Sponges are among the simplest and most primitive types of multicellular animals, having appeared before the Cambrian period and continued to thrive to the present day. They evolved from single-celled organisms; their cells have differentiated, but tissues and organs have not yet formed. Sponges can be solitary or colonial, and their shapes are diverse. Solitary sponges can be goblet-shaped, bottle-shaped, spherical, or cylindrical. The body wall of a sponge has many pores through which water channels run, and there is a central cavity inside the body, with an opening at the top forming the exhalation pore for the entire individual.
Most sponges have skeletons. The skeletons are divided into two categories: one is a needle-like or spiky skeleton made of calcium or silica, called a spicule; the other is a filamentous skeleton made of organic matter, called a filament.
Bone filaments are not easily preserved as fossils, while bone spicules can. Some bone spicules can interlock to form a skeleton, and such a skeleton can retain the original shape of the sponge after fossilization.
Scientists classify sponges primarily based on the nature and composition of their skeletons. Sponges are generally divided into four classes: Calcareous Sponges, Common Sponges, Hexacoral Sponges, and Heterocoral Sponges.
Scientists have documented over 1,000 genera of fossil sponges, the earliest representatives including calcareous sponges found in Precambrian strata in the Congo, Africa; spicules of siliceous sponges found in Middle Proterozoic strata in Karelia and Yenisei Mountains, Russia; and scattered representatives from the Precambrian period in southern my country. From the Cambrian onwards, many representatives of the classes of common sponges, hexaradiate sponges, and heteroradiate sponges appeared, with the heteroradiate sponges becoming extinct after the Middle Triassic. Calcareous sponges, on the other hand, began to appear in the Devonian period.
sponge fossils
Sponges have inhabited a wide range of environments since ancient times, resulting in their fossils being preserved in various types of rocks, including limestone, sandstone, shale chalk, and deep-water claystone. Because Paleozoic and Mesozoic sponges often coexisted with typical shallow-sea animals such as corals and brachiopods, scientists generally believe they primarily lived in shallow seas at that time. Cenozoic sponges inhabited similar environments to modern sponges, but the overall trend was towards an increasingly wider range of adaptations.
Modern sponges, except for the family Monopodiomyidae within the class Echinops which live in freshwater, are all marine. Calcareous sponges are distributed from coastal waters to depths of 2195 meters, but their main distribution area is in waters less than 100 meters deep. Common marine sponges are distributed from coastal to semi-deep seas. Hexafossors, however, primarily inhabit the deep seabed at greater depths, on continental slopes and below; they are rarely found in waters less than 90 meters deep, and only a few individuals are found in depths between 90 and 200 meters. Only a small number of hexafossors live in the shallow waters of Antarctica.
Modern sponges can be found in oceans at all latitudes on Earth. Common sponges are mainly distributed in warm oceans and are less common in high-latitude regions. Hexacoral sponges are more concentrated in tropical and subtropical seas, especially abundant in the waters off Indonesia, the Philippines, and the South China Sea, with a few also found in the waters near Antarctica. Residual calcareous sponges are distributed in warm waters.
