I came up with an idea about this 20 years ago; it's simple. When an ET craft warps to or from Earth, it creates waves in space/time that cause past time to overlap current time, making extinct creatures materialize and then dematerialize.
A trail goes two ways and looks different in each direction - There is no such thing as a timid woodland creature - Whatever does not kill you leaves you a survivor - Jesus is NOT a bad word - MSB