Summary
This page illustrates fossils that date from the transition, lasting around 250 million years, from the earliest jawed vertebrates to the first appearance of the crown group of the sharks, skates and rays. Visual comparison of these fossils shows no marked discontinuities apart from the loss of a bony exoskeleton between the stem-group gnathostomes and the stem-group chondrichthyans. That discontinuity is the only part of the transition not adequately documented by fossils.
Introduction
The elasmobranches (Class Elasmobranchii, superclass Chrondrichthyes) represent the sharks, skates and rays and comprise one of the two clades into which the chondricthyians, or cartilaginous fishes, are subdivided. The Elasmobranchii are divided into two clades, the Batoidea (the rays and skates) and the Selachimorpha (the sharks). The Selachimorpha are also known as the Selachii. A phylogenetic tree of the extant Elasmobranchii, showing their phylogenetic context, is presented below:
The crown group of the Elasmobranchii, represented above by the black dot, comprises all living sharks, skates and rays, together with all extinct species descended from the last common ancestor of all living sharks, skates and rays.
We will now consider the evolution of the stem lines of the elasmobranchs and of the higher-order clades to which they belong, the Chondrichthyes and the Gnathostomes. These stem lines are depicted as red, blue and magenta lines respectively in the above phylogenetic tree.
We will now consider the evolution of the stem lines of the elasmobranchs and of the higher-order clades to which they belong, the Chondrichthyes and the Gnathostomes. These stem lines are depicted as red, blue and magenta lines respectively in the above phylogenetic tree.
The stem group of the jawed vertebrates
The stem group of the Gnathostomes is very well represented in the fossil record. Many phylogenetic studies have been carried out, one of the most recent and comprehensive of which is that by Sallan et al (2018). The following tree is taken, with the modification mentioned in the figure, from their article:
The above tree represents only a small proportion of the total number of stem-group gnathostomes. Many more species are listed in the database.
The stem gnathostomes appeared during the Darriwilian age of the Middle Ordovician. Three genera are known from that time interval: Arandaspis, Sacabambaspis and Pteraspis. Of these, the first two have been described at the species level, and can be considered as the oldest-known representatives of the Stem-Gnathostomata:
Owing to the large number of stem-Gnathostomata, we will deal with the stem line in the following stages (refer to the clade names shown in the phylogenetic tree above):
Pteraspidomorphi
Examples of species from this most basal part of the phylogenetic tree are shown below (click on image for a larger view; this applies to all fossil image sets further below on this page):
The stem gnathostomes appeared during the Darriwilian age of the Middle Ordovician. Three genera are known from that time interval: Arandaspis, Sacabambaspis and Pteraspis. Of these, the first two have been described at the species level, and can be considered as the oldest-known representatives of the Stem-Gnathostomata:
- Arandaspis prionotolepis, a primitive jawless fish, belonging to the Arandaspida (the sister group of the Heterostraci), from the Middle Ordovician Stairway Sandstone of Mount Watt and Mount Charlotte, Northern Territory, Australia (Ritchie and Gilbert-Tomlinson, 1977; Benton et al, 2015).
- Sacabambaspis janvieri, another jawless fish belonging to the Arandaspida and found in the Middle Ordovician Anzaldo Formation at Sacabamba in the department of Cochabamba, Bolivia (Gagnier et al, 1986; Sallan et al, 2018).
Owing to the large number of stem-Gnathostomata, we will deal with the stem line in the following stages (refer to the clade names shown in the phylogenetic tree above):
- The Pteraspidomorphi
- The Anaspida
- The Thelodonti and their sister species
- The Galeaspida
- The Osteostraci
- The Antiarchi and their sister species
- The remaining, most crownward, stem-group species
Pteraspidomorphi
Examples of species from this most basal part of the phylogenetic tree are shown below (click on image for a larger view; this applies to all fossil image sets further below on this page):
* after name indicates that the image represents a life restoration.
The pteraspidomorphs were all jawless, and were essentially armor-plated, with a massive head-shield and scaly tails. The armor and scales were composed of an acellular bone known as as aspidin (Blieck and Elliott, 2017; Keating et al, 2018).
The above images, together with those in the following image sets for the gnathostomes, are numbered in order from the most basal to the most crownward, according to the analysis of Sallan et al (2018). No obvious trends can be seen, except possibly for the hint of a progressively greater head width relative to the width of the rest of the body.
Anaspida
Some examples of the anaspid stem-Gnathostomata are illustrated in the following images:
The above images, together with those in the following image sets for the gnathostomes, are numbered in order from the most basal to the most crownward, according to the analysis of Sallan et al (2018). No obvious trends can be seen, except possibly for the hint of a progressively greater head width relative to the width of the rest of the body.
Anaspida
Some examples of the anaspid stem-Gnathostomata are illustrated in the following images:
Names in red indicate that the fossil is younger than the oldest known crown-group fossil.
* after name indicates that the image represents a life restoration.
* after name indicates that the image represents a life restoration.
These species are also jawless and armored, but they have a form that more closely resembles that of modern fish. One characteristic of the anaspids, lacking in the pteraspidomorphs, that makes them bear a superficial resemblance to modern fishes is the fact that they possess paired fins (Janvier, 1987). Most do not have the massive head-shield seen in many of the more basal forms.
Note that one of the fossils illustrated above (Lasanius problematicus), and many in the illustrations below, have red labels. This indicates that their first appearance age is less than that of the gnathostome crown group, which appeared in the Early Silurian. These post-crown stem-group fossils represent descendants of ancestors that would have separated from the stem line during or before the Early Silurian.
The Thelodonti and their sister species
These stem-group gnathostomes comprise the clade Thelodonti and its sister species. As with the anaspids, the armor of the thelodonts comprises scales, but these were generally dispersed after death, such that their anatomy is poorly understood (Donoghue and Smith, 2001). The images available in the public domain are shown below:
Note that one of the fossils illustrated above (Lasanius problematicus), and many in the illustrations below, have red labels. This indicates that their first appearance age is less than that of the gnathostome crown group, which appeared in the Early Silurian. These post-crown stem-group fossils represent descendants of ancestors that would have separated from the stem line during or before the Early Silurian.
The Thelodonti and their sister species
These stem-group gnathostomes comprise the clade Thelodonti and its sister species. As with the anaspids, the armor of the thelodonts comprises scales, but these were generally dispersed after death, such that their anatomy is poorly understood (Donoghue and Smith, 2001). The images available in the public domain are shown below:
Names in red indicate that the fossil is younger than the oldest known crown-group fossil.
* after name indicates that the image represents a life restoration.
* after name indicates that the image represents a life restoration.
These species are, like the more basal forms, jawless and armored, but they do not show any clear trend through the series shown above. Species with massive heads seem to alternate with shapes that very like modern fishes.
Galeaspida
This jawless clade, illustrated by the following images, comprises species that all have massive head-shields:
Galeaspida
This jawless clade, illustrated by the following images, comprises species that all have massive head-shields:
* after name indicates that the image represents a life restoration.
The shape and nature of the head-shield is superficially similar to the shield of many of the Heterostraci (images 4 and 5 above).
Osteostraci
The next clade to separate from the gnathostome stem line was the Osteostraci, another group of jawless fish. They are generally similar to the galeaspids, but their bone type is cellular (Janvier, 1984) while at least part of the bone in the galeaspids is acellular (Keating et al, 2018) and the bone structure of the head differs in detail. Here are some examples:
Osteostraci
The next clade to separate from the gnathostome stem line was the Osteostraci, another group of jawless fish. They are generally similar to the galeaspids, but their bone type is cellular (Janvier, 1984) while at least part of the bone in the galeaspids is acellular (Keating et al, 2018) and the bone structure of the head differs in detail. Here are some examples:
Names in red indicate that the fossil is younger than the oldest known crown-group fossil.
* after name indicates that the image represents a life restoration.
* after name indicates that the image represents a life restoration.
The more basal species are generally similar in having a head-shield with a characteristic pointed lateral extension on each side (e.g. Ateleaspis tesselata and Hemicyclaspis murchinsoni).
The Antiarchi and their sister species
This group together with all the more crownward species comprise the placoderms (the jawed stem gnathostomes; Zhu et al, 2016). Species for which public-domain images are available are illustrated below:
The Antiarchi and their sister species
This group together with all the more crownward species comprise the placoderms (the jawed stem gnathostomes; Zhu et al, 2016). Species for which public-domain images are available are illustrated below:
Names in red indicate that the fossil is younger than the oldest known crown-group fossil.
* after name indicates that the image represents a life restoration.
* after name indicates that the image represents a life restoration.
Besides having jaws, the antiarchs and their sister species are also different from more basal stem gnathostomes in being more heavily armored, with a box-like thoracic shield and elongate pectoral fins encased in bone (Zhu et al, 2012). A good example of the bony pectoral fins can be seen in Yunnanolepis chii.
The remaining, most crownward, stem-group species
These species represent placoderms that progressively separated from the gnathostome stem line crownward of the antiarchs. Some examples are shown below:
The remaining, most crownward, stem-group species
These species represent placoderms that progressively separated from the gnathostome stem line crownward of the antiarchs. Some examples are shown below:
Names in red indicate that the fossil is younger than the oldest known crown-group fossil.
* after name indicates that the image represents a life restoration.
* after name indicates that the image represents a life restoration.
These species are characterized by armor that comprises large plates and scales. Four of the fossils shown below (Kujdanowiaspis, Dicksonosteus arcticus, Coccosteus cuspidatus and Buchanosteus confertituberculatus) belong to the Arthrodira. This clade differs from more basal stem-Gnathostomata in having a movable joint between the head and the body armor. Like the antiarchs, they also had teeth (Rücklin et al, 2012).
Summary of the stem-Gnathostomata
In summary, all the stem gnathostomes were armored fishes, although the nature of the armor - whether consisting of plates or scales - varied through the development of the stem group. Furthermore, the fossil record shows a transition from jawless fishes without teeth to jawed fish with teeth. Other events in the development of the stem group were the appearance of paired fins and articulation between the head and the body.
This series of transitions is represented by fossils that appeared over a long period of time, from the Middle Ordovician to the Middle Devonian, as shown in the two-part time tree shown below:
Summary of the stem-Gnathostomata
In summary, all the stem gnathostomes were armored fishes, although the nature of the armor - whether consisting of plates or scales - varied through the development of the stem group. Furthermore, the fossil record shows a transition from jawless fishes without teeth to jawed fish with teeth. Other events in the development of the stem group were the appearance of paired fins and articulation between the head and the body.
This series of transitions is represented by fossils that appeared over a long period of time, from the Middle Ordovician to the Middle Devonian, as shown in the two-part time tree shown below:
The above time tree indicates that the time between the origin of the gnathostome stem group and the initiation of the crown group was no more than about 34 million years. Most of the known stem-group fossils, those with red labels in the above figures, are younger than the crown group; there was obviously an explosion of new genera during the Early Devonian, but this must have been derived from ancestors that separated from the stem line much earlier, as indicated in the time tree.
The stem group of the cartilaginous fishes
The stem-Chondricthyes are fairly well represented in the fossil record. An interpretation of their phylogeny is illustrated below:
The oldest known stem-group chondrichthyan is Elegestolepis conica, found in Early Silurian (Llandovery) sediments in the Angara-Jlim area (Niuya River outcrops and Niuya-Berresova area) of the Siberian Platform in Russia (Broughton et al, 2013). It is known only from fossilized scales, and no public-domain images are available. It is not represented in the above tree.
Relatively few images, especially of fossils, exist in the public domain for the species shown in the above tree. The available images are reproduced below (click on any image to see a larger version):
Relatively few images, especially of fossils, exist in the public domain for the species shown in the above tree. The available images are reproduced below (click on any image to see a larger version):
Names in red indicate that the fossil is younger than the oldest known crown-group fossil.
* after name indicates that the image represents a life restoration.
* after name indicates that the image represents a life restoration.
The images shown above are numbered in order from most basal towards the crown group; no clear trends can be seen. These fishes are nearly all similar in having spiny fins, and most have a rounded, rather than sharp, shark-like, face. However, there are less-obvious characteristics that changed through this series; see Coates et al (2018) for details. These authors conclude that Gladbachus displays “morphological incongruence with its phylogenetic position”, from which they suggest that the early stem-Chondrichthyes probably had greater morphological disparity than that seen in the limited pre-Devonian fossil record of the stem group. This disparity might also be reflected in the difference between the most basal chondrichthyan shown above (Nerepisacanthus) and the most crownward stem-group gnathostomes shown in the previous section (Entelognathus and Qilinyu). The latter are armoured fishes, while Nerepisacanthus lacked a bony exoskeleton, as do all chondrichthyans (Brazeau et al, 2020).
Nearly all of the chondrichthyan stem group genera appeared during the Devonian, with the majority appearing in the Early Devonian, as illustrated in the following time tree:
Nearly all of the chondrichthyan stem group genera appeared during the Devonian, with the majority appearing in the Early Devonian, as illustrated in the following time tree:
This time tree indicates that the transition from the stem group to the crown could have lasted as long as 64 million years.
The stem group of the sharks, skates and rays
The figure below shows a fairly recent phylogenetic tree of the stem-Elasmobranchii which has been pruned (i.e. species have been removed) to shorten the side branch that is in a sister relation to the Euselachii:
The oldest known representative of the elasmobranch stem group is Phoebodus sophiae, found in the Middle Devonian (Middle/Late Givetian) Middle through Upper Polygnathus varcus conodont zones at Renanué village in the Aragonian Pyrenees, Spain (Ginter et al, 2008; Frey et al, 2019). No image is available in the public domain for this species, but other members of the genus are shown below together with other genera for which images are available (for a larger view, click on image):
Names in red indicate that the fossil is younger than the oldest known crown-group fossil.
* after name indicates that the image represents a life restoration.
* after name indicates that the image represents a life restoration.
The images are numbered in order from the most basal to those closest to the crown group. A general trend towards a more shark-like form can be clearly seen (for example, compare Phoebodus saidselachus with Tristychius arcuatus) and changes also occurred in the braincase, as described by Coates and Tietjen (2017). Most of the stem-Elasmobranchii appeared from the Late Devonian to the Triassic, as shown below:
The above time tree indicates that the time between the origin of the elasmobranch stem group and the initiation of the crown group could have been as long as 144 million years.
References
Benton, M. J., Donoghue, P. C., Asher, R. J., Friedman, M., Near, T. J., & Vinther, J. (2015). Constraints on the timescale of animal evolutionary history. Palaeontologia Electronica, 18(1), 1-106.
Blieck, A., & Elliott, D. K. (2017). Pteraspidomorphs (Vertebrata), the Old Red Sandstone, and the special case of the Brecon Beacons National Park, Wales, UK. Proceedings of the Geologists' Association, 128(3), 438-446.
Brazeau, M. D., Giles, S., Dearden, R. P., Jerve, A., Ariunchimeg, Y. A., Zorig, E., ... & Castiello, M. (2020). Endochondral bone in an Early Devonian ‘placoderm’ from Mongolia. Nature Ecology & Evolution, 4(11), 1477-1484.
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.
Coates, M. I., & Tietjen, K. (2017). The neurocranium of the Lower Carboniferous shark Tristychius arcuatus (Agassiz, 1837). Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 108(1), 19-35.
Coates, M. I., Finarelli, J. A., Sansom, I. J., Andreev, P. S., Criswell, K. E., Tietjen, K., ... & La Riviere, P. J. (2018). An early chondrichthyan and the evolutionary assembly of a shark body plan. Proceedings of the Royal Society B: Biological Sciences, 285(1870), 20172418.
Donoghue, P. C., & Smith, M. P. (2001). The anatomy of Turinia pagei (Powrie), and the phylogenetic status of the Thelodonti. Earth and Environmental Science Transactions of The Royal Society of Edinburgh, 92(1), 15-37.
Frey, L., Coates, M., Ginter, M., Hairapetian, V., Rücklin, M., Jerjen, I., & Klug, C. (2019). The early elasmobranch Phoebodus: phylogenetic relationships, ecomorphology and a new time-scale for shark evolution. Proceedings of the Royal Society B, 286(1912), 20191336.
Gagnier, P. Y., Blieck, A. R., & Rodrigo, G. (1986). First Ordovician vertebrate from South America. Geobios, 19(5), 629-634.
Giles, S., Friedman, M., & Brazeau, M. D. (2015). Osteichthyan-like cranial conditions in an Early Devonian stem gnathostome. Nature, 520(7545), 82-85.
Ginter, M., Liao, J. C., & Valezuela-RÍios, J. I. (2008). New data on chondrichthyan microremains from the Givetian of the Renanué section in the Aragonian Pyrenees (Spain). Acta Geologica Polonica, 58(2), 165-172.
Janvier, P. (1984). The relationships of the Osteostraci and Galeaspida. Journal of Vertebrate Paleontology, 4(3), 344-358.
Janvier, P. (1987). The paired fins of anaspids: one more hypothesis about their function. Journal of Paleontology, 61(4), 850-853.
Keating, J. N., Marquart, C. L., Marone, F., & Donoghue, P. C. (2018). The nature of aspidin and the evolutionary origin of bone. Nature ecology & evolution, 2(9), 1501-1506.
Ritchie, A., & Gilbert-Tomlinson, J. (1977). First Ordovician vertebrates from the southern hemisphere. Alcheringa, 1(4), 351-368.
Rücklin, M., Donoghue, P. C., Johanson, Z., Trinajstic, K., Marone, F., & Stampanoni, M. (2012). Development of teeth and jaws in the earliest jawed vertebrates. Nature, 491(7426), 748-751.
Sallan, L., Friedman, M., Sansom, R. S., Bird, C. M., & Sansom, I. J. (2018). The nearshore cradle of early vertebrate diversification. Science, 362(6413), 460-464.
Zhu, M., Yu, X., Choo, B., Wang, J., & Jia, L. (2012). An antiarch placoderm shows that pelvic girdles arose at the root of jawed vertebrates. Biology Letters, 8(3), 453-456.
Zhu, M., Ahlberg, P. E., Pan, Z., Zhu, Y., Qiao, T., Zhao, W., ... & Lu, J. (2016). A Silurian maxillate placoderm illuminates jaw evolution. Science, 354(6310), 334-336.
Blieck, A., & Elliott, D. K. (2017). Pteraspidomorphs (Vertebrata), the Old Red Sandstone, and the special case of the Brecon Beacons National Park, Wales, UK. Proceedings of the Geologists' Association, 128(3), 438-446.
Brazeau, M. D., Giles, S., Dearden, R. P., Jerve, A., Ariunchimeg, Y. A., Zorig, E., ... & Castiello, M. (2020). Endochondral bone in an Early Devonian ‘placoderm’ from Mongolia. Nature Ecology & Evolution, 4(11), 1477-1484.
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.
Coates, M. I., & Tietjen, K. (2017). The neurocranium of the Lower Carboniferous shark Tristychius arcuatus (Agassiz, 1837). Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 108(1), 19-35.
Coates, M. I., Finarelli, J. A., Sansom, I. J., Andreev, P. S., Criswell, K. E., Tietjen, K., ... & La Riviere, P. J. (2018). An early chondrichthyan and the evolutionary assembly of a shark body plan. Proceedings of the Royal Society B: Biological Sciences, 285(1870), 20172418.
Donoghue, P. C., & Smith, M. P. (2001). The anatomy of Turinia pagei (Powrie), and the phylogenetic status of the Thelodonti. Earth and Environmental Science Transactions of The Royal Society of Edinburgh, 92(1), 15-37.
Frey, L., Coates, M., Ginter, M., Hairapetian, V., Rücklin, M., Jerjen, I., & Klug, C. (2019). The early elasmobranch Phoebodus: phylogenetic relationships, ecomorphology and a new time-scale for shark evolution. Proceedings of the Royal Society B, 286(1912), 20191336.
Gagnier, P. Y., Blieck, A. R., & Rodrigo, G. (1986). First Ordovician vertebrate from South America. Geobios, 19(5), 629-634.
Giles, S., Friedman, M., & Brazeau, M. D. (2015). Osteichthyan-like cranial conditions in an Early Devonian stem gnathostome. Nature, 520(7545), 82-85.
Ginter, M., Liao, J. C., & Valezuela-RÍios, J. I. (2008). New data on chondrichthyan microremains from the Givetian of the Renanué section in the Aragonian Pyrenees (Spain). Acta Geologica Polonica, 58(2), 165-172.
Janvier, P. (1984). The relationships of the Osteostraci and Galeaspida. Journal of Vertebrate Paleontology, 4(3), 344-358.
Janvier, P. (1987). The paired fins of anaspids: one more hypothesis about their function. Journal of Paleontology, 61(4), 850-853.
Keating, J. N., Marquart, C. L., Marone, F., & Donoghue, P. C. (2018). The nature of aspidin and the evolutionary origin of bone. Nature ecology & evolution, 2(9), 1501-1506.
Ritchie, A., & Gilbert-Tomlinson, J. (1977). First Ordovician vertebrates from the southern hemisphere. Alcheringa, 1(4), 351-368.
Rücklin, M., Donoghue, P. C., Johanson, Z., Trinajstic, K., Marone, F., & Stampanoni, M. (2012). Development of teeth and jaws in the earliest jawed vertebrates. Nature, 491(7426), 748-751.
Sallan, L., Friedman, M., Sansom, R. S., Bird, C. M., & Sansom, I. J. (2018). The nearshore cradle of early vertebrate diversification. Science, 362(6413), 460-464.
Zhu, M., Yu, X., Choo, B., Wang, J., & Jia, L. (2012). An antiarch placoderm shows that pelvic girdles arose at the root of jawed vertebrates. Biology Letters, 8(3), 453-456.
Zhu, M., Ahlberg, P. E., Pan, Z., Zhu, Y., Qiao, T., Zhao, W., ... & Lu, J. (2016). A Silurian maxillate placoderm illuminates jaw evolution. Science, 354(6310), 334-336.
mage credits - Sharks, Skates and Rays
- Header (Leopard shark, Triakis semifasciata: Matthew Field, www.photography.mattfield.com, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Sacabambaspis janvieri (1a): Ghedoghedo / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Sacabambaspis janvieri (1b): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Arandaspis prionotolepis (2a): Michal Maňas [Public domain]
- Arandaspis prionotolepis (2b): By 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
- Astraspis desiderata (3a): Field Museum, Chicago, Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0)
- Astraspis desiderata (3b): Philippe Janvier / CC BY (https://creativecommons.org/licenses/by/3.0)
- Athenaegis chattertoni: Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Tolypelepis undulata: Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Jamoytius kerwoodi: Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Lasanius problematicus (7a): Ghedoghedo / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Lasanius problematicus (7b): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Birkenia elegans (8a): Ghedoghedo / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Birkenia elegans (8b): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Archipelepis turbinata: Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Pezopallichthys ritchiei: Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Loganellia scotica (11a): Ghedoghedo / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Loganellia scotica (11b): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Pituriaspis doylei: Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Neeyambaspis enigmatica: Apokryltaros at English Wikipedia, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons
- Hanyangaspis guodingshanensis (14a): Open Access article Shan, X., Zhu, M., Zhao, W., Pan, Z., Wang, P., & Gai, Z. (2020). A new genus of sinogaleaspids (Galeaspida, stem-Gnathostomata) from the Silurian Period in Jiangxi, China. PeerJ, 8, e9008.
- Hanyangaspis guodingshanensis (14b): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Dayongaspis hunanensis (15a): Open Access article Shan, X., Zhu, M., Zhao, W., Pan, Z., Wang, P., & Gai, Z. (2020). A new genus of sinogaleaspids (Galeaspida, stem-Gnathostomata) from the Silurian Period in Jiangxi, China. PeerJ, 8, e9008.
- Dayongaspis hunanensis (15b): Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Aceraspis robustus: Museum of Geology, University of Tartu, Estonia, Attribution-NonCommercial 3.0 Unported (CC BY-NC 3.0)
- Ateleaspis tesselata: Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Hemicyclaspis murchisoni (18a): Ghedoghedo / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Hemicyclaspis murchisoni (18b): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Auchenaspis salteri: Natural History Museum. London: Attribution 4.0 International (CC BY 4.0)
- Shimenolepis graniferus: Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Silurolepis platydorsalis (21a): The Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/
- Silurolepis platydorsalis (21b): Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Yunnanolepis parvus (22a): Open access article Wang, Y., & Zhu, M. (2018). Redescription of Phymolepis cuifengshanensis (Antiarcha: Yunnanolepididae) using high-resolution computed tomography and new insights into anatomical details of the endocranium in antiarchs. PeerJ, 6, e4808.
- Yunnanolepis chii (22b): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Zhanjilepis aspratilis: Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Chuchinolepis dongmoensis: Apokryltaros, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Minicrania lirouyii: Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Palaeacanthaspis vasta (26a): Open access article Dupret, V., Szaniawski, H., Dec, M., & Szrek, P. (2021). New cranial material of the acanthothoracid placoderm Palaeacanthaspis vasta from the Lower Devonian of Podolia—phylogenetic and taxonomic significance. Acta Palaeontologica Polonica, 66(2), 337-347.
- Palaeacanthaspis vasta (26b): Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Diadongpetalichthys liaojiaoshanensis: Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Romundina stellina (28a): Uppsala University in Sweden / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Romundina stellina (28b): Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Tlamaspis inopinatus: Open access article Vaškaninová, V., & Ahlberg, P. E. (2017). Unique diversity of acanthothoracid placoderms (basal jawed vertebrates) in the Early Devonian of the Prague Basin, Czech Republic: A new look at Radotina and Holopetalichthys. Plos one, 12(4), e0174794.
- Radotina kosoriensis: Open access article Vaškaninová, V., & Ahlberg, P. E. (2017). Unique diversity of acanthothoracid placoderms (basal jawed vertebrates) in the Early Devonian of the Prague Basin, Czech Republic: A new look at Radotina and Holopetalichthys. Plos one, 12(4), e0174794.
- Sudaspis chlupaci: Open access article Vaškaninová, V., & Ahlberg, P. E. (2017). Unique diversity of acanthothoracid placoderms (basal jawed vertebrates) in the Early Devonian of the Prague Basin, Czech Republic: A new look at Radotina and Holopetalichthys. Plos one, 12(4), e0174794.
- Kujdanowiaspis sp. (32a): Entelognathus, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Kujdanowiaspis sp. (32b): Entelognathus, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Dicksonosteus arcticus (33a): Muséum national d'Histoire naturelle, Paris: Attribution 4.0 International (CC BY 4.0)
- Dicksonosteus arcticus (33b): Entelognathus, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Coccosteus cuspidatus (34a): Ghedoghedo / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Coccosteus cuspidatus (34b): Estonian Museum of Natural History / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Buchanosteus confertituberculatus: Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Qilinyu rostrata: Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Entelognathus primordialis (37a): Morosaurus millenii / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Entelognathus primordialis (37b): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Nerepisacanthus denisoni: Danielle Dufault [CC BY 2.5 (https://creativecommons.org/licenses/by/2.5)]
- Ischnacanthus gracilis: Nobu Tamura (http://spinops.blogspot.ca/) [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Diplacanthus longispinus (40a): FunkMonk [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Diplacanthus crassissimus (40b): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Mesacanthus pusillus: Liopleurodon93 at English Wikipedia [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]
- Cheiracanthus sp. (42a): Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Cheiracanthus murchisoni (42b): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Acanthodes sp. (43a): Momotarou2012 [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Acanthodes bronni (43b): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Gladbachus adentatus: DiBgd [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Brochoadmones milesi: DiBgd [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Climatius reticulatus: Nobu Tamura (http://spinops.blogspot.com) [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)]
- Parexus recurvus: Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Gyracanthides murrayi: DiBgd [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Thrinacodus dziki (49a): Ginter, M., Duffin, C.J., Dean, M.T., and Korn, D. / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Thrinacodus gracia (49b): DiBgd, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Phoebodus bifurcatus (50a): Ginter, M. & Ivanov, A., CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Phoebodus saidselachus (50b): DiBgd, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Diplodoselache woodi: Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Onychoselache traquairi: GB3D Type Fossils, licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
- Hamiltonichthys mapesi: Ghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Egertonodus fraasi (54a): Haplochromis [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Egertonodus fraasi (54b): Nobu Tamura (http://spinops.blogspot.com) [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Tristychius arcuatus (55a): Natural history museum of Neuchâtel, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Tristychius arcuatus (55b): Nobu Tamura email:nobu.tamura@yahoo.com http://spinops.blogspot.com/ http://paleoexhibit.blogspot.com/ [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Acronemus tuberculatus: Fanboyphilosopher (Neil Pezzoni), CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
- Synechodus jurensis: Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]