Scientists recently discovered tiny, 500-million-year-old fossils in southern China that contain embryos of an unknown animal.

The fossils are from the Cambrian era, a period dating from 540 million to 485 million years ago. Some of the organisms that appeared during that period were bug-like trilobites that had exoskeletons that fossilized well, in addition to sponges and worms, which rarely fossilize.

Researchers James Schiffbauer of the University of Missouri and Jesse Broce of Virginia Tech were searching for soft-tissue fossils in southern China when they found the tiny spheres, including some with polygonal patterns, CBS News reports. The researchers reported in the Journal of Paleontology that the small fossils are most likely fossilized embryos.

"We found over 140 spherically shaped fossils, some of which include features that are reminiscent of division-stage embryos, essentially frozen in time," Schiffbauer said in a statement.

The researchers began investigating the fossils by trying to dissolve them out of limestone with acid, but the method damaged or completely destroyed the spherical fossils. Researchers then decided to chisel the rock into small chunks, allowing them to expose the fossil surfaces.

The researchers investigated the spheres by slicing them into thin sections so they could be viewed under a microscope. The scientists also imaged the fossils with X-ray and scanning electron microscopy techniques.

The investigations proved that the specimens have a phosphate-rich envelope surrounding a ball of calcite. Some of the spheres had patterns similar to those seen on fossilized embryos from Markuelia, a Cambrian, worm-like creature.

Researchers believe that the specimens are blastulas, which are early, multicellular stages of embryonic development.

It is still not known what animals the embryos could have become.