creatures prowling among earth and sea. There were dragonflies with two-and-a-half foot wingspans and six-foot-long millipedes scampered about. But the species that dominated them all was pterygotid eurypterid -- the giant sea scorpion.
Eurypterids are an extinct family of arthropods believed to be the ancestors of today's scorpions, and possibly all arachnids -- the class that includes spiders and other eight-legged arthropods. During the 27 million years giant sea scorpions lived, they took their place at the top of the food chain, thriving without any natural predators. Giant sea scorpions were some of the fiercest creatures on Earth, devouring large fish -- as well as one another. Eurypterids were cannibals, fighting and eating one another in competition for resources, prey and mates. They evolved into enormous size, but science's conception of just how enormous has recently changed.
A few years ago, paleontologist Markus Poschmann was digging for fossils in a quarry in Germany. He and his colleagues were removing slabs of siltstone that had been sediment in a lake or lagoon hundreds of millions of years ago. This ancient lake bed was the home of the largest arthropod ever discovered.
Poschmann noticed "a dark patch of organic matter" on one of the slabs of siltstone, and upon further investigation, discovered it was part of a claw belonging to Jaekelopterus rhenaniae, a species of sea scorpion. The claw piece was more than 18 inches (46 centimeters) long, and by calculating the proportion to the rest of the body, it showed that the scorpion it belonged to was more than eight feet long. This is almost a foot and a half longer than anyone estimated the sea scorpion ever grew.
Paleontologists differ in their opinions of how the giant sea scorpions grew to be so large. The atmosphere during the Devonian period featured a higher concentration of oxygen (35 percent, compared to 21 percent today). Some paleontologists believe this accounts for the gigantism that is characteristic of the period.
Others believe that sea scorpions grew to be so big out of necessity. In order to pierce the ever-evolving armor of their fish prey, their claws (and bodies) had to evolve into a gigantic size. The authors of the paper that introduced Poschmann's find discount this explanation and believe that it was a combination of factors -- including the lower gravity found underwater -- which allowed sea scorpions to become gigantic.
Regardless of how they became so large, scientists generally believe that sea scorpions met their match in the form of large fish with strong jaws and teeth. These fish forced most sea scorpions onto land, where they evolved into smaller size, leaving fossils behind as a reminder of the time when they ruled the Earth.

 

The largest known in this group was Jaekelopterus rhenaniae at 2.8 metres (9 ft) in length
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Jaekelopterus ("Otto Jaekel's wing) is an extinct genus of sea scorpion. Jaekelopterus lived approximately 390 million years ago. At an estimated length of 2.5 – 3.2 metres , it is the largest known arthropod ever discovered, surpassing the Pterygotus and even the millipede-like Arthropleura. There are two species, one being the type species, J. rhenaniae, from freshwater strata in the Rhineland, and J. howelli from estuarine strata in Wyoming. Jaekelopterus is currently the largest known eurypterid and the largest known arthropod. The J. rhenaniae claw was found near Prüm in Germany. To work out the size of the arthropod it belonged to, Braddy and colleagues collected information on closely related sea scorpions and the ratio between their claw size and body length. This turned out to be relatively constant, leading the researchers to conclude that a creature with a 46-centimetre claw probably had a body length between 233–259 cm (7 ft 8 in–8 ft 6 in) (246 cm (8 ft 1 in) on average). When extended, the chelicerae would have added another 1 metre (3.3 ft) to its length.
Although Jaekelopterus is called a "sea scorpion", the strata in which it was found suggest that the living animal dwelled in fresh-water systems and estuaries, rather than in marine environments proper. The animal was described in November 2007 by Simon Braddy and Markus Poschmann of the University of Bristol in the journal Biology Letters: they found a 46-centimetre (18 in) chelicera (claw-like mouth part), and estimated the total size of the animal based on the proportions of this claw.
The holotype of J. rhenaniae was discovered in the early Middle Devonian (Eifelian) Klerf Formation Lagerstätte of Willwerath near Prüm, Germany.
Jaekelopterus lived in freshwater lakes and rivers and probably never ventured into the ocean. The animal would have been the top predator of its environment, preying on anything it could catch such as fish and other arthropods.

 

A close contender was Pterygotus at 2.3 metres (7.5 ft) in length.
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As the second largest known eurypterid, Pterygotus remains have been recovered from the world over suggesting that it was a highly successful predator. Pterygotus had four compound eyes with two smaller ones on top of the head and two larger ones at the front. This would have given Pterygotus exceptionally good vision for its time with a potentially good degree of depth perception as can be seen in jumping spiders today.
Reaching lengths of 2 ½ meters with a body supported by well-developed legs, and armed with a pair of forward-facing claws laden with sharp projecting spines, they seem like the Tyrannosaurus rex among the invertebrates.
Pterygotus  had a pair of large compound eyes, as well as another pair of smaller eyes in the center of its head. It had 4 pairs of walking legs, a fifth pair modified into swimming paddles, and a pair of large chelae (pincers) for subduing prey. The foremost 6 tergites, or tail sections, contained gills and the reproductive organs of the animal.

Pterygotus could use its paddle like appendages for swimming but could also move its tail for propulsion, perhaps to provide an extra burst of speed when attacking prey. It probably would have focused its attention on other arthropods such as trilobites which would have been very common at the time.
Numerous species have been attributed to Pterygotus - P. anglicus (type), P. arcuatus, P. barrandei , P. bolivianus, P. carmani, P. cobbi, P. denticulatus, P. floridanus, P. gaspesiensis, P. kopaninensis, P. lanarkensis, P. lightbodyi, P. ludensis, P. marylandicus, P. monroensis. , but some have been questioned. The above species mentioned are generally accepted, but may not feature all.


Acutiramus
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Acutiramus was one of the largest eurypterids with pincers 5 cm and length about 2 m. It was related to another large eurypterid, Pterygotus.
Pterygotidae, which lived from the Ordovician to Devonian Eras, were characterized by small to large exoskeletons with semilunar scales. The telson, (tail) was expanded, or flatter than it was tall. Pterygotidae also had chelicerae (claws in front of the mouth) that were large and long, with strong, well developed teeth on the claws. Their walking legs were small and slender, without spines.
Acutiramus is distinguishable from other Pterygotidae by the distal margin of the chelae, where the final tooth is at an acute angle to the rest of the claw (hence the name Acutiramus, or “acute arm”). The large tooth in the center of the claw is distally inclined, which is to say it points forwards. The prosoma (head) is subquadrate, with compound eyes located at the edge of the front corners. The telson has a low row of knobs running down its center.


Hibbertopterus
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Hibbertopterus is a genus of giant sea scorpion (order Eurypterida) that inhabited the swamps of Scotland during the Carboniferous.
Hibbertopterus is a member of the family Hibbertopteridae, large bizarre Eurypterids found from the Upper Devonian to the end of the Permian period.They were sweep feeders, inhabiting freshwater swamps and rivers, feeding by raking through the soft sediment with blades on their anterior appendages to capture small invertebrates.Their morphology was so unusual that they have been thought to be an order separate to Eurypterida.Recent work however confirms them to be a derived member of the suborder Stylonurina, with the genus Drepanopterus being a basal member of their superfamily.
Hibbertopterus is believed to have been one of the first aquatic animals to exhibit terrestrial locomotion, as tracks indicating a dragging movement have been found in West Lothian, Scotland .The track found was roughly six metres long and a metre wide, and suggests that the eurypterid was 1.6 metres in length.


Pentecopterus
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Pentecopterus decorahensis is an extinct sea scorpion that lived during the Middle Ordovician period, as early as 467.3 million years ago.It is the oldest described eurypterid, and, at an estimated length of up to 1.83 metres (6 ft 0 in),one of the largest known arthropods ever discovered, similar in size to the famous millipede-like Arthropleura. Other notable sea scorpions include the more recent Silurian-aged Acutiramus, and Devonian Jaekelopterus, the only arthropod currently known to surpass P. decorahensis in size.According to researcher James Lamsdell of Yale University, P. decorahensis "is the first real big predator".
The generic name refers to the "penteconter", an ancient Greek warship, due to similarities in shape and predatory behavior.It is the oldest described eurypterid, living as early as 467.3 million years ago; at an estimated length of 1.7 metres (5 ft 7 in), or up to 1.83 metres (6 ft 0 in), it is also one of the largest known arthropods.

 

Megalograptus
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Megalograptus (Greek for "big writing") is a 5-foot-long (1.6 m) Ordovician eurypterid (sea scorpion), and was among the earliest known genera. The generic name is derived from the fact that its first fossils were of its very spiny legs, which were mistaken for massive graptolites. It lived from 460 to 445 mya. Megalograptus preyed on fish, trilobites, other sea scorpions, and smaller orthocones, using the spines on their claws to feel for their prey hiding in the sand and mud. Adult Cameroceras, along with the bigger sea scorpion species, preyed on it. Megalograptus did not have a stinger, but may have curled its tail and sword-like telson forward as a threat pose, much like a scorpion.
Megalograptids were characterized by large exoskeletons with ovate to triangular scales. The prosoma (head) was subquadrate, with a tongue-like anterior process bearing marginal spines, and compound eyes on the top front of the head. The chelicerae (claws in front of the mouth) were small and short. The first and third pairs of walking legs were short, with diverging or closely spaced spines. The second pair of walking legs was enormously developed, with long paired spines. The fourth pair of walking legs was nearly spineless. The preabdomen, the front portion of the body, was narrow with axial furrows, while the postabdomen was moderately narrow with broad, flat and curved appendages on the last body segment. The telson was short and lanceolate. Megalograptus is distinguishable from other members of the family by the third walking legs, which are characterized by short diverging spines.



















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Автор: valentint
Категория: Политика
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