Several times in the evolution of reptiles bipedal forms have phylogenetically preceded marine forms. Yes, marine forms. It’s bizarre, but true.
Here’s the list, more or less. Did I miss any?
Huehuecuetzpalli (bipedal capable) > Dinocephalosaurus (marine)
Huehuecuetzpalli, a small speedy lizard with short fore limbs and long hind limbs evolves to become Dinocephalosaurus, a giant long-necked sit-and-wait predator via Macrocnemus, something in between.
Eudibamus (biped) > Claudiosaurus (marine)
Eudibamus, a small lizard-like diapsid with a long neck and long hind limbs evolves to become Claudiosaurus, a long-necked marine undulating marine predator of tiny prey. Thereafter descendants evolve to become ichthyosaurs and plesiosaurs.
Storks (biped) > Penguins (marine)
Flying bipeds evolve to fly underwater.
Australopithecines (biped) > Humans (Homo, marine capable)
(grassland roamers evolve to become able to swim)
Evidently it all comes down to
Conscious control of breathing — if you want to become a marine animal you have to hold your breath.
Not sure what the bipedal connection is with reptiles, because lots of marine reptiles never had a bipedal phase. I just wanted to throw the idea out there.
More on Humans:
Anthropologist Elaine Morgan on TED talks about the origin of bipedal humans from aquatic apes here. Step-by-step: Apes all walk bipedally when they cross streams. For insulation humans have fat migration to a subdermal position, that’s why obesity is possible for humans, not for apes. The nostrils open ventrally, keeping water out by air pressue in humans. Ape nostrils don’t keep out water, except for the proboscis monkey (the most aquatic of primates). Ability to speak comes by way of the conscious control of their breath, according to Morgan.