World's oldest primitive animal fossil found in China

The rice grain-sized primitive sponge fossil.

Animals have been on Earth for at least 600 million years, research led by Chinese scientists confirmed Monday.

The study, published in the U.S. journal Proceedings of the National Academy of Sciences, described a well-preserved, rice grain-sized primitive sponge fossil recovered from 600-million- year rocks in southwest China's Guizhou Province.

"It's the world's earliest and most credible fossil record of primitive animal bodies," lead author Maoyan Zhu of the Nanjing Institute of Geology and Palaeontology, which is part of Chinese Academy of Sciences, told Xinhua. "The discovery will help remove doubts as to whether animals have emerged on Earth 600 million years ago."

The fossilized animal, slightly more than 1.2 mm wide and 1.1 mm tall, displays multiple independent characters of modern adult sponges, according to an analysis based on advanced imaging techniques including scanning electron microscope and synchrotron X-ray tomography.

The specimen is composed of hundreds of thousands of cells, and has a gross structure consisting of three adjacent hollow tubes sharing a common base, the researchers said.

Flat tile-like features on the fossil's external surface, punctuated with small pores, resemble pinacocytes on modern sponges, whereas the fossil's inner surface is covered with a regular pattern of uniform pits, with many pits surrounded by collars, similar to sponge choanocytes.

In addition, it has a simple canal system for water inflow and outflow.

"These features showed it's a primitive animal very similar to modern sponges and could have lived a filter-feeding life through its simple water canal system on the shallow sea floor," Zhu said.

Sponges are the most primitive living animals. They have only differentiated cells, but no real organs or tissues. The prior oldest known primitive sponge fossil only dated to 530 million years ago, in early Cambrian period.

There are other isolated objects that are claimed to be pre- Cambrian fossilized sponges, including a 760-million-year-old fossil from Namibia, but Zhu said they decided to leave aside other discoveries because those objects "did not preserve typical structural characteristics of modern sponges."