Dinosaurs represent a diverse and widely successful group of vertebrates that dominated terrestrial ecosystems for much of the Mesozoic era, from approximately 230 to 65 million years ago. During this long span of time, dinosaurs evolved into an incredible array of forms, ranging from massive long-necked sauropods and plated stegosaurs to ferocious meat-eating tyrannosaurs and bird-like dromaeosaurs.
Although non-avian dinosaurs became extinct at the end of the Cretaceous period, they are closely related to two living groups of reptiles that survive today – crocodilians and birds. Determining which of these two groups is more closely related to dinosaurs has long been a topic of scientific inquiry and debate. Evidence from the fields of paleontology, comparative anatomy, and developmental biology have all been brought to bear on this question.
The issue remains relevant not just for better understanding dinosaur origins and relationships, but for elucidating the evolutionary pathways that led to the morphological diversity seen in modern vertebrate groups. Resolving dinosaur relationships informs our knowledge about the evolution of key adaptations like feathers, flight, and endothermy.
In this review, we will summarize the major lines of evidence regarding the phylogenetic placements of crocodilians and birds among archosaurs. We will highlight similarities and differences in the anatomy, physiology, and development of these groups in order to assess their evolutionary relationships with dinosaurs.
Basic Archosaur Relationships and Origins
To understand the evolutionary relationships between dinosaurs, crocodilians, and birds, it is necessary to briefly consider the larger context of archosaur evolution. Archosaurs are a diverse group of diapsid reptiles that include not just dinosaurs and their living relatives, but also extinct clades like pterosaurs and non-avian dinosaurs.
The earliest known archosaurs emerged during the Late Triassic period, diversifying rapidly and giving rise to a variety of carnivorous and herbivorous forms. Archosaurs shared numerous derived characteristics that united them as a monophyletic group, such as elongated ankle joints and additional fenestrae (openings) in the skull.
Two major branches on the archosaur family tree diverged early on – the Crurotarsi and the Ornithodira. The Crurotarsi gave rise to crocodylomorphs, including modern crocodilians as well as extinct taxa more closely related to dinosaurs like the rauisuchians. The Ornithodira encompassed both pterosaurs and dinosaurs (including birds).
This basic outline of archosaur relationships was established through anatomical studies conducted over many decades. The advent of molecular phylogenetics has upheld this framework, confirming that crocodilians form the sister group to the combined clade of pterosaurs and dinosaurs.
Therefore, when investigating dinosaur relationships, crocodilians serve as an evolutionary outgroup against which birds and non-avian dinosaurs can be compared. Deciphering similarities and differences between crocodilians and these other archosaurs can elucidate dinosaur origins and diversification.
Evidence from Comparative Skeletal Anatomy
Skeletal anatomy provides abundant evidence bearing on the relationship of extinct dinosaurs to their living crocodilian and avian relatives. Overall, non-avian dinosaurs share a greater number of signature skeletal features with birds, supporting their closer affinity.
A key line of evidence relates to the structure of the hip and legs. Dinosaurs (including living birds), share a characteristic “pillar erect” hip architecture where the femur or thigh bone aligns vertically beneath the acetabulum. This allows for more upright limb posture. By contrast, the sprawling gait of crocodilians is dictated by their laterally pointing knees and elbows.
The ankle joint shows further telling similarities between dinosaurs and birds. In both groups, a primitive two-bone ankle joint evolved into a more advanced mesotarsal ankle joint, where a third astragalus bone takes on the main weight-bearing function. The ankle bones are spool-shaped and tightly integrated, permitting improved running capabilities. Crocodilians retain the primitive two-bone ankle configuration.
In the skull and jaws, birds inherited a distinctive dinosaurian anatomy, including toothless beaks and enlarged openings for the antorbital sinus. The palate is secondary or “derived” in type, unlike the retained primary palate of crocodilians. Other shared features of dinosaur and bird skulls relate to the orientation of the quadrate bone and reduced number of vertebrae.
The vertebral column shows detailed similarities between birds and meat-eating theropod dinosaurs. Both groups typically had hollow, pneumatized vertebrae as well as saddle-shaped cervical or neck vertebrae with prominent neural spines for ligament attachment. These neck vertebrae allowed for greater mobility and flexibility.
Forelimb anatomy also allies birds with coelurosaurian theropods, to the exclusion of crocodilians. This is most dramatically demonstrated in the wrist, where a half-moon shaped carpal bone enabled folding of the wrist to withdraw the forelimbs. Feathered forelimbs represent a hallmark dinosaurian innovation, providing further anatomical ties to birds.
Shared Physiological Attributes of Dinosaurs and Birds
Fossilized remains can only reveal so much about the soft tissue anatomy and physiology of extinct animals. Fortunately, modern anatomical, developmental, and genetic studies of crocodilians and birds provide another lens into dinosaur biology, illuminating physiological similarities between dinosaurs and birds.
Perhaps most striking is the evidence that many dinosaur lineages were endothermic, capable of internally regulating their own body temperature as birds and mammals do today. This ranges from polar dinosaurs in Alaska exhibiting insulating integument, to bone histology studies showing fast growth rates comparable to modern warm-blooded animals.
Dinosaurs also likely possessed an advanced four-chambered heart and efficient lung system analogous to modern birds. Unidirectional airflow and aerobic respiration would have fueled high metabolism and activity levels. This represents an evolutionary departure from the more primitive ectothermic metabolism of crocodilians.
Similar to birds, many dinosaurs also possessed sophisticated sensory systems and large brains relative to their body size. Encephalization quotients measured from endocasts confirm that advanced cognitive abilities had evolved in multiple dinosaur groups including dromaeosaurs, troodontids, and tyrannosaurs.
Reproductive strategies also allied certain dinosaurs and birds. Plentiful fossil egg clutches confirm that dinosaurs had high fecundity rates comparable to modern ground-nesting birds. Some dinosaurs such as the oviraptorosaurs protected and brooded their nests in a bird-like manner. The transition from reptilian egg-laying to modern avian-style parental care plausibly took place deep in the dinosaur lineage.
Developmental Evidence from Embryology and Bone Formation
In addition to adult anatomy, the embryonic development of birds and crocodilians can offer clues to dinosaur relationships, since traits evident during ontogeny often reflect deeper evolutionary histories.
Studies of musculoskeletal development have shown that crocodilian embryos form scales and bony armor at a much earlier stage than birds. By contrast, birds develop feathers very early in embryogenesis, echoing evidence that feathers originated deep in dinosaurian history.
The pattern of limb bone ossification also allies dinosaurs and birds. In most vertebrates, the bone tissue in limbs forms from cartilage as a single growth point that spreads outward – known as metaplasia. In birds and similarly in dinosaurs, bone tissue arises from multiple growth points along the limb – known as periosteal ossification.
Researchers have also mapped developmental signaling molecules in crocodilian and chicken embryos. The expression patterns of regulatory genes like BMP4 and SOX9 in the face are more akin between birds and dinosaurs. Molecular data reinforces skeletal analysis showing crocodilians are an anatomical outgroup to dinosaurs and birds.
Evidence from Molecular Phylogenetics
Alongside anatomical and developmental data, phylogenetic studies using DNA sequence data have also shed light on archosaur relationships. Molecular phylogenetics remains limited for extinct dinosaurs, but analysis of living crocodilians and birds can help resolve dinosaur origins.
Early molecular studies were based on a limited set of mitochondrial and ribosomal RNA genes. These weakly supported a sister relationship between crocodilians and birds to the exclusion of dinosaurs. However, with the advent of genome sequencing, huge genomic datasets encompassing thousands of protein-coding genes tell a different story.
Robust phylogenomic studies overwhelmingly group birds with other theropod dinosaurs, with crocodilians placed as the nearest outgroup. This molecular evidence decisively aligns with the majority of anatomical and developmental data supporting crocodilians as phylogenetic outgroups to Dinosauria (the clade encompassing both birds and non-avian dinosaurs).
Lineage | Key Shared Derived Traits with Dinosaurs |
---|---|
Birds | Mesotarsal ankle, vertebral pneumaticity, endothermy, feathers |
Crocodilians | None |
Conclusion
The evolutionary origins of birds among maniraptoran theropod dinosaurs is one of the great scientific detective stories of the past decades. Anatomical, developmental, and molecular evidence converges decisively on theropod dinosaurs, not crocodilians, representing the closest living relatives of birds. Similarities in skeletal anatomy, physiology, reproduction, and growth patterns all support this conclusion.
While a few morphological holdouts have argued for a crocodilian affinity for birds, this hypothesis lacks support from the majority of comparative biology datasets. At a deep evolutionary level as evidenced by molecular phylogenetics, crocodilians represent the nearest living outgroup to the combined clade Dinosauria. Within Dinosauria, the subgroup known as Theropoda gave rise to birds through the lineage Maniraptora.
Deciphering the dinosaur-bird connection illuminates key adaptations like powered flight, feathers, and endothermy that evolved along the lineage. And it cements birds as direct surviving descendants of dinosaurs, allowing us to understand the biology of their remarkable extinct relatives through the lens of avian biology today. The study also reinforces how traits evident in living animal groups can have deep evolutionary origins, even if subsequently modified over millions of years.
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