哺乳類は、恐竜絶滅後に、そのニッチを埋めるように繁栄したことが知られていますが、その大きさは、およそ2500万年で1000倍ほどにまで進化したようです。

Size of mammals exploded after dinosaur extinction Researchers demonstrate that the extinction of dinosaurs 65 million years ago made way for mammals to get bigger - about a thousand times bigger than they had been. The study, which is published in the prestigious journal Science, is the first to show this new pattern of increased body size of mammals after the exit of the dinosaurs.

恐竜がいた当時、哺乳類はせいぜい10キログラム程度がせいぜいでした。それが、マンモスのような大きなものにまで進化しました。下記は日本語の記事。

ほ乳類は恐竜絶滅後、いかに大型化していったか サイエンス誌

ニッチが生まれれば、そこを埋める生物が登場する。仮にゾウがいなくなれば、きっと別の動物がそのニッチを埋めることになるのでしょう。オリジナル論文は下記。

The Evolution of Maximum Body Size of Terrestrial Mammals The extinction of dinosaurs at the Cretaceous/Paleogene (K/Pg) boundary was the seminal event that opened the door for the subsequent diversification of terrestrial mammals. Our compilation of maximum body size at the ordinal level by sub-epoch shows a near-exponential increase after the K/Pg. On each continent, the maximum size of mammals leveled off after 40 million years ago and thereafter remained approximately constant. There was remarkable congruence in the rate, trajectory, and upper limit across continents, orders, and trophic guilds, despite differences in geological and climatic history, turnover of lineages, and ecological variation. Our analysis suggests that although the primary driver for the evolution of giant mammals was diversification to fill ecological niches, environmental temperature and land area may have ultimately constrained the maximum size achieved.