Approximately 145 million: That’s the number of specimens – including plants, animals, minerals and human artifacts – curators estimate are held in the Smithsonian National Museum of Natural History. However, these estimates do not reflect the billions of tiny individual specimens contained on microscope slides – thin pieces of glass that fix objects in place for observation – each representing a record of a species at a specific place and time.
Microscope slide collections are an underused part of natural history collections because they are small, fragile and generally not well cataloged. One slide is usually recorded as a single specimen, even though it may contain hundreds of thousands of identifiable samples. They play a significant role in documenting life both present and past, and they are also a core educational resource for training future scientists.
Our team of plant paleontologists and evolutionary biologists use microscopy techniques to extract the full potential of natural history collections. In our recently published research in the journal PLoS One, we developed a way to digitally image whole microscope slides and make the specimens they contain available to scientists and students around the world.
Unseen troves of specimens
The Denver Pollen Collection contains about 70,000 slides of fossilized pollen extracted from rocks of many geological ages. The collection, now housed at the Smithsonian National Museum of Natural History, represents over 60 years of effort by scores of geologists and paleontologists working for the U.S. Geological Survey, gathering specimens from all over the continental U.S., Alaska and many other parts of the world.
Presented with one of the most complete fossil records of plant life in existence, scientists have used this collection to understand how vegetation and climate changed over geological time.
A snapshot of the Denver Pollen Collection. Slides are contained in boxes, top left, and drawers, bottom right.
Ingrid C. Romero, CC BY-ND
For example, through studying the Denver Pollen Collection, researchers discovered that the North Slope of Alaska had a temperate to subtropical climate about 50 to 56 million years ago that allowed palm trees to grow north of the Arctic Circle.
The collection was also critical in determining how quickly vegetation recovered from the asteroid impact that caused mass extinctions 66 million years ago.
Despite its scientific value, the number of specimens in the Denver Pollen Collection had never been estimated. When the Smithsonian received the collection in 2021, our team began digitally imaging some of these slides over the course of several years.
We estimate this collection holds approximately 4.3 billion microfossils – four times more specimens than were previously estimated to exist in all the collections of the world’s 73 largest natural history museums combined.
