Posted on 05/17/2005 11:53:21 AM PDT by doc30
Berkeley - The bizarre plates and spikes that lined the backbones of the long-extinct stegosaurs were probably extreme examples of the often elaborate and colorful displays developed by animals to recognize fellow members of their species, according to an international team of paleontologists.
The team's analysis of stegosaur plates lends support to a growing consensus among paleontologists that the weird adornments of many dinosaurs - the horns of triceratops, the helmet-like domes of the pachycephalosaurs, and the crests of the duck-billed hadrosaurs - likely served no function other than to differentiate species, akin to birds' colorful feather ornamentation.
"Our studies of bone histology are telling us a lot about dinosaur social behavior and lifestyle," said Kevin Padian, professor of integrative biology at the University of California, Berkeley, and a curator in the campus's Museum of Paleontology. "We cut up and compared the internal structures of stegosaur plates and the smaller scutes of their ancestors, and found that a functional explanation for these plates doesn't make sense for all the stegosaurs. So we think that they're more likely involved in some type of species recognition, as with many African antelopes - you have to be different from all animals in the area so you don't get mixed up with other species."
"When people see bizarre structures, they always want to give them bizarre functions," said co-author Russell Main, a former UC Berkeley undergraduate now in graduate school in Harvard University's Department of Organismic and Evolutionary Biology. "But in the case of stegosaurs or even ceratopsians, like triceratops, and also in modern bovids and some other artiodactyls, where you see a number of different types of horn or antler arrangements, you don't necessarily need to apply functional explanations. They can be relatively easily explained by talking about species or mate recognition."
Padian, Main and coauthors John R. Horner of the Museum of the Rockies in Bozeman, Mont., and Armand de Ricqlès of the University of Paris report their analysis of dinosaur scutes and stegosaur plates in the spring issue of the journal Paleobiology, to be published later this month.
Stegosaurs were elephantine plant eaters that populated the world during the Jurassic period, about 210 to 144 million years ago, alongside ferocious predators like Allosaurus. Growing up to 20 feet from nose to tip of tail, the most recognized stegosaur, Stegosaurus stenops, had a double row of plates down the back with two or three pairs of spikes on the tail. Other stegosaurs had smaller plates, spikes instead of plates or some other combination. The thin plates and spikes, called scutes, were bony outgrowths of the skin, or osteoderms, and probably were covered with a horny keratin.
Previous paleontologists had proposed that the plates were like the ears of African elephants, designed for heat exchange. They would radiate heat on hot days to cool the animal, or absorb heat from the sun to warm the blood on cool days. Others suggested that they were for protection or for sexual display. In their paper, Padian, Main and their colleagues tend to reject each of these arguments as general explanations.
"These plates wouldn't offer much protection - they consist of a layer of dense bone surrounding a latticework of bone that would be like biting through a sandwich," Padian said. "Plus, we don't see a clear distinction between male and female stegosaurs. Without sexual dimorphism, you have no evidence for sexual selection, so you can't invoke sexual display as an explanation."
As for heat exchange, one major reason earlier scientists proposed such a function for stegosaur plates is that these plates have large blood vessels piercing their interior, perhaps channels to carry blood to be cooled or heated. But it turns out that these "pipes" lead to dead ends, so their roles as major blood vessels are difficult to establish.
To probe the possibility that the plates and spikes were heat exchangers, the paleontologists looked at the evolution of these skin growths in the thyreophoran family, which included the stegosaurs. The team obtained fossils from a half-dozen different species of thyreophorans, ranging from the stegosaurs' earliest ancestors - "armored" dinosaurs that lived 200 million years ago - to the first stegosaurs and related ankylosaurs - which had bony plates or scutes all over their bodies - to the last stegosaurs, which died out in the Early Cretaceous period more than 120 million years ago. All were plant eaters with formidable flat or erect plates on the neck, back and tail. The team sliced through about 10 fossil scutes to study their internal structure.
The earliest thyreophorans, such as the North American dinosaur Scutellosaurus, which measured about four feet from nose to tail, had small bony plates lying flat over their backs and tails, each with a slightly raised keel. These scutes, about a half-inch across, had an internal structure similar in some aspects to the much larger plates of the stegosaurs, yet were obviously useless in regulating the internal temperature of the animal, Main said. The same is true of the later Scelidosaurus, a 13-footer covered with larger scutes with bigger keels; the scutes had the same type of blood vasculature as stegosaur plates and spikes. Ankylosaurs, a sister group to the stegosaurs that survived into the late Cretaceous and went extinct with the rest of the dinosaurs 65 million years ago, had more diverse scutes and ossicles that nevertheless were plumbed in the same way as those on stegosaurs.
Based on this analysis, the team argued that it was unlikely that the larger plates that evolved in the stegosaur ancestors of Scutellosaurus and Scelidosaurus were used for heat exchange.
Padian and Main point out, too, that the horns or antlers of many living animals contain large vessels to supply blood needed for fast growth. None of these horns or antlers function as heat exchangers. A possible role of the large "pipes" in the scutes of stegosaurs and their ancestors was to carry the large blood supply needed for the fast growth that was thought to be typical of dinosaurs.
In addition, not all stegosaurs living at the end of the Jurassic had the big, flat plates of Stegosaurus stenops that most people associate with stegosaurs. Kentrosaurus of Africa and the Asian Huayangosaurus, which were about the same size as Stegosaurus, had mostly spikes with a few "dinky" plates, Main said. These spikes and small plates would have been useless for heat exchange.
"You get quite a large variety in the types of osteoderm arrangements in these animals, but they are not specialized in the way that one would expect if they were built specifically for a thermoregulatory function," he said. "What it looks like is the scutes simply show hypertrophic growth of the keel region, it's simply a modification of an already existing growth pattern."
"There is a natural tendency that leads to elaborate displays for social group recognition, like the calls of birds," Padian said. "This underscores the importance of behavior to evolution."
De Ricqlès cautioned, however, that "an accessory role in thermoregulation cannot be ruled out for the Stegosaurus plates. Being so large, well vascularized (and available) they may have been inevitably exapted for such a function. This is so even if the primary explanation of their occurrence in an evolutionary context may be elsewhere: namely in some sort of 'display' (mate or species recognition), as suggested by the comparative, phylogenetic, context of plates development among Stegosauria."
To investigate further whether the elaborate horny displays of stegosaurs and other dinosaurs are involved in sexual displays, Padian is going to South Africa in May and June to measure skulls and bodies of African antelopes to look at the range of sexual dimorphism. Such studies have never been done on a full range of African bovids, he noted. Meanwhile, Main at Harvard is studying bone growth and skeletal mechanics in animals such as goats and emus to see how they change with age.
"We know more about growth in some dinosaurs than we do about growth in large living mammals," Padian said.
### The work was supported by the National Science Foundation, the Merck Family Fund, Jim and Bea Taylor, the Charlotte and Walter Kohler Charitable Trust, the Museum of the Rockies, the National Geographic Society and UC Berkeley's Museum of Paleontology. Fossils sliced up by the team came from the University of California Museum of Paleontology, the Museum of the Rockies, London's Natural History Museum, Brigham Young University and the Smithsonian Institution's National Museum of Natural History.
I'm not sure what you mean. A mutation can certainly dissapear from a species if the mutation has a negative effect.
Baloney.
Developing those plates required a lot of energy and if there is anything biology demonstrates it is that living organisms don't waste energy needlessly - its too expensive from a cost effective perspective.
Even rudimentary spurs at the rear of Booid snakes, the remnants of the pelvic girdle, serve a function - they "stimulate the female during courtship.
These guys should go back to the drawing board. If something exists, it exists for a reason - even if we haven't been able to figure it out definitively.
God doesn't place dice with the Universe" - Albert Einstein.
Okay - thanks. Like natural selection in reverse?
No, just plain old natural selection. A mutation that has a negative effect puts an individual with that mutation at a disadvantage when it comes to surviving and passing on that mutation.
Might have something to do with their speed and/or agility or their position when they eat. Seems if you were a bending down to eat critter, you'd want those obviously exposed parts protected. And then too, perhaps it's just some sort of structural support system.
Well you just didn't read it carefully enough, without the plating how could a bird tell another bird of the same species from a stegosaur? It's all bloody obvious.
Name some "neutral traits". There are not a lot of superfluous features found in nature that I can see.
If you look at a stegosaur skeleton, it becomes apparent that the size of those plates and spikes relative to the mass of the entire animal was significant.
If the sole purpose was sex or species identification, I'd say it was overkill. We're not talking a rack of antlers here.
Lizards and snakes have no problem using just color, and bird plumage has other funtions than merely sexual or species identification.
I think these guys are off the wall in their analysis.
In humans? Hair color. Detached versus attached earlobes. Eye color. Varying thickness of body hair.
We all await the the lucid, logical and consistent creationist explanation of Stegosour Plates.
I agree. A few hundred million years are plenty of time to come up with a more economical way of species identification than developing, nourishing, and lugging around huge plates.
Hair color- sunlight absorbtion, different lattitudes?
Detached versus attached earlobes- heat radiation, retention
Eye color- light thing again?
Varying thickness of body hair- environmental protection, heat/cold, job working in blackberry bushes?
Not sure that fits. Dark hair is more common in warm temperatures. What's the advantage of absorbing extra heat in your head in Africa or Australia?
Detached versus attached earlobes- heat radiation, retention
How much of a difference would the couple of square inches involved really make?
Eye color- light thing again?
Not that I'm aware of.
Varying thickness of body hair- environmental protection, heat/cold, job working in blackberry bushes?
Human hair is never thick enough to make a difference when it comes to temperature or protection.
I'll mention another one- the ability to "fold" one's tongue.
Thanks for the support.
Why wouldn't the better heat absorbing darker haired people be in the northern latitudes and the blonds be in the zero area?
I have parrots, too, and I agree with your line of thinking. Since birds and dinosaurs have a close evolutionary relationship, I would not be surprised if some bird behaviour would be found in dinosaurs as well.
Or the horns of a triceratops were used like deer and moose use their today - rutting season jousting for mates.
And that useful feature may be nothing more than to be attractive to a mate. In other words, a purely cosmetic adaptation that lacks a physical function.
Darker hair may not relate to temperature, but it sure would help protect from UV - i.e. sunburn and scin cancer. The top of the head and shoulders gets direct sun exposure.
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