Giants among us: Paper explores evolution of the world's largest mammals

Indricotherium, based on new skeletal reconstruction.
Indricotherium, based on new skeletal reconstruction. Image: Wikipedia.

The largest mammal that ever walked the Earth -- Indricotherium transouralicum, a hornless rhinoceros-like herbivore that weighed approximately seventeen tons and stood about eighteen feet high at the shoulder -- lived in Eurasia almost 34 million years ago. In a paper just published in the journal Science, an international team of researchers has compiled and analyzed an enormous database of information about the largest mammals across time and around the globe, revealing striking patterns in their evolution.

The research, funded by a National Science Foundation Research Coordination Network grant, was led by scientists at the University of New Mexico, who brought together paleontologists, and macroecologists from universities around the world. University of Georgia Odum School of Ecology dean John Gittleman and postdoctoral researcher Patrick Stephens were among them.

"We were invited to participate because the group wanted to take an explicitly evolutionary approach to size," said Gittleman, whose research focuses on large-scale ecological and evolutionary problems, from disease to extinction to organism characteristics.

"John and Patrick were indispensable members of our team," said Felisa Smith, associate professor of biology at the University of New Mexico and the paper's lead author. "This really was a project that took all of us to accomplish."

The goal of the research was to revisit key questions about size, specifically in mammals. "Size impacts everything, from reproduction to extinction," said Gittleman. "And mammals are a good test case. There is so much variation—everything from mice to elephants—and there is also far more data available about mammals than other taxonomic groups."

"There is a much better fossil record for mammals than for many other groups," Stephens said. "That's partly because mammals' teeth preserve really well. And as it happens, tooth size correlates well with overall body size."

The researchers spent two years assembling the data. "The database is powerful and unique," said Gittleman. "It includes information on the size of all mammals, living and fossil, from around the world."

With access to so much information, the group was able to test a hypothesis about the evolution of mammal size.

"During the Mesozoic, mammals were small," said Gittleman. "Once dinosaurs went extinct, mammals evolved to be much larger as they diversified to fill ecological niches that became available. This phenomenon has been well-documented for North America; we wanted to know if the same thing happened all over the world."

The researchers found that the pattern was indeed consistent, not only globally but across time and across trophic groups and lineages—that is, animals with differing diets and descended from different ancestors—as well. The maximum size of mammals began to increase sharply about 65 million years ago, peaking in the Oligocene Epoch (about 34 million years ago) in Eurasia, and again in the Miocene Epoch (about 10 million years ago) in Eurasia and Africa.

"Having so many different lineages independently evolve to such similar maximum sizes suggests that there were similar ecological roles to be filled by giant across the globe," said Gittleman. "The consistency of the pattern strongly implies that biota in all regions were responding to the same ecological constraints."

Global temperature and the amount of land available as an animal's range are two ecological factors that appear to correlate with the evolution of maximum body size, but Gittleman warned against assigning cause and effect. "A big part of science is seeing patterns, and then producing new hypotheses and testing them," he said. "We have now identified this pattern very rigorously."

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Dec 03, 2010
11. Why did the previous mammals have small bodies?
12. Why the body size of the mammals are different?

Relationship between Gravity and Evolution
(The theory of the increasing of the gravity)
BY: Ramin Amir Mardfar

I've expressed initials of my theorem in 1986. I sent my first article to different associations of zoology, paleontology, and geology in different countries such as London Zoology Association. My first article was published by "Ettelaate Elmi" magazine in 1996. Then my other fifteen articles was published there. My first book consisting a collection of these articles was printed and published in 2000. I published translation of my articles on a website in 2001 and on Dec.2001 I created Paleogravity group in Yahoo Groups for discussions on this respect. Before my book has been printed I asked related questions from myself and wrote their answer in my book. Many people has discussed with me about this theorem

Dec 03, 2010
Many people has discussed with me about this theorem but no one has been able to ask a new question except those in my book.

Despite the progress of science in different scientific fields, there are so many unanswered questions about paleontology, geology and zoology. So many theories have been posed to answer this question, but they could not be accepted by scientists, as they should be. These kinds of question include: the causes of the extinction of dinosaurs, the causes of extinction of different species of animals in past era, the causes of the extinction of primitive giant plants belonged to carbonifer period, the causes of having small bodies for some animals and large bodies for the others, the reason why animals can not grow more than their present size and which factor prevents them to grow more, the reason, why some sea mammals come to coast and die there, the causes of physiologic changes when the gravity is zero, the causes of eruption and outflow of molten materials

Dec 03, 2010
the causes of eruption and outflow of molten materials from the ocean beds, the causes of insufficient growth of some animals and their early sexual maturity and other questions like these.

These are the questions, which are not answered by scientists of different sciences. Although they have tried to solve them with detailed and long experiments, but they could not find clear and simple answers.

The theory of the increase of gravity tries to find satisfactory answers to these questions by using the phenomenon of increase of gravity and by citing evidences. Obviously, a theory is not able to solve all the problems clearly and without mistakes, at first, and they need more discussion and criticism to be solved. However, this theory tries to find logical answer to these questions as far as possible.
So far there have been no discussion that whether the gravity has changed by the passage of time or it has remained unchanged. And so far no one claimed that the gravity have been u

Dec 03, 2010
And so far no one claimed that the gravity have been unchanged and fixed during the past periods up to now. May be it is better to have a careful look at this theory and consider the evidences and proofs carefully, perhaps this theory can be able to pave the way for us and help us and help us to reach our main goal which is the improvement of our knowledge.

I'll be glad knowing your openions in this respects.

Ramin A.Mardfar

1. Why the insects have so small sizes?
2. Dose the force of gravity increase?
3. Plants, other witnesses for the increase of gravity
4. Megatherium and the increase of gravity
5. How can the gravity increase?
6. Gravity variation from the equator to poles?
7. The sea animals and the increase of gravity
8. The relationship between the blood system and air pressure
9. The extinction of dinosaurs and the gravity increase
10. The circulatory system in zero gravity
11. Why did the previous mammals have small bodies?
12. Why the bod

Dec 03, 2010
12. Why the body size of the mammals are different?
13: Relationship between Pressure & Growth speed
14: If the Earth Expanded, where did all the extra mass come from?
15: A physical question
16: New Hearts in the future
17: Axolotl
18: The new class of animals
19: Why the body of Coelacanth grow several time?
20: The relation between blood circulation system and animals body size
21: Were dinosaurs warm-blooded or cold-blooded?
22: The method of finding gravity in the past time
23: The graph of land animals' size during the tim
24: Pterodactyl

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