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Sexual dimorphism in Stegosaurus ?
Topic Started: Jan 12 2014, 04:05 AM (1,987 Views)
Allodaposuchus
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I have recently noticed something. In nearly every quarry and paleostratogrphic zone of Morrison formation there are two tipes of Stegosaurus : mostly S.armatus and S.stenops. Could this be sexual dimorphism ? I think yes. If that is true , then S.stenops would be synonym with S.armatus (type species). What do you think ?
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ThePrehistoricMaster
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Jan 12 2014, 04:05 AM
I have recently noticed something. In nearly every quarry and paleostratogrphic zone of Morrison formation there are two tipes of Stegosaurus : mostly S.armatus and S.stenops. Could this be sexual dimorphism ? I think yes. If that is true , then S.stenops would be synonym with S.armatus (type species). What do you think ?
I have never really understanded what a "synomyn" means. I know the word, but i'm not sure what it means, when you talk about extinct species. Can you please explain it to me?
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Hatzegopteryx
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ThePrehistoricMaster
Jan 13 2014, 08:50 PM
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Jan 12 2014, 04:05 AM
I have recently noticed something. In nearly every quarry and paleostratogrphic zone of Morrison formation there are two tipes of Stegosaurus : mostly S.armatus and S.stenops. Could this be sexual dimorphism ? I think yes. If that is true , then S.stenops would be synonym with S.armatus (type species). What do you think ?
I have never really understanded what a "synomyn" means. I know the word, but i'm not sure what it means, when you talk about extinct species. Can you please explain it to me?
he is saying that S. armatus and S. stenops could be synonyms which means they might be the same animal.
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Taipan
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Stegosaurus' Bony Plates May Reveal Dino's Sex

by Laura Geggel, Staff Writer | April 22, 2015 02:40pm ET

Posted Image
A comparison of the largest wide plate (A) next to the largest tall plate (B) of the studied Stegosaurus mjosi plates.
Credit: Saitta ET (2015) Evidence for Sexual Dimorphism in the Plated Dinosaur Stegosaurus mjosi (Ornithischia, Stegosauria) from the Morrison Formation (Upper Jurassic) of Western USA. PLOS ONE 10(4): e0123503. doi:10.1371/journal.pone.0123503

The plates of the Stegosaurus — the large, bony discs that lined the dinosaur's neck, back and tail in two staggered rows — may have differed between males and females, a new study finds.

An analysis of the 150-million-year-old remains of the species Stegosaurus mjosi show that some individuals had wide plates, whereas others had tall plates. These anatomical differences may distinguish males and females — a concept known as sexual dimorphism, said the study's author, Evan Saitta, a graduate student of paleobiology at the University of Bristol in the United Kingdom who began the study as part of his senior thesis at Princeton University.

"It's the most convincing evidence we have so far of sexual dimorphism in a dinosaur," excluding birds, the living descendants of dinosaurs, Saitta told Live Science.

Saitta has studied Stegosaurus fossils since 2009, when he began visiting the Judith River Dinosaur Institute (JRDI) in Montana while still in high school. The JRDI is the only known dinosaur graveyard to have multiple Stegosaurus fossils from the same time period buried together, Saitta said. He became intrigued by the dinosaurs' plates, and wondered whether the different dimensions were signs of sexual dimorphism.

He took CT scans and measurements of 10 plates in good condition from the Montana quarry, and included 30 Stegosaurus plates from earlier research. He also looked at four tail spikes from the Montana quarry, but didn't find evidence of sexual dimorphism.

The wide plates were up to 45 percent larger in surface area than the tall plates, he found. The largest wide plate was 35.8 inches by 25.5 inches (91 centimeters by 65 centimeters). In contrast, the largest tall plate is 21.2 inches by 33.5 inches (54 cm by 85 cm), he said.

Sexual dimorphism

Next, Saitta looked for other possible causes of the plate differences, including that they came from different species.

Posted Image
The plates of Stegosaurus mjosimay offer clues about the dinosaur's sex, a study suggests.

The dinosaur graveyard in Montana held a jumble of bones, and it wasn't always clear which plates belonged to which skeleton. Yet "every other bone is the same," besides the plates, suggesting that all of the individuals were S. mjosi, Saitta said. In addition, the dinosaurs were buried together, suggesting they lived together, he said.

"Do we have co-existence? You can't say for sure, but I wouldn't be surprised if these individuals in Montana were a social group," Saitta said.

He also checked whether the size of the plates was simply variation within individuals. But if this were the case, the plates would come in all sizes along the spectrum, from wide to tall.

"We have two distinct varieties of plates, and we don't see intermediates," he said.

He also checked whether the different sizes were a product of age. But he found one adult with tall plates and another adult with wide plates, suggesting that growth wasn't the reason for the differences.

Which is the girl?

Sexual dimorphism remained a real possibility. However, it's extremely difficult to determine the sex of dinosaurs. Some fossils can be identified as female because they contain eggs or calcium deposits in the long bones that were likely a preparation for egg laying, Saitta said. But unless these clues are present, scientists can't know a dinosaur's sex.

None of the Stegosaurus fossils in the study come from animals with a known sex, he said. But Saitta shared an idea in the study: Perhaps the males had the wide plates, and the females had the tall ones.

Male animals today typically invest more energy in their ornamentation than females do, and the wider plates were larger and thus would have required more energy to grow, he said. Two staggered rows of wide plates may have been attractive to females, he said.

The tall plates may have belonged to the females, which possibly used them as a prickly deterrent against predators, he said. The males likely didn't use the plates to fight each other, as they may have been deadly weapons, he added.

The study is "intriguing," but more work is needed before the plates can be looked at as reliable predictors of sexual dimorphism, said Andrew Farke, a paleontologist at the Raymond M. Alf Museum of Paleontology in Claremont, California, who was not involved with the study.

"One of the Holy Grails of dinosaur paleontology is trying to distinguish male and female dinosaurs," Farke said. "I think this is one of those cases that's quite suggestive. I wouldn't say it's necessarily airtight, but it's fairly suggestive of something going with these animals."

In future work, scientists could examine even more fossils, Farke said. "I would like to see additional analyses done on this," he added. "The shape analysis that was done is pretty simple. There's much more powerful analyses out there nowadays."

However, the Stegosaurus plates found in Montana could have been damaged by amateur excavators, meaning their true dimensions would never be known, said Kenneth Carpenter, director of the Utah State University Eastern Prehistoric Museum, who was not involved in the study.

But Saitta said that was not the case, and that he "personally prepared many of the plates. "One can examine the edges to see if they are broken," he said, "which is usually apparent on close inspection."

http://www.livescience.com/50569-stegosaurus-plates-sexual-dimorphism.html




Journal Reference:
Saitta ET (2015). Evidence for Sexual Dimorphism in the Plated Dinosaur Stegosaurus mjosi (Ornithischia, Stegosauria) from the Morrison Formation (Upper Jurassic) of Western USA. PLoS ONE, 2015 DOI: 10.1371/journal.pone.0123503

Abstract
Conclusive evidence for sexual dimorphism in non-avian dinosaurs has been elusive. Here it is shown that dimorphism in the shape of the dermal plates of Stegosaurus mjosi (Upper Jurassic, western USA) does not result from non-sex-related individual, interspecific, or ontogenetic variation and is most likely a sexually dimorphic feature. One morph possessed wide, oval plates 45% larger in surface area than the tall, narrow plates of the other morph. Intermediate morphologies are lacking as principal component analysis supports marked size- and shape-based dimorphism. In contrast, many non-sex-related individual variations are expected to show intermediate morphologies. Taphonomy of a new quarry in Montana (JRDI 5ES Quarry) shows that at least five individuals were buried in a single horizon and were not brought together by water or scavenger transportation. This new site demonstrates co-existence, and possibly suggests sociality, between two morphs that only show dimorphism in their plates. Without evidence for niche partitioning, it is unlikely that the two morphs represent different species. Histology of the new specimens in combination with studies on previous specimens indicates that both morphs occur in fully-grown individuals. Therefore, the dimorphism is not a result of ontogenetic change. Furthermore, the two morphs of plates do not simply come from different positions on the back of a single individual. Plates from all positions on the body can be classified as one of the two morphs, and previously discovered, isolated specimens possess only one morph of plates. Based on the seemingly display-oriented morphology of plates, female mate choice was likely the driving evolutionary mechanism rather than male-male competition. Dinosaur ornamentation possibly served similar functions to the ornamentation of modern species. Comparisons to ornamentation involved in sexual selection of extant species, such as the horns of bovids, may be appropriate in predicting the function of some dinosaur ornamentation.

http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0123503
Edited by Taipan, Dec 28 2017, 03:03 PM.
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DinosaurFan95
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Intresting, but I do not believe (shocking, I know) that S. stenops and S. armatus are different genders of the same species, for the simple fact that plate shape alone may not necessarily be caused by different genders, such factors as "racial" characteristics could easily account for plate shape. As the broad rounded plates may be a trait connected with live on the open "grassland", allowing the cool breeze a larger area to which it can cool off the blood of the Stego. While the taller, narrow plates may be an adaptation to life in the forest, where there is a reduced wind flow, and continuous shade.

However all of this is pure speculation, as we do not know for sure exactly what these plates are for, being a sexy Stego may not have had anything to do with how big your plates are. For all we know, the plates could have been for keeping pterosaurs from pecking at you. And even if this plates= gender thing is true, how do we know that the males had the tall plates?
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Ceratodromeus
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I suppose you don't have any hard evidence to back any of that up?
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DinosaurFan95
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Like I said in my post, we have no indisputable proof that plate size correlates to gender.
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Ceratodromeus
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so i'm guessing that means no?
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DinosaurFan95
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I'm not trying to prove anything! If any, I'm trying to DISprove the theory that Stego plates have a function in gender differences.
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Ceratodromeus
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you need evidence to do that, your opinion holds no ground for an argument.
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DinosaurFan95
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Don't need proof to fight a theory that itself dosnt have proof.

Until we find a Stego with eggs in the abdominal region, AND has round/pointed plates, we can't say that plate shape differs between the sexes.
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Ceratodromeus
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That's a horrible argument.
- you've provided nothing against the idea plate size is a sexual characteristic
- nor have you provided any evidence for your hypothesis in earlier posts.
Bearing this in mind, which is more likely -- the theory brought forward in a published paper, or the musings that have no evidence supporting them of some kid on a forum?

Edited by Ceratodromeus, Apr 26 2015, 04:11 AM.
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DinosaurFan95
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Just because it's published, dosn't make it true, you and the paper also have not provided any solid proof. So do not throw then"you have no proof" card at me.
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Ceratodromeus
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the fact of the matter is you've brought no evidence for your statements. belief is not valid here; i do not care which you believe to be more likely. show me hard evidence contradicting the paper, or stop spouting nonsense.
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Fist of the North Shrimp
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Hate to come to help for Allosaurusatrox(Hell he defends a guy who want to bomb my country into oblivion), but there is valid critique of this paper: The specimens used for this study Were all jumbled up to the point where discerning which part belonged to which dinosaur is nigh impossible. Ken Carpenter already said that there actually is a specimen displaying both types of plates. Stegosaurine Stegosaurians and their plante problems xD
Edited by Fist of the North Shrimp, Apr 26 2015, 06:39 AM.
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