hd animal pictures 2013: Amazing Animal Pictures

Tuesday 3 December 2013

Amazing Animal Pictures

Source Link:-google.com.pk

The coelacanths constitute a rare order of fish that includes two extant species in the genus Latimeria: the West Indian Ocean coelacanth (Latimeria chalumnae) and the Indonesian coelacanth (Latimeria menadoensis). They follow the oldest known living lineage of Sarcopterygii (lobe-finned fish and tetrapods), which means they are more closely related to lungfish, reptiles and mammals than to the common ray-finned fishes. They are found along the coastlines of the Indian Ocean and Indonesia.Since there are only two species of coelacanth and both are threatened, it is the most endangered order of animals in the world. The West Indian Ocean coelacanth is a critically endangered species.

Coelacanths belong to the subclass Actinistia, a group of lobed-finned fish related to lungfish and certain extinct Devonian fish such as osteolepiforms, porolepiforms, rhizodonts, and Panderichthys.Coelacanths were thought to have gone extinct in the Late Cretaceous, but were rediscovered in 1938 off the coast of South Africa.Traditionally, the coelacanth was considered a “living fossil” due to its apparent lack of significant evolution over the past millions of years;and the coelacanth was thought to have evolved into roughly its current form approximately 400 million years ago.However, several recent studies have shown that coelacanth body shapes are much more diverse than is generally said.In addition, it was shown recently that studies concluding that a slow rate of molecular evolution is linked to morphological conservatism in coelacanths are biased on the prior hypothesis that these species are living fossils.Coelacanths are a part of the clade Sarcopterygii, or the lobe-finned fishes. Externally, there are several characteristics that distinguish the coelacanth from other lobe-finned fish. They possess a three-lobed caudal fin, also called a trilobate fin or a diphycercal tail. A secondary tail that goes along and extends past the primary tail separates the upper and lower halves of the coelacanth. Cosmoid scales act as thick armor that protects the exterior of the coelacanth. There also are several internal traits that aid in differentiating coelacanths from other lobe-finned fish. At the back of the skull, the coelacanth possesses a hinge, the intracranial joint, which allows it to open its mouth extremely wide. Coelacanths also retain a notochord, a hollow, pressurized tube which is replaced by the vertebral column early in embryonic development in most other vertebrates. The heart of the coelacanth is shaped differently from that of most modern fish; the heart's chambers are arranged in a straight tube. The coelacanth braincase is 98.5% filled with fat; only 1.5% of the braincase contains brain tissue. The cheeks of the coelacanths are unique because the opercular bone is very small and holds a large soft-tissue opercular flap. The coelacanth also possesses a unique rostral organ within the ethmoid region of the braincase.Also unique to extant coelacanths is the presence of a "fatty lung" or a fat-filled single-lobed vestigial lung.Latimeria chalumnae and L. menadoensis are the only two known living coelacanth species.The word coelacanth is dervied from the Greek for “hollow spine”, because of its unique hollow spine fins.Coelacanths are large, plump, lobe-finned fish that grow up to 1.8 meters. They are nocturnal piscivorous drift-hunters.The body is covered in cosmoid scales that act as armor. Coelacanths have 8 fins – 2 dorsal fins, 2 pectoral fins, 2 pelvic fins, 1 anal fin, and 1 caudal fin. The tail is very nearly equally proportioned and is split by a terminal tuft of fin rays that make up the caudal lobe of the tail. The eyes of the coelacanth are very large, while the mouth is very small. The eye is acclimatized to seeing in poor light by having rods that absorb mostly low wavelengths. The vision of coelacanths has evolved to a mainly blue-shifted color capacity.Pseudomaxillary folds surround the mouth, which replace the maxilla, a structure that is absent in coelacanths. There are two nostrils along with four other external openings that appear between the premaxilla and lateral rostral bones. The nasal sacs resemble those of many other fish and do not contain an internal nostril. The rostral organ of the coelacanth is contained within the ethmoid region of the braincase. It has three unguarded openings into the environment. The rostral organ is used as a part of the coelacanth's laterosensory system.The coelacanth's auditory reception is mediated by its inner ear. The inner ear of the coelacanth is very similar to that of tetrapods because it is classified as being a basilar papilla.