Tuesday, 12 January 2016

Steller's sea cow

Reconstruction of Steller's sea cow (Hydrodamalis gigas)
By Emőke Dénes (Natural History Museum in London) (CC BY-SA 2.5)
When first described by Steller in 1741, the range of this sirenian was already restricted. Intensification of hunting led to extinction of the species within another 27 years.

A recent paper with a "stellar" cast of authors (here) uses molecular data to firmly establish the position of Steller's sea cow within crown Afrotheria; with additional morphological data it also shows shows how the species fits into the fossil record.

Steller's sea cow (Hydrodamalis gigas) belongs with the dugong (Dugong dugon) in Family Dugongidae whereas the three species of manatee belong to Trichechidae. Together they make up the Order Sirenia, which is sister to the elephant Order Proboscidea.

Endotheliochorial placenta of the Amazonian manatee
Fetal (fc) and maternal (mc) capillaries are present
Elephants have an endotheliochorial placenta. Therefore it perplexed us that Wislocki had described the placenta of the West Indian manatee (Trichechus manatus) as haemochorial. Therefore we reopened the matter by studying placentas from the Amazonian manatee (T. inunguis). We were able to demonstrate two sets of capillaries (shown above) and conclude this placenta to be endotheliochorial (here).

The allantoic sac of elephants and manatees has four chambers and we have shown this to be a shared, derived character of the superorder Afrotheria (here).

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