Dinosaurs dug for mammalian prey

Thursday, March 15, 2012

Mounting evidence suggests dinosaurs preyed upon our mammalian ancestors (Source: brotherxii/Flickr)

Larry O'Hanlon
Discovery News

Fossilised mammal burrows that appear to have been clawed out by a predator suggests dinosaurs dug into mammal dens to get furry morsels.

Since there were no large mammal predators 80 million years ago, the most likely candidates are theropod dinosaurs.

Making the connection even stronger is that claw marks in the burrows are a pretty good match to the claws of dinosaur fossils found in rocks nearby, though slightly later in time.

"It's pretty tight," says palaeontologist Professor Edward Simpson of Kutztown University of Pennsylvania. "We can't say whether it's a troodont or a velociraptor," because the claw bones of those found nearby have lost their nails, or cuticles. But otherwise the match is a good one, he says.

Simpson and his students have published their 'trace fossil' discovery - that is, fossilised evidence of animal behaviours rather than the animals themselves - in the August issue of the journal Geology.

"To me there is almost no doubt," says trace fossil expert Professor Anthony Martin of Emory University. "It's very good circumstantial evidence."

No actual mammal bones or teeth have been found, though the burrows match the complexity of those of other mouse- or shrew-like mammals of that time and their present-day counterparts.

The criteria for calling something a fossilised mammal burrow were laid out a few years ago in a separate paper by Martin.

Other evidence

With that criteria in mind, the team feels confident that they did find a mammal's subterranean abode. The researchers could go even further to make their case.

"One of the things we could do is to take the burrows and cut them up," says Simpson. There might be mammal teeth in them, which make an even stronger case.

But, he's hesitant to do that since it's a destructive process. There is also only a very small chance they will find any mammal bones or teeth.

So far Simpson and his students haven't even extracted the fossil burrow from the cliff in southern Utah in which it was found.

As for the dinosaur involved, it probably wasn't very big, judging by the claw marks in the burrows, says Martin.

Other evidence that has been reported to support the dinosaurs-bites-mammal story include the specialised jaw, teeth and skulls of some dinosaurs; mammal bones with bite marks; fossilised gut content; and faeces and trackways.

This case is different in that it points directly at how the dinosaurs hunted rather than just the fact that they ate mammals.

"This is an excellent example of how trace fossils can reveal animal behaviour," says Martin.

Dino demise supersized the mammals

Abbie Thomas

The demise of the dinosaurs kick-started a growth spurt in mammals that would see them become supersized within a mere 25 million years, new research has found.

The international team led by Felisa Smith from the University of New Mexico publish its findings today in the journal Science.

The mass extinction event at the end of the Cretaceous period, 65 million years ago, wiped out all the non-flying dinosaurs virtually overnight, as well as many other animals, plants and insects.
Suddenly there was room and resources for the mammals to flourish.

The researchers tracked how mammal body size changed over time by identifying the largest mammal from each geological period since the end of the Cretaceous. All major mammal groups including elephants, cats and horses were included.

For the extinct animals, often only fossil teeth were available. To work out how large the body might have been, researchers used modern day animals as a comparison. They calculated the ratio of tooth size to body size for these modern species, and then extrapolated this to extinct mammals.

When size was tracked over time, it was revealed that mammals eventually grew to a thousand times larger than they had been when they shared the Earth with dinosaurs.

The pinnacle of land mammal size was achieved by the bizarre Indricotherium, a hornless rhinoceros-like herbivore that lived around 34 million years ago. At 17 tonnes and standing five and a half metres at the shoulder, it would have dwarfed today's African elephant.

This pattern of increasing size in mammals after the demise of the dinosaurs repeated itself across all continents, including North America, Africa, Eurasia and to a lesser extent, South America, say the researchers.

So how did the dinosaurs keep the mammals at bay for so long?

Keeping mammals at bay

"It was most likely competition for resources [rather than direct predation]," says Dr Alistair Evans, a palaeontologist at Monash University in Melbourne and a co-author on the paper.

"The dinosaurs were there first, so they were able to fill the ecological niches very effectively, for example feeding on plants and carnivory: They could do it better than the mammals could," says Evans. "So there would have been limited opportunity for these smaller mammals to evolve into larger sizes."

He says, the researchers found that larger animals evolved whenever the Earth got cooler. A big body helps conserve heat, last longer without food and travel further to find it.

But mammals can't keep growing forever. The researchers say that mammal body size will always be limited by environmental temperatures and available land area.

Being big also means slower reproduction rates and a certain vulnerability to changes in the environment.

"We're talking over tens of millions of years, but it may be that if the world gets warmer in the future, the larger mammals may well go extinct again, because they are adapted to cooler climates," says Evans.

How the world's biggest mouth evolved

Blue whales are the largest creatures that have ever lived (Source: Carl Buell)

Anna Salleh

An Australian palaeontologist has figured out a missing step in the evolution of giant filter-feeding mouths characteristic of blue whales.

Dr Erich Fitzgerald from Museum Victoria in Melbourne reports his argument in today's issue of Biology Letters.

"You could fit an average garden-variety kombi van in the mouth of a blue whale," says Fitzgerald, adding that blue whales are the largest animal ever known to inhabit the earth.
They have no teeth but, like other such whales, live on a diet of krill and other marine organisms that they filter out from seawater, using bristles on the roof of their mouths, called baleen.
Central to this baleen whale filter-feeding system is a cavernous mouth with a wide upper jaw and an elastic lower jaw that can open up wide to allow more than the whale's own bodyweight in sea water to enter in one gulp.

"[Modern baleen whales] have extremely mobile lower jaws, which is quite frankly bizarre because no other mammals have that sort of specialisation," says Fitzgerald.

This elastic lower jaw, in which the left and right hand sides are able to stretch apart, was until now believed to be a feature of all baleen whales, even fossil ones.

Scientists have long wondered how ancestral baleen whales, which used their teeth to catch large prey (like killer whales do) evolved into toothless filter feeders.

"This is a huge evolutionary jump," says Fitzgerald.

He now believes he has found the evolutionary missing link in the story.

Missing link

Fitzgerald has found the first fossil evidence of a toothed baleen whale that has no elastic lower jaw.
The newly-described jaw belonged to a tiny 25 million-year-old primitive baleen whale called Janjucetus hunderi, which was at most just three metres long, the size of a bottlenose dolphin.

"This is the clearest evidence yet that the earliest baleen whales could not filter feed and that's interesting because it had previously been thought that all baleen whales were filter feeders," says Fitzgerald.

He first analysed and named this creature in 2006, but at that stage he only had an incomplete lower jaw.

Fitzgerald then came across missing lower jaw bones in the collection of an amateur fossil hunter, by the name of Brian Crichton, who originally found them in the 1970s on a beach near Torquay in Victoria.

These new bones showed that the two halves of the lower jaw bone in Janjucetus hunderi were fused, and unable to open up to allow filter feeding.

Yet, Fitzgerald had previously found the animal had evolved another feature thought to be essential for the filter feeding - a wide upper jaw that creates a large space inside the mouth.

So why did this toothed whale evolve a wide upper jaw?

Suction feeding

Fitzgerald finds a clue in the mouths of modern dolphins, which also lack an elastic lower jaw. Those with really wide upper jaws feed by sucking in large individual prey, he says.

"They generate a vacuum [helped by the wide upper jaw] and hoover up fish and squid, sucking them in through a relatively small opening at the front of their mouths," says Fitzgerald.

"I argue that the big mouth of baleen whales possibly originally evolved to enhance the ability to generate suction."

He says it would be less of an evolutionary leap to go from baleen whales that catch large prey with their teeth to those that suction feed, than directly to those that filter feed of lots on tiny organisms.
After being decimated by past whaling the numbers of blue whales remain low with only about 10,000 individuals left, mainly in the Southern Ocean, says Fitzgerald.

Although they are now protected, he says they remain under threat due to changes in the ocean ecosystem that may affect levels of krill.

Mammals 'thrived despite dinosaurs'

Multituberculate mammals like this evolved teeth that were suited to eating flowering plants, researchers say (Source: Jude Swales/Burke Museum of Natural History and Culture)

Anna Salleh

Mammals did not need the dinosaurs to die off in order to thrive, new research on fossil teeth suggests.

Evolutionary biologist Dr Alistair Evans, of Monash University in Melbourne, and colleagues, report their findings today in the journal Nature.

Until now, most scientists believed it took the extinction of the dinosaurs, around 66 million years ago, before mammals were able to develop, says Evans. But his latest research suggests this is not the case.

Before the death of the dinosaurs, the most common mammals were multituberculates - so called because of their teeth.

"Many of the multituberculates have very bumpy teeth and each of the bumps are called a tuberculate," says Evans. "It's a very long name for 'bumpy teeth'".

Evans and colleagues carried out a comprehensive survey of teeth from about 48 multituberculate species held in fossil collections around the world.

They used software normally used to analyse land topography to create high-resolution 3D images of the teeth.

By analysing the complexity of the teeth bumps, Evans and colleagues were then able to map the evolution of multituberculates.

170 million years ago, their teeth were very simple but about 90 million years ago - long before the dinosaurs died off - their teeth started getting more complex.

Previous research has shown that increasing complexity in teeth enables animals to shift from eating insects and other meat to plant material.

Plants are easier to come by, but they also take longer to digest so a shift to herbivory is also generally associated with an increase in body size. Larger animals have a lower relative metabolic rate and have the time to digest slowly.

Significantly, Evans and colleagues found the increase in multituberculate teeth complexity coincided with the evolution of flowering plants.

"There's a nice correlation there between the ecological dominance of flowering plants and the rise of herbivory in multituberculates," says Evans.

At the same time multituberculates increased in size from that of a mouse to a beaver, also supporting the move to herbivory.

Evans and colleagues think these changes gave multituberculates a way of thriving even during the time of dinosaurs.

"We think it might be because they were exploiting this new food source that perhaps dinosaurs weren't using as much as they could have," says Evans.

These changes also gave multituberculates an evolutionary head start compared to other mammals, meaning they were able to do better than others once the dinosaurs did go extinct.

Multituberculates maintained an edge over other mammals for another 30 million years after the dinosaur extinction, and only died out 35 million years ago, due to competition from primates and rodents.


Curator of Vertebrate Palaeontology at Museum Victoria, Dr Tom Rich describes the research as "a new and imaginative approach" to quantifying the adaptation of the multituberculates.

"The authors show quite persuasively that the widely held conventional view that mammalian evolution was 'held back' until the non-avian dinosaurs became extinct is a generalisation that does not hold in detail," he says.

Rich says although there are literally thousands and thousands of multibuerculate fossils in museum collections from sites in the Northern Hemisphere, there is just one specimen known from Australia.
"But we've got one so we know they were here, too! That's all it takes," he says.