"Pinnipeds evolved about thirty million years ago. They showed up first in the colder parts of the northern hemisphere, then in the Antarctic, then in temperate zones."
That's an interesting sequence, given that the continents then were about where they are now.
Maybe they evolved in both places independently, or maybe it took a couple million years for a pod of seals to make the journey from north to south, and then start dominating the local ecosystem.
Wikipedia says pinnipeds are monophyletic, so there's only a single evolutionary branch they originated from.
The travel question is much more interesting. Napkin math puts the distance between the polar circles at 12500 miles, the max swim speed of a seal at 25mph, giving a travel time of ~3 weeks of nonstop swimming as the crow flies. How does that happen? What would a pod of Arctic seals have eaten along the way, and what would have compelled them to make the trip out of familiar territory? How and why did they cross the temperate equatorial seas with a load of blubber?
Look up monkey raft theory. How did monkeys from Africa get to South America? Also roughly 30 MYA.
Life travels on the ocean a lot farther than people realize, and often species you'd find improbably.
Ants spread around the world by hitching rides on coconuts or waterbirds. This now means there's something like a global war among various ant colonies that invade each other this way.
I thought Wind, Wings and Waves to be a pretty interesting description of how species made it to Hawaii. Basically it either gets picked up in storms or large clumps of land make it across the ocean. Life only has to make the journey seldom or almost never and then take hold once to get to a new place.
It's my understanding the ant spreading was more related with increase in navigation than birds/floating. that'd be why the Argentine ant has spread everywhere in Europe in the last century rather than being ubiquitous since the dawn of time.
Could an iceberg with a few seal families on it have drifted from the Arctic to the Antarctic? Presumably in the tropics the seals would have had a bad time and gone hungry and overheated, but once they got to cooler waters again, they would have been surrounded by food they love with zero competitors.
The distance is not surprising, animal trackers have shown they routinely travel vast distances in the ocean [1, 2, 3].
All you need is a fluke event, once in a thousand years where they for some reason make, and survive, this trip (I say thousand to give us some scale, could be actually once in a million years).
[1] https://www.ocearch.org/tracker/detail/nukumi
[2] https://www.ocearch.org/tracker/detail/dr-brent
[3] https://www.wur.nl/en/show/seal-telemetry.htm
Agree, this stuff is fun to imagine. Check out the many species that landed (and then kept evolving in isolation) in Hawaii. Seals, many birds, etc.
Well the ocean was filled with free floating ammonites, so maybe food wasn't an issue. Another commenter suggests a volcanic winter to lower the sea temps enough for the seals to cross.
It takes one volcanic winter for them to cross the oceans and emerge on the other side, without being fit for the warm waters.
It is possible that they fed on extremophiles which thrived in really cold regions and later evolved to feed on other food sources.
That’s the sequence of notable populations; individuals did who knows what.