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There are several examples where the largest animals were killed off preferentially due to a major ecological shift:

  • The KT extinction event
  • The Holocene extinction, as well as the major megafauna humans killed off

The common ancestor of modern mammals is thought to be "shrew-like" in size, and it differentiated into most of the large animal species in the world today. I imagine that a similar statement can be made for dinosaurs. In general it seems that a major extinction event is likely to be best weathered by smaller creatures which can later, over time, increase in size to fill the niche that larger animals, and evolution dictates it will be profitable to do so.

Does this limit large animals to only the branches of the tree of life? Any given set of (Multicellular) species obviously share a common ancestor that is much smaller than them. Is it possible for many species to share a common ancestor that is considerably larger than any of them?

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To give another example: insects from the Carboniferous period were huge (e.g. Meganeura). Nowadays, apart from some stick insects, I don't think there are other big terrestrial arthropods... –  user132 Dec 15 '11 at 17:41
    
@J.M. That would be a counter-example to my examples, and it could answer the question as yes, they do. But we also need to establish that modern insects actually evolved from the past's large insects, instead of evolving from small insects that lived among the large insects. –  AlanSE Dec 15 '11 at 17:46
    
I'm not up to date with whether Meganeura is phylogenetically related to modern-day dragonflies, so maybe we have to wait for a (palaeo-)entomologist to chime in on that subject. –  user132 Dec 15 '11 at 17:54
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Define "often"; island dwarfism is a seemingly-repeatable effect. –  Nick T Dec 15 '11 at 17:54
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As written, it is confusing as to what you are asking. In one sense you seem to be asking about the environmental factors that would drive the evolution of size but since evolution is never progressive then there can be no expectation of a evolution-based temporal trend. I am not sure if the idea that large animals are more vulnerable to extinction is completely accepted, but if we accept it, then it reduces your question to a truism if applied to mass extinctions - if only small animals remain then non-directional evolution would give the impression of progression. –  KennyPeanuts Dec 15 '11 at 18:05

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Your question brings up several important issues with regards to the evolution of body size. The rationale for concluding that the ancestral mammal had a small body size is that all of the taxa in that area of the tree tend to be small. In contrast, if all of those taxa had been cow-sized, then the most parsimonious conclusion would be that the ancestral mammal was cow-sized.

Identification of ancestors is difficult

Identification of a fossil species as the most recent common ancestor of a pair of sister taxa is exceedingly difficult. The way that different species are diagnosed in fossils in by their unique derived characteristics (autapomorphies). So if a fossil is found with no autapomorphies, then it is plausibly an ancestor (i.e., at the split between two sister taxa). Some would argue that if you don't find any autapomorphies, then you just haven't looked closely enough.

Cope's Rule

The idea that lineages tend to evolve toward larger body size is known as Cope's Rule, named for the paleontologist Edward Drinker Cope. The success of Cope's Rule has been highly variable, with some lineages following and others not (Polly, 1998).

Some clades following Cope's Rule

Some clades not following Cope's Rule

Note that many results depend on the methods used.

Evidence from experimental evolution

Several studies have addressed body size evolution in vertebrates both directly and indirectly. MacArthur (1944a, 1944b) directly selected for large and small body mass in laboratory mice. Within 8 generations, mean body masses had increased or decreased by over 50%. Even when body mass is not directly selected upon, mass can change in a correlated way. Selection for high levels of voluntary activity has led to an approximately 25% reduction in body mass.

Based on these studies, evolution of body size (larger or smaller) is certainly possible within relatively few generations.

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i'd heard that miniaturization of animals is also a recognized effect of subpopulations on islands. gigantism can also occur, but is the result of the organism differentiating into new niches. the hobbits are examples of this effect, do i remember right? –  shigeta Dec 15 '11 at 20:50
    
It probably goes equally both ways. Examples of miniturization are balanced by examples of gigantism (e.g., lots of South American taxa during its "island" period). –  kmm Dec 15 '11 at 22:17
    
yes - i think its a matter of niches being open for just a few critters to fit into and sizes can change drastically. in crowded systems, the time it takes to change size would be prohibitive. –  shigeta Jan 16 '12 at 17:35

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