This page deals with the stem group of the clade that comprises the lobe-finned fishes and the tetrapods (superclass Sarcopterygii, Infraphylum Gnathostomata). Sarcopterygians are characterized by their fleshy pectoral and pelvic (paired) fins that articulate with the pectoral (shoulder) and pelvic (hip) girdles via a single bone (University College London web-book “Vertebrate Diversity”, www.ucl.ac.uk/museums-static/obl4he/vertebratediversity). Most sarcopterygians at the present day are tetrapods: only 8 extant species of sarcopterygian fishes are known in the two non-tetrapod classes Coelacanthi and Dipnoi.
There is currently a lot of debate about the phylogeny of the stem-Sarcopterygii. The discussion centers around a related group of fossil fishes (Achoania, Guiyu, Psarolepis and Sparalepis) that have been placed by some researchers (e.g. Lu et al, 2017; MondéJar-Fernández, 2018) in the stem-Osteichthyes and by others (e.g. Broughton et al , 2013; Zhu et al, 2013; Giles et al, 2015; Lu et al, 2016; Qiao et al, 2016; Choo et al, 2017; Cui et al, 2019) in the stem-Sarcopterygii. Others suggest that this difference in placement results from the application of different methods of phylogenetic analysis (Clement et al, 2018; King, 2019). Given the preponderance of articles, some of which are quite recent, placing these fishes in the Sarcopterygian stem group, that interpretation will be presented here. A representative phylogenetic time tree of the stem-Sarcopterygii is shown below:
There is currently a lot of debate about the phylogeny of the stem-Sarcopterygii. The discussion centers around a related group of fossil fishes (Achoania, Guiyu, Psarolepis and Sparalepis) that have been placed by some researchers (e.g. Lu et al, 2017; MondéJar-Fernández, 2018) in the stem-Osteichthyes and by others (e.g. Broughton et al , 2013; Zhu et al, 2013; Giles et al, 2015; Lu et al, 2016; Qiao et al, 2016; Choo et al, 2017; Cui et al, 2019) in the stem-Sarcopterygii. Others suggest that this difference in placement results from the application of different methods of phylogenetic analysis (Clement et al, 2018; King, 2019). Given the preponderance of articles, some of which are quite recent, placing these fishes in the Sarcopterygian stem group, that interpretation will be presented here. A representative phylogenetic time tree of the stem-Sarcopterygii is shown below:
Figure 1. Time tree of the stem-Sarcopterygii
The oldest known stem-Sarcopterygii are Guiyu oneiros and Sparalepis tingi, which are of the same age. They are both found in the mid-Silurian (Ludlow) Kuanti Formation near Xiaoxiang Reservoir, Qujing, Yunnan, China (Zhu et al, 2009; Choo et al, 2017). Fossils and life restorations of these species are shown below, together a restoration of the only other stem-group Sarcopterygian for which a public-domain image is available, Psarolepis romeri (for a larger view, click on image):
Figure 2. Images of stem-group Sarcopterygii
Very few of the fossils interpreted as stem-group sarcopterygians (see Figure 1) display any of the synapomorphies listed above. For instance, an important transition within the stem-Sarcopterygii is the appearance of muscular fins that articulate with the shoulder and hip girdles via a single bone. But what has been described in Psarolepis and Achoania is articulation via multiple bones, as is seen in basal actinopterygians (Zhu and Yu, 2009).
Several other crown-group synapomorphies are absent from the stem-group fossils depicted in Figure 1. For example, tooth enamel is absent in all of the fossils except Ptyctolepis brachynotus, which does have enamel on its teeth (Lu et al, 2017). Furthermore, Psarolepis does not have a squamosal bone (Zhu et al, 1999) and both Psarolepis and Achoania have a massive scapulocoracoid which is not triradiate (Zhu and Yu, 2009).
In summary, there is a lack of stem-group sarcopterygian fossils that display many of the characteristics that characterize the crown group. In other words, the stem-line transition is not well documented in the currently known fossil record.
An impression of the nature of the transition from the stem group to the crown group of the sarcopterygians can be derived from a comparison of the above images with the examples of early crown-Sarcopterygii shown below:
Several other crown-group synapomorphies are absent from the stem-group fossils depicted in Figure 1. For example, tooth enamel is absent in all of the fossils except Ptyctolepis brachynotus, which does have enamel on its teeth (Lu et al, 2017). Furthermore, Psarolepis does not have a squamosal bone (Zhu et al, 1999) and both Psarolepis and Achoania have a massive scapulocoracoid which is not triradiate (Zhu and Yu, 2009).
In summary, there is a lack of stem-group sarcopterygian fossils that display many of the characteristics that characterize the crown group. In other words, the stem-line transition is not well documented in the currently known fossil record.
An impression of the nature of the transition from the stem group to the crown group of the sarcopterygians can be derived from a comparison of the above images with the examples of early crown-Sarcopterygii shown below:
Figure 3. Examples of early crown-Sarcopterygii
The stem-to-crown transition of the Sarcopterygii appears to have lasted no more than 11 million years million years, from the mid-Silurian to the Early Devonian (Figure 1).
References
Benton, M. J. (2015). Vertebrate Palaeontology - Fourth edition. John Wiley & Sons, 468 pages.
Broughton, R. E., Betancur-R, R., Li, C., Arratia, G., & Ortí, G. (2013). Multi-locus phylogenetic analysis reveals the pattern and tempo of bony fish evolution. PLoS currents, 5.
Choo, B., Zhu, M., Qu, Q., Yu, X., Jia, L., & Zhao, W. (2017). A new osteichthyan from the late Silurian of Yunnan, China. PloS one, 12(3), e0170929.
Clement, A. M., King, B., Giles, S., Choo, B., Ahlberg, P. E., Young, G. C., & Long, J. A. (2018). Neurocranial anatomy of an enigmatic Early Devonian fish sheds light on early osteichthyan evolution. Elife, 7, e34349.
Cui, X., Qiao, T., & Zhu, M. (2019). Scale morphology and squamation pattern of Guiyu oneiros provide new insights into early osteichthyan body plan. Nature Scientific reports, 9(1), 4411.
Giles, S., Darras, L., Clément, G., Blieck, A., & Friedman, M. (2015). An exceptionally preserved Late Devonian actinopterygian provides a new model for primitive cranial anatomy in ray-finned fishes. Proceedings of the Royal Society B: Biological Sciences, 282 (1816), 20151485.
King, B. (2019). Which morphological characters are influential in a Bayesian phylogenetic analysis? Examples from the earliest osteichthyans. Biology letters, 15(7), 20190288.
Lu, J., Zhu, M., Ahlberg, P. E., Qiao, T., Zhu, Y. A., Zhao, W., & Jia, L. (2016). A Devonian predatory fish provides insights into the early evolution of modern sarcopterygians. Science advances, 2(6), e1600154.
Lu, J., Giles, S., Friedman, M., & Zhu, M. (2017). A new stem sarcopterygian illuminates patterns of character evolution in early bony fishes. Nature communications, 8(1), 1-8.
MondéJar-Fernández, J. (2018). On cosmine: its origins, biology and implications for sarcopterygian interrelationships. CYBIUM, 42(1), 41-65.
Qiao, T., King, B., Long, J. A., Ahlberg, P. E., & Zhu, M. (2016). Early gnathostome phylogeny revisited: multiple method consensus. PLoS One, 11(9), e0163157.
Zhu, M., & Yu, X. (2009). Stem sarcopterygians have primitive polybasal fin articulation. Biology Letters, 5(3), 372-375.
Zhu, M., Yu, X., & Janvier, P. (1999). A primitive fossil fish sheds light on the origin of bony fishes. Nature, 397(6720), 607-610.
Zhu, M., Zhao, W., Jia, L., Lu, J., Qiao, T., & Qu, Q. (2009). The oldest articulated osteichthyan reveals mosaic gnathostome characters. Nature, 458(7237), 469-474.
Zhu, M., Yu, X., Ahlberg, P. E., Choo, B., Lu, J., Qiao, T., ... & Zhu, Y. A. (2013). A Silurian placoderm with osteichthyan-like marginal jaw bones. Nature, 502(7470), 188.
Broughton, R. E., Betancur-R, R., Li, C., Arratia, G., & Ortí, G. (2013). Multi-locus phylogenetic analysis reveals the pattern and tempo of bony fish evolution. PLoS currents, 5.
Choo, B., Zhu, M., Qu, Q., Yu, X., Jia, L., & Zhao, W. (2017). A new osteichthyan from the late Silurian of Yunnan, China. PloS one, 12(3), e0170929.
Clement, A. M., King, B., Giles, S., Choo, B., Ahlberg, P. E., Young, G. C., & Long, J. A. (2018). Neurocranial anatomy of an enigmatic Early Devonian fish sheds light on early osteichthyan evolution. Elife, 7, e34349.
Cui, X., Qiao, T., & Zhu, M. (2019). Scale morphology and squamation pattern of Guiyu oneiros provide new insights into early osteichthyan body plan. Nature Scientific reports, 9(1), 4411.
Giles, S., Darras, L., Clément, G., Blieck, A., & Friedman, M. (2015). An exceptionally preserved Late Devonian actinopterygian provides a new model for primitive cranial anatomy in ray-finned fishes. Proceedings of the Royal Society B: Biological Sciences, 282 (1816), 20151485.
King, B. (2019). Which morphological characters are influential in a Bayesian phylogenetic analysis? Examples from the earliest osteichthyans. Biology letters, 15(7), 20190288.
Lu, J., Zhu, M., Ahlberg, P. E., Qiao, T., Zhu, Y. A., Zhao, W., & Jia, L. (2016). A Devonian predatory fish provides insights into the early evolution of modern sarcopterygians. Science advances, 2(6), e1600154.
Lu, J., Giles, S., Friedman, M., & Zhu, M. (2017). A new stem sarcopterygian illuminates patterns of character evolution in early bony fishes. Nature communications, 8(1), 1-8.
MondéJar-Fernández, J. (2018). On cosmine: its origins, biology and implications for sarcopterygian interrelationships. CYBIUM, 42(1), 41-65.
Qiao, T., King, B., Long, J. A., Ahlberg, P. E., & Zhu, M. (2016). Early gnathostome phylogeny revisited: multiple method consensus. PLoS One, 11(9), e0163157.
Zhu, M., & Yu, X. (2009). Stem sarcopterygians have primitive polybasal fin articulation. Biology Letters, 5(3), 372-375.
Zhu, M., Yu, X., & Janvier, P. (1999). A primitive fossil fish sheds light on the origin of bony fishes. Nature, 397(6720), 607-610.
Zhu, M., Zhao, W., Jia, L., Lu, J., Qiao, T., & Qu, Q. (2009). The oldest articulated osteichthyan reveals mosaic gnathostome characters. Nature, 458(7237), 469-474.
Zhu, M., Yu, X., Ahlberg, P. E., Choo, B., Lu, J., Qiao, T., ... & Zhu, Y. A. (2013). A Silurian placoderm with osteichthyan-like marginal jaw bones. Nature, 502(7470), 188.
Image credits - stem-Sarcopterygii
- Figure 2 (Guiyu oneiros, fossil, A): Open Access article Zhu, M., Yu, X., Choo, B., Qu, Q., Jia, L., Zhao, W., ... & Lu, J. (2012). Fossil fishes from China provide first evidence of dermal pelvic girdles in osteichthyans. PloS One, 7(4), e35103.
- Figure 2 (Guiyu oneiros, fossil, B): Open Access article Zhu, M., Yu, X., Choo, B., Qu, Q., Jia, L., Zhao, W., ... & Lu, J. (2012). Fossil fishes from China provide first evidence of dermal pelvic girdles in osteichthyans. PloS One, 7(4), e35103.
- Figure 2 (Guiyu oneiros, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 2 (Sparalepis tingi, fossil, A): Open Access article Choo, B., Zhu, M., Qu, Q., Yu, X., Jia, L., & Zhao, W. (2017). A new osteichthyan from the late Silurian of Yunnan, China. PloS one, 12(3), e0170929.
- Figure 2 (Sparalepis tingi, fossil, B): Open Access article Choo, B., Zhu, M., Qu, Q., Yu, X., Jia, L., & Zhao, W. (2017). A new osteichthyan from the late Silurian of Yunnan, China. PloS one, 12(3), e0170929.
- Figure 2 (Sparalepis tingi, life restoration): Open Access article Choo, B., Zhu, M., Qu, Q., Yu, X., Jia, L., & Zhao, W. (2017). A new osteichthyan from the late Silurian of Yunnan, China. PloS one, 12(3), e0170929.
- Figure 2 (Psarolepis romeri): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 3 (Miguashaia spp.): DiBgd, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 3 (Diplocercides heiligostockiensis): Robert Gess, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 3 (Porolepis sp.): Dmitry Bogdanov, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 3 (Glyptolepis paucidens, life restoration): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license