Femora from an exceptionally large population of coeval ornithomimosaurs yield evidence of sexual dimorphism in extinct theropod dinosaurs
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eLife assessment
This is an important contribution to the field of dinosaur palaeontology. The authors provide convincing evidence for sexual dimorphism in dinosaurs, based on limb bones of ornithomimosaurs from the Cretaceous of France. The article makes several valuable and important contributions -- including the use of a large dataset and robust statistical approaches -- and will serve as a benchmark for future studies on dinosaurs and other fossil reptiles.
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Abstract
Sexual dimorphism is challenging to detect among fossils due to a lack of statistical representativeness. The Angeac-Charente Lagerstätte (France) represents a remarkable ‘snapshot’ from a Berriasian (Early Cretaceous) ecosystem and offers a unique opportunity to study intraspecific variation among a herd of at least 61 coeval ornithomimosaurs. Herein, we investigated the hindlimb variation across the best-preserved specimens from the herd through 3D Geometric Morphometrics and Gaussian Mixture Modeling. Our results based on complete and fragmented femora evidenced a dimorphism characterized by variations in the shaft curvature and the distal epiphysis width. Since the same features vary between sexes among modern avian dinosaurs, crocodilians, and more distant amniotes, we attributed this bimodal variation to sexual dimorphism based on the extant phylogenetic bracketing approach. Documenting sexual dimorphism in fossil dinosaurs allows a better characterization and accounting of intraspecific variations, which is particularly relevant to address ongoing taxonomical and ecological questions relative to dinosaur evolution.
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Author Response
Reviewer #1 (Public Review):
This is one of the most careful analyses of sexual dimorphism in dinosaurs, based on a remarkable assemblage of 61 ornithomimosaur fossils from the Early Cretaceous of western France. The dimorphism is expressed in variations in the shaft curvature and the distal epiphysis width, analysed appropriately here and plausible because these are the kinds of morphological features that vary between males and females among birds and crocodilians, among others.
In the Introduction, it is right to highlight the shortage of convincing cases of demonstrated sexual dimorphism (SD) in dinosaurs. But note the points made by Hone, Saitta and others that SD can exist in many species today without major morphological differences, making it hard to demonstrate in fossils with such types of dimorphism. …
Author Response
Reviewer #1 (Public Review):
This is one of the most careful analyses of sexual dimorphism in dinosaurs, based on a remarkable assemblage of 61 ornithomimosaur fossils from the Early Cretaceous of western France. The dimorphism is expressed in variations in the shaft curvature and the distal epiphysis width, analysed appropriately here and plausible because these are the kinds of morphological features that vary between males and females among birds and crocodilians, among others.
In the Introduction, it is right to highlight the shortage of convincing cases of demonstrated sexual dimorphism (SD) in dinosaurs. But note the points made by Hone, Saitta and others that SD can exist in many species today without major morphological differences, making it hard to demonstrate in fossils with such types of dimorphism. Also, some proposed statistical tests to ensure that SD has been convincingly demonstrated in fossils are so stringent they would be hard ever to pass (requiring enormous and constant morphological distinctiveness). In other words, we are conditioned not to find SD in dinosaurs, and yet may be massively under-reporting it because of preservation difficulties (of course) but also because of some overly rigorous demands for proof. These issues help argue that the current study is especially valuable because the data set is large (itself a rarity), and 3D bone shape analysis and proper statistical testing have been applied.
We are grateful that Reviewer 1 raised this point regarding the occurrence of many subtle sexual dimorphism among modern populations, and added a sentence in the introduction, to further emphasize the importance of a large dataset composed of coeval organisms.
It's interesting the dinosaur example shows the same two dimorphic traits (femoral obliquity = bicondylar angle; width of distal epiphysis = bicondylar breadth) seen in mammals (MS, lines 117-123), where the femur angle may vary because of the need for broader hips in the female to accommodate the birth canal, and yet dinosaurs laid eggs. These are small dinosaurs, so perhaps their eggs were relatively large in proportion to body size. Perhaps the authors could comment on this. There is some discussion with regard to modern birds at MS lines 187-199.
We agree with comments from Reviewer 1 and we raise the question of egg possibly constraining the pelvic and proximal hindlimb morphology from line 170 to 189 and how it relates to modern archosaurs from line 189 to 202. We also originally intended to discuss how the Kiwi hindlimb morphology accommodates large eggs, but no significant dimorphism was demonstrated in the pelvic and hindlimb morphology of this bird.
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eLife assessment
This is an important contribution to the field of dinosaur palaeontology. The authors provide convincing evidence for sexual dimorphism in dinosaurs, based on limb bones of ornithomimosaurs from the Cretaceous of France. The article makes several valuable and important contributions -- including the use of a large dataset and robust statistical approaches -- and will serve as a benchmark for future studies on dinosaurs and other fossil reptiles.
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Reviewer #1 (Public Review):
This is one of the most careful analyses of sexual dimorphism in dinosaurs, based on a remarkable assemblage of 61 ornithomimosaur fossils from the Early Cretaceous of western France. The dimorphism is expressed in variations in the shaft curvature and the distal epiphysis width, analysed appropriately here and plausible because these are the kinds of morphological features that vary between males and females among birds and crocodilians, among others.
In the Introduction, it is right to highlight the shortage of convincing cases of demonstrated sexual dimorphism (SD) in dinosaurs. But note the points made by Hone, Saitta and others that SD can exist in many species today without major morphological differences, making it hard to demonstrate in fossils with such types of dimorphism. Also, some proposed …
Reviewer #1 (Public Review):
This is one of the most careful analyses of sexual dimorphism in dinosaurs, based on a remarkable assemblage of 61 ornithomimosaur fossils from the Early Cretaceous of western France. The dimorphism is expressed in variations in the shaft curvature and the distal epiphysis width, analysed appropriately here and plausible because these are the kinds of morphological features that vary between males and females among birds and crocodilians, among others.
In the Introduction, it is right to highlight the shortage of convincing cases of demonstrated sexual dimorphism (SD) in dinosaurs. But note the points made by Hone, Saitta and others that SD can exist in many species today without major morphological differences, making it hard to demonstrate in fossils with such types of dimorphism. Also, some proposed statistical tests to ensure that SD has been convincingly demonstrated in fossils are so stringent they would be hard ever to pass (requiring enormous and constant morphological distinctiveness). In other words, we are conditioned not to find SD in dinosaurs, and yet may be massively under-reporting it because of preservation difficulties (of course) but also because of some overly rigorous demands for proof. These issues help argue that the current study is especially valuable because the data set is large (itself a rarity), and 3D bone shape analysis and proper statistical testing have been applied.
It's interesting the dinosaur example shows the same two dimorphic traits (femoral obliquity = bicondylar angle; width of distal epiphysis = bicondylar breadth) seen in mammals (MS, lines 117-123), where the femur angle may vary because of the need for broader hips in the female to accommodate the birth canal, and yet dinosaurs laid eggs. These are small dinosaurs, so perhaps their eggs were relatively large in proportion to body size. Perhaps the authors could comment on this. There is some discussion with regard to modern birds at MS lines 187-199.
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Reviewer #2 (Public Review):
The dinosaur literature from the 1970s through 1990s is rife with claims purporting to have identified sexual dimorphism in the skeletons of dinosaurs. Mallon (2017) penned a critical review of these claims and showed that nearly all of them are without statistical support. He also suggested some more appropriate methods that might be fruitfully applied to the matter. This contribution from Pintore et al. heeds Mallon's prior recommendations and, I think, fairly convincingly demonstrates the existence of sexual dimorphism in the femora of the ornithomimosaurs they investigated. Their argument is bolstered by the numerous examples they cite of similar dimorphism seen in the femora of various tetrapod groups. I believe this manuscript holds much merit.
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