Mark Degler was Ellie Degler's husband during the events of the third film. It is unknown when his relationship with Ellie started, but in 1998 they got their first child Charlie. In 2001 they had a baby daughter whose name is unknown.
Mark Degler has a parallel in the The Lost World novel. In the novels, Ellie Sattler married Dr. Reiman, a physicist from Berkeley. They married in 1990, one year after the events of the first novel. By 1995, they had two children - a son and daughter.
It is unknown if this subplot from the novels inspired the break up of Alan and Ellie in the film canon.
Mark's last name is never revealed in the film or credits.