pectoral arch vs shoulder girdle

pectoral arch and shoulder girdle both are nouns.

Word	Noun	Adjective	Verb	Adverb
pectoral arch	Yes	No	No	No
shoulder girdle	Yes	No	No	No

As nouns, pectoral arch and shoulder girdle are synonyms defined as:

pectoral arch and shoulder girdle: the bony arch formed by the collarbones and shoulder blades in humans

pectoral arch (noun)	shoulder girdle (noun)
the bony arch formed by the collarbones and shoulder blades in humans	the bony arch formed by the collarbones and shoulder blades in humans