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Dietary factors & giant land mammals

I don't think the lack of large-dinosaur-sized land mammals is due to physiology, but more likely the result of the spread of grasslands and other angiosperm groups. Even dinosaurs would have probably been smaller if their environment had been dominated by angiosperms (which tend to have more sophisticated ways to deter the wholesale gobbling bigger animals would require).
Grasses in particular aren't terribly nutritious or easy to digest. And even large mammoths quickly evolved dwarf forms in environments where food became relatively scarce and/or less nutritious.
So I would say it is the nature of the food supply, including spreading grasslands as climates became drier. If the Mesozoic climate and flora (angiosperm-poor) had persisted into the Cenzoic, dinosaur-sized land mammals would probably have evolved. And larger vegetarians would have been able to sustain bigger mammalian carnivores (both marsupial and eutherian).
In the oceans the mammals (particularly whales) have gotten very large, but luckily they don't have to depend on angiosperms for the bulk of their energy. The flora has changed more drastically on land than in the oceans. Not that the nature of flora is the only factor at work, but I think it is a big one (and has a major impact on organisms further up the food chain).
From: "Matthew Bonnan" <mbonnan@hotmail.com>
Reply-To: mbonnan@hotmail.com
To: dinosaur@usc.edu
Subject: Waiting for a giant bird and dino physiology revisted
Date: Mon, 07 May 2001 22:02:47 -0000

David Marjanovic writes:

"> and they [birds] have IMHO not yet had a chance to grow
> to gigantic sizes."

I don't see the mystery here about the absence of multi-ton birds.

1) Most birds fly and so there has been an evolutionary "pressure" to keep
most of them small enough to get off the ground. I am aware of giant flying
condors, etc., but there is probably an upper limit to how big you can get
and still fly. Before I am pointed to large pterosaurs, keep in mind that
pterosaurs and birds have a different wing "membrane": one is skin and
tendon, one is feathers. Maybe this has something to do with how big one
group got over the other as far as flying vertebrates are concerned. I
don't know, but it seems like an interesting thing to look at. To fully
expose my ignorance, has anyone working on pterosaur and/or bird flight
explained size differences in terms of feathers vs. skin? Anything about
bats?? And let's not forget skeletal differences either. Hmmm ... I'm
interested now.

2) In the case of ground birds, their hindlimbs were already modified so
that they walk or run with bent knees all the time. This means that their
femur is constantly being loaded in tension, not in compression. As bone is
stronger in compression than tension, maybe there is a size limit to how big
you can get with bent legs. After all, the biggest animals we know of on
land have columnar limbs, not bent legs: check out elephants or sauropods.
In contrast, even moas have bent knees. Further, all of these big ground
birds had no easy option for returning to all fours. Whereas sauropods were
probably quadrupedal secondarily and able to modify the forelimbs of their
bipedal ancestors back to support structures, the wings of these giant
ground birds are pretty reduced.

Thus, size in birds is probably limited by two factors of their evolutionary
history: flight and bent knees. The contingencies of history can sometimes
limit the development of certain body plans. A good example of this is
turtles: they develop the shell early, are successful and radiate into
numerous forms, but in over 200 million years there have been no flying
turtles or running turtles. Evolutionary contingency is probably also at
work on limiting bird size.

Thus, we can wait for a giant bird to tern up (pun intended), or think about
this: the two groups of tachyaerobic endotherms living today do not approach
sauropod sizes, and even though some extinct mammals did get very large
(small sauropod size), dinosaurs have everyone licked in terms of average
size, hands down. Even factoring in the estimated masses of the biggest
extinct mammals, the average mass of most mammals is under a kilogram,
whereas the average mass of most dinosaurs was in the 10kg to 100kg range
(from Farlow et al., 1995).

Now, play devil's advocate with yourself. What is going on here? Why are
the dinosaurs getting so big and no one else? If physiology is the answer,
what's the deal with mammals and birds? How much time do they need to get
dino-sized? Or are we waiting in vain? Maybe dinosaur size has to do with
MANY factors, not just physiology, and maybe tachyaerobic endothermy alone
can't explain dinosaur success at large size. I'm not saying dinosaurs were
cold-blooded (boy, I miss that hyphenated word) gigantotherms. What I am
saying is that we have to consider many, many factors, including functional
morphology, before deciding that dinosaurs are gettin' really big simply
because they are warm-blooded (boy, I miss that hyphenated word, too).
Dinosaurs are DIFFERENT from mammals and birds in many ways, especially the
big ones, and we do these fantastic animals a disservice by trying to
shoehorn them into a mammal or bird pigeon hole (pun intended again).

I am very interested in WHY and HOW dinosaurs got so big on the average, and
why some became incredibly gigantic ON LAND. The minute we settle on a
single factor as the reason for dinosaur success at large size, we blind
ourselves to so much else. Sure, maybe dinosaurs were warm-blooded and that
was a part of their success. But that's the point: a PART. Life history,
reproduction, functional morphology, historical contingencies, the
archosaurian body plan, climate, etc., must have all played a part.
Otherwise, we should have gigantic land mammals and birds, right? Why don't
we? That's what's interesting to me and why I remain cautious about putting
so much stake in physiological models: they are a good start, but we
shouldn't stop there.

Matt "warm-blooded" Bonnan
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