Shelf Life 06: The Tiniest Fossils

What we can do with the geological past is use Earth’s history to learn how life reacted to events in the past, to help to predict how we are dealing with the future.

- Ellen Thomas, Micropaleontologist

Fossil Indicators 

Whether it’s insight into the anatomy of an ancient animal or the ecology of plants that died out eons ago, fossils can offer up a vast array of information.

The video in this episode of Shelf Life explores the Museum’s fossil collection of tiny marine organisms known as foraminifera or—when even scientists admit six syllables is a mouthful—forams. These are abundant, widely-scattered, single-celled creatures that still fill oceans today. The fossilized shells left behind by their foram ancestors are widely used as time capsules for ancient climate data.

Model of a foram species.
Early microbiology researchers mistook foraminifera for ammonites. This model of a foram species—Elphidium crispum—was at one time on display in the Museum. 
© AMNH/321129

When forams were first discovered, their intricate forms were mistaken for another kind of marine organism—the long-extinct group of animals known as ammonites. While the forams’ chambered shells function in very different ways than those of the ammonites, they both contain climate clues. 

Ammonites are a group of ancient mollusks related to modern animals like nautiluses, which most closely resemble their extinct cousins, squid, and octopuses. Like forams, ammonites were a phenomenally successful group, branching out into a wide variety of distinct species and diverse forms all over the world's oceans, where they endured for more than 300 million years. They also had hard shells—most frequently coiled, though some species sported spiral helices and U-shaped shells—and hard jaws, an extraordinary number of which survived as fossils. The Museum's invertebrate fossil collection, one of the largest in the world, has more than 2 million ammonite specimens. (The recently accessioned Mapes Collection of marine fossils, which pushed the Museum's total holdings past 33 million specimens and artifacts last year, added about 150,000 more.)

Ten ammonite specimens packed together.
Once among the most common invertebrates in the oceans, ammonites went extinct around 65 million years ago. 
S. Thurston/© AMNH

Their abundance, broad geographic distribution, and a lengthy but limited stay on the planet make ammonites very useful markers of geologic time. They're also great indicators of ancient climate. Ammonite shells and jaws consist mostly of calcium carbonate, the same substance that makes up the tiny shells of foraminifera. Depending on the temperature of the surrounding water when it forms, calcium carbonate contains different amounts of two oxygen isotopes. The ratio of these isotopes, says Neil Landman, curator in the Division of Paleontology, make the shells “very sensitive indicators of the environments and temperatures in which they were formed.” And since shells from one period can be compared against those from another, they can be used to track changes in climates over time. 

Ammonite specimen.
The Mapes Collection includes fossil ammonites from about 400 million years ago, during the Devonian period. 
S. Thurston/© AMNH

Ammonite shells could provide other clues about the ancient world as well. The fossil record shows that the first ammonites appeared during the Devonian period, around 400 million years ago. After thriving in ancient oceans for hundreds of millions of years, nearly all ammonites fell victim to the mass extinction at the end of the Cretaceous period that also wiped out the dinosaurs and more than half the species on the planet.

“Ammonites are everywhere toward the end of the Cretaceous period,” says Landman. “There’s no decrease in the number of individuals or the number of species leading up to their sudden disappearance.”

Museum diorama depicting Cretaceous era ocean floor, including a chambered nautilus and ammonites.
An exhibit in the Milstein Hall of Ocean Life depicts life in a Cretaceous sea that covered present-day Tennessee and was home to ammonites.
D.Finnin/© AMNH

Their vanishing act can tell us more about the event that killed off so many forms of life, which is why Landman studies ammonite fossils that occur at the Cretaceous-Paleogene (K/Pg) boundary—the thin slice of geologic time immediately after the extinction.

This slice is found in just a few dozen places around the world, including sites in Morocco where Landman and colleagues traveled on a recent Constantine S. Niarchos expedition. Working with local geologists and university professors, Landman and other Museum paleontologists conducted the first big survey for ammonites around the K/Pg boundary in sedimentary rock layers on Morocco’s eastern coast. The result was a treasure trove of fossilized ammonites.

Seven people in the foreground wearing hard hats and most wearing bright vests stand on rocky hill, with 5 more people sitting in the background.
Neil Landman and colleagues search for ammonite fossils along the K/Pg boundary in Morocco. 
J. Sessa/© AMNH

“We knew ammonites existed in this area,” Landman says. “But there is not much information known or published about them in this site.” The new specimens from the Moroccan expedition are still being studied, and Landman expects several papers will come out of the research. In addition to ammonites, the team gathered new specimens of foraminifera to add to the Museum’s collection and looked at levels of the element iridium, which was scattered across Earth during the meteorite impact, in samples of surrounding sediment.

Previous studies of ammonites have produced one consistent finding with implications for ocean life today. On the Cretaceous side of the K/Pg Boundary, ammonites are plentiful, while related nautilids are less common. After the extinction event, ammonites mostly disappear, while nautilid populations persist, largely unaffected. Their fates, Landman suggests, could have been clinched by the animals’ respective life cycles.

Iridescent rainbow colored ammonite specimen.
Some ammonites were formidable animals more than 2 feet in diameter, such as this spectacular 75-million-year-old specimen.
C. Chesek/© AMNH

Ammonites hatched very small—less than a millimeter in diameter for some species—and would have made their homes among plankton and similar creatures in warm surface waters. Nautilids, meanwhile, were born larger and would have spent more time in deeper waters. If a meteorite impact caused rapid acidification of surface waters around the world, as some suggest, that would explain why ammonites, which used those waters as a kind of crib, would have been devastated, while young nautilids could have soldiered on through the catastrophe, sheltered in deeper water. (Deep-sea forams, known as benthic foraminifera, similarly weathered this extinction more successfully than surface-skimming planktonic foraminifera species, which were mostly wiped out at the end of the Cretaceous, then made a comeback in the early Miocene). 

Chambered nautilus underwater, with a large eye, curved body and many appendages.
Chambered nautiluses are sometimes called living fossils because they so closely resemble ancient cephalopods like ammonites. 
Image courtesy of T.B. Smith 

As today’s oceans become increasingly acidic due to climate change, learning more about the ammonite extinction is more than an academic concern. The details of the catastrophe that struck 65 million years ago could inform how we deal with similar environmental issues in the modern era.

“Calcium carbonate shells on modern animals are getting thinner, and some evidence suggests the calcium carbonate spikes of sea urchins are getting smaller as well,” Landman says. “Understanding how ocean acidity affects marine species is very pertinent to where we are today.”