The fossil record shows that the climate of North America warmed and

IDENTIFYING THE PARADOX

The warm and dry climate was universal for all species of mammals of all sizes. Therefore, it was expected that all of them would be affected by the climate.
The line of reasoning here is: One might think that larger mammals which were likely physically more equipped to survive would survive better than their smaller counterparts. However, the opposite was witnessed. The smaller mammalian species survived and majority of the larger ones became extinct. This is the paradox that needs to be resolved.

RESOLVING THE PARADOX
Second line of reasoning
The paradox rests on the size of the mammalian species. It is the size which is expected to give the larger ones advantage over the smaller ones. But what if the size actually proves to be a disadvantage during warmer and drier climate?
Larger size means more food and water required for survival. If food and water became difficult to get during the period, then the larger mammals would be the first to get more affected.

ANSWER CHOICE ELIMINATION
A. Individual large mammals can, in general, travel further than small mammals and so are more able to migrate in search of a hospitable environment.
(This choice actually increases the discrepancy. Per this information, the larger mammals should have been able to survive. This choice is the opposite of the required answer.)

B. The same pattern of comparative success in smaller, as opposed to larger, species that is observed in mammals is also found in bird species of the same period.
(This choice simply tells us that such paradoxes also occur in other cases. It does nothing to actually resolve the paradox. This choice is out of scope)

C. The fossil record from the end of Pleistocene period is as clear for small mammals as it is for large mammals.
(The passage tells us what the records indicate. This choice merely supports the dependability of the records but does nothing to explain the paradox. Out of scope choice.)

D. Larger mammals have greater food and space requirements than smaller mammals and are thus less able to withstand environmental change.
(This choice is quite in line with the second line of reasoning provided above. Correct choice)

E. Many more of the species of larger mammals than of the species of smaller mammals living in North America in that period had originated in climates that were warmer than was that of North America before the end of the Pleistocene period.
(Per this choice, many more larger mammalian species -than the smaller ones- originated in climates warmer than the climate before the end of the Pleistocene period. We are not concerned with the temperatures and survival of mammals before the end of the Pleistocene period. If anything, this choice deepens the paradox by indicating that the larger species were better equipped to deal with warmer climates. Mostly out of scope)

Ans: D

(D) Larger mammals have greater food and space requirements than smaller mammals and are thus less able to withstand environmental change.

This explains why the large mammals extinct.

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