Where have all the ornithischians gone?

[Danvarner@aol.com]

I'm posting this from vrtpaleo. I seem to  remember Mesozoic crocs with 
ornithichian-like teeth being illustrated years  ago. Can anyone refresh my 
ageing 
memory? DV

<< Where have all the  ornithischians gone?
Petrified Forest find wipes out record
of these  Triassic dinosaurs outside South America 

FOR IMMEDIATE RELEASE  

See full story with images  at
http://www.berkeley.edu/news/media/releases/2005/06/24_revuelto.shtml  

Berkeley - The fossilized skeleton of a small crocodile relative  excavated
last year at Petrified Forest National Park in Arizona throws a  wrench into 

theories of how and where the dinosaurs arose more than 210  million years
ago at the end of the Triassic Period. 

The animal, one  of many creatures from the Late Triassic known only from
their teeth, was  thought to be an ancestor of the plant-eating
ornithischian dinosaurs like  Stegosaurus and Triceratops, which roamed the
world millions of years later  in the Jurassic and Cretaceous periods. 

The fact that this presumed  dinosaur, Revueltosaurus callenderi, is instead 

a crocodile ancestor  does not merely disappoint rockhounds, who sell the
abundant teeth as  "dinosaur teeth," but it also throws into question the
identity of other  presumed dinosaur ancestors known only from teeth, which
includes all Late  Triassic ornithischians outside South America. 

"Because the teeth look  like those we know from herbivorous ornithischians, 

people assigned them  to the dinosaurs," said Randall Irmis, a graduate
student in the Department  of Integrative Biology and the Museum of
Paleontology at UC Berkeley. "We  think we've shown that you can't rely on
the dentition to determine what is  an early dinosaur, which casts doubt on
all the ornithischians from the  Triassic of North America." 

This suggests, he said, that the herbivorous  ornithischians and the
meat-eating theropods, like Tyrannosaurus rex, did not  evolve together in
the Late Triassic as many paleontologists thought. Rather,  the theropods
were well established worldwide before the ornithischians  spread out into
North America, Europe and Africa at the very end of the  Triassic, perhaps
from possible origins in South America. 

"We have  pretty much erased the record of Triassic ornithischian dinosaurs
from North  America, Europe and worldwide, except for South America," said
the fossil's  discoverer, park paleontologist William Parker. "Even the
fossils in South  America aren't very well preserved, and people argue
whether they're  dinosaurian as well. If the South American animals are
indeed ornithischians,  then it argues that the group possibly arose there." 

Parker, Irmis and  their colleagues - graduate student Sterling J. Nesbitt
of the Lamont-Doherty  Earth Observatory at Columbia University in New York, 

graduate student  Jeffrey W. Martz of Texas Tech University in Lubbock, and
Lori S. Browne of  the South Dakota School of Mines and Technology in Rapid
City - reported  their find in the May issue of the journal Proceedings of
the Royal Society,  Biological Sciences. 

Dinosaurs are divided into two main groups, the  ornithischians and
saurischians, both of which, based on fossil finds,  appeared to arise in
the Late Triassic, between 225 and 200 million years  ago. The early
ornithischians were thought to have evolved into animals such  as the
stegosaurs of the Jurassic and the frilled and duck-billed dinosaurs  of the 

Cretaceous, while the saurischians split into the sauropodomorphs  - the
long-necked browsers like Brachiosaurus - and the carnivorous  theropods,
like T. rex. Only the theropods survive to the present as birds.  

If, as the team concludes, the first ornithischians outside South  America
did not appear until 25 million years later than people thought,  the
picture of dinosaur evolution radically shifts. 

"Basically, you  have two groups that were thought to appear at the same
time and co-evolve,  and now you have one completely wiped away," Parker
said. "So where did the  ornithischian dinosaurs come from? We don't know
right now. That's something  we are going to address in the future." 

Irmis acknowledges that early  ornithischians may have been around, yet were
rare and living in places not  conducive to the preservation of fossils. But
they definitely were not as  common and diverse in the Late Triassic as some
paleontologists have claimed.  

"Sauropodomorphs and theropods are geographically widespread by the end  of
the Triassic, though the theropods at this time also were small and  rare,"
Irmis said. "What we found suggests that ornithischian dinosaurs did  not
diversify and get large until the beginning of the Jurassic."  

"People have argued that there is a great diversity of dinosaurs in  the
Late Triassic, and this shows that they are not as diverse as  many
thought," Parker said. "Diversity is only found among the ancestors of  the
sauropods and theropods. Exactly how diverse were these other  groups?
That's something we also want to test." 

Revueltosaurus is a  relatively recent discovery, having been named only in
1989 from teeth found  in Revuelto Creek, New Mexico, by Adrian Hunt. Kevin
Padian, professor of  integrative biology at UC Berkeley, found and
identified Revueltosaurus teeth  from Petrified Forest National Park in
1990, though an earlier UC Berkeley  paleontologist, Charles Camp, had found 

but not identified teeth from  these creatures in the early 1930s.
Unknowingly, Hunt, Padian and Camp had  also found other bones of
Revueltosaurus, but because these remains were not  found with the teeth,
the association could not be made. 

The newly  discovered full skeleton of a Revueltosaurus makes it clear that
the teeth  are not of a plant-eating dinosaur, but of a herbivorous or
perhaps  omnivorous crocodilian ancestor living a mostly terrestrial life in 

the  uplands of the Late Triassic. It may have been one source of meat for
the  developing theropods around the world. 

"This find is a great thing for  the crocodilian record, too. Here's this
totally unrecognized group of  possibly herbivorous crocodilians," Parker
said. "The convergent evolution of  the teeth is what makes them look like
herbivorous dinosaurs. That's the only  thing similar in the entire
skeleton. There are no other dinosaur characters  in the entire animal." 

Parker picked up the first fossils in March of  2004 at a promising outcrop
in an area of the park that evidently had not  been scouted by previous
paleontologists, many of them from UC Berkeley. The  campus has been
involved in the park, located in northeastern Arizona near  Holbrook, since
John Muir first picked up fossil bones and teeth while  passing through in
1906 and deposited them with the University of California.  

Parker's initial fossil finds were crocodilian-like armor, but he  didn't
realize they were from Revueltosaurus until the next day, when he  brought a 

colleague to the site and she picked up a jaw with teeth.  During May and
June of 2004, Parker, Irmis and their coauthors excavated one  complete
Revueltosaurus skeleton from the 6-square meter quarry they dug,  plus bones 

from more than a dozen other individuals. In June of 2005, a  second
relatively complete skeleton was uncovered from the same site.  

Irmis, who first started working in the park with Parker in 2002 as  an
undergraduate at Northern Arizona University, said that this  crocodilian
ancestor was only three to four feet long, with a stubbier, less  flattened
skull and a less sprawling leg posture than today's crocodiles  have. Also,
the armor did not cover the entire body, but was restricted to  two lines
down the back. The creature is most similar, he said, to a group  of
Triassic archosaurs - the group that includes crocodiles, dinosaurs  and
birds - called the aetosaurs. 

Irmis, Parker and Nesbitt are  continuing their look at the alleged
ornithischians of the Late Triassic to  ascertain the quality of the fossil
record and what it says about early  dinosaur evolution. 

The work was supported by the Recreational Fee Demo  Program of Petrified
Forest National Park and by the Petrified Forest Museum  Association. 

### 

NOTE: Randall Irmis can be reached at (510)  643-2109 or irmis@berkeley.edu. 

Bill Parker is at (928) 524-6228 x262 or  william_parker@nps.gov. 


Bob Sanders
Manager of Science  Communications
UC Berkeley Office of Media Relations
(510)  643-6998
(510) 642-7289  Fax
rsanders@berkeley.edu
http://newscenter.berkeley.edu/ >>