Yunnanozoon lividum is an extinct animal with body parts on both sides. It lived during the Cambrian period in Yunnan province, China. The name comes from Yunnan, the Greek word "zôion" meaning "animal," and the Latin word "lividum," which means "lead-colored" and describes the color of preserved specimens. Scientists have long debated what kind of animal Yunnanozoon lividum is related to.

Description

The body of Yunnanozoon was spindle-shaped, with lengths between 2 to 6 centimeters (0.79 to 2.36 inches). The body was flattened from side to side, making it taller than it was wide. A segmented structure covered the top of the body. The first segment of this structure was triangular, while the other segments were nearly rectangular. Stripes ran along the body below this top structure. Near the front of the animal, there were 7 pairs of thin, arch-like structures. These arches were covered by sac-like shapes with openings between them. Toward the back of the body, a tube-like structure was present, possibly the gut. Four pairs of mysterious, round structures were located in the middle of the body. Rod-like structures ran along the top and bottom of the front half of the body. These were likely hardened.

Taxonomy

Yunnanozoon was first described in 1991 by Hou, Ramskold, and Bergstrom. They placed it in a group called incertae sedis within Metazoa, which means its exact classification was uncertain. Scientists have debated where Yunnanozoon belongs, with different studies suggesting it might be related to cephalochordates, stem-chordates, hemichordates, stem-vertebrates, stem-deuterostomes, ambulacrarians, bilaterians of uncertain placement, protostomes of uncertain position, or Ecdysozoa.

In 1995, Jerzy Dzik classified Yunnanozoon into its own class, Yunnanozoa. In 1999, a new genus called Haikouella was discovered in the same rock layers as Yunnanozoon, with the species name Haikouella lanceolata. Another species, Haikouella jianshanensis, was described in 2003. Scientists who studied Haikouella noted differences in the number of filamentous arches, circular structures, and the placement of features thought to be pharyngeal teeth compared to Yunnanozoon. However, a 2015 study found that all yunnanozoans had the same number of filamentous arches and circular structures. It also concluded that the supposed "pharyngeal teeth" were actually remnants of other structures, indicating that Haikouella species were not distinct from Yunnanozoon. Instead, the study suggested that only one species, Yunnanozoon lividum, existed, with Haikouella species being later names for the same species.

The 2015 study also placed Yunnanozoon within Bilateria, a group of animals with bilateral symmetry, but its exact position remained unclear. A 2022 study used advanced imaging techniques to examine the filamentous arches and found they were made of cartilage surrounded by a network of microscopic fibers. This discovery suggested that yunnanozoans were stem-vertebrates.

A 2024 study, inspired by a new interpretation of the fossil Pikaia, placed Yunnanozoon as a transitional form between a group called Vetulicolia and the more advanced group Chordata. This relationship is shown in the simplified cladogram below.

Ecology

Scientists believe Yunannozoon used thread-like structures to help it eat. Water was pushed out through openings on the sides of its body.