Shikamaia akasakaensis was a giant bivalve that lived during the mid-Permian (~274-267 million years ago), in the shallow tropical waters of a carbonate platform atop an oceanic seamount in the region of what is now Japan.
It had a long flat shell up to about 1m (3'3") long, with a raised triangular hump at the front and wing-like flanges at the sides — a shape that helped to spread out its weight on the soft seafloor sediment.
Its shell was once thought to have been translucent, allowing it to host symbiotic photosynthetic algae inside its body tissues and providing it with the extra sustenance needed to grow to such a huge size. However, more recent studies suggest its shell was actually opaque to sunlight, so instead it may have hosted chemosynthetic bacteria similar to some modern clams.
…Or, considering that it shared its habitat with some other big invertebrates such as a large species of the marine snail Euconospira and a giant crinoid, Shikamaia could simply have been living in such an incredibly food-rich environment that it was able to attain gigantic sizes purely from normal filter-feeding.
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