| When did it live? | The Late Cretaceous (100.5-66 million years ago) |
| Where did it live? | Central USA |
| What does the name mean? | Placenticeras means “flat horn” for the shape of its shell |
| Why does this fossil have holes? | The rows of holes are puncture marks from the teeth of a mosasaur, a giant sea-going lizard |
Placenticeras was a genus of ammonite that lived during the Late Cretaceous, from 100.5-66 million years ago, and can be found in marine deposits from that time, especially in North America and Europe. Ammonites are distantly related to modern cephalopods like squids and octopus, having evolved from nautiloids (the group that includes the living genus Nautilus) during the Devonian Period 409 million years ago. Part of the larger group, Ammonoidea, which were very abundant and diverse during the Mesozoic Era, then died out at the end of the Cretaceous Period 66 million years ago in the same mass extinction event that killed off dinosaurs apart from birds.
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With soft parts rarely preserved and with no close living relatives, not much is known for certain about how ammonites lived. We do know that they lived in the open ocean, filling chambers in their shells with gas to maintain buoyancy and keep upright. Some were efficient swimmers with streamlined shells. They likely fed on plankton, and may have squirted ink like modern cephalopods to escape predators.
Outside of the fossilized shells that remain of these ancient animals, there are other clues about how they interacted within ancient ecosystems. This is done through the study of trace fossils: fossils which are not themselves parts of the organism’s body. This field is formally known as ichnology, and the specimens are often called ichnofossils. Trace fossils may include burrows, tracks, bite marks, and other traces left behind by an organism. Ichnology can show us that an animal was present in an area even if its body fossils aren’t preserved. It can also tell us about how that animal lived. The rows of holes in this specimen of Placenticeras are puncture marks from the teeth of a mosasaur. Mosasaurs were giant, predatory marine reptiles that lived in the seas of the Cretaceous. The bite marks on this Placenticeras specimen show that ammonites were prey for mosasaurs.
Additional reading:
- Digital Atlas of Ancient Life: Ammonoidea
- Digital Atlas of Ancient Life: Body Fossils and Trace Fossils
- Mosasaur: Apex Predator of the Western Interior Seaway
Spanish Translation
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The information presented in this module comes from UMMP Student Seamus Callaghan, reviewed by Jen Bauer, UMMP Invertebrate Collection Manager
