In a paper released June 5 in the journal Nature, Virginia Tech geobiologist Shuhai Xiao and collaborators report a 550 million-year-old sea sponge from the "lost years" and propose that the earliest sea sponges had not yet developed mineral skeletons, offering new parameters to the search for the missing fossils.

The mystery of the missing sea sponges centered on a paradox.

Molecular clock estimates, which involve measuring the number of genetic mutations that accumulate over time, indicate that sponges must have evolved about 700 million years ago. And yet there had been no convincing sponge fossils found in rocks that old.

For years, this conundrum was the subject of debate among zoologists and paleontologists.

This latest discovery fills in the evolutionary family tree of one of the earliest animals, explaining its apparent absence in older rocks and connecting the dots back to Darwin's questions about when it evolved.

Xiao, who recently was inducted into the National Academy of Sciences, first laid eyes on the fossil five years ago, when a collaborator texted him a picture of a specimen excavated along the Yangtze River in China.