Researchers discover 'missing' 550 million-year-old sea sponge

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A recent study published in Nature addresses a significant gap in the fossil record of sea sponges, revealing a 550 million-year-old sponge that bridges a 160 million-year gap in sponge fossils. Virginia Tech geobiologists discovered this ancient sponge fossil along the Yangtze River in China. This finding is pivotal as it fills a longstanding gap between the estimated molecular evolution of sponges around 700 million years ago and the much later appearance of convincing sponge fossils around 540 million years ago.

The missing sponge fossils puzzled scientists because molecular clock estimates indicated that sponges should have existed much earlier than the oldest fossils. The discovery suggests that the earliest sea sponges did not have mineral skeletons, which may explain their absence in older fossil records. Instead, these early sponges were likely soft-bodied and composed primarily of organic material, which did not fossilize well except under rare conditions.

The newly discovered sponge is about 15 inches long and exhibits a complex conical body plan with a unique pattern of regular boxes, resembling a glass sponge. This large size and complexity challenge previous assumptions about the early appearance of sponges. The fossil was preserved in marine carbonate rocks known for preserving soft-bodied animals, indicating that such conditions can capture early animal forms that lack hard parts.

This find fills a crucial gap in the sponge evolutionary timeline and guides future searches for early sponge fossils. It suggests that researchers should broaden their approach to include soft-bodied organisms. This discovery enhances our understanding of early animal evolution and provides new insights into the characteristics of ancient sea sponges.