The Trilobite was a strange creature created as the result of the black liquid infecting Holloway and he impregnating Elizabeth Shaw during sex, despite her infertility. It is not made clear if the creature was conceived by the pair or if the infant creature merely moved into Shaw from the infected Holloway.
Ten hours after Elizabeth Shaw and Holloway have sex, the Trilobite's embryo begins to grow inside Shaw's womb. This is revealed during David's examination inside Shaw's womb, who deducts that she seems to be three months pregnant. When David wants to freeze her in cryo-stasis, to preserve the creature inside, Shaw escapes and finds her way to the Med Pod 720i in Vickers' private life boat and begins to initiate a caesarian section to extract the creature before it can burst out of her womb. Shaw attempts to decontaminate the medpod and kill the infant Trilobite, but it is merely rendered unconscious and later grows to an enormous size.
Shaw later released the Trilobite in hopes of subduing the Engineer pursuing her, which it does. After a struggle, it impregnates the Engineer with an embryo and renders it unconscious. The Trilobite dies shortly after impregnating the Engineer.