Myllokunmingia is an extinct group of early jawless fish that lived during the Cambrian period, about 518 million years ago, in what is now Yunnan Province, China. It was found together with Haikouichthys, another group of jawless fish, in 1999. Myllokunmingia is believed to be one of the oldest known vertebrate animals in the fossil record. The holotype, which is the only specimen scientists agree is a true example of this species, was discovered in a specific layer of rock called the Yuanshan member of the Qiongzhusi Formation. This layer is located in the Eoredlichia Zone near Haikou, Ercaicun, Kunming City, Yunnan Province, China.

Description

The holotype measures 28 millimeters in length and 6 millimeters in height. The animal has a clear head and body, with a forward-facing, sail-shaped dorsal fin that is 1.5 millimeters tall and a ventral fin fold located farther back (likely paired). The head contains five or six gill pouches with hemibranchs. Along the body, there are 25 muscle segments (myomeres) marked by backward-pointing, double-V-shaped patterns. The animal has a notochord, a pharynx, and a digestive tract that may extend to the end of the body. The mouth is not clearly visible. A pericardial cavity may be present. There are no fin radials. The tail tip in the holotype is covered with sediment.

Taxonomy

Although usually considered a separate group, scientists have had difficulty determining how this name fits with other myllokunmingiid fossils.

In 2002, Hou and others argued that Haikouichthys is a later name for the same group (based on rules about naming when two groups are described on the same date). Their reasoning included the idea that certain features in Haikouichthys fossils, such as fin rays, might not be real but instead formed by the rock surrounding the fossils. They also noted that gill structures, which were missing in fossils from both groups, were present in the fossils they studied. Additionally, they believed the differences in shape between the two groups were not significant enough to separate them.

However, later research by paleontologist Simon Conway Morris showed that the two groups are different. He found differences in how gills were arranged, the absence of branchial rays in Myllokunmingia, and the more pointed shape of muscle segments in Haikouichthys.

In 2017, Hou and colleagues in their book The Cambrian Fossils of Chengjiang, China suggested that fossils labeled as Myllokunmingia, Haikouichthys, and Zhongjianichthys might represent the same animal at different stages of decay. Because they believed the features used to distinguish these groups were not reliable, they followed the earlier view from 2002 and used the name Myllokunmingia for all of them.