An Ancient Mammal Paves the Way for Modern Rodents

Paleontologists identify an ancient mammal species and ancestor to modern animals like rats and mice

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Illustration by April Isch / University of Chicago

Like most early nocturnal mammals, the Rugosodon was active at night. This reconstruction shows the Rugosodon searching for food among ferns and cycads on the lakeshores in the darkness

On the geological timescale, humans’ stay on the planet earth amounts to little more than the blink of an eye. Before we emerged, earlier creatures enjoyed golden ages for more than twice as long as primates have existed, and hundreds of times longer than the paltry few hundred thousand years Homo sapiens have walked the earth. One example is the order of small, furry mammals known as multituberculates, which were as common as rodents are now for roughly 130 million years — including much of the age of dinosaurs. They disappeared 35 million years ago, leaving no living descendants — the longest run of any mammalian lineage ever. Their species ranged from mouse- to beaver-size, and could be found digging burrows, scampering along the ground or scaling trees like squirrels.

Though bits and pieces of their fossilized remains have been discovered around the world, intact skeletons of early multituberculates are rare. So four years ago, when scientists at the Chinese Academy of Geological Sciences received a multituberculate fossil from a local who had unearthed it at the Tiaojishan fossil bed, it was cause for excitement — especially when, after examining the sediment layers the fossil came from, they realized it was a very old one. The scientists dated it to 160 million years ago, making it one of the earliest multituberculates ever, and in a report published this week in Science, they reveal key details of the life of a long-extinct creature and its descendants.

(MORE: Hola, Olinguito! The Smithsonian Discovers a New Mammal)

The most notable discoveries relate to the teeth and ankles of the mouse-size animal, a previously unknown species of multituberculate the team has named Rugosodon. They managed to remove a loose tooth from the rock; after taking detailed images using a scanning electron microscope, the researchers found that the tooth has a ridged surface similar to that of omnivorous modern mammals like the African dormouse. That evidence suggests that this creature ate both plants and insects. “This helps us to bridge a gap — the later herbivorous multituberculates really went through a stage of omnivory,” says Zhe-Xi Luo, a professor of vertebrate paleontology at University of Chicago and an author of the Science paper. “The vast majority of early mammals are mostly insectivores … and the latest multituberculates are perfectly good herbivores.”

Examining Rugosodon’s limbs, though, they made an even more surprising discovery. Although its bone structure made it clear that the animal ran along the ground — a behavior that usually requires stiff ankles like our own — Rugosodon was incredibly flexible, capable of rotating 180 degrees. They were ankles you’d sooner expect to see in a tree-climbing species, where suppleness is a boon — see modern squirrels, which have similar flexible ankles. Luo says seeing flexible ankles in this very early specimen suggests that the feature may have been common to all multituberculates.

The discovery of such a well-preserved, early multituberculate skeleton is thrilling, says Richard Cifelli, a professor of vertebrate paleontology at University of Oklahoma. “Most of the fossils of multis, and for that matter most mammals from the age of dinosaurs, are known from jaw fragments and teeth,” he says. “In fact, my own specialty, by default, has ended up being working mostly with isolated teeth. I kid you not, I have established new species on not only single teeth, but teeth that are incomplete. So this is really something.”

(MORE: Cockroaches, Sponges and Snakes: The Top 10 New Species)

These particular fossil beds are likely home to many more specimens dating to the early history of mammals. Last week, two papers in Nature — one of them co-authored by Luo — described two mammal fossils found in the Tiaojishan beds, which are in the same geographic area as those that yielded China’s famous feathered dinosaurs. “To many paleontologists, the so-called middle Jurassic, about the time period of this formation, is the sweet spot on the fossil record,” says Cifelli, who compares Tiaojishan to the Burgess Shale, a famous fossil-hunting spot in western Canada. “This is when you’re first getting explosive diversification of mammals, and that is demonstrated in spades by the stuff that’s coming out of this formation.”

But for all their bounty, these beds in China provide but the merest peephole into what was living on the planet more than 100 million years ago. There are many unanswered questions: Did this furry, mouselike creature run along the shores of the Sundance Sea in what is now North America? Which of the conifers and ferns that covered the planet before flowering plants evolved did it eat? And why, after surviving for so long, did they finally die out? Until paleontologists find more fossils, our imaginations will have to suffice.

2 comments
BabuG.Ranganathan
BabuG.Ranganathan

NATURAL LIMITS TO EVOLUTION: Only micro-evolution, or evolution within biological "kinds," is genetically possible (such as the varieties of dogs, cats, horses, cows, etc.), but not macro-evolution, or evolution across biological "kinds," (such as from sea sponge to human). How could species have survived if their vital tissues, organs, reproductive systems, etc. were still evolving? A partially evolved trait or organ that is not complete and fully functioning from the start would be a liability to a species, not a survival asset. Plants and animals in the process of macro-evolution would be unfit for survival. For example, “if a leg of a reptile were to evolve (over supposedly millions of years) into a wing of a bird, it would become a bad leg long before it became a good wing” (Dr. Walt Brown, scientist and creationist). Survival of the fittest actually would have prevented evolution across biological kinds! Read my Internet article: WAR AMONG EVOLUTIONISTS! (2nd Edition).

What about natural selection? Natural selection doesn't produce biological traits or variations. It can only "select" from biological variations that are possible and which have survival value. The term "natural selection" is a figure of speech. Nature doesn't do any conscious selecting. If a variation occurs in a species (i.e. change in skin color) that helps the species survive then that survival is called being "selected." That's all it is. Natural selection is a passive process in nature, not a creative process.

The real issue is what biological variations are possible, not natural selection. Only limited evolution, variations of already existing genes and traits, is possible. Nature is mindless and has no ability to design and program entirely new genes for entirely new traits. Evolutionists believe and hope that over, supposedly millions of years, random genetic mutations caused by environmental radiation will generate entirely new genes. This is total blind and irrational faith on the part of evolutionists. Read my articles.

 
Visit my latest Internet site: THE SCIENCE SUPPORTING CREATION .

I discuss: Punctuated Equilibria, "Junk DNA," genetics, mutations, natural selection, fossils, dinosaur “feathers,” the genetic and biological similarities between various species, etc., etc.

Babu G. Ranganathan*
B.A. Bible/Biology
 
Author of popular Internet article, TRADITIONAL DOCTRINE OF HELL EVOLVED FROM GREEK ROOTS
 
*I have given successful lectures (with question and answer period afterwards) defending creation before evolutionist science faculty and students at various colleges and universities. I've been privileged to be recognized in the 24th edition of Marquis "Who's Who in The East" for my writings on religion and science.





CottonMcKnight
CottonMcKnight like.author.displayName 1 Like

"Like most early nocturnal mammals, the Rugosodon was active at night." Who wrote this caption? Doesn't the definition of nocturnal mean active at night?