Final answer:
Radiolarians have skeletons made of silica and are predominantly found throughout the ocean. Their silica skeletons lead to extensive deposits on the ocean floor, and they are also well-preserved in the fossil record.
Step-by-step explanation:
The skeleton of radiolarians is made of silica, specifically glassy silica, which gives them intricate exteriors with radial or bilateral symmetry. Radiolarians are a subtype of Rhizaria, a group of mostly microscopic protists. Needle-like pseudopods extend from their cell bodies and function to capture food particles. These pseudopods are supported by microtubules that extend from the central body through the siliceous skeleton.
Radiolarians are primarily found in the ocean, from surface waters to the deep sea. When radiolarians die, their silica skeletons sink to the ocean floor, where they can accumulate to great depths, sometimes up to 100 meters thick. These deposits contribute to the sediment layer and are very common in the fossil record due to their excellent preservation, much thanks to the durability of silica.
Other marine organisms also have skeletons made of silica or similar compounds. For example, sponges can have spicules composed of silica, calcium carbonate, or spongin. Moreover, diatoms, another group of microscopic organisms, possess an external skeleton made of silica dioxide, similar to glass. Comparatively, echinoderms like starfish have an endoskeleton of calcareous ossicles, and fish vary between those with bony skeletons and those with cartilaginous ones, like rays. These diverse skeletal materials reflect the varied evolutionary adaptations and ecological niches different marine organisms occupy.