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| Ammonitic Suture Detail |
Like other cephalopods, ammonites had sharp, beak-like jaws inside a ring of squid-like tentacles that extended from their shells. They used these tentacles to snare prey, — plankton, vegetation, fish and crustaceans — similar to the way a squid or octopus hunt today. Catching a fish with your hands is no easy feat, as I'm sure you know. But the ammonites were skilled and successful hunters. They caught their prey while swimming and floating in the water column.
Within their shells, they had a number of chambers, called septa, filled with gas or fluid that were interconnected by a wee air tube. By pushing air in or out, they were able to control their buoyancy in the water column.
They lived in the last chamber of their shells, continuously building new shell material as they grew. As each new chamber was added, the squid-like body of the ammonite would move down to occupy the final outside chamber.
They were a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids — octopuses, squid, and cuttlefish) than they are to shelled nautiloids such as the living Nautilus species.
Ammonites first appeared about 240 million years ago, though they descended from straight-shelled cephalopods called bacrites that date back to the Devonian, about 415 million years ago, and the last species vanished in the Cretaceous–Paleogene extinction event.
Ammonites were prolific breeders that evolved rapidly. If you could cast a fishing line into our ancient seas, it is likely that you would hook an ammonite, not a fish. They were prolific back in the day, living (and sometimes dying) in schools in oceans around the globe. We find ammonite fossils (and plenty of them) in sedimentary rock from all over the world. In some cases, we find rock beds where we can see evidence of a new species that evolved, lived and died out in such a short time span that we can walk through time, following the course of evolution using ammonites as a window into the past. For this reason, they make excellent index fossils. An index fossil is a species that allows us to link a particular rock formation, layered in time with a particular species or genus found there. Generally, deeper is older, so we use the sedimentary layers rock to match up to specific geologic time periods, rather the way we use tree-rings to date trees.
Ammonites have intricate patterns on their shells called sutures. The different suture patterns tell us what time period the ammonite is from. If they are geometric with numerous undivided lobes and saddles and eight lobes around the conch, we refer to their pattern as goniatitic, a characteristic of Paleozoic ammonites.
If they are ceratitic with lobes that have subdivided tips; giving them a saw-toothed appearance and rounded undivided saddles, they are likely Triassic. If they have lobes and saddles that are fluted, with rounded subdivisions instead of saw-toothed, they are likely Jurassic or Cretaceous.
The Ammonoidea can be divided into six orders:
- Agoniatitida, Lower Devonian - Middle Devonian
- Clymeniida, Upper Devonian
- Goniatitida, Middle Devonian - Upper Permian
- Prolecanitida, Upper Devonian - Upper Triassic
- Ceratitida, Upper Permian - Upper Triassic
- Ammonitida, Lower Jurassic - Upper Cretaceous
In some classifications, these are left as suborders, included in only three orders: Goniatitida, Ceratitida, and Ammonitida. Once you get to know them, ammonites in their various shapes and suturing patterns make it much easier to date a rock formation at a glance.
Heteromorphs come in a variety of shapes and sizes. They must have intrigued and mystified those who were first to find them as they do not have an intuitive shape at all for a marine predator.
The beautiful plate you see on the upper left here showing two ammonites is from Sowerby (1837) and is one of the very first scientifically accurate studies of heteromorph ammonites. We see similar species to the heteromorph on the right of the plate in the Nanaimo Group of Vancouver Island, British Columbia.
Ammonite shells have been collected for millennia. During medieval times they were believed to be snakes that had been turned into stone and were sold to people going on pilgrimages. They have been found in archaeological sites in many parts of the world. We find them in archaeological remains spanning human history, across cultures and civilizations.
Ammonites are prized for their scientific and aesthetic value and have been used as building materials, jewellery, amulets, charms to aid in the hunt, religious totems amongst other things. The original discus used by the ancient Greeks in their Olympics was a fossilized ammonite.
A great temple to the god Amon was built at Karnak in Upper Egypt around c. 1785. It is from Amon that we get his cephalopod namesake, the ammonites and also the name origin for the compound ammonia or NH3.
