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Hands
- Semilunate wristbone shifted to the lateral side. Homeotic shift.
- Bones in the hands fused together
- Alula evolved to allow for stable flight at low speeds
Shoulder/Chest
- Sternum lengthens. More space for muscles to attach.
- Evolution of flapping flight stroke. Appearance of the musculoskeletal configuration of the m. supracoracoideus (SC)
- These traits developed around 115 million years ago
- In Theropods, forelimb (ulna,radius,humerus) lengthened over the course of evolution
- Advantageous when it came to grabbing prey
- Long forelimbs made it easy for wings to evolve
- Scapula (shoulder blade) and Coracoid became longer and narrower. Better movement
- Clavicles fused to form wishbone. Muscle anchor point
- All these changes = strengthened skeleton
- Many of these characteristics were already present in pre-avian Theropods
- Hands became three-fingered
- Wristbone became half-moon shaped
- Clavicles fused into a wishbone
- Forelimbs become longer, although there are exceptions
- Ancestors of Theropods had 5-fingered hands
- Lost digits IV and V to form 3-fingered hands
- Basal Theropods had a flat wristbone. (carpus) Overlapped metacarpus.
- Soon became crescent in shape (semilunate)
- Flexibility increased; flexing sideways became possible.
- Advantageous for catching prey.
- More flexible wrist = more efficient flight stroke
- Discovered 1861 (soon after Darwin's Species)
- Widely accepted as the first known avialan
- Transitional fossil between Theropods and Birds
- Lived in Late Jurassic; 150 million years ago
- Feathered
- Could not fly well.
- Lacked a m. supracoracoideus (SC) muscle.
- But how did we get here?