Taxonomy
- Kingdom: Animalia
- Phylum: Mollusca
- Class: Cephalopoda
- Subclass: Ammonoidea
- Order: Ammonitida
- Superfamily: Acanthoceratoidea
- Family: Placenticeratidae
- Genus: Placenticeras (type species Placenticeras mixtum)
Geological range
- Upper Cretaceous, primarily from the Campanian to the Maastrichtian (approximately 83.6 – 66 million years ago).
Morphological characteristics
- Shells are generally large, ranging from 20 cm to over 1 m in diameter.
- The shell is involute and laterally compressed, with a narrow umbilicus.
- Whorl sections are typically rounded to slightly depressed.
- Ornamentation is usually fine, closely spaced ribbing; some species display weak tubercles or growth lines.
- The suture pattern is highly complex, exhibiting numerous lobes and saddles typical of late Cretaceous ammonites.
Stratigraphic and geographic distribution
- Fossils of Placenticeras have been recovered from marine sedimentary sequences in North America (Western Interior Seaway), Europe (France, England, Denmark), and parts of Asia (Japan, Kazakhstan).
- The genus is common in chalk and shale deposits associated with offshore, relatively deep marine settings of the Late Cretaceous epicontinental seas.
Paleoecology
- As a nektonic cephalopod, Placenticeras is interpreted as an active swimmer and predator, likely feeding on planktonic and small nektonic organisms.
- The large size and complex suture lines suggest a well‑developed buoyancy control system, enabling vertical migration within the water column.
Significance
- Placenticeras is an important index fossil for Late Cretaceous biostratigraphy, particularly in North American Campanian–Maastrichtian sequences.
- Its widespread occurrence and rapid evolutionary turnover assist in correlating marine strata across different paleogeographic provinces.
References
- Cobban, W. A., & Hook, S. C. (1983). Upper Cretaceous ammonites of the Western Interior, United States. Geological Survey Professional Paper 1320.
- Wright, C. W., with Friedrich, A. & Pinzari, K. (1996). Treatise on Invertebrate Paleontology, Part L, Mollusca 4, Cephalopoda – Ammonoidea. Geological Society of America and University of Kansas Press.
- Paleobiology Database (accessed 2026). Entry for Placenticeras.
All information presented reflects current consensus from peer‑reviewed paleontological literature and recognized databases.
