The mammals (class Mammalia in Superclass Tetrapoda) belong to a clade, the Synapsida, which is in a sister relationship to the Sauria and as such comprises all non-saurian amniotes. The Mammalia are actually a crown group of the synapsid clade (Asher, 2018).
As synapsids, the mammals are characterized by having only one opening, or fenestra, in the skull behind each eye in contrast to the diapsids (a clade that contains the reptiles and birds), which have two openings.
The mammalian stem group contains a huge number of fossil species and represents a complex phylogeny. Many different phylogenetic trees have been published; these differ in detail, but are generally in agreement when summarized to show only the direct mammal line. Such a summary is shown in the following composite tree assembled from a number of analyses:
As synapsids, the mammals are characterized by having only one opening, or fenestra, in the skull behind each eye in contrast to the diapsids (a clade that contains the reptiles and birds), which have two openings.
The mammalian stem group contains a huge number of fossil species and represents a complex phylogeny. Many different phylogenetic trees have been published; these differ in detail, but are generally in agreement when summarized to show only the direct mammal line. Such a summary is shown in the following composite tree assembled from a number of analyses:
Figure 1. Summarized composite phylogenetic tree of the stem-Mammalia
As indicated in the above tree, the Synapsida, which is a sister clade to the Sauria, represent the mammalian total group. All the synapsid fossils are more closely related to the mammalian crown group than to modern saurians (reptiles and birds) but many of the side branches shown above (e.g. suborder Caseasauria and family Ophiacodontidae) contain fossil species that at least superficially appear quite unlike modern mammals. However, these branches represent evolutionary dead-ends that diverged from the mammalian stem line and we will not consider them exhaustively here. Our focus will be on stem fossils lying close to the red line of descent shown in the tree above.
In order to understand the phylogenetic relationships of these stem fossils, we need to see a more detailed tree that depicts individual stem species. However, such a tree would be too big to show all at once, so we will divide it into the following four parts, each representing a phase of evolution of the stem group mammals:
These four parts of the stem-Mammalia phylogenetic time tree are shown below:
In order to understand the phylogenetic relationships of these stem fossils, we need to see a more detailed tree that depicts individual stem species. However, such a tree would be too big to show all at once, so we will divide it into the following four parts, each representing a phase of evolution of the stem group mammals:
- From the basal node (root) of the Synapsida to the root of the Therapsida;
- From the Therapsida root to the root of the Cynodontia;
- From the Cynodontia root to the root of the Mammaliamorpha;
- From the Mammaliamorpha root to the appearance of the Crown-Mammalia.
These four parts of the stem-Mammalia phylogenetic time tree are shown below:
Figure 2a. Time tree of the stem-Mammalia (Part 1)
Figure 2b. Time tree of the stem-Mammalia (Part 2)
Figure 2c. Time tree of the stem-Mammalia (Part 3)
Figure 2d. Time tree of the stem-Mammalia (Part 4)
From the Synapsida basal node to the root of the Therapsida
The earliest known fossils in the stem line of mammals are Archaeothyris florensis and Echinerpeton intermedium, both found in the Morien Group, less than 7.62 meters above the Lloyd Cove coal seam of Late Carboniferous (Late Moscovian) age in the Dominion Coal Company strip mine Number 7, 3.2 kilometers north of Florence, Cape Breton County, Nova Scotia, Canada (Mann and Patterson, 2020). These members of the family Ophiacodontidae are illustrated below, together with other fossils for which images are available in the public domain (click on image for a larger view):
The earliest known fossils in the stem line of mammals are Archaeothyris florensis and Echinerpeton intermedium, both found in the Morien Group, less than 7.62 meters above the Lloyd Cove coal seam of Late Carboniferous (Late Moscovian) age in the Dominion Coal Company strip mine Number 7, 3.2 kilometers north of Florence, Cape Breton County, Nova Scotia, Canada (Mann and Patterson, 2020). These members of the family Ophiacodontidae are illustrated below, together with other fossils for which images are available in the public domain (click on image for a larger view):
Figure 3a. Images of stem-group mammals (Synapsida basal node to the root of the Therapsida)
Most of the fossils illustrated above appear very lizard-like and generally resemble the stem-Sauria from which they split. The images are ordered from most basal to most crownward, and no trend of development is obvious, although species with neural spines that are thought to have provided “sails” (possibly for thermal regulation) on their backs appear in the Edaphosauridae and Sphenacodontidae families in the more crownward part of the tree.
From the Therapsida basal node to the root of the Cynodontia
Fossils from this part of the tree (Figure 2b) with public-domain images available are illustrated below (to see a larger view, click on image):
From the Therapsida basal node to the root of the Cynodontia
Fossils from this part of the tree (Figure 2b) with public-domain images available are illustrated below (to see a larger view, click on image):
Figure 3b. Images of stem-group mammals (Therapsida basal node to the root of the Cynodontia)
As always in this web site, for the image sets representing stem groups, the images are placed in order from most basal to most crownward. In this case, there is a general trend towards more massive body forms and larger size. Some of the life restorations of the most crownward species (Diictodon in the Anomodontia, and Ictidosuchoides and Microgomphodon in the Therocephalia) indicate that the animals had hair, but there is no fossil evidence for that.
From the Cynodontia basal node to the root of the Mammaliamorpha
The fossils from this part of the tree (Figure 2c) for which images are available in the public domain are illustrated below (click to see larger view):
From the Cynodontia basal node to the root of the Mammaliamorpha
The fossils from this part of the tree (Figure 2c) for which images are available in the public domain are illustrated below (click to see larger view):
Figure 3c. Images of stem-group mammals (Cynodontia basal node to the root of the Mammaliamorpha)
The above series of images suggests that all of the stem mammals in this part of the tree had a somewhat dog-like appearance without any obvious trend. However, the suggestion that they had a furry coat is again interpretative and is not based on direct fossil evidence. The oldest known in-situ fossil hair was described from Castorocauda lutrasimilis, a Late Jurassic member of the Docodonta, depicted in the next part of the tree below (Ji et al, 2006; Benoit et al, 2016), although putative hair has been described from coprolites of Late Permian age in Russia (Bajdek et al, 2016).
From the Mammaliamorpha basal node to the appearance of the Crown-Mammalia
The fossils for which images are available in the public-domain are illustrated below:
From the Mammaliamorpha basal node to the appearance of the Crown-Mammalia
The fossils for which images are available in the public-domain are illustrated below:
Names in red indicate that the fossil is younger than the oldest known crown-group fossil.
Figure 3d. Images of stem-group mammals (Mammaliamorpha basal node to the appearance of the Crown-Mammalia)
The above images suggest that this most crownward part of the tree comprises smaller, rodent-like species. Again, no trend is obvious, but there is evidence that at least some of the Docodonta (e.g. Castorocauda lutrasimilis, Ji et al, 2006) did have hair.
Note that most of the fossils illustrated above have red labels. This indicates that their first appearance age is less than that of the mammal crown group, which appeared in the Late Triassic. These post-crown stem-group fossils represent descendants of ancestors that would have separated from the stem line during or before the Late Triassic.
One comment should be made before closing this consideration of the mammal stem group, as illustrated in the above four sets of images. Many of the changes that occurred through the mammalian stem line are not obvious from pictures such as those above. The main developments had to do with transformations of the mammalian middle ear and the jaw hinge (Luo, 2007) and also in the morphology of the shoulder (Luo, 2015), but there were other modifications to elements such as brain size, nasal cavity elaboration, secondary palate and evolution of a diaphragm (Kemp, 2007).
Some idea of the nature of the transition from the stem group to the crown group of the mammals can be derived from a comparison of the above images with the examples of early crown-Mammalia shown below:
Note that most of the fossils illustrated above have red labels. This indicates that their first appearance age is less than that of the mammal crown group, which appeared in the Late Triassic. These post-crown stem-group fossils represent descendants of ancestors that would have separated from the stem line during or before the Late Triassic.
One comment should be made before closing this consideration of the mammal stem group, as illustrated in the above four sets of images. Many of the changes that occurred through the mammalian stem line are not obvious from pictures such as those above. The main developments had to do with transformations of the mammalian middle ear and the jaw hinge (Luo, 2007) and also in the morphology of the shoulder (Luo, 2015), but there were other modifications to elements such as brain size, nasal cavity elaboration, secondary palate and evolution of a diaphragm (Kemp, 2007).
Some idea of the nature of the transition from the stem group to the crown group of the mammals can be derived from a comparison of the above images with the examples of early crown-Mammalia shown below:
Figure 4. Examples of early crown-Mammalia
Making allowance for the ghost lineage on the mammalian stem line (Figure 2a), the time between the origin of the mammal stem group and the initiation of the crown group was between 100 and 115 million years.
References
Asher, R. (2018). Diversity and Relationships within Crown Mammalia. In Zachos, Frank. De Gruyter, Mammalian Evolution, Diversity and Systematics. [Book chapter]. https://doi.org/10.17863/CAM.27234.
Bajdek, P., Qvarnström, M., Owocki, K., Sulej, T., Sennikov, A. G., Golubev, V. K., & Niedźwiedzki, G. (2016). Microbiota and food residues including possible evidence of pre‐mammalian hair in Upper Permian coprolites from Russia. Lethaia, 49(4), 455-477.
Benoit, J., Manger, P. R., & Rubidge, B. S. (2016). Palaeoneurological clues to the evolution of defining mammalian soft tissue traits. Nature scientific reports, 6(1), 1-10.
Benton, M. J. (2015). Vertebrate Palaeontology - Fourth edition. John Wiley & Sons, 468 pages.
Ji, Q., Luo, Z. X., Yuan, C. X., & Tabrum, A. R. (2006). A swimming mammaliaform from the Middle Jurassic and ecomorphological diversification of early mammals. Science, 311(5764), 1123-1127.
Kemp, T. S. (2007). The origin of higher taxa: macroevolutionary processes, and the case of the mammals. Acta Zoologica, 88 (1), 3-22.
Luo, Z. X. (2007). Transformation and diversification in early mammal evolution. Nature, 450 (7172), 1011.
Luo, Z. X. (2015). Origin of the mammalian shoulder. Great Transformations: Major Events in the History of Vertebrate Life. The University of Chicago Press, Chicago, Illinois, 167-187.
Mann, A., & Paterson, R. S. (2020). Cranial osteology and systematics of the enigmatic early ‘sail-backed ‘synapsid Echinerpeton intermedium Reisz, 1972, and a review of the earliest ‘pelycosaurs’. Journal of Systematic Palaeontology, 18(6), 529-539.
Bajdek, P., Qvarnström, M., Owocki, K., Sulej, T., Sennikov, A. G., Golubev, V. K., & Niedźwiedzki, G. (2016). Microbiota and food residues including possible evidence of pre‐mammalian hair in Upper Permian coprolites from Russia. Lethaia, 49(4), 455-477.
Benoit, J., Manger, P. R., & Rubidge, B. S. (2016). Palaeoneurological clues to the evolution of defining mammalian soft tissue traits. Nature scientific reports, 6(1), 1-10.
Benton, M. J. (2015). Vertebrate Palaeontology - Fourth edition. John Wiley & Sons, 468 pages.
Ji, Q., Luo, Z. X., Yuan, C. X., & Tabrum, A. R. (2006). A swimming mammaliaform from the Middle Jurassic and ecomorphological diversification of early mammals. Science, 311(5764), 1123-1127.
Kemp, T. S. (2007). The origin of higher taxa: macroevolutionary processes, and the case of the mammals. Acta Zoologica, 88 (1), 3-22.
Luo, Z. X. (2007). Transformation and diversification in early mammal evolution. Nature, 450 (7172), 1011.
Luo, Z. X. (2015). Origin of the mammalian shoulder. Great Transformations: Major Events in the History of Vertebrate Life. The University of Chicago Press, Chicago, Illinois, 167-187.
Mann, A., & Paterson, R. S. (2020). Cranial osteology and systematics of the enigmatic early ‘sail-backed ‘synapsid Echinerpeton intermedium Reisz, 1972, and a review of the earliest ‘pelycosaurs’. Journal of Systematic Palaeontology, 18(6), 529-539.
Image credits – stem-Mammalia
- Figure 3a (Eocasea martini, fossil): Robert R. Reisz, Jörg Fröbisch, CC BY 2.5 <https://creativecommons.org/licenses/by/2.5>, via Wikimedia Commons
- Figure 3a (Eocasea martini, life restoration): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3a (Vaughnictis smithae): Brocklehurst N, Reisz RR, Fernandez V, Fröbisch J (2016), CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 3a (Casea broilii, fossil): Smokeybjb, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 3a (Casea broilii, life restoration): Nobu Tamura, Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 3a (Oedaleops campi): Theropsida, Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported (CC BY-NC-ND 3.0)
- Figure 3a (Euromycter rutenus): Euromycter rutenus 73643.jpg: Ghedoderivative work: Taxon34, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 3a (Eothyris parkeyi, fossil): Frederik Spindler, Jocelyn Falconnet, and Jörg Fröbisch, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
- Figure 3a (Eothyris parkeyi, life restoration): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3a (Ennatosaurus tecton, fossil): Гав-Гав2020, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 3a (Ennatosaurus tecton, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 3a (Echinerpeton intermedium, fossil): Skye McDavid, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 3a (Echinerpeton intermedium, life restoration): Smokeybjb, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 3a (Archaeothyris florensis, skull reconstruction): Gretarsson, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 3a (Archaeothyris florensis, life restoration): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3a (Varanosaurus acutirostris): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3a (Ophiacodon mirus, fossil): Dallas Krentzel, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
- Figure 3a (Ophiacodon mirus, life restoration): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3a (Ianthasaurus hardestiorum): Nobu Tamura, Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 3a (Edaphosaurus boanerges): Daderot, CC0, via Wikimedia Commons
- Figure 3a (Edaphosaurus pogonias): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3a (Haptodus garnettensis, skull reconstruction): Amson, E. and Laurin, M., CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 3a (Haptodus garnettensis, life restoration): Nobu Tamura, Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 3a (Pantelosaurus saxonicus, fossil): DagdaMor, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 3a (Pantelosaurus saxonicus, life restoration): Nobu Tamura, Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 3a (Cutleria wilmarthi): ДиБгд, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 3a (Secodontosaurus obtusidens): Nobu Tamura, Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 3a (Dimetrodon limbatus fossil): Photographed by Bob James (owner of website) at American Museum of Natural History, New York, May 2024
- Figure 3a (Dimetrodon limbatus, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 3b (Sinophoneus yumenensis, fossil): Jonathan Chen, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 3b (Sinophoneus yumenensis, life restoration): Nobu Tamura, Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 3b (Estemmenosuchus uralensis): Radim Holiš, Wikimedia Commons, CC BY-SA 3.0 CZ <https://creativecommons.org/licenses/by-sa/3.0/cz/deed.en>, via Wikimedia Commons
- Figure 3b (Estemmenosuchus mirabilis): Nobu Tamura, Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 3b (Titanophoneus potens, fossil): Pavel Bochkov from Moscow, Russia, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
- Figure 3b (Titanophoneus potens, life restoration): Nobu Tamura, Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 3b (Syodon biarmicum, fossil): PaleoKent, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 3b (Syodon biarmicum, life restoration): Nobu Tamura, Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 3b (Styracocephalus platyrhynchus): Karkemish, Public domain, via Wikimedia Commons
- Figure 3b (Jonkeria sp.): Smokeybjb, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 3b (Jonkeria truculenta): Nobu Tamura, Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 3b (Hipposaurus boonstrai, fossil): Ghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 3b (Hipposaurus boonstrai, life restoration): Karkemish, Public domain, via Wikimedia Commons
- Figure 3b (Herpetoskylax hopsoni): Mojcaj, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons
- Figure 3b (Leucocephalus wewersi): Bluedwarf, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 3b (Lemurosaurus pricei, fossil): Kyler kru, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 3b (Lemurosaurus pricei, life restoration): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3b (Moschops capensis, fossil): Daderot, CC0, via Wikimedia Commons
- Figure 3b (Moschops capensis, life restoration): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3b (Patranomodon nyaphulii, fossil): Lamnhi, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 3b (Patranomodon nyaphulii, life restoration): Theropsida, Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported (CC BY-NC-ND 3.0)
- Figure 3b (Eodicynodon oosthuizeni): Nobu Tamura, Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 3b (Diictodon feliceps, fossil): Nkansahrexford, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
- Figure 3b (Diictodon feliceps, life restoration): El fosilmaníaco, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 3b (Lycaenops ornatus, fossil): Photographed by Bob James (owner of website) at American Museum of Natural History, New York, May 2024
- Figure 3b (Lycaenops ornatus, life restoration): Nobu Tamura, Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 3b (Aloposaurus gracilis): Theropsida, Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported (CC BY-NC-ND 3.0)
- Figure 3b (Euchambersia mirabilis, fossil): https://www.si.edu/object/euchambersia-mirabilis-broom-1931:nmnhpaleobiology_3584361, CC0, via Wikimedia Commons
- Figure 3b (Euchambersia mirabilis, life restoration): Theropsida, Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported (CC BY-NC-ND 3.0)
- Figure 3b (Ictidosuchoides intermedius): Rept0n1x, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 3b (Ictidosuchoides sp.): Smokeybjb, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 3b (Microgomphodon oligocynus, fossil): Broom, Robert, 1866-1951., Public domain, via Wikimedia Commons
- Figure 3b (Microgomphodon oligocynus, life restoration): Dmitry Bogdanov, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
- Figure 3c (Procynosuchus delaharpeae, fossil): Momotarou2012, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 3c (Procynosuchus delaharpeae, life restoration): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3c (Galesaurus planiceps, fossil): Henry Alleyne Nicholson and Richard Lydekker, Public domain, via Wikimedia Commons
- Figure 3c (Galesaurus planiceps, life restoration): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3c (Thrinaxodon liorhinus, fossil): Claire H. from New York City, USA, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
- Figure 3c (Thrinaxodon liorhinus, life restoration): Nobu Tamura, Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 3c (Platycraniellus elegans): Theropsida, under Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License
- Figure 3c (Lumkuia fuzzi): Theropsida, under Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License
- Figure 3c (Cynognathus crateronotus, fossil): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 3c (Cynognathus crateronotus, life restoration): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3c (Diademodon tetragonus, fossil): https://www.si.edu/object/diademodon-tetragonus-seeley-1894:nmnhpaleobiology_3450096, CC0, via Wikimedia Commons
- Figure 3c (Diademodon tetragonus, life restoration): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3c (Trirachodon berryi): Smokeybjb, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 3c (Ecteninion lunensis): Theropsida, Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported (CC BY-NC-ND 3.0)
- Figure 3c (Chiniquodon theotonicus, fossil): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 3c (Chiniquodon theotonicus, life restoration): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3c (Probainognathus jenseni, fossil): Ghedoghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 3c (Probainognathus jenseni, life restoration): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3c (Prozostrodon brasiliensis): Martinelli AG, Soares MB, Schwanke C, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 3c (Therioherpeton cargnini): Theropsida, Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported (CC BY-NC-ND 3.0)
- Figure 3c (Pachygenelus monus): Smokeybjb, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 3c (Riograndia guaibaensis): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3d (Pseudotherium argentinus): Wallace RVS, Martínez R, Rowe T (2019), CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 3d (Oligokyphus triserialis): Nobu Tamura, Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 3d (Tritylodon longaevus, fossil): Arthur Smith Woodward, Public domain, via Wikimedia Commons
- Figure 3d (Tritylodon longaevus, life restoration): Nobu Tamura, Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 3d (Kayentatherium wellesi, fossil): 5of7, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
- Figure 3d (Kayentatherium wellesi, life restoration): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3d (Botucaraitherium belarminoi): Open Access article Soares, M. B., Martinelli, A. G., & OLIVEIRA, T. V. (2014). A new prozostrodontian cynodont (Therapsida) from the Late Triassic Riograndia Assemblage Zone (Santa Maria Supersequence) of Southern Brazil. Anais da Academia Brasileira de Ciências, 86, 1673-1691.
- Figure 3d (Brasilodon quadrangularis, fossil): Martinelli AG, Soares MB, Schwanke C, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 3d (Brasilodon quadrangularis, life restoration): Theropsida under Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License
- Figure 3d (Brasilitherium riograndensis): Open Access article by Martinelli, A. G., Soares, M. B., De Oliveira, T. V., Rodrigues, P. G., & Schultz, C. L. (2017). The Triassic eucynodont Candelariodon barberenai revisited and the early diversity of stem prozostrodontians. Acta Palaeontologica Polonica, 62(3), 527-542.
- Figure 3d (Sinoconodon rigneyi): Sir Beluga, CC0, via Wikimedia Commons
- Figure 3d (Morganucodon oehleri): Hemiauchenia, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 3d (Morganucodon watsoni): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3d (Megazostrodon sp.): Theklan, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 3d (Hadrocodium wui): Stephan Lautenschlager, Pamela Gill, Zhe-Xi Luo, Michael J. Fagan, Emily J. Rayfield, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
- Figure 3d (Castorocauda lutrasimilis): Nobu Tamura, Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 3d (Docofossor brachydactylus): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3d (Agilodocodon scansorius): Kaek, Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License
- Figure 4 (Pseudotribos robustus): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 4 (Haramiyavia clemmenseni): Zhe-Xi Luo, Illustration by April Neander, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
- Figure 4 (Volaticotherium antiquum): Zhao Chuang, under a Creative Commons Attribution-NonCommercial 4.0 International License.