Pauly D better learn to duck and cover ... 'cause his baby mama had no qualms about beating the crap out of another guy who knocked her up ... according to police records obtained by TMZ.
We've learned ... Amanda Markert was arrested in July 2009 for opening a can of whoopass on her first baby daddy -- Lance Gerbino -- leaving him with bloody lips, a nasty gash above his eye and injuries inside his mouth.
Gerbino tells TMZ ... Amanda had been out drinking with friends ... and was pissed because she had to come home to help take care of their infant son.
He claims Markert just started "going crazy" ... wailing on his face with her fists while she was holding their son. Gerbino felt the baby was in danger ... and that's why he called police.
According to police records, Markert turned herself in to cops --booked under her legal name, Amareda Markert -- but the assault charge was eventually dismissed after she completed an anger management course.
As we've reported, Markert and Pauly D are involved in an ugly custody fight over their 6-month old daughter, Amabella ... and they've both been very clear about how much they hate each other.
We contacted Amanda for comment on the domestic violence case ... she hung up on us.