What is the participial of sentence riding a horse sheriff looked as if he had stepped out history book?

The participial phrase in the sentence "Riding a horse, the sheriff looked as if he had stepped out of a history book" is "Riding a horse".

Here's why:

* Participial Phrases are verb forms ending in "-ing" or "-ed" that function as adjectives, modifying nouns or pronouns. They often act like descriptive phrases.

* In this case, "Riding a horse" describes the sheriff, telling us how he appears.

Let me know if you have any other sentences you'd like to analyze!