The ammonites of the Mesozoic seas were mostly swimming animals that moved freely about in the ocean waters chasing small swimming creatures that they caught with their tentacles. In Cretaceous times they developed into a number of different forms. There were those with heavy shells produced in irregular coils, that spent their time crawling along the seabed. Others were freely drifting animals, filtering microscopic food particles from the water using very fine tentacles. This second trend has reached a pinnacle with the modern kraken.
The shell of the kraken is truly enormous, some specimens reaching 4 meters (13 feet) in diameter. The shell acts as a protective armor, as well as a float to keep the animal in the fertile surface waters. It has twelve tentacles that it spreads out around itself, and each of these has thousands of trailing fibers that are armed with stings and hooks. The whole arrangement forms a deadly net that covers an area of about 20 meters (67 feet) in diameter. The kraken will eat almost anything that becomes entangled in its traps, from microscopic floating plants, to fairly large fish. Many krakens often drift in the same area of productive waters, their floating shells acting as perches for migrant birds and pterodactyloid pterosaurs.