Zhongjianichthys

Zhongjianichthys Temporal range: Cambrian Stage 3, 518  Ma [1] PreꞒ Ꞓ O S D C P T J K Pg N ↓
Fossil
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Infraphylum:	Agnatha
Order:	† Myllokunmingiida
Family:	† Myllokunmingiidae
Genus:	† Zhongjianichthys Shu, 2003
Species:	†Z. rostratus
Binomial name
†Zhongjianichthys rostratus Shu, 2003 [2]

Restoration

Zhongjianichthys is an extinct genus of primitive jawless fish that lived in the Cambrian Period, approximately 518 million years ago, in what is now Southwest China. As a myllokunmingiid, it is considered one of the earliest known vertebrates in the fossil record alongside the other two identified genera, Haikouichthys and Myllokunmingia . Like its confamilials, its fossils were found in the Maotianshan Shales.

The type species, Zhongjianichthys rostratus, is named is named after paleontologist Yang Zhongjian (1897-1979), who was widely honored as the "Father of Chinese Vertebrate Paleontology"; although the specific epithet rostratus means "beak-like" in Latin, Shu states in the describing paper that it is in reference to the rarity of the fossils. ^[2]

Description

Zhongjianichthys had a small, elongated, eel-like body. Notably, its head extends into a blunt, beak-like projection termed the antero-dorsal lobe, a feature also carried by Haikouichthys but it is smaller. Moreover, the eyes are located posteriorly to the antero-dorsal lobe relative to Haikouichthys. It also had thicker skin than Haikouichthys or Myllokunmingia, which is evidenced by the lack of visible myomeres.

The posterior positioning of its eyes and the thickness of its skin are considered to be advanced characters by Shu, contrasting the former with the anterior position of the eyes in Haikouichthys and the later Jamoytius being the primitive condition and comparing the latter with the condition of modern lampreys and hagfish, which have an epidermis consisting of multiple layers of cells versus the probable singular layer thereof in the other myllokunmingiids. ^[2]

In comparison to Haikouichthys, it is similar in having large eyes, a pair of smaller structures near the eyes interpreted as nasal sacs — or, less likely, a pineal/parapineal complex (an identity that would later be confirmed in 2026 by Lei and colleagues in their study of specimens of Haikouichthys and indeterminate myllokunmingiids, who interpret them as an additional set of camera-type eyes) ^[3] , a singular nostril, a pair of rostral plates anterior to its antero-dorsal lobe, a singular nostril, and a set of simple gill arches. ^[2]

Possible vertebral elements have also been interpeted in the fossils. ^[2]

Taxonomy

Although generally accepted to be valid alongside the other two myllokunmingiid taxa, ^[4] whether Zhongjianichthys is a distinct taxon is not entirely certain. Conway Morris and Caron in 2012 suggest that the lack of evident preservation of myomeres may actually be a taphonomic artifact and raise the possibility of Zhongjianichthys being synonymous with Haikouichthys. ^[5] Similarly, Hou and colleages in 2017 suggest in their book, The Cambrian Fossils of Chengjiang, China, that all myllokunmingiid specimens referred to the three known taxa — including Zhongjianichthys — may be taphonomic variants of the same animal and use the name Myllokunmingia for all of them due to what they argue to be a lack of certainty in the reliability of the distinguishing characters between the three taxa. ^[6]

Paleoecology

Zhongjianichthys' elongated body and reduced fins suggest limited swimming ability, indicating a likely benthic lifestyle with intermittent burrowing. ^[2]

References

1. Yang, C.; Li, X.-H.; Zhu, M.; Condon, D. J.; Chen, J. (2018). "Geochronological constraint on the Cambrian Chengjiang biota, South China" (PDF). Journal of the Geological Society. 175 (4): 659–666. Bibcode:2018JGSoc.175..659Y. doi:10.1144/jgs2017-103. ISSN   0016-7649. S2CID   135091168.
2. Shu, Degan (2003). "A paleontological perspective of vertebrate origin". Chinese Science Bulletin. 48 (8): 725–735. Bibcode:2003ChSBu..48..725S. doi:10.1007/BF03187041. S2CID   85163902.
3. Lei, X.; Zhang, S.; Cong, P.; Vinther, J.; Gabbott, S.; Wei, F.; Xu, X. (2026). "Four camera-type eyes in the earliest vertebrates from the Cambrian Period". Nature. 650 (8100): 150–155. Bibcode:2026Natur.650..150L. doi:10.1038/s41586-025-09966-0.
4. Zhao, Jun; Li, Guo-Biao; Selden, Paul A. (April 2019). "A poorly preserved fish-like animal from the Chengjiang Lagerstätte (Cambrian Series 2, Stage 3)". Palaeogeography, Palaeoclimatology, Palaeoecology. 520: 163–172. doi:10.1016/j.palaeo.2019.02.005.
5. Morris, Simon Conway; Caron, Jean‐Bernard (2012). "Pikaia gracilens Walcott, a stem‐group chordate from the Middle Cambrian of British Columbia". Biological Reviews. 87 (2): 480–512. doi:10.1111/j.1469-185X.2012.00220.x. ISSN   1464-7931.
6. Hou, Xian‐guang; Siveter, David J.; Siveter, Derek J.; Aldridge, Richard J.; Cong, Pei‐yun; Gabbott, Sarah E.; Ma, Xiao‐ya; Purnell, Mark A.; Williams, Mark (2017-04-12). The Cambrian Fossils of Chengjiang, China: The Flowering of Early Animal Life (1 ed.). Wiley. doi:10.1002/9781118896372.ch24. ISBN   978-1-118-89638-9.