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Anomalocaris

Page history last edited by xanthine@... 8 years, 5 months ago

Anomalocaris


Anomalocarids (meaning "unusual shrimp") are a group of very early marine animals known from fossils found in Cambrian deposits in China, North America, and Australia. Anomalocarids are the largest Cambrian animals known (some Chinese forms grew up to 2 m, and most of them were probably active carnivores (although recent thought posits one genus, Laggania, as a plankton-eating animal).

 

Anomalocarids were flat free-swimming segmented animals, which in front of their mouths had two appendages that look like the bodies of shrimps. The mouth is a peculiar circular structure like a pineapple slice, but with a ring of hard sharp teeth in the central orifice. The mouth was more rectangular than round, and the teeth did not meet in the middle. This would still let it crack open shells of small arthropods and other like animals, such as trilobites. Indeed, many trilobites have been found with bite marks on them. Anomalocarids also had large eyes and a body half-flanked with a series of swimming lobes. Parapeytoia yunnanensis, one species of anomalocarid, may even have legs.

 

Compared with many of the other sea-dwelling creatures of its time, anomalocarids were extremely agile. The flaps along its body could probably be moved in a wave-like formation, allowing it to move at great speeds or to 'hover'. This motion could be compared to present-day Batoidea (rays). The shell of the anomalocarids was more flexible than those of its prey, allowing it easier movement.

 

The anomalocarids thrived in the Early and Mid Cambrian and then apparently died out.

 

Dinocarida

Dinocarida is a proposed group of fossil arthropod-like marine animals found in the early and middle Cambrian. Two subgroups are known, the anomalocarids and the opabinids. The name of this group means terrible or tyrant shrimp, due to their shrimp-like appearance.

 

Anomalocaris

Anomalocaris was a swimming creature which possibly used flexible lobes on the sides of its body to propel itself through the water. Its large head had big eyes with multiple lenses and a round mouth. Two large 'arms' with barb-like spikes were positioned in front of the mouth; Anomalocaris probably used these to grab prey and bring it to its mouth.

 

For the time in which it lived Anomalocaris was a truly gigantic creature, reaching lengths of an already large 60 cm (2 ft) to a staggering 2 m (6 ft 8 in).

 

Laggania

In contrast to Anomalocaris canadensis, Laggania shows a wide, parabolic head, with eyes placed well behind the mouth and anterior appendages. Collins (1996) argues that the mouth of Laggania is more oval, with long axis oriented anterior-posterior, and with a roughly rectangular aperture. The anterior appendages, shown lying to each side of the mouth, are not as large as those of Anomalocaris canadensis, and bear long, delicate spines. Further, there is no trace of a fantail, and the lateral lobes reach maximum width well behind the front half of the body.

 

Amplectobelua

Compared to other anomalocarids, Amplectobelua is smaller and more compact (although still much bigger than most, if not all other animals of its time). It had big eyes, which were positioned laterally of the mouth, and several extended lobes on both sides of its segmented body, used for swimming. It had a pair of string-like fibers on its backside and two claw-like appendages on the front of its face. The latter were probably used to capture prey and deliver it to Amplectobelua's mouth, located at the bottom of its face. It is believed to be a carnivore.

 

Opabina

Opabinia, a possible relative of anomalocaris, was a highly unusual extinct animal found in early Cambrian fossil deposits. The animal was segmented and had a soft-shelled exoskeleton. The head carried an array of five, fully functional eyes that would have given their owner a range of vision of almost 360°, and a long, flexible, hose-like proboscis or snout which appears to be in no way homologous to the head appendages of any other known contemporary lifeform. At the end of the proboscis were grasping spines; these are theorised to have served as a grab to catch prey, which would then be brought to the mouth, which was located underneath the head, behind the base of the proboscis.

 

The body segments each featured a set of gills and a pair of flap-like appendages that are also dissimilar to other known animals of the time, save Anomalocaris. The three rearmost flaps formed the tail. Unlike known arthropods, the head does not appear to be formed from fused segments. The animal was covered with what seems to be a soft, flexible, uncalcified shell with no joints between the segments. Opabinia has no known relatives, except possibly Anomalocaris.

 

Branchiopods

Branchiopoda is a group of primitive and primarily fresh water crustaceans, mostly resembling shrimp. There are over 900 known species worldwide. A few are well-known, including Artemia (brine shrimp, called Sea-Monkeys when marketed as novelties), and Daphnia, both of which are raised as aquarium food or as interesting pets in and of themselves. Branchiopods should not be confused with Brachiopods.

 

Branchiopods in general are equipped with a ventral food groove, useful for suspension and filter feeding. The water current in the ventral food groove, used for breathing and feeding in most species, is produced by a battery of unspecialised legs. This is thought to resemble a very original way of living among the crustaceans. It's likely that contemporary branchiopods could give good insights into the lifestyle of the prehistoric dinocarida.

 

Anostraca (Fairy Shrimp)

Fairy shrimp are possibly the best living example of how anomalocarid locomotion would have worked. The similarity between the two species is striking, which could possibly imply a shared ancestry. Indeed, as fairy shrimp are also seen as the most original and primitive members of the branchiopod class, and the oldest known species is a 500 million year old fossil fairy shrimp called Rehbachiella kinnekullensis, it doesn't seem an unreasonable inferrence.

 

They are capable of three states of mobility. Resting at the bottom of the pools, darting rapidly and drifting slowly. The shrimp propel themselves with a wave-like anterior-posterior beating motion of their legs. This action is complemented by the propeller motion of the outermost part of the legs (the "exopodites". By changing the angle of these exopodites the speed of motion can be changed. The leg movements also serve the purpose of collecting algae, bacteria, protozoa, rotifers and floating detritus from the water. Food is then filtered from the water and scraped by sets of appendages to be eaten using a mandible mouth. Fairy shrimp have been observed gnawing on larger matter such as dead tadpoles, mollusks and amphibian eggs. The leg movements of the fairy shrimp also serve the purpose of taking the oxygen the animal needs from the water.

 

Although they live in fresh or saltwater they do not live in oceans or seas. They are well-adapted to living in arid areas where water is present for only part of the year. Their eggs will survive drought for several years and hatch about 30 hours after rains fill the pools where they live. Some eggs may not hatch until going through several wet/dry cycles, ensuring the animals' survival through times that the pools don't last long enough for the shrimp to reproduce. Male shrimp possess an enlarged second antenna used to clasp the female during mating. Female fairy shrimp often have a brood sack on their abdomen. Female fairy shrimp usually outnumber males.

 

Although most fairy shrimp are small (under ½ inch, 1 cm), the largest species are over 6 inches (15 cm) long and are predatory on other fairy shrimp.

 

Branchinecta Gigas

The largest living branchiopod is Branchinecta gigas, a fairy shrimp that reaches a length of 10 centimetres (3.9 inches). Some members of the fossil order Kazacharthra also grew to a length of 10 centimetres. B. Gigas is found in the playas of California's southern deserts. This species traps the much smaller alkali fairy shrimp (Branchinecta mackini) with its large antennae. This seems reminiscent of the manner in which anomalocaris is believed to have captured it's prey.

 

Branchinecta Raptor

Branchinecta raptor n. sp. is highly adapted to a predatory mode of life. In both sexes, the first four pairs of thoracopods bear elongated, curved, heavily chitinized endopods (bearing a highly modified type of spines), which are modified for grasping prey. Both sexes have elongated, “whip-like” cercopods nearly as long as the abdomen that are used for detecting prey. This new species appears most closely related to B. gigas Lynch 1937. Both species are large in size, have small eyes and sensory papillae, and share similar habitats and food preferences. However, B. raptor has some unusual predatory behaviors that differ from B. gigas. B. raptor is separated from all other Branchinecta by the unique form of the second antennae, the form of the cercopods, and the male genitalia. Large branchiopod records from Idaho are also discussed.

 

B. Raptor's predatory modifications (including the absence of filtering or scraping setae) are currently assumed to have "probably evolved secondarily within the Anostraca". This too seems convergent with anomalocaris, if indeed such adaptions did evolve after and not before other branchiopod features.

 

Notostracans (Triops)

 

 

Unsorted

(B. Gigas)

The class Branchiopoda is divided into 10 orders, two of which are extinct and known only through the fossil record. The eight living orders show a great diversity of form. In the Laevicaudata, for example, the number of trunk segments remains constant; there are 12 pairs of trunk limbs in the female and 10 pairs in the male. In the Spinicaudata, however, the number of paired trunk segments varies among its members from 12 up to 32 in some species. A carapace is present in the orders Ctenopoda and Anomopoda, but it encloses only the trunk, leaving the head free. In the orders Onychopoda and Haplopoda the carapace does not enclose the trunk limbs but forms a brood pouch on the dorsal surface. The anostracans (fairy shrimps and brine shrimps) lack a carapace and have stalked eyes, in contrast to the other living group, whose eyes are set into the head.

 

The two fossil groups as well differ markedly from each other. The order Lipostraca lacked a carapace and had 13 pairs of trunk limbs and a pair of large antennae, which appear to have been used in swimming. The order Kazacharthra had a well-developed carapace and six pairs of large thoracic limbs. The main structural feature linking these diverse forms, both living and fossil, is the flattened, or paddlelike, trunk limb, which often but not always is used in filter feeding. In the orders Onychopoda and Haplopoda even this feature is modified, and the trunk limbs have become specialized for grasping prey.

 

 

 

Links

Anomalocarids

Branchiopods

 

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