Another look at the O’Leary et al. hypothetical ancestor of placentals

This post takes another look at
O’Leary et al. (2013) who created a hypothetical ancestral placental. Now that several dozen mammals have been added to the LRT (including the extant basal placental, Monodelphis), hopefully now we can look at the situation with more insight. Let’s see how close to Monodelphis O’Leary’s team got in creating their own hypothetical ancestor. Comparisons can be made both in the bones and soft tissues between the LRT ancestor, Monodelphis, and the imagined ancestor of the O’Leary team (see below).

BTW, the O’Leary team nested Didelphis and Monodelphis as sisters without descendants using nucleotide data. The LRT found that both were good plesiomorphic models for as yet undiscovered ancestors to the Metatheria and the Eutheria (=Placentalia) respectively.

Figure 1. The marsupial, Monodelphis domestica, nests as a sister to Eomaia, the oldest known placental.

Figure 1. The purported marsupial without a pouch, Monodelphis domestica, nests as the most primitive tested placental in the LRT.

O’Leary et al. (2013) sought to bring new insight into the earliest radiation of placental mammals by creating a hypothetical ancestor to all placentals. They reported this happened in a great radiation of clades right after the K-T extinction event. This counters earlier claims that undiscovered placentals may have been present during the Cretaceous based on data mentioned below. Monotremes and metatherians, the ancestors of today’s egg-laying and marsupial mammals, were present during the Cretaceous. Other lineages of mammals, like Morganucodon, were present as far back as the Triassic. So the O’Leary et al. hypothesis requires relative stasis throughout much of the Mesozoic for mammals, followed by an explosive radiation in the first third of the Paleocene following the K-T extinction event.

Figure 2. Hypothetical ancestor to placental mammals as imagined by O'Leary et al. 2013.

Figure 2. Hypothetical ancestor to placental mammals as imagined by O’Leary et al. 2013.

The O’Leary et al. abstract reports: To discover interordinal relationships of living and fossil placental mammals and the time of origin of placentals relative to the Cretaceous-Paleogene (K-Pg) boundary, we scored 4541 phenomic characters de novo for 86 fossil and living species. Combining these data with molecular sequences, we obtained a phylogenetic tree that, when calibrated with fossils, shows that crown clade Placentalia and placental orders originated after the K-Pg boundary. Many nodes discovered using molecular data are upheld, but phenomic signals overturn molecular signals to show Sundatheria (Dermoptera + Scandentia) as the sister taxon of Primates, a close link between Proboscidea (elephants) and Sirenia (sea cows), and the monophyly of echolocating Chiroptera (bats). Our tree suggests that Placentalia first split into Xenarthra and Epitheria; extinct New World species are the oldest members of Afrotheria.”

Figure 2. Monodelphis skull in three views. Note the supra occipital is narrower than the exoccipitals, like other mammals, not like the data from the figure previously used.

Figure 3. Monodelphis skull in three views. Note the supra occipital is narrower than the exoccipitals, like other mammals, not like the data from the figure previously used. From Digimorph.org and used with permission.

What the O’Leary team imagined vs what the LRT recovered:

  1. Molecular clock analyses indicate crown Placentalia were present by the late Early Cretaceous, 100 mya. The LRT confirms that.
  2. Fossil evidence does not corroborate the presence of early primates and early rodents in the Late Cretaceous. The LRT recovered an Early Cretaceous carnivore, Vinceletes and a Late Jurassic rodent precursor, Shenshou, a sister to the extant Solenodon clade that includes moles and shrews, and a Late Triassic sister to multituberculates, Haramiyavia, which nest as derived rodents in the LRT. 
  3. Phenomic phylogenies incorporating fossils have placed ordinal and intraordinal speciation of Placentalia after the K-Pg (K-T) extinction event. The LRT found two raditions: small arboreal basal mammals in the Triassic and Jurassic, larger Tenreccetaceans and Condylarthra (ungulates and kin) in the Cenozoic. 
  4. Ukhaatherium, Mailestes and Zalambdalestes, all from the Late Cretaceous, nest outside the Placentalia. In the LRT the first two nest with Leptictis a basal Tenreccetacean. The last one indeed does nest outside with wombats in the Marsupialia. So, one out of three. 
  5. The first members of modern placental orders began appearing 2 to 3 million years (My) later during the Paleocene.All recent clock-based estimates for the ages of key clades, with few exceptions, are substantially older than indicated by the fossil record.  The first placentals in the LRT appear in the Late Triassic, which agrees with the clocks, not O’Leary et al.
  6. We recognize Protungulatum donnae as the oldest undisputed species within crown Placentalia (Fig. 1), and this species dates to the earliest Paleocene. Haramiyavia (Late Triassic) was tested in the LRT, but later deleted due to its low number of traits.
  7. We find with 100% jackknife support that Eomaia falls outside of Eutheria as a stem taxon to Theria. So does the LRT.
Figure 4. Entire skeleton of Monodelphis from Digimorph.org and used with permission.

Figure 4. Entire skeleton of Monodelphis from Digimorph.org and used with permission.

O’Leary et al. reconstructed the placental ancestor.
Let’s see how well it conforms to the LRT placental ancestor, Mondodelphis (M).

 

  1. It weighed between 6 and 245 g – M: 58 (f) to 95(m) grams
  2. insectivororous – M: omnivorous – gives rise to carnivores and insectivores, notably it holds prey with its hands, which is the fast track toward the primaries
  3. scansorial – M: scansorial
  4. single young born hairless, eyes closed – M: 6-11 young, hairless, eyes closed
  5. uterus with two horns – M:?
  6. placenta with trophoblast – M:?
  7. sperm with flat head – M:?
  8. abdominal testes just caudal to kidneys M: scrotum after 24 days; several placental clades do not produce a scrotum (see below)
  9. brain has corpus callosum, encephalizatin quotient >0.25 – M:?
  10. facial nerve fibers passed ventral to the trigeminal sensory column – M:?
  11. gyrencephalic cerebral cortex – M:?
  12. separate olfactory bulbs – M: single bulb
  13. hemichorial placenta – M: a rudimentary placenta develops then disappears
  14. separate openings for anus and urogenitalia – M: single cloaca
  15. triangular perforated stapes – M: perforation triangular, externally rectangular
  16. lacked epipubic bones – M: epipubes present, as in other basal placentals
  17. internal carotid artery present, but did not leave its mark on bones – M:?
  18. seven post canine teeth, four premolars and three molars – M: four molars, also in Asrioryctes, Leptictis and other Tenreccetaceans.
  19. premolar 3 is lost in Theria, so:p1,2,4,5, m1,2,3 – M: 3 premolars, 4 molars; in many other placentals: 3 premolars, 3 molars (2-4). Pachygenelus has 6 post canine teeth. Morganucodon has 4 premolars, 4 molars. Juramaia has 5 premolars, 3 molars. Amphitherium has 5 premolars and 6 molars. So did certain mammals add teeth? Or did most lose teeth? Depends on where you start counting…

Look again at #4 above. 
Consider the survival advantage that Monodelphis presents: Six to eleven young born at a time feeding on up to 13 retractable nipples without a pouch, versus one joey in a pouch, the marsupial constraint. Female opossums, including Didelphis, often give birth to very large numbers of young, most of which fail to attach to a teat.

Paleogeography
O’Leary et al. state: “Our relatively younger age estimate for Placentalia means that there is no basis for linking placental interordinal diversification to the Mesozoic fragmentation of Gondwana [thus] Afrotheria did not originate in Africa.” The LRT provides a chronology by which placentals could spread worldwide before the breakup of Pangaea and Gondwana.

The O’Leary et al. cladogram
(Fig. 5) does not resemble the LRT. Gradual accumulations of derived traits are hard to find in the O’Leary et al. cladogram. They found the anteaters and sloths were the most primitive placentals and nested whales with hooved mammals, among other odd pairings.

Figure 5. Simplified version of the O'Leary et al 2013 cladogram showing placental relations exploded after the K-T boundary.

Figure 5. Simplified version of the O’Leary et al 2013 cladogram showing placental relations exploded after the K-T boundary.

The LRT provides
a gradual accumulation of derived traits unmatched by the O’Leary et al topology. This is the best test of tree topology validity as it echoes actual evolutionary events, micro step by micro step.

O’Leary et al. taxa not validated by the LRT

  1. Epitheria – all placentals sans Xenarthra.
  2. Sundatheria – (= Scandentia + Dermoptera)
  3. Afrotheria 
  4. Paenungulata
  5. Tethytheria
  6. Boreoeutheria
  7. Laurasiatheria
  8. Euarchontoglires
  9. Euarchonta

A postscript on testicles
According to slate.com, “Platypus testicles, and almost certainly those of all early mammals, sit right where they start life, safely tucked by the kidneys. Nearly all marsupials today have scrotums, Marsupials’ testicles hang in front of their penises.” In the opossum they are about where the ovaries are in females. In some placentals like “elephants, mammoths, aardvarks, manatees, and groups of African shrew- and mole-like creatures…retain their gonads close to their kidneys. Scrotums bounce along between the hind limbs (behind the penis) of primates, cats, dogs, horses, bears, camels, sheep, and pigs. Hedgehogs, moles, rhinos and tapirs, hippopotamuses, dolphins and whales, some [wait a minute, only some?] seals and walruses, and scaly anteaters have gonads inside and away from the kidneys.” You can read the various hypotheses of why testicles descend or not the slate website.

References
Wible JR, Rougier GW, Novacek MJ, Asher RJ 2007. Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary Nature 447: 1003-1006
O’Leary, MA et al. 2013. The placental mammal ancestor and the post-K-Pg radiation of  placentals. Science 339:662-667. abstract

ArchibaldEtAl.pdf
protungulatum-donnae website

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