This page covers the stem group of the vertebrates (Subphylum Vertebrata, phylum Chordata). A summary of the phylogeny of this clade is shown below:
Figure 1. Summarized phylogenetic tree of the vertebrates
As for each page of this website that belong to the Animals and Land Plants categories, this page will deal with a single branch of the phylogenetic tree. For the vertebrates, the branches are numbered as follows:
Figure 2. Branch numbering scheme
The pages corresponding to the branches shown can be found by clicking the links in the following table:
Branch number |
Branch name |
Page link |
1 |
Stem-Vertebrata |
This page |
1.1 |
Stem-Cyclostomata |
|
1.2 |
Stem-Myxiniformes |
|
1.3 |
Stem-Petromyzontiformes |
|
1.4 |
Stem-Gnathostomata |
|
1.5 |
Stem-Chondrichthyes |
|
1.6 |
Stem-Holocephali |
|
1.7 |
Stem-Elasmobranchii |
|
1.8 |
Stem-Osteichthyes |
|
1.9 |
Stem-Actinopterygii |
|
1.10 |
Stem-Cladistia |
|
1.11 |
Stem-Actinopteri |
No stem-group fossils known |
1.12 |
Stem-Chondrostei |
|
1.13 |
Stem-Neopterygii |
|
1.14 |
Stem-Teleostei |
|
1.15 |
Stem-Holostei |
|
1.16 |
Stem-Sarcopterygii |
|
1.17 |
Stem-Coelacanthi |
|
1.18 |
Stem-Rhipidistia |
No stem-group fossils known |
1.19 |
Stem-Dipnoi |
|
1.20 |
Stem-Tetrapoda |
This page thus covers Branch 1, along which are found the stem-group vertebrates.
In addition to chordate synapomorphies (notochord not attached to gut, dorsal nerve chord, pharyngeal gill slits, tail used for swimming, myomeres, endostyle organ (equivalent to thyroid gland in vertebrates)) crown-group vertebrates have the following synapomorphies that represent the features of a true head (Benton, 2014):
These characteristics would have appeared along the vertebrate stem line. Few fossils have been found that display these synapomorphies. 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 phylogenetic time tree shown below:
In addition to chordate synapomorphies (notochord not attached to gut, dorsal nerve chord, pharyngeal gill slits, tail used for swimming, myomeres, endostyle organ (equivalent to thyroid gland in vertebrates)) crown-group vertebrates have the following synapomorphies that represent the features of a true head (Benton, 2014):
- Well-defined sensory organs (nose, eyes, ears)
- Cranial nerves
- Olfactory, optic and auditory (otic) regions that make up a true brain.
These characteristics would have appeared along the vertebrate stem line. Few fossils have been found that display these synapomorphies. 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 phylogenetic time tree shown below:
Figure 3. 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 4. 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. 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 3).
A preview of the vertebrate crown group
The stem groups that make up the vertebrate crown group are discussed in the following pages. However, it is appropriate at this point to synthesize the results in a single phylogenetic time tree, as shown below:
Figure 5. Time tree of basal stem groups in the crown-Vertebrata
Several interesting observations can be made about the above tree. Firstly, it is noteworthy that the tetrapod stem group appeared in the Early Devonian, long before the teleostean clade that dominates the fishes of the world today. Indeed, the tetrapods appeared before all of the terminal clades of the vertebrates except for the Dipnoi (lungfishes) and the coelacanths.
Another point of interest concerns the great length of some of the stem lines (e.g. the Cyclostomata and the Gnathostomata) and the sparseness of the fossil record of many stem groups, represented by ghost lineages. The latter are highlighted only for the non-terminal taxa in Figure 5 above, but some long ghost lineages also exist for the terminal taxa (e.g. the Cladistei and the Myxiniformes).
The overall picture is of slow change over a long period of time, but an exception to this is seen in the appearance and division of the Sarcopterygii. From the separation of this clade from the Actinopterygii to the appearance of the tetrapod stem group, no more than 18 million years passed. This rapid rate of evolution appears to be related to the colonization of an empty new (terrestrial) environment (Ruta et al, 2006; Coates et al, 2008; Standen et al, 2014).
Another point of interest concerns the great length of some of the stem lines (e.g. the Cyclostomata and the Gnathostomata) and the sparseness of the fossil record of many stem groups, represented by ghost lineages. The latter are highlighted only for the non-terminal taxa in Figure 5 above, but some long ghost lineages also exist for the terminal taxa (e.g. the Cladistei and the Myxiniformes).
The overall picture is of slow change over a long period of time, but an exception to this is seen in the appearance and division of the Sarcopterygii. From the separation of this clade from the Actinopterygii to the appearance of the tetrapod stem group, no more than 18 million years passed. This rapid rate of evolution appears to be related to the colonization of an empty new (terrestrial) environment (Ruta et al, 2006; Coates et al, 2008; Standen et al, 2014).
References
Benton, M. J. (2014). Vertebrate Palaeontology. John Wiley & Sons, 480 pages.
Coates, M. I., Ruta, M., & Friedman, M. (2008). Ever since Owen: changing perspectives on the early evolution of tetrapods. Annual Review of Ecology, Evolution, and Systematics, 571-592.
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.
Ruta, M., Wagner, P. J., & Coates, M. I. (2006). Evolutionary patterns in early tetrapods. I. Rapid initial diversification followed by decrease in rates of character change. Proceedings of the Royal Society B: Biological Sciences, 273(1598), 2107-2111.
Standen, E. M., Du, T. Y., & Larsson, H. C. (2014). Developmental plasticity and the origin of tetrapods. Nature, 513(7516), 54-58.
Coates, M. I., Ruta, M., & Friedman, M. (2008). Ever since Owen: changing perspectives on the early evolution of tetrapods. Annual Review of Ecology, Evolution, and Systematics, 571-592.
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.
Ruta, M., Wagner, P. J., & Coates, M. I. (2006). Evolutionary patterns in early tetrapods. I. Rapid initial diversification followed by decrease in rates of character change. Proceedings of the Royal Society B: Biological Sciences, 273(1598), 2107-2111.
Standen, E. M., Du, T. Y., & Larsson, H. C. (2014). Developmental plasticity and the origin of tetrapods. Nature, 513(7516), 54-58.
Image credits - Stem-Vertebrata
- Header (Kentrosaurus aethiopicus skeleton): H. Zell [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia CommonsFigure 2 (fossil): Degan Shu, Northwest University, Xi'an, China. Cropped by User:Andrew Dalby, Attribution, via Wikimedia Commons
- Figure 4 (fossil, Myllokunmingia fengjiaoa): Degan Shu, Northwest University, Xi'an, China. Cropped by User:Andrew Dalby, Attribution, via Wikimedia Commons
- Figure 4 (restoration, Myllokunmingia fengjiaoa): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 4 (Haikouichthys ercaicunensis): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 4 (Metaspriggina walcotti): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license