''Zhongjianichthys rostratus'' is an extinct basal

chordate A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fiv ...

that lived in the

Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...

period, approximately 518 million years ago. It is sometimes regarded as an early fish, and therefore as one of the first vertebrates. ''Zhongjianichthys'' is named after Zhongjian in China.

Physical characteristics

The eyes are located behind the antero-dorsal lobe and the mouth apparently did not have a jaw. It lacked scales, and had a thick skin. It is known to have had a thicker skin than other chordates at that time because, unlike ''

Myllokunmingia ''Myllokunmingia'' is a genus of basal chordate from the Lower Cambrian Maotianshan shales of China 518 to 490 mya and is thought to be a vertebrate, although this is not conclusively proven. The species M. fengjiaoa is 28 mm long and 6&nbs ...

,'' no impressions of its

myomere Myomeres are blocks of skeletal muscle tissue arranged in sequence, commonly found in aquatic chordates. Myomeres are separated from adjacent myomeres by connective fascia (myosepta) and most easily seen in larval fishes or in the olm. Myomere ...

s have been found. The ventral fin is low in height, and runs much of the body's length. Although ''Zhonjianichthys'' appears more advanced than its relatives due to the back-positioned eyes and the thicker skin, this may be

convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...

and it may not, therefore, be particularly closely related to the ancestors of modern chordates.

Ecology

''Zhongjianichthys''' reduced fins may indicate that it was mainly bottom-dwelling and did not swim much. This could have been a factor in its evolution of thicker skin, as protection from predators.

Locations

''Zhongjianichthys'' specimens have been found in the early Cambrian Maotianshan shales of China.

Notes

References

Maotianshan shales fossils Prehistoric jawless fish genera Cambrian fish Fossil taxa described in 2003 {{cambrian-animal-stub Cambrian chordates Cambrian genus extinctions