|My reconstruction of the early vertebrate Haikouichthys ercaicunensis, based on Zhang & Hou, 2004.|
Early vertebrates: the Myllokunmingiidae
Molecular data of extant fauna places the divergence of vertebrates (animals with a backbone, including mammals, birds, reptiles, amphibians and fish) from their closest relatives, the cephalochordates (Amphioxus) as far back as 751 MYA (Hedges, 2001) during the Cryogenian period of the proterozoic. This is well before the Ediacaran biota (575 MYA) and the so-called Cambrian explosion (542 MYA). However, it was recently shown that the molecular clock ran some five times faster during the Cambrian than during any other period that followed (Lee, 2013). A more conservative and reasonable estimate would therefore make the vertebrates appeared at the very end of the Proterozoic or during the Lower Cambrian along many other phyla. What does the fossil record says? For a while, the earliest undisputed vertebrate fossil remains consisted of some isolated dermal bones dating from the Early Ordovician (480 MYA) of central Australia and belonging to a group of jawless fish called Arandaspida. This situation changed quite a bit in 1999 with the discovery and description of two fossils from the famous Chengjiang biota of the Maotianshan Shale in the Yunnan Province of China, dating from the middle of the Lower Cambrian (525-520 MYA). Two species were erected, Myllokunmingia fengjiaoa Shu et al., 1999 and Haikouichthys ercaicunensis Luo et al., 1999, regrouped into the family Myllokunmingiidae. A third species, Zhongjianichthys rostratus Shu, 2003 has been added to the list four years later.
Haikouichthys (“Haikou fish”) is by far the best known of the three. Originally based on a single incomplete specimen, it is today known from more than 500 specimens from the same fossil locality near Haikou, in the Kunming prefecture of Yunnan. Measuring about 2.5 cm in length, Haikouichthys has an elongated fish-like body with a single dorsal, ventral and caudal fin, as shown from a remarkably well-preserved specimen (Zhang & Hou, 2004). It is not easy to interpret faint impressions within the fossils but structures and internal organs such as vertebrae, paired eyes, guts, heart, and possibly a nostril, an olfactory organ have been identified. Haikouichthys had a number of gill pouches and series of W shape myomeres (muscle blocks that are typical in fish). The presence of a mouth can only be inferred as it is not clearly visible in the fossils. Haikouichthys was certainly an active swimmer but probably not a good one because of the lack of paired fins. Haikouichthys and the other Myllokunmingiids appear to be the most primitive agnathans (jawless fish). Phylogenetic analysis indicates that this is a stem vertebrate more primitive than lampreys and any other known jawless fish.
Myllokunmingia (“Kunming fish”) is known from a single 2.8 cm long specimen. It is usually seen as being a bit larger and bulkier than Haikouichthys. However, a new fossil (Hou et al., 2002) showing a combination of characters found in Haikouichthys and Myllokunmingia, may indicate that the two constitute in fact a single animal (in that case, the name Myllokunmingia would have precedence over Haikouichthys) and any observed differences may rather reflect preservation bias. This view is however not universally recognized. Zhongjianichthys is a problematic animal that has been classified as a Myllokunmingiid, but not enough is known about it for this attribution to be certain. Another Maotianshan Shale animal, Haikouella lanceolata Chen, Huang & Li, 1999, known from more than 300 specimens, is often considered as another possible stem vertebrate. However it looks so similar to the contemporaneous Yunnanozoon lividum, a possible hemichordate or stem chordate, that this attribution is somewhat unlikely. Most specimen of Haikouella measured 2.5 to 3 cm in length with some individuals reaching 4 cm.
Chen, J., Huang, D., & Li, C. (1999). An early Cambrian craniate-like chordate. Nature, 402(December), 518–522.
Hedges, S. (2001). Molecular evidence for the early history of living vertebrates. In Major Events in Early Vertebrate Evolution, 119–134.
Lee, M. S. Y., Soubrier, J., & Edgecombe, G. D. (2013). Rates of phenotypic and genomic evolution during the Cambrian explosion. Current Biology, 23(19), 1889–95.
Shu, D., Luo, H., Morris, S., Zhang, X., & Hu, S. (1999). Lower Cambrian vertebrates from south China. Nature, 402(November), 42–46.
Shu, D., Morris, S., Han, J., & Zhang, Z. (2003). Head and backbone of the Early Cambrian vertebrate Haikouichthys. Nature, 421(January), 526–529.
Xian-guang, H., Aldridge, R. J., Siveter, D. J., Siveter, D. J., & Xiang-hong, F. (2002). New evidence on the anatomy and phylogeny of the earliest vertebrates. Proceedings. Biological Sciences / The Royal Society, 269(1503), 1865–9.
Zhang, X.-G., & Hou, X.-G. (2004). Evidence for a single median fin-fold and tail in the Lower Cambrian vertebrate, Haikouichthys ercaicunensis. Journal of Evolutionary Biology, 17(5), 1162–6.