Few stem-group fossils have been found for the vertebrates. However, morphological analysis has resulted in three species being assigned to the vertebrate stem in a polytomy (Morris and Caron, 2014). Furthermore, in the phylogenetic tree presented by Miyashita et al (2019), the species are resolved in the stem-Vertebrata together with Haikouella lanceolata. However, given that the latter is now considered to be a yunnanozoan (Cong et al, 2015), it has been excluded from the time tree shown below:
Figure 1. Phylogenetic time tree of the stem-Vertebrata
Images available in the public domain for the species represented in the above figure are shown below (click on image for a larger version):
Figure 2. Images of stem-group vertebrates
The above species are generally similar in being soft-bodied, with a notochord, prominent eyes and gill slits (Morris and Caron, 2014). They are the only known transitional fossils between the division of the vertebrate and tunicate stem lines and the appearance of the vertebrate crown group. Some idea of the nature of this transition can be gained by comparing the above images with the examples of early crown vertebrates shown below:
Names in red indicate that the fossil is younger than the oldest known crown-group fossil.
Figure 3. Examples of early crown-Vertebrata
This transition took place in the Early and Middle Cambrian over a period of about 20 million years, as indicated in the phylogenetic time tree above (Figure 1).
References
Benton, M. J. (2015). Vertebrate Palaeontology - Fourth edition. John Wiley & Sons, 468 pages.
Cong, P. Y., Hou, X. G., Aldridge, R. J., Purnell, M. A., & Li, Y. Z. (2015). New data on the palaeobiology of the enigmatic yunnanozoans from the Chengjiang Biota, Lower Cambrian, China. Palaeontology, 58(1), 45-70.
Miyashita, T., Coates, M. I., Farrar, R., Larson, P., Manning, P. L., Wogelius, R. A., ... & Currie, P. J. (2019). Hagfish from the Cretaceous Tethys Sea and a reconciliation of the morphological–molecular conflict in early vertebrate phylogeny. Proceedings of the National Academy of Sciences, 116(6), 2146-2151.
Morris, S. C., & Caron, J. B. (2014). A primitive fish from the Cambrian of North America. Nature, 512(7515), 419-422.
Cong, P. Y., Hou, X. G., Aldridge, R. J., Purnell, M. A., & Li, Y. Z. (2015). New data on the palaeobiology of the enigmatic yunnanozoans from the Chengjiang Biota, Lower Cambrian, China. Palaeontology, 58(1), 45-70.
Miyashita, T., Coates, M. I., Farrar, R., Larson, P., Manning, P. L., Wogelius, R. A., ... & Currie, P. J. (2019). Hagfish from the Cretaceous Tethys Sea and a reconciliation of the morphological–molecular conflict in early vertebrate phylogeny. Proceedings of the National Academy of Sciences, 116(6), 2146-2151.
Morris, S. C., & Caron, J. B. (2014). A primitive fish from the Cambrian of North America. Nature, 512(7515), 419-422.
Image credits - Stem-Vertebrata
- Figure 2 (fossil, Myllokunmingia fengjiaoa): Degan Shu, Northwest University, Xi'an, China. Cropped by User:Andrew Dalby, Attribution, via Wikimedia Commons
- Figure 2 ((life restoration, Myllokunmingia fengjiaoa): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 2 (Haikouichthys ercaicunensis): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 2 (Metaspriggina walcotti): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3 (Promissum pulchrum): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3 (Clydagnathus windsorensis): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3 (Sacabambaspis janvieri): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Figure 3 (Arandaspis prionotolepis): Nobu Tamura (http://spinops.blogspot.com) [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], from Wikimedia Commons