I am reading Charlotte's Web with my daughter. I am glad to be reading E. B. White's book. We pay close attention to the construction of sentences so that we can learn from such exemplary writing. We are reading the 60th anniversary edition.
One sentence read,
The sheep lay motionless.
and after a while she asked whether it should be "The sheep lay motionlessly". We reasoned that since the word is telling us more about the sheep's action, we should choose an adverb.
We checked Merriam-Webster's dictionary and dictionary.com and they both defined motionlessly as the adverb, keeping motionless only as an adjective.
Was motionless once used as an adverb as well?
