Ammonites belong to the extinct subclass Ammonoidea

Ammonites belong to the extinct subclass Ammonoidea, marine mollusks related to modern squid, octopus, and the Nautilus. They first evolved roughly 409 million years ago during the Devonian Period from earlier straight-shelled cephalopods known as bactritoid nautiloids. This spiral shell became the defining feature of ammonites and likely provided improved hydrodynamics, buoyancy control, and protection. After several extinction events in the late Paleozoic, ammonites diversified dramatically during the Triassic, Jurassic, and Cretaceous periods, dominating marine ecosystems for more than 140 million years. Because species evolved quickly and were widespread, ammonites are extremely important index fossils, allowing geologists to date sedimentary rock layers very precisely.

The ammonites commonly sold today from Madagascar formed during the Early Cretaceous, when large parts of the island were covered by warm shallow seas. Sediments deposited in this marine environment preserved large numbers of ammonite shells in what is now the Mahajanga Basin. These fossils are typically dated to the Albian Stage of the Early Cretaceous (~113–100 million years ago). Cleoniceras Age: Lower to middle Albian Stage, about 113–100 million years ago. Where found: Madagascar, Europe, and parts of Central Asia.

Shell characteristics tightly coiled spiral shell narrow central opening (umbilicus) paired ribs emerging from nodes along the inner coil complex ammonitic suture patterns These animals likely lived as active swimmers, using jet propulsion similar to modern cephalopods. Because the shell interior was divided into chambers filled with gas and fluid, they could regulate buoyancy while swimming. “Goat Horn” Ammonite (Douvilleiceras) Douvilleiceras Often sold in the fossil trade as “Goat Horn ammonites.” Age: Early–middle Albian (~107–105 million years ago). Key features thick spiral shell strong ribbing across the whorls large rounded tubercles (“horns” or knobs) on the ribs These tubercles are what give them the “goat horn” appearance.

Both genera lived at roughly the same time (~110–105 million years ago) and often occur in the same marine sedimentary deposits. They belong to different ammonite families but share the typical ammonite spiral shell. Ammonites survived for hundreds of millions of years but disappeared during the Cretaceous–Paleogene extinction event, about 66 million years ago, the same catastrophe that ended the age of the dinosaurs. Parts of the Ammonite Shell (Useful for Explaining Fossils) When showing a cut or polished ammonite, these are the key anatomical features. Whorl The spiral turn of the shell.

Each new whorl represents growth of the animal. Umbilicus The central hollow where the inner whorls are visible. Chambers (Camerae) Internal compartments inside the shell used for buoyancy. Septa Curved walls separating each chamber. Suture Lines Complex patterns where septa meet the outer shell wall.

These intricate patterns strengthen the shell and are used by paleontologists to identify species. Living Chamber The outermost and largest chamber where the animal’s body lived. Siphuncle A tube running through the chambers that controlled fluid and gas balance to regulate buoyancy. 2. Carboniferous–Permian Ammonoids (359–252 million years ago) One major group from this time was: Goniatites These had relatively simple suture lines and represent an earlier stage of ammonoid shell complexity.

All ammonites disappeared during the Cretaceous–Paleogene extinction event about 66 million years ago. This asteroid-related catastrophe eliminated about 75% of Earth’s species, including ammonites and the dinosaurs. Ammonites were the hunters of their time, snapping up tiny crustaceans and plankton with their tentacles and beak. But even they weren’t safe—giant mosasaurs and plesiosaurs loved to crunch ammonite shells, leaving bite marks that we still see in fossils today! Even within the Mahajanga Basin, the black ammonites usually come from a slightly different layer or nearby locality than the more familiar orange/brown ones.

In that area: groundwater carried manganese the manganese altered the fossil after burial the chambers and shell filled with dark minerals That’s what creates the black coloration. Because only certain parts of the basin had that chemistry, miners find far fewer black specimens.