Nigeria Ammonites

Nigerian Ammonites: A Geological and Paleontological Overview
Introduction to Nigerian Ammonites

Nigerian ammonites are fossilized remains of extinct marine mollusks belonging to the subclass Ammonoidea, which thrived in the Earth’s oceans during the Paleozoic and Mesozoic Eras. In Nigeria, ammonites are predominantly found in Cretaceous sedimentary deposits, particularly within the Benue Trough, a geologically significant region that records extensive marine transgressions during the Cretaceous period. These ammonites provide valuable insights into the paleoenvironments, biostratigraphy, and evolutionary history of marine life during that era.

Geological Context of Nigerian Ammonites

Nigeria’s ammonite fossils are primarily found in the Benue Trough, a rift basin extending from the Niger Delta in the south to the Chad Basin in the north. This trough is divided into Lower, Middle, and Upper Benue Trough, each containing well-preserved fossiliferous formations. The presence of ammonites in these formations indicates that the region was once covered by shallow seas, allowing for their proliferation.

Key formations where ammonites are commonly found include:

Albian to Cenomanian-aged marine formations such as the Odukpani Formation (Southern Benue Trough) and the Asu River Group (Lower Benue Trough).
Turonian to Maastrichtian formations such as the Eze-Aku Formation, the Nkporo Formation, and the Santonian Awgu Formation.
These formations consist of marine shales, limestones, and sandstones, which have preserved ammonites alongside other marine fossils, including bivalves, gastropods, and foraminifera.

Species and Taxonomy of Nigerian Ammonites

Several genera and species of ammonites have been identified from the Nigerian fossil record. Some of the most notable include:

Mortoniceras spp. – A genus commonly found in Cretaceous deposits worldwide, also present in Nigeria’s Albian to Turonian strata.
Puzosia spp. – Another Cretaceous genus characterized by smooth shells and prominent whorls.
Neolobites spp. – Known for its distinctive ribbed shell structure, common in the Cenomanian-Turonian stages.
Eotissotia spp. – Found in Late Cretaceous deposits, often associated with deep marine environments.
Vascoceras spp. – One of the most prominent ammonite genera in Nigeria, occurring widely in Turonian deposits.
These ammonites are important for biostratigraphy, helping geologists date rock formations and correlate sedimentary layers across West Africa.

Morphology and Preservation

Nigerian ammonites exhibit a variety of shell shapes and sizes, including:

Planispiral shells (flat-coiled, with a single plane of coiling).
Highly ornamented shells with ribs, nodes, and tubercles.
Pyritized and silicified preservation, where mineral replacement enhances fossil durability.
Septal sutures with complex patterns, distinguishing species and aiding in classification.
Some ammonites are found fully preserved, while others occur as fragmentary remains embedded in sedimentary rock matrices.

Paleoenvironmental Significance

The ammonites found in Nigeria suggest a warm, shallow marine environment with periodic sea-level fluctuations during the Cretaceous. This aligns with global Cretaceous paleoclimate trends, characterized by greenhouse conditions and high marine biodiversity. Their presence in black shale deposits also indicates episodes of anoxia, where oxygen levels were low, preserving organic-rich sediments.

Economic and Scientific Importance

Biostratigraphic Correlation – Ammonites are key index fossils that help geologists date sedimentary sequences and correlate rock layers across Nigeria and beyond.
Oil and Gas Exploration – The presence of ammonites in petroleum-bearing basins aids in stratigraphic mapping, guiding oil exploration efforts.
Paleoclimate Studies – Isotopic analysis of Nigerian ammonite shells provides insights into ancient ocean temperatures and Cretaceous climate fluctuations.
Fossil Trade and Collection – Some Nigerian ammonites are highly sought after by collectors, especially well-preserved specimens with intricate suture patterns or pyritized preservation.