Opinion

Science

The Mystery of Life on Earth Encrypted in Fossils
—Evolution and Paleoecology of Ammonites—

Kazuyoshi Moriya
Associate Professor, Faculty of Education and Integrated Arts and Sciences, Waseda University

The History of Life on Earth

Since the beginning of its history approximately 4.6 billion years ago, Earth and life forms that inhabit it have gone through a dramatic history to reach their current form. Life forms that are large enough to be seen with the naked eye, like those on Earth today, first appeared about 600 million years ago. This means that for the first 4 billion years, i.e. about 87% of its entire history, there were no life forms on Earth large enough to be visible to the naked eye. However, this does not mean that Earth was a barren, lifeless planet during this period. For about the first 4 billion years of Earth's history, microorganism such as bacteria thrived.

To put it in human perspective, the clade of human and those of chimpanzees were blanched about 7 million years ago. In other words, humans have only existed on Earth for a mere 0.15% of its entire history.

Records of Life Forms That Have Flourished in the Past

Now, let us take a look at how long other animals have existed on Earth. The ancient animal that many of you are most familiar with is probably the dinosaur. The dinosaur group first appeared on Earth about 250 million years ago during the Triassic and went extinct about 66 million years ago at the end of the Cretaceous. They dominated the planet for about 180 million years. However, even these dinosaurs only existed on Earth for about 4% of its history. The extinction of dinosaurs is thought to have been caused by an asteroid impact. Another life form that became extinct at the same time was the ammonite, which thrived in the oceans. Ammonites first appeared about 410 million years ago, and existed for about 340 million years until their extinction at the end of the Cretaceous, surviving for a total of about 7.5% of Earth's history. Since life forms that are large enough to be observed with the naked eye have existed on Earth for about 13% of its history, this means that ammonites existed for over half of this period.

Furthermore, in the 600 million years of evolutionary history of animals, five mass extinction events have been identified. Extinction is defined as when a species disappears from the planet without leaving any offspring. A mass extinction, as its name suggests, is defined as an event in which a massive number of life forms become extinct at once. Ammonites experienced four of these events. They survived the first three, but the fourth extinction event at the end of the Cretaceous finally terminated their evolutionary history. As organisms that adapted to the earth's environmental changes and continued to survive until their eventual extinction, ammonites can be considered as witnesses to the dynamic history of Earth and the evolution of its life forms.

The Biology of Ammonites

So, what kind of organisms were the ammonites? The closest relatives to ammonites that can be found on Earth today are cephalopods (mollusks), such as squids and octopuses. Ammonites possessed shells with gas-filled chambers called phragomocones. This structure is thought to have given them neutral buoyancy to remain in sea water without floating to the surface or sinking to the seafloor (Figure 1). At first glance, they may not appear similar to squids at all, but some squids such as cuttlefish have much in common with ammonites, particularly the shells used to control buoyancy. However, squids have internal shells while ammonites had external shells. Also, the appearance of ammonites is similar to that of nautiluses, which can be commonly found in pet shops. While it is known that nautiluses are ancestors of ammonites, ammonites and squids share more common anatomical characters than ammonites and nautiluses do. On a side note, although ammonites have become extinct, nautiluses, which are their ancestors, still survive to this day, which is a mystery in and of itself. However, I will leave this topic for another time.

Figure 1: Photographs of ammonite, nautilus, cuttlefish, and spirula shells (taken by the author)
These are shells of ammonite (A), and present-day cephalopods: nautilus (B), cuttlefish (C), and spirula (D). A1: Ammonite (Gaudryceras) fossil from the Cretaceous. The shell is external. A2: Cross-section of A1. The shell contains many phragmocones, with a living chamber at the end that houses the body. The living chamber is now filled with mud. B1: Shell of a present-day nautilus. The shell is external. B2: Cross-section of B1. The shell contains many phragmocones, with a living chamber at the end that houses the body. C1: Shell of a present-day cuttlefish. The shell is internal. C2: Cross-section of a cuttlefish shell. It is not in a spiral form like the nautilus, but it is similar in that it contains many phragmocones. Each section separated by lines (shell walls) represents the phragmocone. The cuttlefish has a particularly high number of phragmocones. D1: Shell of a present-day spirula. The shell is internal. D2: Cross-section of a spirula shell. The phragmocones are filled with resin as it was embedded with resin when cutting the cross-section. The spirula also has a spiral shell containing many phragmocones.

Although I have mentioned that ammonites had neutral buoyancy to stay in sea water, it was thought that they were able to swim freely in seawater due to the similarities in external morphology with present-day nautiluses. Furthermore, from the discovery of many cephalopod beaks found in the stomach of a plesiosaur, it was thought that they hunted ammonites swimming within a water column.

The Undiscovered Mystery of Ammonites

However, scientific evidence that ammonites did in fact actively swim in the ocean did not surface after over 200 years of research into these organisms. Research up until now has focused on taking measurements of the various shapes of ammonite fossils and comparing them to present-day nautiluses and cuttlefish, as well as conducting hydrodynamic experiments, but none led to a decisive conclusion. This is where I decided to take a new approach in an attempt to solve this problem. Ammonite shells are composed of calcium carbonate, similar to present-day bivalves and gastropods. As the name suggests, it contains carbonate ions (CO32−) and calcium (Ca2+). Oxygen (O) constitute the carbonate ions have stable isotopes of varying mass numbers, 16O, 17O, and 18O. The ratio of these stable isotopes contained in ammonite shells have a particularly interesting property, in that the ratio between 16O and 18O is determined by the seawater temperature when the shell was formed. In other words, by measuring the ratio of 16O and 18O, we can calculate the seawater temperature of where and when the ammonite lived. Furthermore, if we know the temperature gradient from the surface to the seafloor at the time the ammonites lived, we can compare it to the water temperature calculated from the ammonite shell, and determine the ocean depth that they inhabited.

The Secret Finally Revealed

By applying this technique and determining the depth of habitat of ammonites at the time just before their extinction, an unexpected discovery was revealed. Before the analyses, I thought, like many researchers that came before me, that ammonites swam freely in the ocean. Therefore, I expected the water temperature to correspond to the surface or intermediate depth. However, after analyzing nine species of ammonite of various morphologies, all indicated a water temperature corresponding to the area near the ocean floor. This revealed that ammonites did not swim freely in the ocean, but inhabited close to the ocean floor. This was a huge surprise at first, but after considering the fact that ammonites are close relatives of squids and octopuses, it started to make more sense. Present-day squids with shells (such as species of cuttlefish) and some octopuses do not swim actively, but drift near the seafloor. This led me to conclude that these nine species of ammonite analyzed must have drifted along the seafloor. Therefore, it can be hypothesized that the plesiosaurs did not hunt the ammonites, but rather scooped them up from the seafloor by using their mouths like spoons.

New Mysteries and a Challenge to Unveil the History of Ancient Life on Earth

Figure 2: The phylogeny of ammonites whose depth of habitat is known
This figure shows the phylogeny of ammonites from the Jurassic to the Cretaceous, and their known depth of habitat. The white symbols indicate groups that swam freely from the surface to medium depth. The gray symbols indicate groups that inhabited close to the seafloor. The vertical axis indicates years past (in millions), and the symbols are placed in positions that align with the age from which data was obtained for each lineage. The star symbols indicate results obtained from the author's data, and the circles indicate results obtained from subsequent research.

Thereafter, many researchers from around the world followed this research by analyzing more species of ammonite. Surprisingly, many of them were discovered to have inhabited near the seafloor (Figure 2). However, this raises another profound mystery. It is now known that although most of the ammonites that became extinct along with the dinosaurs in the Cretaceous were seafloor-dwelling organisms, it was also discovered that some ammonites swam freely in the ocean as previous researchers hypothesized. In other words, ammonites with completely different ecology—those that inhabited the seafloor and those that swam freely in the ocean—all became extinct at the same time. This suggests that the extinction of ammonites was unrelated to their mode of life that can be inferred from the depth of habitat, but was potentially caused by other factors. A new journey to unlock this new ancient mystery has begun.

Kazuyoshi Moriya
Associate Professor, Faculty of Education and Integrated Arts and Sciences, Waseda University

Kazuyoshi Moriya graduated from the Department of Earth Sciences, School of Education at Waseda University in 1997. He went on to obtain a Ph.D. from the Department of Earth and Planetary Science, Graduate School of Science at the University of Tokyo in 2002. He has a Ph.D. in earth science. He took up his current position in 2015 after working as a post-doctoral researcher at the Japan Society for the Promotion of Science (PD), a visiting research fellow at the National Oceanography Centre, Southampton in Great Britain, a visiting research fellow at the University of British Columbia in Canada and a research associate at the School of Education, Waseda University. His expertise is in evolutionary paleobiology and paleoceanography.

Recent publications include: Ammonoid Paleobiology: From Anatomy to Ecology (Springer, co-author)