Потребителски вход

Запомни ме | Регистрация
Постинг
17.12.2015 14:21 - Encyclopedia Largest prehistoric animals Vol.1 Vertebrates part1 Mammals ch.7 Hippos and Hyraxes
Автор: valentint Категория: Забавление   
Прочетен: 5395 Коментари: 0 Гласове:
0

Последна промяна: 23.05 23:18


Hippopotamuses, Choeropsis & Hexaprotodons
image1. Kenyapotamus 2. Siamotherium 3. Archaeopotamus 4. Anthracotherium 5. Merycopotamus

Until 1909, naturalists grouped hippos with pigs, based on molar patterns. Several lines of evidence, first from blood proteins, then from molecular systematics and DNA and the fossil record, show that their closest living relatives are cetaceanswhales, dolphins and porpoises.
The common ancestor of hippos and whales branched off from Ruminantia and the rest of the even-toed ungulates; the cetacean and hippo lineages split soon afterwards. The most recent theory of the origins of Hippopotamidae suggests that hippos and whales shared a common semiaquatic ancestor that branched off from other artiodactyls around 60 million years ago.This hypothesized ancestral group likely split into two branches around 54 million years ago.
One branch would evolve into cetaceans, possibly beginning about 52 million years ago, with the protowhale Pakicetus and other early whale ancestors collectively known as Archaeoceti, which eventually underwent aquatic adaptation into the completely aquatic cetaceans.The other branch became the anthracotheres, a large family of four-legged beasts, the earliest of which in the late Eocene would have resembled skinny hippopotamuses with comparatively small and narrow heads. All branches of the anthracotheres, except that which evolved into Hippopotamidae, became extinct during the Pliocene without leaving any descendants.
image

A rough evolutionary lineage can be traced from Eocene and Oligocene species: Anthracotherium and Elomeryx to the Miocene species Merycopotamus and Libycosaurus and the very latest anthracotheres in the Pliocene.Merycopotamus, Libycosaurus and all hippopotamids can be considered to form a clade, with Libycosaurus being more closely related to hippos. Their common ancestor would have lived in the Miocene, about 20 million years ago. Hippopotamids are therefore deeply nested within the family Anthracotheriidae.
The Hippopotamidae are believed to have evolved in Africa; the oldest known hippopotamid is the genus Kenyapotamus, which lived in Africa from 16 to 8 million years ago. While hippopotamid species spread across Asia and Europe, no hippopotamuses have ever been discovered in the Americas, although various anthracothere genera emigrated into North America during the early Oligocene. From 7.5 to 1.8 million years ago, an ancestor to the modern hippopotamus, Archaeopotamus, lived in Africa and the Middle East.
While the fossil record of hippos is still poorly understood, the two modern genera, Hippopotamus and Choeropsis (sometimes Hexaprotodon), may have diverged as far back as 8 million years ago. Taxonomists disagree whether or not the modern pygmy hippopotamus is a member of Hexaprotodon – an apparently paraphyletic genus, also embracing many extinct Asian hippopotamuses, that is more closely related to Hippopotamus – or of Choeropsis, an older and basal genus.


The largest artiodactyl was Hippopotamus gorgops with a length of 4.3 metres (14 feet) and a height of 2.1 metres (6.9 feet).

image
Hippopotamus gorgops ("Gorgon-eyed river horse") is an extinct species of hippopotamus. It first appeared in Africa during the late Miocene, and eventually migrated into Europe during the early Pliocene (where its fossils were first discovered). It became extinct prior to the Ice Age.
With a length of 4.3 metres (14 ft) and a shoulder height of 2.1 metres (6.9 ft) and with a weight of 3,900 kilograms (8,600 lb) H. gorgops was much larger than its living relative, H. amphibius. Another feature setting it apart from H. amphibius were its eyes. Modern hippos have eyes placed high on the skull, but H. gorgops had orbits extruding above its skull, making it even easier for the creature to see its surroundings while (almost) fully under water.


European Hippopotamus - Hippopotamus antiquus
image

Hippopotamus antiquus, sometimes called the European Hippopotamus, was a species of hippopotamus that ranged across Europe, becoming extinct some time before the last ice age at the end of the Pleistocene epoch. H. antiquus ranged from the Iberian Peninsula to the British Isles to the Rhine River to Greece.
Similar in size and form to Hippopotamus gorgops, H. antiquus on average was larger than the modern common hippopotamus (Hippopotamus amphibius). H. antiquus is believed to have first appeared around 1.8 million years ago, compared to 2 million years ago for H. amphibius. Beginning in the Middle Pleistocene, H. amphibius also lived in Europe, but is not considered a European Hippopotamus.
The Cretan Dwarf
 Hippopotamus (H. creutzburgi) is believed to have evolved from H. antiquus through the process of insular dwarfism on the island of Crete.
Fossilised remains of two ancient hippos have been discovered in Norfolk by scientists at the Natural History Museum and Queen Mary, University of London.
The fossil bones of hippos, hyena, fish and rodents are providing a rare glimpse of the landscape of East Anglia 500,000-780,000 years ago. The fossil remains point towards a unique find of animals living in Britian during a warmer climate, never previously recorded.
The hippos and other animals would have lived in the early Middle Pleistocene where exotic species, now found only in African savanna, would have roamed the landscape.
The ancient hippopotamus (Hippopotamus sp.) weighed about six to seven tonnes, much heavier than today"s modern hippos weighing up to four tonnes. The ancient hippos had prominent eyes that acted as periscopes when under water. It is likely the hippos discovered died through natural causes and their bones show evidence of having being gnawed by hyenas.
The site is approximately 15 kilometres from Norfolk’s present-day coast and insect fossils indicate the summer temperature at that time was 2-3°c warmer than today.

Hyraxes
image

1. Titanohyrax 2. Megalohyrax 3. Saghatherium 4. Kvabebihyrax 5. Pachyhyrax
All modern hyraxes are members of the family Procaviidae (the only living family within Hyracoidea) and are found only in Africa and the Middle East. In the past, however, hyraxes were more diverse, and widespread. The order first appears in the fossil record at a site in the Middle East in the form of Dimaitherium, 37 million years ago.For many millions of years, hyraxes were the primary terrestrial herbivores in Africa, just as odd-toed ungulates were in North America.
Through the middle to late Eocene, many different species existed,the largest of them weighing the same as a small horse and the smallest the size of a mouse. During the Miocene, however, competition from the newly developed bovids, which were very efficient grazers and browsers, displaced the hyraxes into marginal niches. Nevertheless, the order remained widespread and diverse as late as the end of the Pliocene (about two million years ago) with representatives throughout most of Africa, Europe, and Asia. The descendants of the giant "hyracoids" (common ancestors to the hyraxes, elephants, and sirenians) evolved in different ways. Some became smaller, and evolved to become the modern hyrax family. Others appear to have taken to the water (perhaps like the modern capybara), ultimately giving rise to the elephant family and perhaps also the sirenians. DNA evidence supports this hypothesis, and the small modern hyraxes share numerous features with elephants, such as toenails, excellent hearing, sensitive pads on their feet, small tusks, good memory, higher brain functions compared with other similar mammals, and the shape of some of their bones.
Hyraxes are sometimes described as being the closest living relative of the elephant,although whether this is so is disputed. Recent morphological- and molecular-based classifications reveal the sirenians to be the closest living relatives of elephants. While hyraxes are closely related, they form a taxonomic outgroup to the assemblage of elephants, sirenians, and the extinct orders Embrithopoda and Desmostylia.
The extinct meridiungulate family Archaeohyracidae, consisting of four genera of notoungulate mammals known from the Paleocene through the Oligocene of South America,is a group unrelated to the true hyraxes.

The largest hyraxes was Titanohyrax ultimus
image
Titanohyrax is a genus of large to very large hyrax from the early Eocene and early Oligocene. Specimens have been discovered in modern-day Algeria, Tunisia and Egypt. The genus was first described by Matsumoto in 1922. Some species, like Titanohyrax ultimus, are estimated to be as large as rhinoceroses.


Kvabebihyrax
image
Kvabebihyrax – a genus of very large fossils of Pliohyracidae family. They lived only in Transcaucasia (Eastern Georgia) in the late Pliocene.
They were distinguished by their large size: the length of their massive bodies reached 1.5 m
and weigh more than 80 kg.
Their eye sockets were small, considerably protruding over the temple and looking sideways, and at the same time set out far beyond the skull.
Judging by the relatively short and very high nasal bones, as well as by the large nasal incisure, notably stretching backwards, Kvabebihyrax could have a small proboscis. Possibly, the noted original combination of characteristics of Kvabebihyrax points to its adaptation to river and lakes habitat, among swampy brushwood of forest thickets. 
The protruding of Kvabebihyrax’ eye sockets over the temple, resembling that of hippopotamus, indicates the ability of Kvabebihyrax to hide underwater. Probably, in water Kvabebihyax searched for shelter in moments of danger.












 

 




Гласувай:
0
0



Няма коментари
Търсене

За този блог
Автор: valentint
Категория: Политика
Прочетен: 922581
Постинги: 225
Коментари: 18
Гласове: 16
Календар
«  Юли, 2020  
ПВСЧПСН
12345
6789101112
13141516171819
20212223242526
2728293031