Introduction
The evolutionary relationships between birds, crocodiles, and dinosaurs have been a subject of extensive research and debate. Understanding these relationships helps clarify the broader evolutionary history of archosaurs, a group that includes birds, crocodiles, and non-avian dinosaurs.
Key Insights
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Common Ancestry of Birds and Crocodiles:
- Some researchers propose that birds and crocodiles share a common ancestor within the Thecodontia, suggesting a closer relationship between these two groups than previously thought .
- The hypothesis that birds and crocodiles are sister groups within the Archosauria is supported by numerous homologous characters, making this relationship more plausible than the alternative Haemothermia hypothesis (birds and mammals as sister groups).
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Birds Evolved from Theropod Dinosaurs:
- There is strong evidence supporting the theory that birds evolved from small theropod dinosaurs, particularly from the group known as saurornithoidids.
- Comparative studies of cranial anatomy and other morphological features strengthen the claim that birds are more closely related to theropod dinosaurs than to crocodiles.
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Physiological and Behavioral Similarities:
- Birds and crocodiles share several physiological traits, such as similar amino acids in β-keratins, oviduct homologies, and forelimb structures, which suggest a common archosaurian heritage.
- Both birds and crocodiles exhibit complex reproductive behaviors, including parental care, which may reflect primitive characteristics of archosaurs, including dinosaurs.
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Differences in Digestive Physiology:
- Birds and crocodiles share certain digestive adaptations, such as the presence of a functional gizzard and the formation of gastric pellets, which are absent in theropod dinosaurs. This suggests that these features either reappeared in birds or were never present in theropod dinosaurs.
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Bone Growth and Metabolic Rates:
- Studies on bone growth rates indicate divergent evolutionary trends: bone growth rates increased from the last common ancestor of Ornithodira (pterosaurs and dinosaurs including birds) to extant birds, while they decreased from the last common ancestor of Crurotarsi (crocodiles) to extant crocodiles.
Conclusion
The majority of evidence supports the conclusion that birds are more closely related to theropod dinosaurs than to crocodiles. While birds and crocodiles share a common archosaurian ancestry and exhibit several physiological and behavioral similarities, the specific evolutionary lineage of birds is more directly linked to theropod dinosaurs. This conclusion is reinforced by morphological, physiological, and paleohistological data.