Потребителски вход

Запомни ме | Регистрация
29.01.2016 22:19 - Encyclopedia Largest prehistoric animals Vol.1 Vertebrates part11 Sharks Ch 1-Carcharocles Sharks - Relatives of Carcharocles megalodon
Автор: valentint Категория: Забавление   
Прочетен: 2753 Коментари: 0 Гласове:

Последна промяна: 08.08.2021 09:23

Long before dinosaurs walked the Earth, sharks were roaming the oceans. Their legacy has been traced back at least 450 million years which is quite amazing. They have been able to evolve as needed to continue surviving.
The first indication of sharks in the fossil record dates all the way back to the late Silurian period, possibly as much as four hundred and fifty million years ago. Unfortunately the only evidence that exists is scales from genera such as Elgestolepis, Mongolepis and Polymerolepis, but these scales are still well enough preserved to indicate that these fish were sharks.
Shark fossils become much more common in deposits from the Devonian, possibly reflecting growing success at survival. A greater number of sharks swimming in the Devonian waters would after all leave a proportionately higher amount of fossils. Already some of these sharks had developed a basic shark profile, with some like Cladoselache having a long torpedo shaped body and well developed fins for fast and agile swimming. However these sharks were still developing in the shade of other predators that meant they kept small and relied on speed for both hunting and avoidance of more powerful hunters like the arthrodire placoderms. However the disappearance of these predators at the end of the Devonian seems to have opened up the way for sharks to move beyond their more humble beginnings.

The Carboniferous & Permian periods saw the sharks evolve into what are possibly some of their most bizarre forms. In fact some of these forms were so strange that they would have made the hammerhead shark that we know today seem totally normal. Most famous of these is probably Helicoprion which had its teeth arranged in a strange spiral ‘tooth whorl’ that is thought to have extended from its lower jaw. It is unknown how exactly this whorl was used and upon what prey, but similar forms include Ornithoprion and Sarcoprion. Both these sharks have semi-circular arrangements of teeth in their jaws, and it’s possible that they may have been for rasping through prey to saw off bite sized chunks.
The mystery of the tooth whorl of Helicoprion may be connected to another shark named Edestus, better known as the ‘scissor-toothed shark’. Edestus acquired this nickname from the fact that its teeth were not shed like in other sharks and the jaws steadily grew throughout its life. This meant that the inner jaws with their teeth were constantly pushed forward throughout it life so that its outer most teeth pointed forwards rather than up and down like other sharks. If Helicoprion"s jaw grew in a similar manner then it may have started out small, but steadily curled round as the shark grew. This might even suggest that rather than being used for hunting it was a display indicating the maturity of the individual.
One group of sharks that were very different to those we know today are the xenacanthids. These sharks developed eel like bodies that were better suited for navigating the freshwater swamps that were clogged with submerged debris. Additionally, rather than having dorsal fins they instead had a single spine that rose up from the back of the head. This is usually interpreted as being for defence from larger fish as well as large amphibians.
Some sharks that did have dorsal fins had highly unusual ones. Briefly going back to the Devonian period and a shark called Stethacanthus had an anvil shaped dorsal fin that had growths of tooth like denticles on top of it (as well as a similar mat of denticles on top of its head). This dorsal fin is why Stethacanthus is sometimes called the ‘anvil shark’, or ‘ironing board shark’, with theories for the unusual dorsal fin ranging from a method of intimidating predators to same species recognition, but it remains hard to be certain without seeing the living creature. Another shark with an unusual dorsal fin is the small thirty centimetre long Falcatus that had a forwards horizontally pointing point projecting from the top of its dorsal fin and over its head. However the function of this dorsal fin is easier to determine with confidence as this dorsal fin only appears to have been on male Falcatus. This makes it almost certain that the dorsal fin of Falcatus was for the purpose of attracting a mate with perhaps the most developed spike being the most attractive to the female Falcatus.
Before moving on another group of fish similar to the sharks are those referred to as ‘spiny sharks’. These actually appeared back in the Silurian period and rose to prominence during the Devonian and Carboniferous periods. As cartilaginous fish the spiny sharks were related to sharks, but were not true sharks themselves. Spiny sharks are so named from the well developed and often numerous spines that projected from their bodies, presumably for the purpose of defence as these spines would make it both difficult and painful for a predator to close its jaws around them. Spiny sharks also appear to have been among the first fish to actually develop jaws, as earlier fish similar to Cephalaspis (which itself is actually known from the early Devonian period) are often referred to as jawless fish.
Spiny sharks were successful fish for their day, producing a variety of forms as well as colonising both sea and freshwater environments. However it would seem that combined competition from true sharks as well as bony fish reduced the numbers of spiny sharks to the point that they disappeared at the end of the Permian, before the start of the Mesozoic era. It’s also plausible that the extinction event that marks the end of the Permian may have also been responsible for their total disappearance.
Between the start of the Triassic and end of the Cretaceous the sharks continued to thrive, some in forms that are much more familiar to us today. Hybodus is probably the greatest success story here with a worldwide distribution as well as temporal range that stretches from the end of the Permian through to the early Cretaceous. In addition to these feats Hybodus also survived the mass extinctions at the end of the Permian and Triassic periods, the latter being a particularly bad one as far as marine life in concerned. Success for Hybodus is thought to have come from its ability to adapt to different types of prey thanks to its combination of sharp piercing teeth as well as rounded crushing teeth.
Other sharks of the time also appear to be quite modern with Scapanorhynchus resembling a goblin shark, while Squalicorax and Cretoxyrhina both appearing to be oceanic sharks well suited to a pelagic life. With more standard forms now established they had almost reached their full potential with successful body forms that are so perfect for aquatic life they have barely changed since. Shark diversification did not come to a complete halt however, as the Mesozoic sharks simply became better adapted to hunt other marine animals. Modifications now were along the lines of such things as different teeth for different kinds of prey such as thin pointed teeth for fish, serrated triangular teeth for large prey or rounded plate like teeth for crushing shells.

Family of  Megalodon-The megatoothed sharks


People have always had a particular fascination with large sharks, although usually they envision the great white shark (Carcharodon carcharias) which is the biggest predatory shark in the ocean today, while others like the whale shark are bigger but are filter feeders of plankton. However both of these sharks pale in comparison to the giant sharks that lived from the Oligocene to Pleistocene periods.
The most famous of these sharks is C. megalodon, often just referred to as megalodon, which is now well known all over the world thanks mainly to it appearing as a star monster in an ever expanding number of films and stories. Despite its current fame C. megalodon is actually late to the party, with it being recognised as a distinct species as far back as 1843, and teeth documented in scientific journals as far back as the end of the renaissance. With minimum size estimates averaging fifteen meters long, C. megalodon was the largest and last of the megatoothed sharks, but the ones that came before it such as Otodus and C. angustidenss are also estimated at being around nine meters long themselves.

Two problems persist with our understanding of the megatoothed sharks, the first one being how big they grew. With the exception of a few vertebrae, only the teeth are known, so size is usually determined by scaling these in comparison to other sharks that are alive in the ocean today. Unfortunately to muddle things further there are a number of different methods of comparison, some such as measuring slant height of a tooth (the length between the tip and outer edge of the tooth) to the amount of enamel that is on it, as well as others. Most of this research has been directed towards C. megalodon as it has the largest teeth, and therefore was the largest shark. C. megalodon is usually compared to the great white shark as it is thought to be the closest thing to a C. megalodon still swimming in the ocean. Depending upon the size of the teeth and the method involved estimates for C. megalodon usually range between fifteen and twenty meters, sometime more, but most researchers agree that the science is not exact and is best for just a more general indication of the total size.
The second problem with the later megatoothed sharks such as C. megalodon, C. auriculatus, C. chubutensis and C. angustidens is exactly which genus they should be placed in. As you may have noticed these species have been referred to as ‘C.’, and this is because while many people place them in the Carcharocles genus, separate from modern sharks. Other people however place them in the Carcharodon genus along with the great white shark, on the grounds of slightly similar teeth and the fact that the great white is the biggest predatory shark in the ocean today. Most today seem to lean towards the Carcharocles placement as while the teeth of sharks like C. megalodon, do bear a superficial resemblance to the great white, closer inspection reveals a number of differences. What can be said however is while it is almost certain that the great white shark is not descended from currently known megatoothed sharks, it may have shared a common ancestor with them.

Otodus obliquus may have reached lengths of 12 m (39 ft) and are estimated to have weighed in at 8 tonnes.

Otodus is an extinct genus of mackerel shark which lived during the Paleocene and Eocene epochs, approximately 60 to 45 million years ago.
This shark is known from the fossilteeth and fossilized vertebral centra. Like other elasmobranchs, the skeleton of Otodus was composed of cartilage and not bone, resulting in relatively few preserved skeletal structures appearing within the fossil record. The teeth of this shark are large with triangular crown, smooth cutting edges, and visible cusps on the roots. Some Otodus teeth also show signs of evolving serrations.
The fossils of Otodus indicate that it was a very large macro-predatory shark. The largest known teeth measure about 104 millimetres (4.1 in) in height.The vertebral centrum of this shark are over 12.7 cm (5 inch) wide.Scientists suggest that this shark at least approached 9 metres (30 ft) in total length ,with a maximum length of 12 metres (39 ft).
Otodus had a worldwide distribution, as fossils have been excavated from Africa, Asia, Europe, and North America.
Otodus likely preyed upon marine mammals, large bony fish, and other sharks. It was among the top predators of its time.
Scientists determined that Otodus evolved into the genus Carcharocles, given substantial fossil evidence in the form of transitional teeth. Some teeth have been excavated from the sediments of the Potomac River in Maryland, USA, Ypres clay in Belgium, and western Kazakhstan, which are morphologically very similar to Otodus teeth but with lightly serrated cusplets and a serrated cutting edge. These transitional fossils suggest a worldwide , event, and support the theory that Otodus eventually evolved into Carcharocles aksuaticus and thus initiated the Carcharocles lineage

Carcharocles chubutensis may have reached lengths of 12 m (39 ft) and are estimated to have weighed in at 10 tonnes.
Carcharocles chubutensis,meaning "glorious shark of Chubut", from Ancient Greek is a prehistoric megatoothed shark that lived during Oligocene, Miocene, and Pliocene epochs, approximately 28 – 5 million years ago.This shark is considered to be a close relative of another prehistoric megatoothed shark, C. megalodon. However, as is the case with C. megalodon, the classification of this species is disputed.
This species is also known from
fossil teeth and some fossilized vertebral centra. Shark skeleton is composed of cartilage and not bone, and cartilage rarely gets fossilized. Hence, fossils of C. chubutensis are generally poorly preserved. Although the teeth of C. chubutensis are morphologically similar to teeth of C. megalodon, they are comparatively slender with curved crown, and with presence of lateral heels feebly serrated.Fossils of this species have been found in North America,South America,Cuba,Puerto Rico,Africa  and Europe.
C. chubutensis was larger than C. angustidens. Teeth of C. chubutensis can approach 130 mm in slant height (diagonal length), which according to size estimation method proposed by Gottfried at al, in 1996, indicate 12.2 m (40 ft) long specimen.
Paleontological research suggests that this species may have changed habitat preferences through time, or it may have had enough behavioral flexibility to occupy different environments at different times.
C. chubutensis was likely an apex predator and commonly preyed upon
fish, sea turtles, cetaceans (e.g. whales)  and sirenids.

Carcharocles auriculatus was about 30 feet long (9 meters)
Carcharocles auriculatus is a large extinct shark species of the family Otodontidae, closely related to the sharks of the genus Otodus, and also closely related to the later species megalodon. Its teeth are large, having coarse serrations on the cutting edge, and also with two large cusplets. The teeth can reach up to 130 mm, and belonged to a large "megatoothed" shark.
It is known that there is at least one genus in the family Otodontidae, that being Otodus. But the names and number of the genera in Otodontidae is controversial and the family"s accepted phylogeny varies among paleontologists in different parts of the world. In the USA and Britain the most widespread genus name for otodontids with serrated teeth is Carcharocles, and the owners of unserrated ones Otodus. In countries of the former USSR, like Ukraine or Russia, all of these genera were attributed to Otodus, because scientists like Zhelezko and Kozlov thought that the absence or presence of tooth serrations is not enough to place these sharks in different taxa.
The tooth length of C.auriculatus is relatively large - from 25 to 114 mm. However, it is smaller than that of megalodon and
Carcharocles angustidens; the tooth length of C. megalodon is 38–178 mm and C. angustidens 25–117 mm.
Most C. auriculatus teeth come from South Carolina. However, many Eocene shark teeth are known from Khouribga Plateau, in Morocco.

Carcharocles auriculatus is the most primitive known member of the genus Carcharocles.
The family of Otodontidae is well known and famous because of its last member - Megalodon. Yet, there were many other genera in this family, first appearing in the Cretaceous. The first one was Cretalamna, one of the largest sharks in the Cretaceous. The teeth basal morphology was similar to the teeth of later Otodus. First, the crown was triangular. This feature helped the shark to bite harder, and this is the feature to which the otodontid evolution was leading for. Second, there was a small protuberance on the labial side of the crown, which is also present in many other species of the family. But there was a feature, which limited the bite force, decreasing the area of the bite, the cusplets. The completely similar absolutely triangular unserrated (all the teeth of Cretalamna had no serrations, the cutting edge was smooth) cusplets were present in a later Otodus obliquus from Paleocene and Eocene. Otodus obliquus is the first large form of otodontid. The maximum size of the teeth was up to 100mm.

C. angustidens may have reached lengths of 9.3 m (31 ft) and are estimated to have weighed in at 4-5 tonnes.
Carcharocles angustidens is a prehistoric megatoothed shark, which lived during the Oligocene and Miocene epochs approximately 35 to 22 million years ago.This shark is believed to be closely related to another extinct megatoothed shark, C. megalodon. However, just as in the case of C. megalodon, the classification of this species is also under dispute.
As is the case with most extinct sharks, this species is also known from fossil teeth and some fossilized vertebral centra. Shark skeleton is composed of cartilage and not bone, and cartilage rarely gets fossilized. Hence, fossils of C. angustidens are generally poorly preserved. To date, the best preserved specimen of this species have been excavated from New Zealand, which comprises 165 associated teeth and about 35 associated vertebral centrum.This specimen is around 26 million years old. C. angustidens teeth are noted for their triangular crowns and small side cusps that are fully serrated. The serrations are very sharp and very well pronounced. C. angustidens was a widely distributed species with fossils found in North America, South America,Europe, Africa, New Zealand, Japan,Australia and Malta.
Like other known megatooth sharks, the fossils of C. angustidens indicate that it was considerably larger than the extant great white shark,
Carcharodon carcharias. The well preserved specimen from New Zealand is estimated at 9.3 metres (31 ft) in length.This specimen had teeth measuring up to 9.87 cm (3.89 in) in diagonal length, and vertebral centra of around 1.10 cm (0.43 in) in diameter. However, there are reports of larger C. angustidens fossils.
C. angustidens was an apex predator and likely preyed upon penguins, fish, dolphins, and
baleen whales.

Megalolamna parodoxodon

Megalolamna is an extinct genus of
 lamniform shark that belongs to the family Otodontidae. Its name comes from the similarity of  its teeth to those of the extant shark genus Lamna. It is known from the early Miocene of Peru, Japan, and California and North Carolina in the United States, implying a cosmopolitan distribution. It is considered to be the sister genus of Otodus. The study of Megalolamna"s taxonomic relationships also demonstrates the possibility that Otodus needs to include the species sometimes assigned to Carcharocles (i.e., the megatoothed lineage, including megalodon) in order to be monophyletic. The biggest of these teeth were 1.77 inches long, so scientists estimate that it could have grown anywhere from 12.1 to 23.6 feet. That’s a bit smaller than the megalodon, which stretched from 33 to 67 feet. Still, M. paradoxodon would have been quite a threat 20 million years ago.




Няма коментари

За този блог
Автор: valentint
Категория: Политика
Прочетен: 1231576
Постинги: 255
Коментари: 24
Гласове: 25
«  Юли, 2022