Animals that can reinvent themselves
Sponges are amongst the most primitive and basic animals alive. They don’t have a nervous, digestive or circulatory system, but feed by filtering the water for organic particles. They come in all shapes and sizes, and date back at least 580 million years.
Sponges have a sac-like body formed from millions of tiny interlocking silica needles. They draw water through the body into a central cavity, remove organic nutrients, then eject the water again through a larger hole usually at the top of the body.
Needles and nodules
Sometimes the overall shape of the sponge is preserved as a fossil. But at other times it’s just the textures, or the doughnut-shaped central cavity. This is because the silica needles are easily dissolved in the ground water that percolates through the rocks.
The gel-like silica then hardens again to create flint in chalk, and chert in limestone or sandstone. Sometimes the silica collects around solid objects such as other sponges, then the sponge breaks down, resulting in hollow flint nodules.
Masters of reinvention
Sponges have remarkable powers of regeneration. If you put one through a blender, mash it to a pulp, then put it back in sea water, the sponge will remake itself!