Friday, 31 August 2018

Litter size in the tailless tenrec and a Jurassic tritylodont

Tailless tenrec (Tenrec ecaudatus) with litter
Photo (C) by H. Schütz. From Goodman, Benstead and Schütz
The Natural History of Madagascar
The record for litter size in mammals - 32 viable fetuses - is held by the tailless tenrec. The source for this is Bluntschli (Revue Suisse de Zoologie 1937; 44: 271-282). The tailless tenrec polyovulates and an even larger number of unimplanted blastocysts has been observed (Nicoll & Racey).

Small litter size is nonetheless typical for mammals and is currently in focus because of a remarkable fossil from the Early Jurassic (Hoffman & Rowe). This comprised a clutch of at least 38 perinates and the presumed mother in the genus Kayentatherium

The mammals derive from the synapsids and Kayentatherium is on a non-mammalian side branch called tritylodontids. The trees in Prothero (previous post) suggest they diverged from the branch leading to mammals way back in the Triassic. So the finding does not exactly pinpoint when a reduction in litter size started to happen. 

The abstract claims that 38 is "well outside the range of litter sizes documented in recent mammals." Except one might add in the tailless tenrec.

Friday, 24 August 2018

Aye-aye captain of its own raft

An aye-aye (Daubentonia madagascariensis) foraging
Joseph Wolf c. 1863 Wikipedia Commons (public domain)
We are used to molecular data shaping our view of evolution, so it is all the more delightful when a morphological study shakes the mammalian tree.  The pleasure is no less for it involving that strange Malagasy primate the aye-aye (Daubentonia madagascariensis).

It is widely accepted that Madagascar was colonized by mammals rafting across the Mozambique Strait (see previous post). The lemurs (Lemuriformes) and the aye-aye (within its own Infraorder Chiromyiformes) have hitherto been thought to be descended from a single founder.

New tree for strepsirrhine primates from Gunnel et al. 
Nature Communications 2018 (here) CC
Now a study in Nature Communications paves the way to a new scenario. The historical background is interesting. George Gaylord Simpson (a proponent of the rafting hypothesis) once proposed a new fossil species of strepsirrhine primate Propotto leakeyi from the early Miocene of Kenya. Subsequently this was reinterpreted as a fruit bat. Now Gunnell and co-authors have unearthed the specimens from the National Museums of Kenya and compared the morphology (especially the dentition) both to the aye-aye and to Plesiopithecus teras from a late Eocene site in Egypt.

Firstly, they show Simpson was right about Propotto. Secondly, they construct a tree (combining morphological and molecular data) showing the split between Lemuriformes and Chiromyiformes occurred in the Eocene. Thirdly, it is most parsimonious to assume two separate rafting events with the ancestor of the aye-aye drifting to Madagascar on its own raft. Finally, both rafting events are likely to have occurred in the Miocene, which explains the lack of fossils of earlier date.

Placentation in the aye-aye

Allantochorion of the aye-aye. From Hill & Burne 1922 (here)
Milne-Edwards briefly described the placenta of the aye-aye (C R Acad Sci 1884), but the first complete account is by Hill and Burne (here). They described the villous nature of the allantochorion and the presence of chorionic vesicles. This contributed to Hill's later characterisation of the "lemuroid stage" of placentation where the placenta is diffuse, non-deciduate and epitheliochorial (reviewed here).