This video says about itself:
Ancient Prediators and Their Razor Jaws
12 November 2013
Hyaenodon (“hyaena-toothed”) is an extinct genus of Hyaenodonts, a group of carnivorous creodonts of the family Hyaenodontidae endemic to all continents except South America, Australia and Antarctica, living from 42—15.9 mya, existing for approximately 26.1 million years.
Some species of this genus were amongst the largest terrestrial carnivorous mammals of their time; others were only of the size of a marten. Hyaenodon was one of the latest genera of the Hyaenodonts and is known from the Late Eocene to Early Miocene. Remains of many species are known from North America, Europe, Asia and Africa (In 1993 42 species were distinguished).
Typical of early carnivorous mammals, the Hyaenodon had a very massive skull but only a small brain. It had a long skull with a narrow snout – much larger in relation to the length of the skull than in canine carnivores, for instance. Its neck was shorter than its skull, while its body was long and robust and terminated in a long tail. Despite the name, these creatures are not related to hyenas.
The average weight of adult or subadult H. horridus, the largest North American species, is estimated to about 40 kilograms (88 lb) and may not have exceeded 60 kilograms (130 lb). H. gigas, the largest Hyaenodon species was much larger, being 500 kilograms (1,100 lb) and around 10 feet. H. crucians from the early Oligocene of North America is estimated to only 10 to 25 kilograms (22 to 55 lb). H. microdon and H. mustelinus from the late Eocene of North America were even smaller and weighed probably about 5 kilograms (11 lb).
Another Eocene mammal group were pantodonts.
From Canwest News Service in Canada:
Prehistoric Arctic ‘hippo’ teeth offer clues to mammal evolution
By Randy Boswell
June 1, 2009
An analysis of the 53-million-year-old fossilized teeth of huge, hippo-like animals found on Canada’s once-temperate Ellesmere Island has produced what scientists are calling a “smoking gun” discovery about the migration and evolution of large mammals in ancient North America.
Researchers have long known that Canada’s northernmost Arctic islands were once relatively warm, lush environments inhabited by alligators and other creatures associated today with southerly latitudes.
But the latest findings, according to Canadian and U.S. researchers who’ve published a study in the latest issue of the journal Geology, shed new light on the diets and movements of Arctic animals in the post-dinosaur age and “may provide the behavioural smoking gun for how modern groups of mammals like ungulates — ancestors of today’s horses and cattle — and true primates arrived in North America.”
The study was led by Saskatchewan paleontologist Jaelyn Eberle, curator of fossil vertebrates at the University of Colorado Museum of Natural History.
The team’s claims are based on an analysis of isotopes found in the teeth of the prehistoric creatures, which reveal information about the locations and kinds of plants they ate and the time of year they migrated and reproduced.
Among the remains analyzed were teeth from the extinct Coryphodon, a semi-aquatic mammal resembling the modern hippopotamus. Teeth from two other species — an extinct ancestor of today’s tapirs and a rhinoceros-like animal called brontothere — also were examined and confirmed the team’s findings.
Each of the animals would have weighed around 500 kilograms, the researchers note.
“We were able to use carbon signatures preserved in the tooth enamel to show that these mammals did not migrate or hibernate,” Eberle said in a summary of the study released on Monday. “Instead, they lived in the high Arctic all year long, munching on some unusual things during the dark winter months.”
The “dietary chemical signatures” found in the animals’ teeth, along with the three extinct species’ “portly shapes and fossil evidence for babies and juveniles in the Arctic,” prove the creatures did not undertake long migrations to the south each winter, the summary stated.
The animals are believed to have fed in summer mainly on flowering plants and deciduous leaves. In winter, the researchers found, they switched to eating twigs, pine needles and fungi.
“In order for mammals to have covered the great distances across land bridges that once connected the continents, they would have required the ability to inhabit the High Arctic year-round in proximity to these land bridges,” said Eberle, referring to Arctic ridges believed to have linked North America and Asia at that time.
“The study has implications for the dispersal of early mammals across polar land bridges into North America and for modern mammals that likely will begin moving north if Earth’s climate continues to warm.”
See also here.
What caused global warming 55 million years ago? Here.
Eocene Canadian plants: here.
Eocene British Columbia: here.
While the recently announced discovery of “Ida,” a remarkably well-preserved early primate fossil, promises insights into the evolution of later primate forms, including humans, the way it has been presented to the public distorts both its significance and the processes of biological evolution: here.
Florida Pleistocene mammals: here.
When did the Pleistocene begin? Here.