The platypuses and echidnas (order Monotremata, subclass Prototheria) are the egg-laying mammals. They comprise the crown group of the prototherians, which contain extinct species that are not monotremes and thus represent a total group (Weisbecker and Beck, 2015).
The phylogenetic relationships of the Prototheria are shown in the following time tree, which is based on a recent article by Mao et al (2021):
The phylogenetic relationships of the Prototheria are shown in the following time tree, which is based on a recent article by Mao et al (2021):
Figure 1. Phylogenetic time tree of the stem-Monotremata
The oldest known stem monotreme is Asfaltomylos patagonicus, found in the Early Jurassic (Toarcian) Cañadón Asfalto Formation at the Queso Rallado locality near the village of Cerro Condor, Chubut, Argentina (Rauhut et al, 2002; Pian et al, 2016). A image of its mandible, or lower jawbone, is shown below together with images of some other stem-Prototheria (for larger versions, click on images):
Figure 2. Images of stem-group monotremes
The images shown above illustrate some aspects of the ancestry of the prototherian crown group. For instance, the restoration of Steropodon galmani suggests that this animal was much more platypus-like than Ambondro or Pseudotribos. There are also some significant changes in the teeth; compare the molar labelled m3 in the jawbones of Shuotherium, Ausktribosphenos and Bishops.
The oldest known monotreme, or crown prototherian, is Obdurodon dicksoni, a platypus from Middle Miocene sediments at the Ringtail Site, Riversleigh World Heritage property, Queensland, Australia (Macrini et al, 2006; Phillips, 2015):
The oldest known monotreme, or crown prototherian, is Obdurodon dicksoni, a platypus from Middle Miocene sediments at the Ringtail Site, Riversleigh World Heritage property, Queensland, Australia (Macrini et al, 2006; Phillips, 2015):
Figure 3. Obdurodon dicksoni, the oldest known crown-group prototherian
References
Benton, M. J. (2015). Vertebrate Palaeontology - Fourth edition. John Wiley & Sons, 468 pages.
Macrini, T. E., Rowe, T., & Archer, M. (2006). Description of a cranial endocast from a fossil platypus, Obdurodon dicksoni (Monotremata, Ornithorhynchidae), and the relevance of endocranial characters to monotreme monophyly. Journal of Morphology, 267(8), 1000-1015.
Mao, F., Zhang, C., Liu, C., & Meng, J. (2021). Fossoriality and evolutionary development in two Cretaceous mammaliamorphs. Nature, 592(7855), 577-582.
Phillips, M. J. (2015). Four mammal fossil calibrations: balancing competing palaeontological and molecular considerations. Palaeontologia Electronica, 18(1), 1-16.
Pian, R., Archer, M., Hand, S. J., Beck, R. M., & Cody, A. (2016). The upper dentition and relationships of the enigmatic Australian Cretaceous mammal Kollikodon ritchiei. Memoirs of Museum Victoria, 74.
Rauhut, O. W., Martin, T., Ortiz-Jaureguizar, E., & Puerta, P. (2002). A Jurassic mammal from South America. Nature, 416(6877), 165.
Weisbecker, V., & Beck, R. M. (2015). Marsupial and monotreme evolution and biogeography. In Marsupials and Monotremes, Editors: A. Klieve, L. Hogan, S. Johnston, P. Murray, Chapter 1.
Macrini, T. E., Rowe, T., & Archer, M. (2006). Description of a cranial endocast from a fossil platypus, Obdurodon dicksoni (Monotremata, Ornithorhynchidae), and the relevance of endocranial characters to monotreme monophyly. Journal of Morphology, 267(8), 1000-1015.
Mao, F., Zhang, C., Liu, C., & Meng, J. (2021). Fossoriality and evolutionary development in two Cretaceous mammaliamorphs. Nature, 592(7855), 577-582.
Phillips, M. J. (2015). Four mammal fossil calibrations: balancing competing palaeontological and molecular considerations. Palaeontologia Electronica, 18(1), 1-16.
Pian, R., Archer, M., Hand, S. J., Beck, R. M., & Cody, A. (2016). The upper dentition and relationships of the enigmatic Australian Cretaceous mammal Kollikodon ritchiei. Memoirs of Museum Victoria, 74.
Rauhut, O. W., Martin, T., Ortiz-Jaureguizar, E., & Puerta, P. (2002). A Jurassic mammal from South America. Nature, 416(6877), 165.
Weisbecker, V., & Beck, R. M. (2015). Marsupial and monotreme evolution and biogeography. In Marsupials and Monotremes, Editors: A. Klieve, L. Hogan, S. Johnston, P. Murray, Chapter 1.
Image credits – stem-Monotremata
- Header (Duck-billed Platypus, Ornithorhynchus anatinus): Brisbane City Council, Creative Commons
- Figure 2 (Pseudotribos robustus): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 2 (Shuotherium dongi): Kielan−Jaworowska, Z., Cifelli, R.L., and Luo, Z.−X., CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2 (Asfaltomylos patagonicus): Open Access article Flannery, T. F., Rich, T. H., Vickers-Rich, P., Veatch, E. G., & Helgen, K. M. (2022). The Gondwanan Origin of Tribosphenida (Mammalia). Alcheringa: An Australasian Journal of Palaeontology, 1-14.
- Figure 2 (Ambondro mahabo, fossil): Open Access article Flannery, T. F., Rich, T. H., Vickers-Rich, P., Veatch, E. G., & Helgen, K. M. (2022). The Gondwanan Origin of Tribosphenida (Mammalia). Alcheringa: An Australasian Journal of Palaeontology, 1-14.
- Figure 2 (Ambondro mahabo, life restoration): Alannis, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 2 (Ausktribosphenos nyktos): Open Access article Flannery, T. F., Rich, T. H., Vickers-Rich, P., Veatch, E. G., & Helgen, K. M. (2022). The Gondwanan Origin of Tribosphenida (Mammalia). Alcheringa: An Australasian Journal of Palaeontology, 1-14.
- Figure 2 (Bishops whitmorei): Open Access article Flannery, T. F., Rich, T. H., Vickers-Rich, P., Veatch, E. G., & Helgen, K. M. (2022). The Gondwanan Origin of Tribosphenida (Mammalia). Alcheringa: An Australasian Journal of Palaeontology, 1-14.
- Figure 2 (Steropodon galmani, fossil): https://www.si.edu/object/steropodon-galmani-archer-et-al-1985:nmnhpaleobiology_11668999, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2 (Steropodon galmani, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
- Figure 3: Open Access article Bino, G., Kingsford, R. T., Archer, M., Connolly, J. H., Day, J., Dias, K., ... & Whittington, C. (2019). The platypus: evolutionary history, biology, and an uncertain future. Journal of mammalogy, 100(2), 308-327.