12, Arnoldia 78/3 « February 2021 
the nuts look as fresh as if they were only a few 
years old. 
Walnut family species with large, animal- 
dispersed fruits are only part of the story. Wing- 
nuts (Pterocarya)—a genus that is now known 
for six extant species—were once dominant 
trees here in Central Europe along rivers and in 
mountain slope forests. These are ancestors of 
the species we now call the Caucasian wingnut 
(P. fraxinifolia), which today runs wild in parks 
and gardens in Central Europe, its root sprouts 
forming dense stands. Some horticulturists 
have argued that we should cease planting this 
species in our gardens, given these invasive 
tendencies, but based on the fossil record, we 
could also view the wingnut as a returnee from 
another era. After all, wingnut leaf fossils in 
the Stuttgart region were found in sediments 
of the Holstein interglacial and date back only 
325,000 years. The few remaining populations 
of this once widely distributed species are 
increasingly threatened in their last refuges in 
the Caucasus. Wheel wingnuts (Cyclocarya) 
and platycarya—both unusual wind-dispersed 
genera now found only in East Asia—are also 
represented in the fossil records in Europe. 
The reason the walnut family went extinct in 
Europe while some species meanwhile survived 
in North America and East Asia is related to 
the geographical shape of the continents. Here 
in Europe, the Alps and the Mediterranean Sea 
form a barrier for the north-south migration 
of plant species. In cold periods, trees could 
survive only in the southernmost corners of 
Europe; therefore, while in America plant spe- 
cies could migrate according to climate condi- 
tions, many European species died out with 
every cooling and warming. The fossil record 
indicates that wingnuts survived this back and 
forth the longest of all Juglandaceae, but in the 
end, they vanished irretrievably, just like the 
European magnolias (Magnolia), kiwis (Actin- 
idia), and sweetgum (Liquidambar). Other gen- 
era of woody plants, including maples (Acer) 
and ashes (Fraxinus), are now represented in 
Europe with only a few species but had much 
greater diversity before the Pleistocene ice ages 
that started about two and a half million years 
ago. The diversity of these genera in Europe was 
similar to their modern-day representation in 
North America and Asia. 
The fossils reveal more than former distri- 
butions and long-extinct species—the record 
also documents how the walnut family evolved 
from an entirely wind-dispersed family to one 
with the charismatic nut-bearing species that 
we know today. Some of the oldest fossils of 
Juglandaceae fruits originate from the United 
States. Fruits of a wheel wingnut named Cyc- 
locarya brownii have been found in different 
sites from the Paleocene, occurring shortly 
after the K-T boundary, the geologic marker 
that separated the Cretaceous and Paleogene a 
good sixty-five million years ago. This event of 
mass extinction was both the end of the era 
of dinosaurs and ammonites and the beginning 
of a new chapter for the walnut family. 
Cyclocarya looks very typical for early mem- 
bers of the family, especially since its fruits are 
spread by the wind and not by birds or mam- 
Fossils document the former abundance of the walnut family in Central Europe, where no members of the family 
naturally occur today. Hickory (Carya) fossils, shown above, were collected from sediments in the Rhine Valley, close 
to Strasbourg, France, and are around five million years old. 
