Fifty-six million years ago, just before earth's carbon dioxide levels and average temperatures soared, many species of primitive primates went extinct in North America for reasons unclear to scientists. Now, a study of fossilized molars appears to exonerate one potential culprit in the animals' demise: competition with primitive rodents for food.
In a study described in an advance online article in the American Journal of Physical Anthropology and conducted primarily by Kristen Prufrock, now a graduate student at the Johns Hopkins University School of Medicine, a team of paleontologists examined dental clues to the diets of so-called stem primates and rodents who plied North America at the end of the Paleocene epoch.
Other paleontologists had suggested many of the stem primates—which superficially resembled rodents but had some primate characteristics, such as long, grasping fingers—died off because rodents outcompeted them for their preferred foods. But when Prufrock, then studying at the University of Toronto Scarborough, and her colleagues took CT scans of 13 rodent and 181 stem primate jaws held in multiple museums, the shapes of their molars revealed that most of the primates ate different types of food than the rodents. The results, says Prufrock, call into doubt the idea that food competition with rodents killed off the primates.
Most of the jaw specimens were collected in Wyoming and Montana and were scanned with a micro CT scanner. Second mandibular molars used for grinding food were isolated using imaging software.
"We found that one genus of stem primates, Chiromyoides, lived at about the same time and had adapted to eat the same foods as primitive rodents, but the others either were adapted to eating different foods or lived at different times," Prufrock says. "So competition from rodents was unlikely to have been the main reason for the decline of most of the stem primates. Something else must have been the driving force."
Archaeologists with The Australian National University (ANU) have discovered fossils of seven giant rat species on East Timor, with the largest up to 10 times the size of modern rats.
Dr Julien Louys of the ANU School of Culture, History and Language, who is helping lead the project said these are the largest known rats to have ever existed.
"They are what you would call mega-fauna. The biggest one is about five kilos, the size of a small dog," Dr Louys said.
"Just to put that in perspective, a large modern rat would be about half a kilo."
The work is part of the From Sunda to Sahul project which is looking at the earliest human movement through Southeast Asia. Researchers are now trying to work out exactly what caused the rats to die out.
Dr Louys said the earliest records of humans on East Timor date to around 46,000 years ago, and they lived with the rats for thousands of years.
"We know they're eating the giant rats because we have found bones with cut and burn marks," he said.
"The funny thing is that they are co-existing up until about a thousand years ago. The reason we think they became extinct is because that was when metal tools started to be introduced in Timor, people could start to clear forests at a much larger scale."
The largest rodent ever to have lived may have used its front teeth just like an elephant uses its tusks, a new study led by scientists at the University of York and The Hull York Medical School (HYMS) has found.
Josephoartigasia monesi, a rodent closely related to guinea pigs, lived in South America approximately 3 million years ago. It is the largest fossil rodent ever found, with an estimated body mass of 1000 kg and was similar in size to a buffalo.
Dr Philip Cox, of the Centre for Anatomical and Human Sciences, a joint research centre of the University's Department of Archaeology and HYMS, used computer modelling to estimate how powerful the bite of Josephoartigasia could be.
He found that, although the bite forces were very large - around 1400 N, similar to that of a tiger - the incisors would have been able to withstand almost three times that force, based on earlier estimates by co-authors, Dr Andres Rinderknecht, of The Museo Nacional de Historia Natural, Montevideo, and Dr Ernesto Blanco, of Facultad de Ciencias, Instituto de F?sica, Montevideo, who first described the fossil in 2008.
Dr Cox said: "We concluded that Josephoartigasia must have used its incisors for activities other than biting, such as digging in the ground for food, or defending itself from predators. This is very similar to how a modern day elephant uses its tusks."
A poisonous surprise under the coat of the African crested rat
Authors:
Kingdon et al
Abstract:
Plant toxins are sequestered by many animals and the toxicity is frequently advertised by aposematic displays to deter potential predators. Such ‘unpalatability by appropriation’ is common in many invertebrate groups and also found in a few vertebrate groups. However, potentially lethal toxicity by acquisition has so far never been reported for a placental mammal. Here, we describe complex morphological structures and behaviours whereby the African crested rat, Lophiomys imhausi, acquires, dispenses and advertises deterrent toxin. Roots and bark of Acokanthera schimperi (Apocynaceae) trees are gnawed, masticated and slavered onto highly specialized hairs that wick up the compound, to be delivered whenever the animal is bitten or mouthed by a predator. The poison is a cardenolide, closely resembling ouabain, one of the active components in a traditional African arrow poison long celebrated for its power to kill elephants.
Some researchers think the Earth is on the brink of its next mass extinction that could hit within the next several centuries, as a result of human-induced habitat destruction and environmental degradation, said Jan Zalasiewicz, a geologist at the University of Leicester in the United Kingdom who studies Earth history. Just in the past several hundred years, thousands of animal species have become endangered, and hundreds have gone extinct, many as a result of human activity.
Zalasiewicz and colleagues have developed a thought experiment in which they consider which animal might be the most likely to survive and repopulate the world if this purported mass extinction were to take place — and they concluded that rats may be the best candidates.
The researchers based their hypothesis on rats' proven ability to infiltrate most major landmasses and islands on the planet, as well as their persistence throughout the world despite widespread attempts to control their populations. Other animals, such as cats and feral pigs, also do well in diverse ecosystems around the world, but they are not as widespread as rats. In the event of a mass extinction caused either by human activity or a catastrophic event, rats are theoretically the most likely mammals to be spared, given their wide extent and ability to cope in varied conditions, Zalasiewicz said.
The earliest Caribbean rodents: Oligocene caviomorphs from Puerto Rico
Authors:
Vélez-Juarbe et al
Abstract:
The Greater Antilles was once home to a diverse array of endemic groups of caviomorph rodents. Many of these became extinct, together with endemic insectivores, sloths, and primates, in what is widely considered an archtypical example of Quaternary extinction on oceanic islands. When and how the ancestors of these land mammals arrived on these islands is debated. Here we report on the discovery, for the first time, of undoubted rodent incisors in Oligocene contexts in Puerto Rico. The enamel microstructure of the Puerto Rican fossils is consistent with caviomorph affinities, particularly with extinct Pleistocene species from Puerto Rico and Anguilla (Lesser Antilles). These fossils represent the oldest rodents now known from the West Indies, as well as the oldest record of caviomorphs north of the Equator. Their presence in the Greater Antilles by the early Oligocene suggests that caviomorph rodents must have begun to disperse throughout South America, and beyond, very soon after their initial radiation (currently pegged to middle or late Eocene). This, together with other lines of evidence, is consistent with the landspan hypothesis for the origin of the Greater Antillean land mammal fauna, although other mechanisms, including overwater transport, cannot be ruled out.
