We Dig Ammonites

Ammonites used to rule the seas for 350M years. They were prolific, and adaptive, with over 10,000 different species, across sizes spanning from a millimeter to 10 feet across. But then they all disappeared with the meteorite impact that took out the dinosaurs.

Ammonites are ancient relatives of octopuses and squids, marine cephalopods with tentacles, beaks, blue blood, and jet propulsion, but they had external shells for buoyancy and protection (as argonaut octopuses still have today).

Ammonites lived in the outermost chamber of their shell. They used the inner, separated chambers to control their buoyancy, filling them with gas or liquid to move up and down in the water column. A viscous tube called a siphuncle ran along the exterior wall through each of the chambers and regulated the gas/fluid mix to maintain near-neutral buoyancy.

This ion-pumping siphon is just co cool IMHO: “Ammonoids were the first scuba divers able to navigate the water’s depths. To rise, this cephalopod absorbed elements from seawater to increase the saltiness of the fluid in the siphuncle. Through osmosis, the less saline liquid in the chamber was drawn into the mineral-rich siphon system. At the same time, gas— likely the combination of nitrogen, oxygen and CO2—diffused from the siphuncle to replace water in the emptying chambers. Now lighter, the cephalopod was able to rise in the water column. To dive, the technique was reversed. Pumping ions—typically sodium and chloride—into the chambers increased the salinity of the internal fluid. Water was then absorbed via osmosis; gases were pushed out, density increased, and the ammonite grew heavier and dropped to deeper ocean depths.” (p.124)

Predator bites: “Any perforation of the shell would have an impact on the ammonite’s buoyancy control. Even a small hole devastated the siphuncle system; the puncture would have allowed water to flow uncontrolled into the conch” (165)

“Mosasaurs were apex predators—sea monsters with a foreboding presence. They ate virtually everything in their path, including ammonites. They were long, sleek, dark, scaly-skinned, air-breathing saurians that likely evolved from reptiles that slithered into the waters to feed on marine life.” (166) See prior post below for a picture.

Ammonites had “eyes like a nautilus and could sense vibration but didn’t have any hearing.” (115) “Ammonoids were among the earliest known creatures to swim above the sea bottom.”(3)

Their shells were made of aragonite, the same mineral found in pearls. It’s the same CaCO3 as calcite but with a different crystal form (slender needlelike crystals). “Undersea, where water pressure can be intense, aragonite is the preferred structure because it is denser and stronger.” (100)

There is some recent micro-fossil research “indicating that the subclass may have been around as early as 522 million years ago. That would mean cephalopods emerged at the very beginning of the evolution of multicellular organisms during the Cambrian explosion.” (11)

They survived the ice age and wild swings in ocean salinity, CO2 intensity and other toxic conditions that wiped out 85% of all Ordovician species.

“When you find a cluster of ammonoids, consider that they were possibly the last creatures to survive in poorly oxygenated waters, slowly suffocating into the big sleep. Ironically, these low-oxygen environments were an ideal locale to become embalmed into fossils.” (79)

“Over time, the fossilizing shell absorbed phosphates, carbonates and other minerals suspended in the seawater via a redox reaction” and over time “minerals replace all parts of the animal’s shell.” (82)

For the ones that look metalized with crystals in the chambers: “as the organic matter decayed, it released sulfides that interacted with the iron ions dissolved in the surrounding waters. This allowed pyrite to invade the shell and fill the chambers, creating a preservation with a brassy luster.” (85)

For the gemmy brown ones: “Calcite and aragonite are replaced by a textural variety of silicates, including opal, agate or quartz. Debris from ancient volcanos seeped into and replicated the remains of a formerly living creature that was buried in the mire.” (87)

“Mineralization of the original shell occurs so gradually that details such as suture patterns and microstructural features are preserved, even with the total loss of the original material.” (100)

Females are 4x larger than males. They ate most smaller animals in the sea. “They likely lived one to five years. All ammonites grew fast. Cephalopods are among the most efficient animals in the world for converting food to growth.” (137)

“As the most prolific fossil in the geologic record, families have been discovered in sedimentary strata from Antarctica to Alaska.” (3)

Medieval people thought they were petrified snakes. In Europe, ammonites were called "serpentstones" or "snakestones" and believed to be coiled snakes turned to stone by saints.

Their suture patterns are like nature’s artwork. The internal walls (septa) dividing shell chambers create incredibly intricate, fractal-like patterns visible when fossils are polished. These "sutures" are unique to each species and help paleontologists identify and date them precisely. Some preserve stunning crystal chambers. When cut and polished, many fossils reveal beautiful quartz or calcite crystals filling the buoyancy chambers, turning them into natural works of art with iridescent or colorful patterns.

In certain Cretaceous deposits in Canada, the outer shell layer fossilized into ammolite, a rare iridescent gemstone with vivid rainbow colors similar to opal, one of the few fossils turned into jewelry.

“Ammonites are exquisite posterchildren for paleontology, evolution and extinction, observes Kirk Johnson, Director of the Smithsonian Natural History Museum.” (p.2)

Thanks to my kids for getting me this book for Xmas — We Dig Ammonites!