At first glance, the simple sea sponge is no creature of mystery. No brain. No gut. No problem dating it back 700 million years. Yet convincing sponge fossils only go back about 540 million years, leaving a 160 million-year gap in the fossil record. At first glance, the simple sea sponge is no creature of mystery. No brain. No gut. No problem dating it back 700 million years. Yet convincing sponge fossils only go back about 540 million years, leaving a 160 million-year gap in the fossil record. Paleontology & Fossils Phys.org – latest science and technology news stories