If it weren't for the clavicle.
The English word clavicle (Latin clavicula) means key (key of a keyboard instrument) or clasp. The free part of upper limbs (via the scapula) are clasped to the trunk (sternum). This state is what is called chest-bearing. Therefore, without the clavicles, the chest cannot be stretched. Without the clavicle, a dog cannot do cross pull-ups in gymnastics. However, bats, which are flapping mammals, have well-developed clavicles. Translated with www.DeepL.com/Translator (free version)