Are there any examples of ovoviviparous species that are developing placenta?

Question

This could be alive currently or in the fossil record. Somewhere along the way, egg laying creatures had to give way to placental creatures. There should be a species that has egg and placental characteristics.

Answer 1

http://pandasthumb.org/cgi-bin/mt/mt-sea...

Since the placenta is soft issue, you're not going to find fossil evidence of its earliest forms.  The creationists make a lot of fuss about this rather obvious fact. However, this link will take you to current peer reviewed research with references showing what current scientific thinking is on this subject, using results derived from molecular biology.

Answer 2

Yes.

There are more examples among the reptiles (sometimes called "placental mammals") than you might think.  These are basically egg-layers who retain the egg for longer and longer time, until the egg begins to act more and more like an internal placenta, the yolk slowly getting replaced with a nutrient link with the mother.

E.g. the Brazilian skink, Mabuya heathi: "This species has evolved a fascinating strategy for raising offspring. Instead of laying eggs like many other lizards, this species retains the eggs within its body. Just as in mammals, the eggs have no yolk to nourish them, and so they implant on a chorioallantoic placenta within the mother's body. The chorioallantois lies against the uterine mucosa, which has large, binucleated cells and microvilli for nutrient transfer to the developing embryos. There are 9-12 eggs produced per brood, and gestation lasts about 9-12 months."

Or the lizard, Pseudemoia pagenstecheri: "one of the most placentotrophic reptiles studied to date":.

Or the garter snakes, Thamnophis ordinoides and Thamnophis sirtalis.

And there is all range from ovoviparity to viviparity (placental links to live-born offspring) among sharks.   Among the sharks with partial placentas are:  hammerheads,  bull sharks, tiger sharks, the basking shark and the smooth dogfish.

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P.S. to codej: Since you use your belief that there are no such examples as evidence that evolution is false ... does the fact that there *are* examples now act as evidence that evolution is true?

Answer 3

Living, none that are known.

In the fossil record it is impossible to know.  Placentas are soft tissue that would not be preserved through fossilization.

Answer 4

There are not. I find this to be yet another valid sign that we were created as the species which exist today, maybe bred out a little (a lot different from developing entirely new sets of functional genetic information - actually, the opposite: losing it).