The Eutheria, or placental mammals (Placentalia), (infraclass Eutheria in subclass Theria) are the sister group of the marsupials (infraclass Metatheria) and comprise the two clades Boreoeutheria and Atlantogenata, which are based primarily not on morphology, but on molecular data (Murphy et al, 2007).
Although there are many publications that depict the phylogeny of modern eutherians, few exist that show the relationships between fossil members of the infraclass. A fairly recent article that does deal with fossil examples is that by Bi et al (2018), on which the following phylogenetic time tree is based:
Although there are many publications that depict the phylogeny of modern eutherians, few exist that show the relationships between fossil members of the infraclass. A fairly recent article that does deal with fossil examples is that by Bi et al (2018), on which the following phylogenetic time tree is based:
Figure 1. Phylogenetic time tree of the stem-Eutheria
The oldest known member of the stem-Eutheria is generally considered to be Juramaia sinensis, supposedly of Middle to Late Jurassic age (Luo et al, 2011). However, while there is consensus that Juramaia is a stem eutherian (King and Beck, 2020), there is some controversy about its age (Meng, 2014). King and Beck (2020) point out the great morphological similarity of Juramaia with Early Cretacous stem-Eutheria from the Yixian Formation of China. We will allow here the possibility that Juramaia is of Early Cretaceous age.
Leaving aside Juramaia, the oldest known stem eutherian would be Ambolestes zhoui, described from the Early Cretaceous Yixian Formation at Xisanjia, Inner Mongolia, China (Bi et al, 2018). These authors suggest that Ambolestes is older than other stem eutherians from the Yixian Formation, but more recent work by Zhong et al (2021) indicates that the entire Yixian Formation is of Late Barremian to Early Aptian age and thus Ambolestes cannot be considered as older than the other Yixian fossils. Two of these fossils, Sinodelphys szalayi and Eomaia scansoria, are illustrated below together with Juramaia sinensis and other stem eutherians for which images are available in the public domain (click on image for a larger view):
Leaving aside Juramaia, the oldest known stem eutherian would be Ambolestes zhoui, described from the Early Cretaceous Yixian Formation at Xisanjia, Inner Mongolia, China (Bi et al, 2018). These authors suggest that Ambolestes is older than other stem eutherians from the Yixian Formation, but more recent work by Zhong et al (2021) indicates that the entire Yixian Formation is of Late Barremian to Early Aptian age and thus Ambolestes cannot be considered as older than the other Yixian fossils. Two of these fossils, Sinodelphys szalayi and Eomaia scansoria, are illustrated below together with Juramaia sinensis and other stem eutherians 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 2. Images of stem-group eutherians
The above images are ordered from most basal to most crownward, but no clear trends in development can be seen by comparing these images, which are few and generally of limited usefulness. The life restorations indicate that the stem group comprised rodent-like animals.
Some idea of the nature of the transition from the stem group to the crown group of the eutherians can be derived from a comparison of the above images with the examples of early crown-Eutheria shown below:
Some idea of the nature of the transition from the stem group to the crown group of the eutherians can be derived from a comparison of the above images with the examples of early crown-Eutheria shown below:
Figure 3. Examples of early crown-Eutheria
Assuming that Juramaia sinensis is of Early Cretaceous (Late Barremian to Early Aptian) rather than Jurassic age, the above time tree (Figure 1) indicates that the tetrapod stem group developed from Early Cretaceous to latest Cretaceous time, representing a stem-to-crown transition of at least 57 million years.
References
Benton, M. J. (2015). Vertebrate Palaeontology - Fourth edition. John Wiley & Sons, 468 pages.
Bi, S., Zheng, X., Wang, X., Cignetti, N. E., Yang, S., & Wible, J. R. (2018). An Early Cretaceous eutherian and the placental–marsupial dichotomy. Nature, 558(7710), 390-395.
King, B., & Beck, R. M. (2020). Tip dating supports novel resolutions of controversial relationships among early mammals. Proceedings of the Royal Society B, 287(1928), 20200943.
Luo, Z. X., Yuan, C. X., Meng, Q. J., & Ji, Q. (2011). A Jurassic eutherian mammal and divergence of marsupials and placentals. Nature, 476(7361), 442-445.
Meng, J. (2014). Mesozoic mammals of China: implications for phylogeny and early evolution of mammals. National Science Review, 1(4), 521-542.
Murphy, W. J., Pringle, T. H., Crider, T. A., Springer, M. S., & Miller, W. (2007). Using genomic data to unravel the root of the placental mammal phylogeny. Genome research, 17(4), 413-421.
Zhong, Y., Huyskens, M. H., Yin, Q. Z., Wang, Y., Ma, Q., & Xu, Y. G. (2021). High-precision geochronological constraints on the duration of ‘Dinosaur Pompeii’ and the Yixian Formation. National Science Review, 8(6), nwab063.
Bi, S., Zheng, X., Wang, X., Cignetti, N. E., Yang, S., & Wible, J. R. (2018). An Early Cretaceous eutherian and the placental–marsupial dichotomy. Nature, 558(7710), 390-395.
King, B., & Beck, R. M. (2020). Tip dating supports novel resolutions of controversial relationships among early mammals. Proceedings of the Royal Society B, 287(1928), 20200943.
Luo, Z. X., Yuan, C. X., Meng, Q. J., & Ji, Q. (2011). A Jurassic eutherian mammal and divergence of marsupials and placentals. Nature, 476(7361), 442-445.
Meng, J. (2014). Mesozoic mammals of China: implications for phylogeny and early evolution of mammals. National Science Review, 1(4), 521-542.
Murphy, W. J., Pringle, T. H., Crider, T. A., Springer, M. S., & Miller, W. (2007). Using genomic data to unravel the root of the placental mammal phylogeny. Genome research, 17(4), 413-421.
Zhong, Y., Huyskens, M. H., Yin, Q. Z., Wang, Y., Ma, Q., & Xu, Y. G. (2021). High-precision geochronological constraints on the duration of ‘Dinosaur Pompeii’ and the Yixian Formation. National Science Review, 8(6), nwab063.
Image credit – stem-Eutheria
- Figure 2 (Sinodelphys szalayi): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 2 (Juramaia sinensis): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 2 (Eomaia scansorial, fossil): Zofia Kielan-Jaworowska and Jørn H. Hurum, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
- Figure 2 (Eomaia scansorial, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 2 (Batodon tenuis): Smithsonian Institution, public domain (free of copyright restrictions)
- Figure 2 (Paranyctoides aralensis): Archibald J.D., Averianov A.O. 2001., CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2 (Uchkudukodon nessovi): McKenna, M.C., Kielan-Jaworowska, Z., & Meng, J., CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2 (Asioryctes nemegtensis): Robert Niedźwiedzki, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2 (Cimolestes incisus): Open Access article Clemens, W. A. (2019). Puercolestes and Betonnia (Cimolestidae, Mammalia) from the early Paleocene (Puercan 3 Interval Zone) of northeastern Montana, USA. PaleoBios, 36.
- Figure 2 (Zalambdalestes lechei, fossil): Robert Niedźwiedzki, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2 (Zalambdalestes lechei, life restoration): Nobu Tamura, under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 2 (Leptictis dakotensis): James St. John, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
- Figure 3 (Head of Ocepeia daouiensis): Open Access article Gheerbrant, E., Amaghzaz, M., Bouya, B., Goussard, F., & Letenneur, C. (2014). Ocepeia (Middle Paleocene of Morocco): the oldest skull of an afrotherian mammal. PLoS One, 9(2), e89739.
- Figure 3 (Ernanodon antelios): Nobu Tamura, under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3 (Metacheiromys marshi): Nobu Tamura, under Creative Commons Attribution- ShareAlike (CC BY-SA) license