XI'AN, July 30 (Xinhua) -- A research team has discovered the earliest cephalopod fossil in northwest China's Shaanxi Province, confirming that the active predatory mollusc originated in the early Cambrian roughly 520 million years ago and shedding new light on the origin and early evolution of the group, according to a study published in Nature Wednesday.

The cephalopod family such as squids, octopuses and nautiluses represent a distinctive lineage within the mollusks. The emergence of the siphuncle is regarded as the key innovation that marked the divergence of cephalopods from basal mollusks.

The earliest universally accepted cephalopod fossil had been discovered in strata of the late Cambrian about 490 million years ago. However, this fossil record left a substantial evolutionary gap when compared with molecular clock estimates, which place the divergence of cephalopods in the early Cambrian.

The team, led by Guo Junfeng from Chang'an University in Xi'an, discovered the new cephalopod fossil in the Shuijingtuo Formation in southern Shaanxi. The new fossil, only millimeter-scale in size, possesses a straight, conical shell.

Guo said that after detailed analysis, the team proposed a new hypothesis that cephalopods may have originated from straight, tubular shelled forms with multiple septa, thereby pushing the empirical origin of cephalopods back from the late Cambrian to the early Cambrian.

According to the study, the discovery extends the earliest known cephalopod fossil record by roughly 30 million years, confirms the molecular clock inference that cephalopods originated in the early Cambrian, and offers new perspectives for reinterpreting the numerous tubular fossils from the Cambrian explosion. ■