Summary
The following figure is a summarized phylogenetic tree for the Elasmobranchii (sharks, skates and rays), showing that the tree is documented by transitional fossils. As will be seen in the following sections, some of the red stars shown below represent multiple transitional fossils.
The major transitions represented by the above tree are the development of jaws along the stem line of the Gnathostomata (the jawed vertebrates) and the separation of fish with skeletons made of cartilage (the Chondrichthyes) from those with skeletons of bone (the Euteleostomi).
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.
This tree is summarized in the context of geological time in the figure below:
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.
This tree is summarized in the context of geological time in the figure below:
This time tree is constrained by fossils that will be discussed below. It indicates that the division between the Cyclostomata (jawless vertebrates) and the Gnathostomata (jawed vertebrates) took place in the late Cambrian, even though the earliest known stem-group gnathostome is of mid-Ordovician age. This difference is reflected in a ghost lineage of about 24 million years in the gnathostomes with respect to the cyclostomes. The stem line of the the Chondricthyes (the cartilaginous fishes) appeared in the Early Silurian, while that of the Elasmobranchii dates from the Late Devonian.
We will now consider the evolution of the elasmobranchs in four stages:
We will now consider the evolution of the elasmobranchs in four stages:
- The stem-Gnathostomata (jawed vertebrates)
- The stem-Chondricthyes (cartilaginous fishes)
- The stem-Elasmobranchii
- Within the crown-Elasmobranchii
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, presented here in two parts, is taken, with the modification mentioned in the figure, from their article:
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):
All stem-group gnathostomes had a cartilaginous internal skeleton (Maisey, 2013) although, as recently documented by Brazeau et al (2020), there is evidence that bone replaced cartilage in the skeletons of at least some of the most crownward 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; Keating and Donoghue, 2016).
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):
- Basal to the Anaspida
- The Anaspida
- Between the Anaspida and the Galeaspida
- The Galeaspida
- The Osteostraci
- The Antiarchi and their sister species
- Between the Antiarchi and the sister species of the Arthrodira
- The remaining, most crownward, stem-group species
All stem-group gnathostomes had a cartilaginous internal skeleton (Maisey, 2013) although, as recently documented by Brazeau et al (2020), there is evidence that bone replaced cartilage in the skeletons of at least some of the most crownward stem-Gnathostomata.
Basal to the Anaspida
Examples of species from this most basal part of the phylogenetic tree, known collectively as the Pteraspidomorpha, are shown below (click on image for a larger view; this applies to all fossil image sets further below on this page):
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 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.
Note that many of the fossils illustrated above 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 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.
Note that many of the fossils illustrated above 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 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. One interesting point is that the two most basal species, according to the analysis of Sallan et al (2018) are Euphanerops longaevus and Endeiolepis aneri, which appear significantly later (Late Devonian) than do all the other anaspids (Early Silurian to Early Devonian). This implies, if the phylogeny is correct, that these two species descended from ancestors that separated from the stem line in the Early Devonian or earlier. Both of these species have gills like those of lampreys, which are cyclostomes (Janvier et al, 2006); this resemblance might be reflected in the alternative interpretation that the anaspids belong to the stem-Cyclostomata (see the page on Jawless Vertebrates).
Between the Anaspida and the Galeaspida
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 look very like modern fishes.
The Galeaspida
This jawless clade, illustrated by the following images, comprises species that all have massive head-shields:
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 shape and nature of the head-shield is superficially similar to the shield of many of the Heterostraci (images 4 through 33 above). Nearly all the species shown above are quite similar to one another, except for the two most crownward species, Sanchaspis magalarostrata and Lungmenshanaspis kiangyouensis, which have marked protuberances on their heads.
The 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. Hemicyclaspis murchinsoni and Stensiopelta pustulata) but the more crownward forms are more variable (compare Tremataspis mammillata with Tauraspis rara).
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, 2016a). 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.
The jaws of the antiarchs and their sister species are composed of bone plates that developed around the mouth, and do not have the more complex jaw structure of the crown-Gnathostomes (Zhu et al, 2019a). This group of placoderms is 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.
Between the Antiarchi and the sister species of the Arthrodira
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, which have jaws that are similar to those of the antiarchs and their sister species, are characterized by armor that comprises large plates and scales.
The remaining, most crownward, stem-group species
These fossils comprise the sister species of the Arthrodira, the Arthrodira clade itself and a few species just below the crown group. 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.
A very important transition occurs within this group. The most crownward species, such as Qilinyu rostrata and Entelognathus primordialis, which are known as the "maxillate placoderms", have a jaw structure that is very similar to that of the crown-group gnathostomes (Zhu et al, 2019a).
All of the above images, except the more basal genera Tlamaspis, Radotina and Sudaspis and the maxillate placoderms Qilinyu and Entelognathus, 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).
All of the above images, except the more basal genera Tlamaspis, Radotina and Sudaspis and the maxillate placoderms Qilinyu and Entelognathus, 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 with cartilaginous internal skeletons (Maisey, 2013) that were replaced by bone in at least some of the most crownward members of the stem group (Brazeau, 2020). 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 figure below:
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 figure 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 26 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.
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.
The fact that the stem-Chondrichthyes are ancestral to the cartilaginous fish should not be taken to mean that a cartilaginous skeleton was a novelty of the stem group. In fact, even the stem-group gnathostomes had internal skeletons of cartilage (Maisey, 2013). The important question is whether the cartilaginous internal skeleton is a primitive characteristic that survived in the chondrichthyans, or whether the chondrichthyes represent more advanced fish that dispensed with bone in their skeleton. A recent article by Brazeau et al (2020) suggests the latter idea. These authors present evidence that the skeleton of the stem-group gnathostome Minjinia turgenensis is composed of endochondral bone (the product of replacement of cartilage by bone). The existence of bony skeletons in the crownward stem-Gnathostomata indicates that the stem-Chondrichthyes formed by the loss of a bony skeleton.
Nearly all of the chondrichthyan stem group genera appeared during the Devonian, with the majority appearing in the Early Devonian:
The fact that the stem-Chondrichthyes are ancestral to the cartilaginous fish should not be taken to mean that a cartilaginous skeleton was a novelty of the stem group. In fact, even the stem-group gnathostomes had internal skeletons of cartilage (Maisey, 2013). The important question is whether the cartilaginous internal skeleton is a primitive characteristic that survived in the chondrichthyans, or whether the chondrichthyes represent more advanced fish that dispensed with bone in their skeleton. A recent article by Brazeau et al (2020) suggests the latter idea. These authors present evidence that the skeleton of the stem-group gnathostome Minjinia turgenensis is composed of endochondral bone (the product of replacement of cartilage by bone). The existence of bony skeletons in the crownward stem-Gnathostomata indicates that the stem-Chondrichthyes formed by the loss of a bony skeleton.
Nearly all of the chondrichthyan stem group genera appeared during the Devonian, with the majority appearing in the Early Devonian:
This time tree indicates that the transition from the stem group to the crown lasted around 51 million years.
The stem group of the sharks, skates and rays
The figure below shows a fairly recent phylogenetic tree of the stem elasmobranches in which some modifications have been made:
The oldest known representative of the elasmobranch stem group is Phoebodus sophiae, described from rocks of Middle Devonian (Givetian) age at a section near Renanué village in the Aragonian Pyrenees, Spain (Ginter et al, 2008). The genus Phoebodus is interpreted by Frey et al (2019a) as being a member of the stem-Elasmobranchii. An image of teeth from Phoebodus bifurcatus is shown below, together with the other species for which public-domain 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. No obvious trends can be seen, but changes did occur in the braincase, as described by Coates et al (2017).
The oldest known member of the crown-Elasmobranchii is Rhomaleodus budurovi, a stem-group selachimorph from the Middle Triassic (Anisian) Babino Formation at the Quarries Section, 1 km north of the town of Belogradchik, Vidin Province, NW Bulgaria (Andreev and Cuny, 2012). No public-domain image is available.
Most of the stem-Elasmobranchii appeared from the Late Devonian to the Triassic, as shown below:
The oldest known member of the crown-Elasmobranchii is Rhomaleodus budurovi, a stem-group selachimorph from the Middle Triassic (Anisian) Babino Formation at the Quarries Section, 1 km north of the town of Belogradchik, Vidin Province, NW Bulgaria (Andreev and Cuny, 2012). No public-domain image is available.
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 140 million years.
Evolution of the crown-Elasmobranchii
In this section we are concerned with the origin of the orders (which have names ending in “formes”) into which the elasmobranch crown group is divided. The phylogenetic relationships between these orders, based on mitochondrial DNA data, is illustrated below:
However, few members of the stem groups of these orders and the higher clades to which they belong (e.g. Galeomorphii, Selachimorpha) have been identified; those that have been so described are shown in the above tree. Most of them are illustrated below (for larger view, click on image):
The above images represent stem-Batoidea (Belemnobatis, Spathobatis and Asterodermus), possible stem-Hexanchiformes (Notidanoides) and stem-Squatiniformes (Pseudorhina).
The time frame of evolution of the crown-Elasmobranchii, at least for the orders for which stem-group fossils have been identified, is shown below:
The time frame of evolution of the crown-Elasmobranchii, at least for the orders for which stem-group fossils have been identified, is shown below:
The above time tree shows that the selachimorph (shark) stem group originated in the Middle Permian and possibly continued until the Early Cretaceous, an interval of 134 million years. The corresponding period for the stem-Batoidea (skates and rays) was apparently somewhat shorter, from the Middle Jurassic until the Late Cretaceous, an interval of around 90 million years.
Image credits - Sharks, skates and rays
- Header (Leopard shark (Triakis semifasciata) at Scripps Aquarium in La Jolla, California, United States) Cropped version of photo by Johan Fredriksson, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Astraspis desiderata (1a) By Eric Hadley-Ives, Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0)
- Astraspis desiderata (1b) By Philippe Janvier / CC BY (https://creativecommons.org/licenses/by/3.0)
- Sacabambaspis janvieri (1a) By Ghedoghedo / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Sacabambaspis janvieri (1b) By Philippe Janvier / CC BY (https://creativecommons.org/licenses/by/3.0)
- Arandaspis prionotolepis (3a) By Michal Maňas [Public domain]
- Arandaspis prionotolepis (3b) 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
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- Sphenonectris turnerae By Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Pituriaspis doylei By Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Dayongaspis hunanensis (53a) From Open-Access article by Shan et al (2020), licensed under Attribution 4.0 International (CC BY 4.0)
- Dayongaspis hunanensis (53b) By Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Shuyu zhejiangensis By Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Sinogaleaspis shankouensis (55a and 55b) From Open-Access article by Shan et al (2020), licensed under Attribution 4.0 International (CC BY 4.0)
- Nochelaspis maeandrine By Captmondo / Copyrighted free use.
- Eugaleaspis changi From Open-Access article by Shan et al (2020), licensed under Attribution 4.0 International (CC BY 4.0)
- Eugaleaspis xujiachongensis By Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Siyingia altuspinosa By Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Laxaspis qujingensis (60a) By Captmondo / Copyrighted free use.
- Laxaspis qujingensis (60b) By Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Sanchaspis magalarostrata (61a) From Open-Access article by Shan et al (2020), licensed under Attribution 4.0 International (CC BY 4.0)
- Sanchaspis magalarostrata (61b) By Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Lungmenshanaspis kiangyouensis By Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Ateleaspis tesselata By Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Hemicyclaspis murchinsoni (64a) By Ghedoghedo / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Hemicyclaspis murchinsoni (64b) By Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Cephalaspis lyelli (65a) By Ghedoghedo / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Cephalaspis lyelli (65b) By Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Parameteoraspis gigas By Muséum national d'Histoire naturelle, Paris: Attribution 4.0 International (CC BY 4.0)
- Parameteoraspis oberon By Muséum national d'Histoire naturelle, Paris: Attribution 4.0 International (CC BY 4.0)
- Parameteoraspis lanternaria By Muséum national d'Histoire naturelle, Paris: Attribution 4.0 International (CC BY 4.0)
- Machairaspis battaili By Muséum national d'Histoire naturelle, Paris: Attribution 4.0 International (CC BY 4.0)
- Stensiopelta pustulata By Ghedoghedo / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Diademaspis poplinae By Muséum national d'Histoire naturelle, Paris: Attribution 4.0 International (CC BY 4.0)
- Auchenaspis sp. By Natural History Museum. London: Attribution 4.0 International (CC BY 4.0)
- Thyestes verrucosus By Walter Holbrook Gaskell. / Public domain
- Didymaspis grindrodi By Natural History Museum. London: Attribution 4.0 International (CC BY 4.0)
- Norselaspis glacialis By Muséum national d'Histoire naturelle, Paris: Attribution 4.0 International (CC BY 4.0)
- Dartmuthia gemmifera By Museum of Geology, University of Tartu, Estonia: Creativecommons.org/licenses/by-nc/3.0/
- Tremataspis mammillata (77a) By Ghedoghedo / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Tremataspis mammillata (77b) By Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Severaspis rostralis By G. Baranov, Geoscience collections of Estonia, Attribution-NonCommercial 3.0 Unported (CC BY-NC 3.0)
- Boreaspis rostrata By Muséum national d'Histoire naturelle, Paris: Attribution 4.0 International (CC BY 4.0)
- Boreaspis macrorhynchus By Muséum national d'Histoire naturelle, Paris: Attribution 4.0 International (CC BY 4.0)
- "Boreaspis" ceratops By Muséum national d'Histoire naturelle, Paris: Attribution 4.0 International (CC BY 4.0)
- Spatulaspis costata By Muséum national d'Histoire naturelle, Paris: Attribution 4.0 International (CC BY 4.0)
- Tauraspis rara (83a) By G. Baranov, Geoscience collections of Estonia, Attribution-NonCommercial 3.0 Unported (CC BY-NC 3.0)
- Tauraspis rara (83b) By Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Belonaspis puella By Muséum national d'Histoire naturelle, Paris: Attribution 4.0 International (CC BY 4.0)
- Shimenolepis graniferous By Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Silurolepis platydorsalis (86a) From Open-Access article by Zhu et al (2019b), licensed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/
- Silurolepis platydorsalis (86b) By Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Yunnanolepis parvus From Open-Access article by Wang & Zhu (2018), licensed under Attribution 4.0 International (CC BY 4.0)
- Yunnanolepis chii By Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Zhanjilepis aspartilis By Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Minicrania lirouyii By Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Palaeacanthaspis vasta By Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Diandongpetalichthys baojiaoshanensis By Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Romundina stellina (93a) By Uppsala University in Sweden / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Romundina stellina (93b) By Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Tlamaspis inopinatus From Open-Access article by Vaškaninová & Ahlberg (2017), licensed under Attribution 4.0 International (CC BY 4.0)
- Radotina tesselata From Open-Access article by Vaškaninová & Ahlberg (2017), licensed under Attribution 4.0 International (CC BY 4.0)
- Radotina kosorensis By Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Sudaspis chlupaci From Open-Access article by Vaškaninová & Ahlberg (2017), licensed under Attribution 4.0 International (CC BY 4.0)
- Antarctaspis mcmurdoensis By Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Wuttagoonaspis fletcheri By Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Lehmanosteus hyperboreuz By Muséum national d'Histoire naturelle, Paris: Attribution 4.0 International (CC BY 4.0)
- Aleosteus eganensis By Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Placolepis budawangensis By Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Austrophyllolepis ritchiei By Apokryltaros at English Wikipedia / CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/)
- Phyllolepis orvini By Dmitry Bogdanov / CC BY (https://creativecommons.org/licenses/by/3.0)
- Actinolepis tuberculata By Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Actinolepis magna By Fossiilid, licensed under CC BY-NC Licence.
- Bryantolepis brachycephala By Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Tiaraspis subtilis By Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Dicksonosteus arcticus By Muséum national d'Histoire naturelle, Paris: Attribution 4.0 International (CC BY 4.0)
- Heintzosteus brevis By Muséum national d'Histoire naturelle, Paris: Attribution 4.0 International (CC BY 4.0)
- Arctolepis decipiens By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Coccosteus cuspidatus (112a) By Ghedoghedo / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Coccosteus cuspidatus (112b) By Estonian Museum of Natural History / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Buchanosteus confertituberculatus By Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Qilinyu rostrata By Apokryltaros / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Entelognathus primordialis (115a) By Morosaurus millenii / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Entelognathus primordialis (115b) By Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Nerepisacanthus sp. By Danielle Dufault [CC BY 2.5 (https://creativecommons.org/licenses/by/2.5)]
- Ischnacanthus gracilis By Nobu Tamura (http://spinops.blogspot.ca/) [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Diplacanthus longispinus By FunkMonk [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Diplacanthus crassissimus By Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Mesacanthus pusillus By Liopleurodon93 at English Wikipedia [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]
- Cheiracanthus sp. By Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Cheiracanthus murchisoni By Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Acanthodes sp. By Momotarou2012 [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Acanthodes bronni By Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Gladbachus adentatus By DiBgd [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Brochoadmones milesi By DiBgd [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Climatius reticulatus By Nobu Tamura (http://spinops.blogspot.com) [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)]
- Parexus recurvus By Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA)
- Gyracanthides murrayi By DiBgd [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Thrinacodus dziki By Ginter, M., Duffin, C.J., Dean, M.T., and Korn, D. / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Phoebodus bifurcatus By Ginter, M. & Ivanov, A. / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Diplodoselache woodi By Ghedoghedo / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Triodus sessilis By Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Orthacanthus senckenbergianus (132a) By Daderot under the Creative Commons CC0 1.0 Universal Public Domain Dedication
- Orthacanthus senckenbergianus (132b) By Nobu Tamura (http://spinops.blogspot.com) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]
- Tristychius arcuatus By 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 By Ghedoghedo / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Lissodus hasleensis By Rees, J. [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Palaeobates sp. By Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Hybodus hauffianus By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Egertonodus fraasi (138a) By Haplochromis [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Egertonodus fraasi (138b) By Nobu Tamura (http://spinops.blogspot.com) [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Acrodus nobilis By Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Asteracanthus ornatissimus By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Synechodus jurensis By Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Paraorthacodus sp. By Haplochromis / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Sphenodus nitidus By Ghedoghedo / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Macrourogaleus hassei By Ghedoghedo / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Belemnobatis sismondae By Ghedoghedo / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Spathobatis bugesiacus By Ghedoghedo / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Asterodermus platypterus By Ghedoghedo / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Notidanoides muensteri By Ghedoghedo / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Pseudorhina sp. By Chillibilli / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)