Bivalves first made their appearance on earth about 500 million years ago during what is called the Cambrian Explosion when there was a great diversification of life. The earliest bivalves burrowed in soft sediments on the bottom of the ocean. They lived just under the surface, not deep down like some modern bivalves. Over time, bivalves evolved structures allowing them to bury deeper such as snorkel-like tubes (siphons) to get water from above; and also evolved modified gills for filter feeding. Bottom-dwelling (benthic) bivalves became fossilized when sediment hardened around them. Fossil evidence is a shell itself or just an impression of the shell left in the sediment (an ichnofossil). Bivalves have evolved into many, distinct forms over time, and survived through major extinction events to become a very diverse group of organisms. Most modern forms are benthic, but others attach themselves to hard surfaces or even swim around a bit (e.g., scallops).