The original mammal - egg or live young

Question

if we great apes share a common ancestor that would mean that if I were to hold hands with my mother, and she were to hold hands with hers and this went on for about 300 miles (according to Richard Dawkins) we would eventually get to the ancestor shared by modern day great apes.

So then if we were to do this for even longer, and a duck-billed platypus were to do the same thing, eventually we would reach a common ancestor between man and platypus. Would this mammal lay eggs or give birth?

If we keep going back we should eventually find a common ancestor for all modern day mammals, what I want to know is has such a creature been discovered yet and if not, what does the oldest traceable mammal look like?

Which came first, eggs or live young?

Answer 1

Short answer: Egg.

So far, the fossil record hasn't yielded direct evidences (that I know of) of the ancient mammalian reproductive mode. It would be extraordinary to find a fossilized "nest" from one of those early shrew-like creatures!

But based on other evidences, the consensus among specialists is that the earliest mammals retained the ancestral mode, which is egg-laying.

This is supported by the following:

** the primitive reproductive mode for the synapsid amniotes ancestral to the first mammals (which are also synapsids, since we all belong to the same clade or natural group) was oviparity.

** The earliest known mammals (both among living forms and in the fossil record), the Prototheria (monotremes - platypus and echidna), which are egg-laying.

Take a look at this cladogram:

http://scienceblogs.com/pharyngula/uploa...

Note that beyond the earliest mammals (Sinoconodon, Morganucodon, etc), the Monotremes stem from the earliest radiation, and that's one reason their reproductive mode is believed to be ancestral.

At the same time, viviparity may have been already present in the Multituberculates, a now-extinct group. This was postulated by researchers who studied the skeleton of these animals.

Incidentally, viviparity (live-bearing) is not exclusive to therian mammals. Many other animals, including sharks, lizards and snakes, are viviparous, which means that this condition has some adaptive advantages that were favored in these groups, so that it evolved independently several times.

Answer 2

the first 'mammal' were more like reptiles and lived in the devonian/silurian periods.. such animals as eurekasaurus..

and i would doubt that the monotremes evolved away from vivipariousness and instead reatined the trait

as such i will guess that it laid eggs

edit..

dykes i was mistaken in my comments of silurian reptile blahblahblah..

what i thought i was refering to were the mammal-reptile animals like Procynosuchus, of the permian period such things as this

http://www.nasmus.co.za/PALAEO/jbotha/Im...

and

http://blogs.discovery.com/.shared/image...

of the early triassic

i acknowlege my mistake..

Answer 3

<<the original mammal - egg or live young>>

The egg-laying habits of living monotremes indicate mammals originally laid eggs, as is the case for most vertebrates.

<<if we great apes share a common ancestor that would mean that if I were to hold hands with my mother, and she were to hold hands with hers and this went on for about 300 miles (according to Richard Dawkins) we would eventually get to the ancestor shared by modern day great apes.>>

You'd have to ask Richard Dawkins about that, assuming he said anything of the kind, which I doubt.  I've no idea what it's supposed to mean.

<<Would this mammal lay eggs or give birth?>>

Eggs.

<<If we keep going back we should eventually find a common ancestor for all modern day mammals, what I want to know is has such a creature been discovered yet and if not, what does the oldest traceable mammal look like?>>

The oldest possible mammals don't back to early during the Upper Triassic.  There's part of a skull from Texas called /Adelobasileus/, and a couple of isolated teeth from India.  From later in the Upper Triassic isolated teeth start turning up in Europe.  More substantial finds date from the Lower Jurassic of about 200 million years ago.  Lots of isolated bones, teeth and jaws have come from sites in Wales.  Southern China, meanwhile, has come up with a number of complete skulls, and Southern Africa has managed a couple of complete but squashed skeletons.

The most basal mammal known, at least according to my understanding of Mammalia, is called /Morganucodon/ - 'Glamorgan tooth'.  Remains come from Upper Triassic-Lower Jurassic deposits in Europe, China and the US.  Logic suggest it went to work on parchment-shelled eggs.  Live birthing developed later.  (Other understandings of Mammalia could push things back somewhat earlier, as indicated above.)

Answer 4

eggs came before live young.