New evidence raises questions about when dinosaurs evolved in North America

August 12, 2014, Massachusetts Institute of Technology
Collage: Jose-Luis Olivares/MIT (original background photograph courtesy of Malka Machlus from Lamont-Doherty Earth Observatory of Columbia University)

The Jurassic and Cretaceous periods were the golden age of dinosaurs, during which the prehistoric giants roamed the Earth for nearly 135 million years. Paleontologists have unearthed numerous fossils from these periods, suggesting that dinosaurs were abundant throughout the world. But where and when dinosaurs first came into existence has been difficult to ascertain.

Fossils discovered in Argentina suggest that the first dinosaurs may have appeared in South America during the Late Triassic, about 230 million years ago—a period when today's continents were fused in a single landmass called Pangaea. Previously discovered fossils in North America have prompted speculation that dinosaurs didn't appear there until about 212 million years ago—significantly later than in South America. Scientists have devised multiple theories to explain dinosaurs' delayed appearance in North America, citing environmental factors or a vast desert barrier.

But scientists at MIT now have a bone to pick with such theories: They precisely dated the rocks in which the earliest dinosaur fossils were discovered in the southwestern United States, and found that dinosaurs appeared there as early as 223 million years ago. What's more, they demonstrated that these earliest dinosaurs coexisted with close nondinosaur relatives, as well as significantly more evolved dinosaurs, for more than 12 million years. To add to the mystery, they identified a 16-million-year gap, older than the dinosaur-bearing rocks, where there is either no trace of any vertebrates, including dinosaurs, in the rock record, or the corresponding rocks have eroded.

"Right below that horizon where we find the earliest dinosaurs, there is a long gap in the fossil and rock records across the sedimentary basin," says Jahan Ramezani, a research scientist in MIT's Department of Earth, Atmospheric and Planetary Sciences. "If the record is not there, it doesn't mean the dinosaurs didn't exist. It means that either no fossils were preserved, or we haven't found them. That tells us the theory that dinosaurs simply started in South America and spread all over the world has no firm basis."

Ramezani details the results of his geochronological analysis in the American Journal of Science. The study's co-authors are Sam Bowring, the Robert R. Shrock Professor of Geology at MIT, and David Fastovsky, professor of geosciences at the University of Rhode Island.

The isotope chronometer

The most complete record of early can be found in Argentina, where layers of sedimentary rock preserve a distinct evolutionary progression: During the Late Triassic period, preceding the Jurassic, dinosaur "precursors" first appeared, followed by animals that began to exhibit dinosaur-like characteristics, and then advanced, or fully evolved, dinosaurs. Each animal group is found in a distinct rock formation, with very little overlap, revealing a general evolutionary history.

At the Blue Mesa locality of the Petrified Forest National Park, a few logs of petrified wood (rust colors) crop out from within sandstones (white).

In comparison, the dinosaur record in North America is a bit muddier. The most abundant fossils from the Late Triassic period have been discovered in layers of rock called the Chinle Formation, which occupies portions of Arizona, New Mexico, Utah, and Colorado, and is best exposed in Petrified Forest National Park. Scientists had previously dated isolated beds of this formation, and determined the earliest dinosaur-like animals, discovered in New Mexico, appeared by 212 million years ago.

Ramezani and Bowring sought to more precisely date the entire formation, including levels in which the earliest dinosaur fossils have been found. The team took samples from exposed layers of sedimentary rock that were derived, in large part, from volcanic debris in various sections of the Chinle Formation. In the lab, the researchers pulverized the rocks and isolated individual microscopic grains of zircon—a uranium-bearing mineral that forms in magma shortly prior to volcanic eruptions. From the moment zircon crystallizes, the decay of uranium to lead begins in the mineral and, as Ramezani explains it, "the chronometer starts." Researchers can measure the ratio of uranium to lead isotopes to determine the age of the zircon, and, inferentially, the rock in which it was found.

A unique but incomplete record

The team analyzed individual grains of zircon, and created a precise map of ages for each sedimentary interval of the Chinle Formation. Ramezani found, based on rock ages, that the fossils found in New Mexico are, in fact, not the earliest dinosaurs in North America. Instead, it appears that fossils found in Arizona are older, discovered in rocks as old as 223 million years.

In this North American mix, the early relatives of dinosaurs apparently coexisted with more evolved dinosaurs for more than 12 million years, according to Ramezani's analysis.

"In South America, there is very little overlap," Ramezani says. "But in North America, we see this unique interval when these groups were coexisting. You could think of it as Neanderthals coexisting with modern humans."

While fascinating to think about, Ramezani says this period does not shed much light on when the very first dinosaurs appeared in North America.

"The fact that our record starts with advanced forms tells us there was a prior history," Ramezani says. "It's not just that advanced dinosaurs suddenly appeared 223 million years ago. There must have been prior evolution in North America—we just haven't identified any earlier dinosaurs yet."

He says the answer to when dinosaurs first appeared in North America may lie in a 16-million-year gap, in the lower Chinle Formation and beneath it, which bears no fossils, dinosaurian or otherwise. The absence of any fossils is unremarkable; Ramezani notes that fossil preservation is "an exceptional process, requiring exceptional circumstances." Dinosaurs may well have first appeared during this period; if they left any fossil evidence, it may have since been erased.

"Every study like this is a step forward, to try to reconstruct the past," Ramezani says. "Dinosaurs really rose to the top of the pyramid. What made them so successful, and what were the evolutionary advantages they developed so as to dominate terrestrial ecosystems? It all goes back to their beginning, to the Late Triassic when they just started to appear."

The new dates provide a framework against which other theories of dinosaur evolution may be tested, says Raymond Rogers, a professor of geology at Macalester College in Saint Paul, Minn., who was not involved in this work.

"This is the kind of careful work that needs to be done before evolutionary hypotheses that relate to the origination and diversification of the can be addressed," Rogers says. "This gap in the Chinle fossil record makes comparing the North American and South American dinosaur records problematic. Existing hypotheses that relate to the timing of dinosaur evolution in North and South America arguably need to be reconsidered in light of this new study."

Explore further: Early dino was turkey-sized, social plant-eater

More information: Paper: "Revised chronostratigraphy of the lower Chinle Formation strata in Arizona and New Mexico (USA): High-precision U-Pb geochronological constraints on the Late Triassic evolution of dinosaurs":

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Aug 12, 2014
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5 / 5 (8) Aug 12, 2014

Why don't you just admit your fundamentalist religious beliefs, not science, is the root of your non-acceptance of evolution?
5 / 5 (7) Aug 12, 2014
Verkle is suggesting that we HAVE To find, and show him each generation of dinos before he'll Consider it.

Vietvet you are right, until he pulls his head out of his nether nose there is no hope for him.

-alpha10bravo -Parseval
4.3 / 5 (12) Aug 12, 2014

Why don't you just admit your fundamentalist religious beliefs, not science, is the root of your non-acceptance of evolution?
-Because then he would have to admit that is beliefs come entirely from a book that describes things which we now know never happened.

And then he would have to admit that he has no business arguing evidence when his god apparently has a complete disregard for ti.
1.4 / 5 (12) Aug 12, 2014
@ -verkle @ -vietvet @ -TheGhostofOtto1923 Everyday, new evidence is found, to substantiate or disprove both religion and science. And, every time, when we think we know the answers - our minds get blown away! As soon as we come up with a theory, we just as quickly, come up with a new and improved theory! So, in essence, we still have a long ways to go "Baby" ! Until then, anything and everything is possible!
5 / 5 (11) Aug 12, 2014

How does this reveal evolutional history? If evolution were true, there would be millions of links between each of these animal groups, because "evolution" is supposed to be slow, over millions and billions of years.

There ARE millions of links! And the sedimentary rock formation also takes place over millions of years. That how they can use them to date the formation of the fossils.

I actually was shocked to have read your comment. Read up a bit before you go making yourself look silly.
5 / 5 (8) Aug 12, 2014
Everyday, new evidence is found, to substantiate or disprove both religion and science . . .

Science doesn't need any substantiation or proof- science is a methodology not an ideology. Religion on the other hand . . .
4.1 / 5 (9) Aug 12, 2014
Everyday, new evidence is found, to substantiate or disprove both religion
No, the evidence is conclusive. It tells us that things were going on back then which make the bible fables impossible. This act of deception would require much more effort than simply committing mass murder by flooding the entire world.

Not only would god have had to obliterate evidence, he would have had to replace it with completely convincing contrary evidence. Massive amounts of it, over the course of many centuries.

"Tel Aviv University archaeologist Ze'ev Herzog:

"This is what archaeologists have learned... the Israelites were never in Egypt, did not wander in the desert, did not conquer the land in a military campaign and did not pass it on to the 12 tribes of Israel. Perhaps even harder to swallow is that the united monarchy of David and Solomon, which is described by the Bible as a regional power, was at most a small tribal kingdom."

-Your god is phony like all the rest. Big surprise.
4 / 5 (8) Aug 12, 2014
No, the evidence is conclusive. It tells us that things were going on back then which make the bible fables impossible. This act of deception would require much more effort than simply committing mass murder by flooding the entire world
But then the lack of absolutely any evidence FOR, and an overwhelming amount of evidence AGAINST, this event tells us it never happened either. Not to mention that the fable was copied verbatim from previous religious myths.

This is exactly the same thing as joseph smith taking standard illustrations of egyptian animal gods, replacing their heads with human ones, and then claiming that it was proof that the patriarchs were in egypt. And thinking nobody would notice.

It is ignorant, crude, insulting, and outrageous.

Joseph smith - almost as beautiful as the caucasian jesus.

-Or muhammud

5 / 5 (4) Aug 13, 2014
@ tGoA- Totally agree- any attempt reconcile science and religion are misbegotten from the start- there is nothing to reconcile when one of them has the same answer for every question.
1 / 5 (1) Sep 13, 2014
Well, I guess it's really late. And this conversation is over. I was notified, to verify my account on this site. When I noticed that I had forgotten this conversation.

So, I only have this to say:

I don't, quite, get what the big deal is. This dispute over Creation vs. Evolution. Other than the fact, of course, the different religious fanatics taking they're agendas to the maximum extreme. And, totally blowing it out of proportion.

(My) point is: All things are being created, and, All things are evolving. All the time!
Science is trying to uncover the mysteries of the universe. It's awesome! The mystery is GOD!

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