A fossilised leafy branch from the early Eocene (54 to 48 million years ago) found in Patagonia and described in 1941 is often cited as the oldest bamboo fossil – but it now appears that it is no such thing,
A new examination by Peter Wilf, from Pennsylvania State University, US, reveals, he says, that Chusquea oxyphylla was actually a conifer.
That’s significant, because it has been the main fossil evidence for a Gondwanan origin of bamboos. The oldest microfossil evidence for bamboo in the Northern Hemisphere belongs to the Middle Eocene (48-38 mya), while other South American fossils are not older than Pliocene (5.3-2.8 mya).
Wilf notes that over the years some scientists have actually doubted whether the fossil was bamboo – or even a grass species for that matter – but little has been published.
Most researchers have only had a chance to study a photograph found in the original publication from 1941. In his recent work, however, he studied the holotype specimen at Argentina’s Museo de La Plata – and came up with some clear findings.
“There is no evidence of bamboo-type nodes, sheaths or ligules. Areas that may resemble any bamboo features consist only of the broken departure points of leaf bases diverging from the twig,” he says.
“The decurrent, extensively clasping leaves are quite unlike the characteristically pseudopetiolate leaves of bamboos, and the heterofacially twisted free-leaf bases do not occur in any bamboo or grass.”
Instead, Wilf links the holotype to the recently described fossils of the conifer genus Retrophyllum from the same fossil site – the prolific Laguna del Hunco fossil lake beds in Argentina’s Chubut Province.
It matches precisely, he says, the distichous fossil foliage form of Retrophyllum spiralifolium, which was described based on a suite of 82 specimens collected from Laguna del Hunco and the early middle Eocene Río Pichileufú site in Río Negro Province.
So if Chusquea oxyphylla has nothing in common with bamboos, it needs a new name, Wilf says.
Preserving the priority of the older name, he combined Chusquea oxyphylla with Retrophyllum spiralifolium to come up with Retrophyllum oxyphyllum.
Perhaps more complicated, is that researchers now have to further rethink the story of the late-Gondwanan vegetation of South America.
“Chusquea joins a growing list of living New World genera that are no longer included in Paleogene Patagonian floras, whose extant relatives are primarily concentrated in Australasia and Malesia via the ancient Gondwanan route through Antarctica,” Wilf writes in a paper in the journal Phytokeys.