Joined: Sep. 2007
|Quote (mitschlag @ Mar. 09 2008,11:56)|
|Otto Heinrich, the cherry-picker:|
|Schindewolf, pp. 291-295 (continuing from the previous excerpt):|
...And yet, this [he means inexorable increase in size] is by no means always the case; extremely often it is just the opposite, that extinct, ancient animal forms are characterized by unusual size, and the layman is indeed inclined to imagine these, without exception, as gigantic monsters.
In fact, we know that among extinct tigers, bears, elephants, rhinoceroses, and so on, there are some extinct species that were considerably larger than those living today. A particularly conspicuous example is the mighty Baluchitherium, from the Oligocene of Asia, which is assigned to the rhinoceros group even though (like most ancient rhinoceroses) it has no horn on its nose (fig. 3.136). The shoulder height of this animal comes to about 5.3 meters, and the length of the torso is as much as 10 meters, making it one of the largest terrestrial mammals that ever lived. The enormous size of this animal is clearly seen in the comparison of a reconstruction of Baluchitherium with the Recent Indian rhinoceros, both shown to scale in figure 3. 137.
These examples, however, by no means represent a contradiction to our rule of phyletic increase in size, for the extinct forms in question are not the immediate predecessors of the smaller Recent species. They are only members of a broader, related group within which they represent the terminal forms of extinct collateral lines (fig. 3.138). To this extent, they thoroughly confirm the general rule that gigantic forms mark the end of evolution.
Unquestionable examples of a once-attained body size being secondarily reduced are almost unknown except in instances where such a reduction is succeeded by a thorough remodeling to a completely new typal structure, which, itself, begins again with small forms. The exceptions occasionally cited are probably only apparent, for in those cases it has not been shown that the forms with the supposed reduction in size really issued from larger ancestral forms of the same genetic lineage; only in such a situation would our rule be contradicted.
Accordingly, the evolution of size is, in general, irreversible. However, it is immediately clear that gigantic forms are indicators of dying lineages, for ultimately a point would be reached beyond which continued increase in size would be impossible for physiological reasons.
However, you are full of it, Schindewolf:
|Simpson, op. cit., pp.137-138:|
In this connection, it is known that many large animals of the past became extinct and are not the ancestors of their smaller living relatives. Mammoths were not ancestral to smaller elephants. (As a matter of fact, most mammoths were no larger than some living elephants, but a few were.) The elephantine ground sloths were not ancestral to the little living tree sloths. The dinosaurs were not the ancestors of the small lizards of later times. But this does not mean that forms that were the ancestors of living animals were not also somewhat larger than the latter at one time or another, and such does appear to be the case for some of them.
Some paleontologists have been so impressed by the frequent trend for animals to become larger as time goes on that they have tried to work it the other way around. If they find, say, a Pleistocene bison that is somewhat larger than a Recent bison (so-called Bison taylori, associate and prey of early man in America, is a good example), then they conclude that it is not ancestral to later bison because it is larger. You can establish any “rule” you like if you start with the rule and then interpret the evidence accordingly.
That last line is a keeper.
I don't think Schindewolf was of the opinion that all lineages always increased in size. I remember him talking about exceptions such as pygmy and dwarf species. He also talked at length, and provided septal and suture line data that showed that some lineages didn't change much at all for long periods of time. He does claim that there's a definite tendency towards gigantism towards the end of many lineages, and your quote form Simpson agrees with that:
|Some paleontologists have been so impressed by the frequent trend for animals to become larger as time goes on that they have tried to work it the other way around.|
Schindewolf might have overstated its extent, but I don't think that means his statements should be ignored altogether. Perhaps they should just be taken with a grain of salt.
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