The therians (subclass Theria in class Mammalia) are the sister group of the Prototheria and comprise the marsupials (infraclass Metatheria) and the placental mammals (infraclass Eutheria).
Several phylogenetic trees of the stem-Theria have been published in recent years (e.g. Mao et al, 2020; Hoffmann et al, 2020; Wang et al, 2021; Morales-García et al, 2021; Mao et al, 2021). They generally agree in most respects, and we will use the tree of Wang et al (2021) as a representative example. For convenience we will divide it into two parts:
A phylogenetic time tree based on the tree of Wang et al (2021) is shown in two parts below:
Several phylogenetic trees of the stem-Theria have been published in recent years (e.g. Mao et al, 2020; Hoffmann et al, 2020; Wang et al, 2021; Morales-García et al, 2021; Mao et al, 2021). They generally agree in most respects, and we will use the tree of Wang et al (2021) as a representative example. For convenience we will divide it into two parts:
- From the basal node (root) of the stem-Theria up to the Trechnotheria
- From the Trechnotheria root to the therian crown node.
A phylogenetic time tree based on the tree of Wang et al (2021) is shown in two parts below:
Figure 1a. Time tree of the stem-Theria (Part 1)
Figure 1b. Time tree of the stem-Theria (Part 2)
From the stem-Theria basal node to the root of the Trechnotheria
The oldest known stem therian is Haramiyavia clemmenseni, described from the Late Triassic (middle Norian to early Rhaetian) Ørsted Dal Member of the Fleming Fjord Formation on the north side of Ærenprisdal at the intersection with Pingel Dal, Jameson Land, East Greenland (Jenkins et al, 1997; Mao et al, 2020). This species is illustrated below, together with other members of the basal stem group for which images are available in the public domain (click on image for a larger view):
The oldest known stem therian is Haramiyavia clemmenseni, described from the Late Triassic (middle Norian to early Rhaetian) Ørsted Dal Member of the Fleming Fjord Formation on the north side of Ærenprisdal at the intersection with Pingel Dal, Jameson Land, East Greenland (Jenkins et al, 1997; Mao et al, 2020). This species is illustrated below, together with other members of the basal stem group for which images are available in the public domain (click on image for a larger view):
Names in red indicate that the fossil is younger than the oldest known crown-group fossil.
Figure 2a. Images of stem-group therians (stem-Theria basal node to the root of the Trechnotheria)
As usual, the images are ordered from most basal to most crownward. There is no obvious trend apart from a possible change from a rather rat-like form to a more squirrel-like shape. However, there is one basal species (Volaticotherium antiquum) that is interpreted to look rather like a modern flying squirrel, as is Maiopatagium furculiferum, which occupies a much more crownward position in the tree.
Note that a couple of the fossils illustrated above have red labels. This indicates that their first appearance age is less than that of the therian crown group, which appeared in the Early Cretaceous. These post-crown stem-group fossils represent descendants of ancestors that would have separated from the stem line during or before the Early Cretaceous.
From the Trechnotheria basal node to the therian crown node
Fossils from this part of the tree (Figure 1b) with public-domain images available are illustrated below (to see a larger view, click on image):
Note that a couple of the fossils illustrated above have red labels. This indicates that their first appearance age is less than that of the therian crown group, which appeared in the Early Cretaceous. These post-crown stem-group fossils represent descendants of ancestors that would have separated from the stem line during or before the Early Cretaceous.
From the Trechnotheria basal node to the therian crown node
Fossils from this part of the tree (Figure 1b) with public-domain images available are illustrated below (to see a larger view, click on image):
Figure 2b. Images of stem-group therians (Trechnotheria basal node to the therian crown node)
There are not enough images to determine the nature of the gross evolutionary trend in this part of the stem line. However, some idea of the nature of the transition from the stem group to the crown group of the therians can be derived from a comparison of the above images with the examples of crown-Theria shown below:
Figure 3. Examples of crown-Theria
The above time tree (Figures 1a and 1b) indicates that the mammalian stem group developed from Late Triassic to Early Cretaceous time, representing a stem-to-crown transition of between 78 and 94 million years.
References
Benton, M. J. (2015). Vertebrate Palaeontology - Fourth edition. John Wiley & Sons, 468 pages.
Hoffmann, S., Beck, R. M., Wible, J. R., Rougier, G. W., & Krause, D. W. (2020). Phylogenetic placement of Adalatherium hui (Mammalia, Gondwanatheria) from the Late Cretaceous of Madagascar: implications for allotherian relationships. Journal of Vertebrate Paleontology, 40(sup1), 213-234.
Jenkins, F. A., Gatesy, S. M., Shubin, N. H., & Amaral, W. W. (1997). Haramiyids and Triassic mammalian evolution. Nature, 385(6618), 715-718.
Mao, F., Hu, Y., Li, C., Wang, Y., Chase, M. H., Smith, A. K., & Meng, J. (2020). Integrated hearing and chewing modules decoupled in a Cretaceous stem therian mammal. Science, 367(6475), 305-308.
Mao, F., Zhang, C., Liu, C., & Meng, J. (2021). Fossoriality and evolutionary development in two Cretaceous mammaliamorphs. Nature, 592(7855), 577-582.
Morales-García, N. M., Gill, P. G., Janis, C. M., & Rayfield, E. J. (2021). Jaw shape and mechanical advantage are indicative of diet in Mesozoic mammals. Nature Communications biology, 4(1), 1-14.
Wang, J., Wible, J. R., Guo, B., Shelley, S. L., Hu, H., & Bi, S. (2021). A monotreme-like auditory apparatus in a Middle Jurassic haramiyidan. Nature, 590(7845), 279-283.
Hoffmann, S., Beck, R. M., Wible, J. R., Rougier, G. W., & Krause, D. W. (2020). Phylogenetic placement of Adalatherium hui (Mammalia, Gondwanatheria) from the Late Cretaceous of Madagascar: implications for allotherian relationships. Journal of Vertebrate Paleontology, 40(sup1), 213-234.
Jenkins, F. A., Gatesy, S. M., Shubin, N. H., & Amaral, W. W. (1997). Haramiyids and Triassic mammalian evolution. Nature, 385(6618), 715-718.
Mao, F., Hu, Y., Li, C., Wang, Y., Chase, M. H., Smith, A. K., & Meng, J. (2020). Integrated hearing and chewing modules decoupled in a Cretaceous stem therian mammal. Science, 367(6475), 305-308.
Mao, F., Zhang, C., Liu, C., & Meng, J. (2021). Fossoriality and evolutionary development in two Cretaceous mammaliamorphs. Nature, 592(7855), 577-582.
Morales-García, N. M., Gill, P. G., Janis, C. M., & Rayfield, E. J. (2021). Jaw shape and mechanical advantage are indicative of diet in Mesozoic mammals. Nature Communications biology, 4(1), 1-14.
Wang, J., Wible, J. R., Guo, B., Shelley, S. L., Hu, H., & Bi, S. (2021). A monotreme-like auditory apparatus in a Middle Jurassic haramiyidan. Nature, 590(7845), 279-283.
Image credits – stem-Theria
- Figure 2a (Volaticotherium antiquum, fossil): Open Access article by Meng, J. (2014). Mesozoic mammals of China: implications for phylogeny and early evolution of mammals. National Science Review, 1(4), 521-542.
- Figure 2a (Volaticotherium antiquum, life restoration): Zhao Chuang, under a Creative Commons Attribution-NonCommercial 4.0 International License.
- Figure 2a (Fruitafossor windscheffeli, fossil): Fanboyphilosopher, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2a (Fruitafossor windscheffeli, life restoration): Nobu Tamura, under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 2a (Repenomamus giganticus, fossil): Open Access article by Meng, J. (2014). Mesozoic mammals of China: implications for phylogeny and early evolution of mammals. National Science Review, 1(4), 521-542.
- Figure 2a (Repenomamus giganticus,life restoration): Nobu Tamura, under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 2a (Gobiconodon ostromi): Ghedoghedo, Public domain, via Wikimedia Commons
- Figure 2a (Gobiconodon sp.): DiBgd, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2a (Jeholodens jenkinsi, fossil): Laikayiu, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons
- Figure 2a (Jeholodens jenkinsi, life restoration): Nobu Tamura, under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 2a (Yanoconodon allini): Nobu Tamura, under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 2a (Liaoconodon hui): Open Access article by Meng, J. (2014). Mesozoic mammals of China: implications for phylogeny and early evolution of mammals. National Science Review, 1(4), 521-542.
- Figure 2a (Priacodon fruitaensis): Open Access article by Jäger, K. R., Cifelli, R. L., & Martin, T. (2020). Molar occlusion and jaw roll in early crown mammals. Scientific reports, 10(1), 1-12. ( http://creativecommons.org/licenses/by/4.0).
- Figure 2a (Haramiyavia clemmenseni, fossil): Zhe-Xi Luo, Illustration by April Neander, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
- Figure 2a (Haramiyavia clemmenseni, life restoration): Zhe-Xi Luo, Illustration by April Neander, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
- Figure 2a (Vintana sertichi): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 2a (Shenshou lui, fossil): Open Access article by Meng, J. (2014). Mesozoic mammals of China: implications for phylogeny and early evolution of mammals. National Science Review, 1(4), 521-542.
- Figure 2a (Shenshou lui, life restoration): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 2a (Maiopatagium furculiferum): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 2a (Sinobaatar lingyuanensis): Jonathan Chen, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2a (Rugosodon eurasiaticus, fossil): Zhe-Xi Luo, Qing-Jin Meng, Di Liu, Yu-Guang Zhang, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
- Figure 2a (Rugosodon eurasiaticus, life restoration): Nobu Tamura, under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 2b (Spalacotherium tricuspidens): British Geological Survey, licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
- Figure 2b (Akidolestes cifellii): Kielan−Jaworowska, Z. and Hurum, J.H., CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2b (Zhangheotherium quinquecuspidens): Laikayiu, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons
- Figure 2b (Maotherium sinensis): Kielan−Jaworowska, Z. and Hurum, J.H., CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2b (Henkelotherium guimarotae): P.Fernandes (Trebaruna), CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
- Figure 2b (Dryolestes priscus): Othniel Charles Marsh, Public domain, via Wikimedia Commons
- Figure 2b (Amphitherium prevostii): British Geological Survey, under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
- Figure 2b (Vincelestes neuquenianus): Adrian W., CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 2b (Peramus tenuirostris): The Trustees of the Natural History Museum, London, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
- Figure 3 (Didelphodon vorax): Nobu Tamura, under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3 (Juramaia sinensis): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 3 (Eomaia scansorial): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license