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Sponge Fossils

Sponge, belonging to the phylum Porifera, is a multicellular creature in the simplest form. The meaning of the word Porifera is 'pore bearer' and it is named so because of the channels and pores present in the Sponge body. Thus, water is circulated throughout the body to help with feeding and breathing. The sponge is found in a variety of colors, sizes and shapes. But it possesses the same body structure in every species. It doesn't possess circulatory, digestive or a nervous system, but instead, all the functions of those systems are carried out by the pores. The constant supply of water through the pores, supply the essential nutrients and oxygen to survive and to expel the waste out of the body. Each day, the water circulated from the Sponge body is expected to be about 20,000 times of its own capacity. The oldest fossil obtained, belongs to a period of 540 million years old.

Anatomy
The body of a Sponge is typically hollow and full of pores. It retains a shape and structure due to the presence of a jelly like substance made of collagen, known as mesohyl, and also has a thick network of fiber made of collagen. Sponges grow only up to a few centimeters. The urn shaped or shapeless Sponges are found less than 1 centimeter is size. The Sponges having vase, branches or tube shapes grow about 1to 2 meters in length, and the mass developed in and round and broad shape attains 1 to 2 meters in diameter. Some grow bushy, some have finger like projecting structures, whereas some grow like branched tree. The Sponge belonging to the Demospongiae class is mostly found in a cushion form or grow as a thin layer on the surface of hard objects. These masses usually develop in amorphous or shapeless structures. Some species have spherical, fan or cup like shapes, whereas some are shaped in a tubular sac form. Members belonging to the Hexactinellida group are found in an erect, stalk-like or cylindrical form. Similar to the structures, the colors of the Sponges also vary depending on the environment and the type of the species. The Sponges grown in the deep water are often seen in neutral or brown color. The shallow water species are found in bright colors like yellow, red, orange, violet, white, green and at times, in black color.


Reproduction
The Sponge has the capability to reproduce sexually and asexually. It is generally a hermaphrodite, but in some species, the occurrence of male and female cells takes place in different time in the same individual. In the sexual reproduction, egg fertilization is very different and is done by the spermatozoa. When it is released from the Sponge's body, the water flow carries the spermatozoa, until the flagellated cell known as collar cell or choanocyte from another Sponge captures it. The collar cell is transformed into a carrier cell that possesses a shape of amoebas, and carries the spermatozoa. Later, it releases the male sex cell to fertilize an egg near the chamber that contains flagella or lash-like long appendages. The larva thus produced flows freely in the water for few hours or days, depending on the species, and is later attached to a suitable substrate that develops into a new Sponge specimen.
Asexual reproduction occurs in many ways, but more commonly, gemmulation method is used by the Sponge to reproduce. In this process, when the archaeocyte cell become loaded with food granules, it gets encircled with a protective layer or covering, thereby getting isolated on the surface of the Sponge. The matured Sponge expels these cells from the body, which helps in the reproductive process.


Habitat
The Sponge is found in a wide variety of oceanic habitats and is equally distributed around the globe. It can be seen in the oceans located from tropic regions to the polar climates. Only one family member, the Spongillidae species are found in the fresh water and the remaining 98% species are marine animals. Usually, it grows in clear and quiet water, where the water currents and waves occur less, because, regular and heavy waves will cause the sediments in the water to block the pores in the Sponge body, through which it breaths and feeds. It grows on a strong base like rocks and is often found growing over the rocky areas, whereas some species with a root like bottom, have the capability to grow on a soft substrate or sediment. The Calcarea species grow in shallow water with less than 100 m depth, whereas the species like glass Sponges grow in deep oceans. The DemoSponges reside in abyssal to intertidal habitat and the carnivorous types are found in 8840 meters depth.


Fossil Records
Though the Sponge possesses a long and varied history, it is not a common fossil, probably due to its delicate skeleton and the low lying substrate to which it is attached. The oldest fossil of the Sponge acquired, belongs to the approximately 550 million years old Precambrian period. Since the Precambrian period, the Sponges have been widespread in the deep seas. It was also a main contributor to the formation of the reef during the Phanerozoic era. During the Cretaceous period, it developed into many new species and attained great diversity. The fossils belonging to the Edicaran period, that is about 540 million years ago, were discovered from the Doushantuo Formations. The Glass Sponge fossils have been discovered from the rocks in China, Mongolia and Australia. The Permian and Pennsylvanian rocks of the eastern parts of the Kansas consist of old Sponge fossils.


Feeding
The members of the Porifera phylum are filter-feeders, and consume the food particles flowing in the water with the help of Choanocytes. The Sponges probably feed on the bacteria, remains of the organic debris and other microorganisms suspended in the surrounding water. It also has the capability to absorb the dissolved substances in the water. Some species are carnivores and feed on tiny insects. The hair like structure covering the Sponge body helps to capture the prey. The Sponge consumes a large quantity of food and water each day. The continuous beating of the flagella helps the movement of the water throughout the Sponge body. The contraction and expansion of the pores present in the Sponge body also help in the movement of the water that helps in feeding and breathing.  The food is transferred from the choanocytes throughout the Sponge body by the thesocytes and the digestion takes place in both, thesocytes and choanocytes.