Myllokunmingiidae

Myllokunmingiidae Temporal range: Cambrian Stage 3,
Reconstruction of Haikouichthys
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Infraphylum:	Agnatha
Order:	†Myllokunmingiida Shu, 2003
Family:	†Myllokunmingiidae Shu, 2003
Species
†Haikouichthys †Myllokunmingia †Zhongjianichthys

Myllokunmingiidae is a family of primitive jawless fish which lived during the Cambrian period.^[2] The myllokunmingiids are considered to be the earliest known vertebrate animals. The group contains only three known genera, Haikouichthys, Myllokunmingia, and Zhongjianichthys. Their fossils have been found only in the Maotianshan Shales lagerstätte.

Myllokunmingiids feature a notochord with vertebral elements, gill arches, fins with radials, zigzag-shaped muscle segments (myomeres) and paired sense organs on the head.^[2][3][4][5] These sense organs include a pair of otic capsules^[3][4] and two pairs of camera-type eyes: one large lateral set and another smaller medial set, which were formerly interpreted as nasal sacs. This was indicated after a study of six specimens of Haikouichthys and four specimens of indeterminate myllokunmingiids in 2026 by Lei and colleagues, who link the two smaller medial eyes to the pineal/parapineal system in crown-group vertebrates.^[6]

The validity of the three known taxa is generally accepted.^{[7][8][9][10]}

Hou and colleages have argued in 2002 that Haikouicthys is synonymous with Myllokunmingia.^[11] Subsequent studies led by the British paleontologist Simon Conway Morris identified the genera to be distinct on the basis of their gill arrangements,^[7] the absence of branchial rays in Myllokunmingia and the myomeres having a more acute shape in Haikouichthys.^[8] In 2017, Hou et al. suggest in their book, The Cambrian Fossils of Chengjiang, China, that Haikouichthys, Myllokunmingia and Zhongjianichthys are taphonomic variants (i.e., of the same animal but in different states of decay) and on the basis of the lack of certainty in the reliability of the distinguishing characters between the three taxa, they prefer to use Myllokunmingia to refer to myllokunmingiid specimens in concurrence with Hou et al., 2002.^[10]

• Order †Myllokunmingiida Shu, 2003^[12][13][14]
  • Family †Myllokunmingiidae Shu, 2003
    • Genus †Haikouichthys Luo, Hu & Shu 1999 sensu Shu et al., 2003
      • Species †Haikouichthys ercaicunensis Luo, Hu & Shu 1999 sensu Shu et al., 2003
    • Genus †Myllokunmingia Shu, Zhang & Han, 1999
      • Species †Myllokunmingia fengjiaoa Shu, Zhang & Han, 1999
    • Genus †Zhongjianichthys Shu, 2003
      • Species †Zhongjianichthys rostratus Shu, 2003

Myllokunmingiids were likely active swimmers^[3] and probably occupied a niche in the pelagic foodchain as suspension feeders.^[6][15] They would have been ideal prey for contemporary predators due to being soft-bodied, muscular and high-calorie targets, driving the evolution of enhanced visual systems (i.e., the four camera-type eyes identified by Lei et al.) that enable detection and evasion of threats. The dense fossil aggregations in which myllokunmingiid specimens often occur may reflect schooling behavior, noted by Lei et al. to be a common anti-predator response in modern teleost fishes.^[6]

Their active swimming lifestyle may explain the rarity of their preservation, which enabled escaple from sediment flows that would bury them.^[3]

Other very early vertebrates or transitional between olfactores and vertebrates are:

• Yunnanozoon (Terrenuevian; junior synonym: Haikouella)
• Nuucichthys (Cambrian)
• Zhongxiniscus (Cambrian Stage 3)
• Metaspriggina (Middle Cambrian)
• Palaeospondylus (Middle Devonian)