Fossil Micro-monkey the Size of a Hamster Discovered in Amazon Rain Forest

Peewee primate, classified by one single tooth the size of two pinheads, doesn’t seem to have been a gummivore like today’s South American micro-monkeys

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Ruth Schuster
Ruth Schuster

A micro-monkey about the size of a hamster has been found in a fossil bed in the Peruvian Amazon, scientists reported late last week.

The one micro-monkey toothCredit: 3D scan by Duke SMIF

Parvimico materdei, or “Tiny monkey from the Mother of God River,” lived 18 million years ago (the Early Miocene). If we ever find more of it, the peewee primate could shed light on monkey evolution in the New World after its arrival to the continent from Africa some 40 million years ago.

The extraordinary story of its discovery — a paean to patience if ever there was one — was reported in the Journal of Human Evolution.

The paleontologists didn’t find the tiny fossil monkey as such. After attentively sieving 907 kilograms (2,000 pounds) of sediment recovered from a river bank, which contained hundreds upon hundreds of fossilized bones from other animals, they spotted a tooth the size of two pinheads. How, they did not reveal. 

“It’s by far the smallest fossil monkey that’s ever been found worldwide,” said first author Richard Kay, a professor of evolutionary anthropology at Duke University and part of the team that made the extraordinary discovery, together with people from the National University of Piura in Peru.

Note that it isn’t the smallest monkey ever, period. The pygmy marmoset, which actually still exists, is probably about the same size or even smaller — “but barely,” Kay says.

Like this long-dead Parvimico, the pygmy marmoset lives in the South American rainforests and weighs about 100 grams — about as much as a tomato or a standard bar of chocolate.

That may be where their similarities end. The subgroup of marmosets — the pygmy marmoset and common marmoset — are unique in the primate world for being gummivores. They live off sap sucked from trees, augmented by the odd bug for dessert.

Parvimico wasn’t a gummivore, the researchers postulate, based on the size and shape of the one tooth they found. They think it ate fruits and insects like other monkeys do.

“Primate fossils are as rare as hen’s teeth,” said Kay, who has been doing paleontological research in South America for nearly four decades.

The find fits neatly into a hole in the fossil monkey record, though not much else can be said at this point.

What else can the tooth tell? Not much. It is thought that monkeys spread from Africa to South America around 40 million years ago, and from 31 million years ago the fossil record went dark.

Mouse lemur, a tiny cousin of primatesCredit: Peter Pröpper / Bikeadventure

The upshot is that, over millions of years, over 150 monkey species developed in South America, some of them impressively large. We do not know how their evolution moved along: there is a huge hole of tens of millions of years in the fossil evidence.

Not all the monkeys in the New World were small. In 1996, the fossil remains of a large monkey with impressive dentition that probably weighed betweenbetween 20 to 25 kilograms was found in Bahia, Brazil. It may have been a predecessor of today’s howler monkeys, which can weigh more than 7 kilograms.

It bears adding that on the other side of the world, on the island of Madagascar, lives the extant mouse lemur, which maxes out at just 30 grams.

Lemurs and monkeys seem to have arisen from a common ancestor some 60 million years ago, though an alternative theory has them arising from the loris. Anyway, lemurs are distant cousins of monkeys and the mouse lemur weighs only about a third of the postulated adult weight of Parvimicos. Also, in contrast to the finicky marmosets supping on tree sap, the mouse lemur will eat anything it can fit in its tiny, tiny mouth.

Pygmy marmoset
Pygmy marmoset: The newly discovered species of fossil micro-monkey may have looked similar.Credit: Edwin Butter /