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Anomalocaris

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on January 31, 2007 at 2:40:24 pm
 

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.

 

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 years old fossil fairy shrimp called Rehbachiella kinnekullensis, it doesn't seem an unreasonable inferrence.

 

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 giant fairy shrimp (Branchinecta gigas) is the largest of the anostracans and 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

The newly-discovered giant fairy shrimp Branchinecta raptor from Idaho has modified raptorial phyllopodia, which it uses to stab and puncture its prey. This too seems akin to anomalocarids.

 

Notostracans (Triops)

 

 

 

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Anomalocarids

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