Citrus fruits might have originated in Australia, New Caledonia and New Guinea around 30 Mya. North-eastern India and Burma - or China - have been suggested, with a demarkation line ("Tanaka line") from Hanoi to Ledo: to the south of this line the citrus evolved into an acidic fruit, to the north it evolved into a sweat one. Citron is considered as the oldest extant citrus.
(Trends in Genetics 17(2001)536; Citrus genetics, Breeding & Biotechnology, I Khan, p. 20)
...Recent molecular studies indicate that the closest relatives to the citron - long-considered to have originated in India and one of the parents of the lemon - are species from New Ireland (off eastern New Guinea) and others from New Caledonia. Conventional wisdom holds that citrus evolved in Southeast Asia but this is based on ignorance about what constitutes a true species, the relationships between plants in Asia and Australasia, and how the two landmasses were related. There are about 25 true species of citrus and some 50% of these are from New Guinea, New Caledonia and Australia, while most commercial species and cultivars originated in Asia. (Mabberley)
The genetics of citrus is bewildering. For example, lemons are hypothesized to be (i) a hybrid of citron and lime, (ii) a trihybrid of citron, pomelo, and finger lime (iii) a hybrid of sour orange and citron, and (iv) a fruit of polyphyletic origin. The most basal of citrus fruits tend to have thicker rinds, and it seems that such rinds are ancestral. An unknown proto-citrus fruit, perhaps on a tropical island, has developed thick, waxy peel around the flesh, for reasons unknown.
Perhaps the animal for which it was designed is long gone. Or maybe (given its island origin) it was something else and truly uncommon: citrus are good at floating and the "fingers" might've worked as a lure for fruit-eating fish (that do not care about the acidity). There are examples of fig tree seed dispersal by tropical fish living in the streams in the rain forest,
Pirhaha fish, actually, subsist on seeds and fruits falling into water and flooded forests.
There are also fruit-eating reptilians (eg, Brazilian caimans) that would not mind acid. Bright color, strong smell via hydrophobic scent molecules make sense from the standpoint of luring an aquatic animal.
Maybe the proto-citrus targeted some extinct fish on the islands? Why do we peel oranges?
Buddha hand citron, one of the basal citrus fruits