'Jersey Shore' Star Ronnie Ortiz-Magro Baby Mama Jen Harley Arrested

Ronnie Ortiz-Magro's baby mama, Jen Harley, has been arrested for a New Year's Eve incident when she allegedly threw an ashtray at the reality star ... TMZ has learned.

Harley was arrested in Vegas early Thursday around 3 AM on a warrant that was issued last month. Law enforcement sources tell us ... Jen actually called the cops early Thursday morning and said someone had a gun. Cops responded, came to the house and when they did a background check found she had an outstanding warrant.

TMZ broke the story ... Ronnie filed a battery report against Harley on January 3 after the two got in a blowout fight at a Vegas club on New Year's Eve. At some point during their argument, Harley allegedly hurled an ashtray at Ronnie, striking him in the face, and cutting his nose, lip and forehead.

We obtained video from the club that night which showed Ronnie leaving and bloodied.

The fight didn't just end there ... when Jen got home that night, she allegedly found her home ransacked ... accusing Ronnie of smashing a flat-screen TV, picture frames and a vase. He was also recorded smashing the security camera at her front door back in December.