I’m going to crowd source this one,
but I think I covered all the bases here. In this subset of the large reptile tree (LRT, 1165 taxa) I’ve divided placental mammals born helpless (blue) from mammals born able to walk, swim and see (pink). I’ll need your help if there are any exceptions, like pangolins, that I missed one way or the other. Fossils are colorized based on phylogenetic bracketing.
Figure 1. Newborn mammals are born either helpless, like humans, or able to keep up with their mother, like horses. I think I located the split correctly here. Let me know I missed a few. Fossil taxa are colored based on phylogenetic bracketing.
Marine taxa need to be ready to go from the first minute.
Apparently so do the large plant-eaters ( including ant and copepod eaters), beginning with long-legged former tree shrew, Onychodectes.
Dens and nests
are associated with basal mammals, like us. Not so much with the derived herbivores (and anteaters) of the plains and forests. All of them get milk from their mothers before they start to dine on meat, plants, ants and copepods. Some of them have to keep up with here. Some of them have to keep up with her underwater.
there also seems to be a behavioral node at Maelestes in which succeeding taxa are all leaving the trees for good. Of course, that also happens exceptionally with the various mole and aquatic clades in more basal mammals.
We have a pregnant
plesiosaur (Fig. 1; O’Keefe and Chiappe 2011; LACM 129639; Late Cretaceous, 78 mya)…
Figure 1 Pregnant Polycotylus (LACM 129639) from O’Keefe and Chiappe 2011.
and a pregnant primate (Fig. 2) very dear to my heart.
Figure 2. My daughter Stephanie one week before giving birth to grandson James (nickname: Jet) and about three years ago.
Being a mom goes way, way back
In our lineage, first cells stuck together, flagella out (Fig. 3). Then four cells stuck together. Then eight. Ultimately hundreds stuck together creating a sphere, or blastula. And little blastulas formed inside until they were large enough to break free.
Figure 3. Blastula from the book, “From the Beginning” by Peters 1991.
Plesiosaurs and primates capable of understanding prehistory
followed shortly thereafter. The basics of being a mother haven’t really changed much in the last few billion years.
O’Keefe FR and Chiappe LM 2011. Viviparity and K-selected life history in a Mesozoic marine plesiosaur (Reptilia, Sauropterygia). Science. 333 (6044): 870–873. doi:10.1126/science.1205689.
Peters D 1991. From the beginning – the story of human evolution. Little Brown. 128 pp. Online here.