Evrim Yazgin
Cosmos science journalist
Hundreds of 52-million-year-old eucalyptus leaf fossils show the tell-tale signs that they were munched on by leaf-eating insects.
The trail marks left on the leaves are identical to those found on eucalypts today in Australia. But these fossils were found in Argentina.
Today, there are more than 700 species of eucalyptus. These flowering plants – the majority of which are trees – are native to Australia and neighbouring islands in the southern Pacific. Native eucalypts are found as far north as the Philippines.
Among them are the largest flowering trees in the world – found in Australia’s southern-most island state of Tasmania.
Tens of millions of years ago, however, eucalypts were common throughout the ancient supercontinent Gondwana which began breaking up about 180 million years ago – a break up which lasted more than 100 million years.
Eucalyptus fossils have been found in New Zealand and South America. Eucalyptus is no longer native to these regions. It is possible that eucalypts even existed in Antarctica when the Earth’s temperature was much warmer, and the continent was not covered by ice.
The oldest eucalyptus fossils are from southern Argentina. The 52-million-year-old fossils come from the Laguna del Hunco formation which dates to the early Eocene epoch, 56–34 million years ago. The formation preserves an ancient eucalyptus rainforest.
New research published in the journal New Phytologist examined 284 ancient eucalyptus fossils from the formation which were compared to 10,000 modern examples of eucalyptus leaves which exhibit insect damage.
“The big stunning result is that every type of insect damage on the fossils survives today on Eucalyptus,” says lead author L. Alejandro Giraldo, a doctoral student at Pennsylvania State University in the US. “It suggests that the insects that were eating the Eucalyptus plants in the past tracked their plant food through time.”
Insect damage preserved in the ancient leaves include external feeding, galls and mines. Galls are tumour-like growths on leaves that result from insect feeding or egg-laying. Mines are tunnels of insect larvae.
“It’s just like today – if you go to a park and pick up a bunch of leaves, you’ll probably see chew marks, galls or mines,” Giraldo says. “We can see these same insect-feeding traces in the fossils, and by doing so we can take a glimpse into the ecology of ancient forests.”
“There is an evolutionary arms race between plants and insects,” Giraldo explains. “Plants evolve new defences, insects find a way to go around them to continue to feed, and this cycle repeats through time. This work shows nicely that the evolutionary association between herbivorous insects and their plant food can be stable through time.”
The insects which caused the damage on the ancient eucalypts haven’t been identified.
In fact, Giraldo says that an estimated 15,000–20,000 insect species feed on eucalyptus today and even the majority of those are yet to be described.
Giraldo believes that studying the fossils could help identify modern eucalypt-eating insects.
“What we do have are the locations of the modern plants we studied. So, if you’re interested in describing a potentially new insect species, you can go to this site and look for this plant, and it’s very likely that you could find a new insect species if you track the same feeding traces we observed in the fossils. I think that’s a neat perspective in how studying the past also allows you to better understand the present.”
