Enchantment, 2UU (4)
Rare
At the beginning of each opponent's upkeep, that player reveals the top card of his or her library. If it's a creature card, the player puts the card into his or her graveyard and Shapeshifter's Marrow becomes a copy of that card. (If it does, it loses this ability.)
Illus. Chippy

5/1/2007: If the revealed card isn't a creature card, it stays on top of that player's library. Shapeshifter's Marrow remains an enchantment.
5/1/2007: If the revealed card is a creature card, Shapeshifter's Marrow becomes a copy of that card permanently. It's no longer an enchantment.
5/1/2007: In a Two-Headed Giant game, this ability triggers separately for each player at the start of the opposing team's turn. If the first ability to resolve reveals a creature card, Shapeshifter's Marrow becomes a creature. If the second ability to resolve also reveals a creature card, the ex-Shapeshifter's Marrow then becomes a copy of that creature.
Legal in Vintage (Type 1)
Legal in Legacy (Type 1.5)
Legal in Classic (MTGO)
Legal in Commander
Legal in Modern