EVOLUTION - THE TRANSITIONAL FOSSILS
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placental mammals

Introduction

The eutherians, or placental mammals (infraclass Eutheria, subclass Theria), are distinguished from the other therian clade, the Metatheria, by having a placental membrane that allows the embryo to grow into a well-developed fetus (Biology Online Dictionary, accessed 7th July, 2018). The crown-Eutheria are known as the Placentalia (Murphy and Eizirik, 2009).

The eutherian crown group (shown below as a red dot), comprises two clades, the Atlantogenata and the Boreoeutheria, whose crown groups are shown as black dots in the following phylogenetic tree:
Picture
The fact that animals with very different morphology, such as for example whales and bats, occur within the same clade on the above tree is a consequence of its being based largely on molecular rather than morphological data (Murphy et al, 2007).

The stem group

​Many phylogenetic trees of the eutherian stem group have been published; one of the less complex ones is shown below:
Picture
​The oldest known fossil representative of the eutherian stem group is Juramaia sinensis, from the Middle - Late Jurassic (Callovian - Kimmeridgian) Tiaojishan Formation of the Daxigou site in Jianchang County, Liaoning Province, China (Luo et al, 2011; Weisbecker and Beck, 2015). This species is illustrated in the Therian section above, and also in the following figures that show other examples of stem-eutherians:
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
​The above series of images illustrates some of the fossils that represent the ancestry of the eutherian crown group. Note that part of this ancestry (represented by names in red) post-dates the appearance of the crown group; these fossils represent branches of the stem line that continued to evolve after the crown-group had appeared.

The fossils are numbered in order from the most basal examples to the fossils that are closest to the crown group, but no obvious trends can be seen by comparing these images. However, significant changes took place in dentition (Luo et al, 2011).

The eutherian stem group evolved from Middle or Late Jurassic time until the Paleocene:
Picture

The crown group

The crown-Eutheria appeared when the stem lines of the Atlantogenata  and the   Boreoeutheria separated from one another:
Picture
Research carried for this website has not encountered any fossils identified in the literature as either stem-Atlantogenata or stem-Boreoeutheria. This accords with the analysis in the Ph.D. thesis of Thomas Halliday, which states that "no taxon was consistently resolved on the stem of either Atlantogenata or Boreoeutheria" (Halliday, 2015). Therefore the best estimate we can make of the timing of the split must be based on the earliest crown-group fossils.

The oldest known crown-group boreoeutherian is Purgatorius sp., a stem-primate found in the Paleocene (Early Danian) Tullock Formation of Garfield County, northeast Montana (Chester et al, 2015; Springer at al, 2017). This fossil is somewhat older than the earliest known member of the crown-Atlantogenata, which is Carodnia vieirai, a stem tethytherian (the elephants and sea cows) from the Late Paleocene Itaboraian mammal zone at São José de Itaborai, Brazil (Paula Couto, 1952;O'Leary et al, 2013).

The relationship between the ages of appearance mentioned above and that of the earliest stem eutherian is shown in the figure below. Also shown are representative images of species from the respective stem and crown groups. The figure also depicts the total uncertainty in the age of the eutherian crown node. This uncertainty represents the maximum period of time for the stem-to-crown transition; the time between the origin of the stem-Eutheria and the initiation of the crown group could have been as long as 102 million years:
Picture
The time frame depicted above implies that the division of the Atlantogenata and the Boreoeutheia into the distinct orders of placental mammals that we see today must have taken place over a period of less than 60 million years. In order to investigate this a little further, we will take a look at the appearance history of each of these orders. Firstly, here is a recent phylogenetic tree:
Picture
The following figure shows the time frame over which the stem and crown groups of these orders appeared:
Picture
As may be seen in above plot, most of the crown groups appeared by the end of the Paleogene, indicating that most of the morphological diversity seen in the placental mammals today developed over a period of no more than 40 million years. For more discussion of this theme, see Tarver et al (2016) and Halliday et al (2019).​

Image credits - Eutherians
  • Headrer: ​Ring-tailed Lemur (Lemur catta) at Berenty Private Reserve in Madagascar    By   Alex Dunkel (Maky) [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)]
  • Juramaia sinensis     By Nobu Tamura http://paleoexhibit.blogspot.com/ http://spinops.blogspot.com/ http://www.palaeocritti.com [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons​
  • Eomaia scansoria     By Zofia Kielan-Jaworowska and Jørn H. Hurum (http://app.pan.pl/article/item/app51-393.html) [CC BY 2.0 (https://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons
  • Eomaia sp.     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
  • Asioryctes nemegtensis     By Robert Niedźwiedzki [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 4.0  (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons
  • Titanoides primaevus      By Creator:Dmitry Bogdanov (dmitrchel@mail.ru) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
  • Stylinodon mirus     By   Apokryltaros at English Wikipedia [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
  • Psittacotherium multifragum     By Smokeybjb [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​
  • Zalambdalestes lechei   (3A)     By Robert Niedźwiedzki [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 4.0  (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons
  • Zalambdalestes lechei   (3B)    By Smokeybjb [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  
  • Carodnia vieirai     By   Apokryltaros at English Wikipedia [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)]
  • Purgatorius unio     By Nobu Tamura [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)], from Wikimedia Commons
  • Home
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