November 23rd, 2007

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Can someone explain this to me?

Hearing about the giant sea scorpion claw has raised again for me one of these questions I can never figure out.

I was always told that for land creatures, and even for sea creatures (although bouyance helps), there are physical limits on how big a critter can get. The thickness and strength of bone needed to support more weight start to hit natural limits. And there is a huge trade off for energy, Even a very efficient organism, partiuclarly one that is in any way self-regulating, needs a lot of energy relative to its body mass. Yes, the larger the creature, generally the greater the ability to store energy and the smaller the amount of raw material needed relative to body mass. Thus horses eat far less then birds of measured as a percentage of body mass, but not on an absolute scale.

Creatures with exoskeletons, like most arthopods, have an even worse time in the trade off of weight to muscle and therfore there is a signifcant limit on the size they can grow. This is one reason why arthorpods tend to be small (like insects) and even the larger ones that live in the sea grow slowly and are not nearly as large as the critters without exoskeletons.

This trend away from gigantism -- especially for land anaimals -- is one of the reasons why we have a relative lack of "megafauna" on land, and even in the sea giant creatures are the exception rather than the rule. Mamoths in the ice age were a somewhat notable exception, because suddenly the ability to conserve body heat by having a much smaller surface area to mass ratio became a significant factor in favor of survival. After the ice age, most critters found it easier to survive as smaller, less prone to injuring themselves with a fall, and needing less to eat. Plus they could avoid those biped hunters better.

But the period prior to the mass extinction of the dinosaurs is marked by a preposterous prevelance of gigantic creatures. Now environmental factors probably mitigated some of the issues that make this less succesful as a survival mechanism today. In softer ground, or in large inland seas, the stresses on giant skeletons would be less. Even if dinosaurs were warm blooded, they would probably require less self-regulation in the gentler climate. But none of that should translate into land creatures as large as was apparently commonn, or sea scorpions able to exceed the current size limit constraints.

Has anyone looked at answers to this question? There must be a literature on it somewhere.