by Greg Mayer
One of the standard things we learn about animals are their modes of reproduction: budding, egg-laying, live-bearing, etc. And one of the standard things we “know” about modes of reproduction is that mammals are live-bearing, and reptiles lay eggs. Neither of these things we “know” is true, though– they are generalities, with exceptions. The platypus and its cousins the echidnas are fairly well known as egg-laying mammals, but that many lizards and snakes are live-bearers is not well known. Lizards and snakes are actually quite adept at evolving viviparity: over 100 instances of independent (i.e. convergent) evolution of live-bearing are known among lizards and snakes, versus only a single (or perhaps two) instances in mammals.
For many years, our foremost student of reptilian live-bearing has been Daniel Blackburn of Trinity College in Connecticut. In a paper in press in the Journal of Morphology, he and Alexander Flemming of Stellenbosch University report the most mammal-like placenta yet found in a reptile.
In most placental reptiles, exchange of nutrients, gases, and wastes occur through juxtaposition of fetal and maternal tissues, but not by direct contact with maternal capillaries. In the African skink Trachylepis ivensi, they have now found that this does occur, a condition previously thought to occur only in mammals. Money quote:
Histological study shows that this species has evolved an extraordinary placental pattern long thought to be confined to mammals, in which fetal tissues invade the uterine lining to contact maternal blood vessels.
This species of skink is not very well known. Blackburn and Flemming did their histological studies on a small series of preserved specimens housed in the scientific collections of the Port Elizabeth Museum in South Africa.
h/t Dominic, Matthew Cobb
Blackburn, D.G. 2006. Squamate reptiles as model organisms for the evolution of viviparity. Herpetological Monographs 20: 131-146. (abstract)
Blackburn, D.G. and A.F. Flemming. 2011. Invasive implantation and intimate placental associations in a placentotrophic african lizard, Trachylepis ivensi (Scincidae). Journal of Morphology in press. (abstract)
Blackburn, D.G., L.J. Vitt and C.A. Beuchat. 1984. Eutherian-like reproductive specializations in a viviparous reptile. Proceedings of the National Academy of Science (USA) 81:4860-4863. (pdf)