Evolution 'favours bigger sea creatures'

Reading down through the article kindly submitted here after first reading the previous comments, I see, "Certain key events, also highlighted by the study's authors, deliver an extra boost. Most obviously, at the stages when land animals like reptiles and, later, mammals returned to the water, the size graph lurched upward.

"It was the return of mammals, after all, that eventually produced the modern giants of the sea: whales.

"'You can see in the data, there is this trend to increasing size - but a real kick comes when you have these air-breathers re-entering the water,' Dr Berenbrink told the BBC.

"'Then you have a sort of a step change in size.'

"Breathing air delivers extra oxygen to an animal's tissues, he explained, allowing them to sustain bigger bodies beneath the waves."

Oxygen-delivery systems indeed have to be crucial for animal life on land or in the sea. Fish and their gills simply can't bring in as much oxygen for an energetic large-mass lifestyle as whales and their lungs can. It would be interesting to find out how much whale blood deoxygenates as it flows through the tissues of the largest whales. The duration of movement on one breath for whales is already quite impressive compared to what most land mammals can achieve, so whales probably have some good adaptations for efficient use of oxygen from each breath.

I have a dream, that someday a science editor will read an article in which the word "average" appears and then suggest that the word "median" should also appear.

TFA: "Today's tiniest sea critter is less than 10 times smaller than its Cambrian counterpart, measured in terms of volume; both are minuscule crustaceans."

Presumably we're talking about Stygotantulus stocki, which is 94 µm long. That's already smaller than a lot of bacteria, so I'm not sure how much more it could possibly shrink? There is an effective lower bound here.

TFA: "But at the other end of the scale, the mighty blue whale is more than 100,000 times the size of the largest animal the Cambrian could offer: another crustacean with a clam-like, hinged shell."

Leaving aside the apples-to-rutabagas comparison of clams and mammals, is there any physical reason why whales are the largest animal we could imagine? They're big, but so is the ocean. There is no effective upper bound in sight.

Average is not a meaningful statistic in this case.

Reminds me about this Memory Palace (a short podcast) about lobsters: http://thememorypalace.us/2010/08/episode-33-lost-lobsters/ – not very long ago they used to be gigantic compared to now. Evolution of the moment often favors animals that aren't as easy prey for humans.

Somewhere else I read, I think a comment in AskScience about how insects used to be gigantic, an evolutionary biologist noted that over time species evolve to become larger. I would infer that given a set of ecological constraints, bigger is usually better for intraspecies competition (having already settled upon a successful niche for the species in general). But then things change, the species itself must compete anew as the ecology changes, and there can be resets, reverting back to smaller more efficient forms.

Another thing alluded to in the article is that there's some complexity to simply maintaining larger forms. E.g., breathing air is more efficient if you have to bring in large amounts of oxygen. The crustacean line simply hasn't explored these areas. So even if there were always advantages to larger forms, not everything can just get bigger. But it always leaves the question, what about dinosaurs? So odd that the only true remnant seems to be birds that went down such a different path.

I like the insight that air breathing animals have stronger energy input systems than those with gills. Respiration in mammals is an energy system evolved for land, where the constraints involve 5 degrees of freedom and normal g. The ocean has 6 degrees of freedom and reduced g. So in the case that an organism's niche can be improved by evolving to be larger and they have an energy system which can support more flesh, doing so is easier in the ocean than on land. It's true for all ocean animals but it's especially true for mammals.

"larger species likely took advantage of being able to move faster, burrow better in sediment, or eat larger prey" -- larger species burrow?

I'm suspicious - maybe larger animals didn't fare so well as fossils? Or maybe atmospheric oxygen has increased or something, that directly drove evoloution. The claim that larger is better is groundless without some engine to drive that claim.