Bones is finally making the leap to introduce Seeley Booth's estranged mother, Marianne Booth, in the April 15th episode. Body of Proof mom, Joanna Cassidy, has been tapped to play the pivotal role of Marianne. As fans know, Booth's mother left her abusive husband and two sons 24 years ago, but her reappearance comes at a very inopportune time. She will set into motion some big surprises for the season 8 finale, set to air April 29th.
Source: TV Guide