Chinese researchers have tracked the "exceptionally well-preserved fossils" to the Middle Triassic of China (235-242 million years ago).
The Triassic period saw the re-establishment of ecosystems after the Permian mass extinction.
The fossils represent new evidence that marine ecosystems re-established more quickly than previously thought.
The Permian mass extinction had a bigger impact on the earth's ecological systems than any other mass extinction, wiping out 90-95% of marine species.
Previous studies have suggested that Triassic marine life developed more quickly than was once thought and that marine ecosystems were re-established more rapidly than terrestrial ecosystems.
The new research, published in Proceedings of the Royal Society B journal, was carried out by scientists from Peking University, the Chinese Academy of Sciences and the Zhejiang Museum of Natural History.