| Summary: | 碩士 === 國立成功大學 === 地球科學系碩博士班 === 100 === Phylogenetic relationship of birds with dinosaurs has recently well-demonstrated by continuous discoveries of feathered dinosaurs from China. In addition to the feathered dinosaurs, recent studies of paired-eggs in pelvic region of an oviraptoroid dinosaur, small theropods and paleognath bird fossils provided further evidences to support the hypothesis that birds are members of the clade of Dinosauria. The understanding of the reproductive behaviors through the study of oological morphology of troodontids or oviraptorians eggs or nests thus are crucial to illustrate the evolutionary history of the reproductive pattern of birds.
Brooding is a peculiar behavior of extant Avian. From biological perspective, the brooding behavior is defined as “maintain constant temperature in nest by direct contact with eggs, sometimes with manipulation”, however, previous researches combined the feathered dinosaur and similar-brooding oviraptoroid fossils to demonstrate brooding had present in Dinosauria. Moreover, comparison of the ratio of clutch-volume to adult-body-mass between Aves and Mesozoic dinosaurs, proposed that the polygamy and paternal-care had been appeared on non-avian theropods. According to the discoveries, paleobiologists came to the conclusion that the brooding, polygamy, paternal-care behaviors of Avian were originated from dinosaurs in Cretaceous. However, our studies on Cretaceous fossil oviraptorids nests from China invalidate the aforementioned hypothesis.
We analyzed five oviraptorids nests excavated from the Cretaceous strata in the Hongcheng Basin, Ganzhou, Jiangxi Province of China. Detailed analysis, measurement of external form of the nests, observation of eggshell thin section through polarization microscope and SEM, and, furthermore, compared the profile structure of the eggshell thin section with that of the extant birds (Ostrich and Gallus), turtles and crocodilians (Tomistoma schlegelli and Crocodylus porosus) demonstrate communal conclusions. All the fossil eggs were laid in pairs and tilted outward. Most of the eggs occur in two “rims”, which are interbedded with matrix, indicating that the rims were not laid en masse. The blunt ends of the long ellipsoid-shape eggs all point inward in the nest. While the surface in sharp end is smooth, the blunt end is characterized by linearituberculate ornamentation, in contrast to the entirely smooth surface of the avian eggshell. Finally, there is a major difference in the accretion characters and layering between the oviraptor and avian eggshells. From cluster analysis through paleoecological perspectives, the eggs in Cretaceous oviraptor’s nest should not be corporately laid by multiple females. Comparison of thickness in different part of oviraptor’s egg reveals possible structure of egg and ecological behaviors. For instance, egg-rotation is common in extant birds, which is beneficial to hatchling; however, the behavior doesn’t exist in Crocodilians due to the lack in chalazae. Detailed comparison of aformentioned analysis implies that oviraptoroid dinosaurs probably hold different incubation and caring behaviors from extant birds.
We propose an alternative hypothesis herein suggesting that the paternal care and brooding behaviors not originated from Mesozoic clade of Dinosauria, which didn’t brood their clutch and show polygamy behaviors, either. The Mesozoic oviraptors dinosaurs adults laid pair eggs each time, arrange them toroidally, buried them in a substrate, and then superpose another rim of eggs. Multi-rim clutch can be also observed in crocodiles and megapode, and both of which do not have brooding behaviors. The structure of egg is alike to crocodile’s, which is lacking in chalazae and air space. Oviraptoroid dinosaurs probably guard their exquisite nests without caring or egg-rotation behaviors, just alike to extant crocodilians which belong to the same clade of Reptiles of Archosauria.
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