Otherlands, Thomas Halliday (excerpt)

[kaberett]

Island dwarfism, one half of the general rule that island animals tend towards a medium size, was first noted in a Cretaceous fossil site in Haţeg, Romania. At the time the limestone of the Gargano caves was being laid down beneath the European seas, Haţeg was a largish island, and housed dwarfed dinosaurs. Their small size was thought to come from the lower resources of islands, with enormous creatures unable to survive on the limited nutrients available. This is not limited to creatures as big as dinosaurs. Over time, in the absence of ordinary predators, many large animals whose size would otherwise offer protection against predation -- such as deer, and, on other islands, hippopotamuses and elephants -- become smaller as food is scarcer. Small animals, which cannot store energy or water as easily, become larger, aiding the survival of the population through periods of scarce resources. [...]

On different Mediterranean mountains, the niche of small mammalian herbivore has been filled by supersized or shrunken versions of whatever happens to have colonized the island upon isolation. On Gargano, there are the herds of Hoplitomeryx. On Mallorca, a minute goat with a disconcerting forward-facing stare prunes the box shrubs. Box is notoriously toxic, containing large amounts of alkaloid compounds that normally deter predators. Myotragus has a behavioural solution to this toxicity, however: it eats small quantities of clay from the riverbed, which neutralizes the toxic alkaloids in the leaves. This abrasive mud antidote wears down their teeth, so they have evolved rodent-like ever-growing incisors, and molars with very high crowns, which explains the meaning of their name, 'mouse antelope'. The pressures of island life often produce such unusual responses. Physiologically, Myotragus is even rather unlike most mammals. To avoid problems with fluctuating nutrient supplies, the dwarf goat can vary its metabolic rate. They grow slowly, speeding up only when times are good, exactly as ectotherms, or 'cold-blooded' creatures do. On Menorca, the role of medium-sized herbivore is filled by a giant rabbit, Nuralagus, wombat-like in its hapless, hopless, rollicking gait and tumbleweed figure.

(Chapter 3, Deluge, ~5.33 million years ago; Wikipedia links my own, because I don't love you enough to type up the actual book's actual list of sources; more generally, see allopatric speciation)