Rastellum Oyster Bivalve Fossil (Alien Form) – Cretaceous Fossil from Marovoay, Mahajanga District, Madagascar

Description

Authentic Rastellum Oyster Fossil from the Cretaceous of Madagascar

This stunning Rastellum oyster bivalve fossil originates from the Cretaceous deposits of Marovoay, Mahajanga District, Madagascar. Its distinctively ridged, fluted shell gives it a striking “alien-like” appearance, making it a favourite among collectors and enthusiasts of ancient marine life. Each specimen is a 100% genuine fossil, presented exactly as discovered and carefully selected for its exceptional preservation and sculptural quality. The photographs show the actual fossil you will receive, accompanied by a 1 cm scale cube for accurate size reference. Every piece comes with a Certificate of Authenticity, ensuring its provenance and scientific integrity.

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Geological and Palaeontological Background

The Rastellum genus belongs to the family Ostreidae, a group of ancient oysters that thrived during the Cretaceous Period, approximately 145 to 66 million years ago. Fossils from the Mahajanga Basin of Madagascar represent one of the most productive and scientifically significant Cretaceous marine deposits in the Southern Hemisphere. This area was once part of a warm, shallow epicontinental sea that bordered the ancient supercontinent Gondwana.

Sedimentary layers in Marovoay consist primarily of limestones, marls, and sandy calcareous deposits, ideal for preserving marine organisms in exquisite detail. These sediments were laid down in a tropical shallow-marine environment, rich in life including ammonites, belemnites, bivalves, and marine reptiles. The Rastellum fossils found here display remarkable three-dimensional preservation, often retaining their natural shell ornamentation and curvature.

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Morphological Features and Identification

Rastellum oysters are instantly recognisable by their elongated, strongly folded shells, giving them an almost otherworldly appearance that has earned them the nickname “fossil aliens” among collectors. The shells exhibit deep radial ridges and wing-like flares, often asymmetrical due to their epifaunal lifestyle—growing attached to hard substrates or other shells on the seafloor.

Key characteristics of Rastellum sp. include:

• Highly corrugated radial ribs across both valves.

• Conspicuous zigzag commissure line, where the two shells meet.

• Irregular, ornate fluting and twisted growth form adapted to firm attachment.

• Calcitic shell composition, aiding in long-term fossilisation.

These morphological adaptations helped the organism withstand strong marine currents and sediment accumulation. The textured surface of this specimen showcases those details beautifully, making it both a scientifically valuable and visually striking display piece.

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Depositional Environment and Paleoecology

During the Cretaceous, the coastal environments of Madagascar were home to flourishing reef and lagoonal ecosystems. Rastellum lived in shallow, high-energy marine zones, attaching itself to coral reefs, rocky outcrops, or larger shells. The species functioned as a filter feeder, drawing plankton and organic material from seawater through gill filtration. Its unusual, contorted shell shape was an evolutionary adaptation to turbulent waters, allowing it to anchor securely while maintaining efficient feeding surfaces.

The fossil’s preservation suggests rapid burial in fine calcareous sediment, protecting it from bioerosion and compaction. Over millions of years, mineral replacement transformed the original shell into solid calcite and iron-rich material, stabilising its structure while maintaining its original form.

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Scientific and Collector Significance

Rastellum fossils are widely studied for insights into bivalve evolution and Cretaceous paleoecology. Their presence in Madagascar highlights ancient marine connectivity between the southern Tethys Ocean and developing Indian Ocean basins. Specimens like this are prized not only for their dramatic “alien-like” morphology but also for their contribution to understanding the adaptive diversity of Cretaceous molluscs.

For collectors, this specimen represents an extraordinary blend of natural artistry and deep geological history. The fluted architecture, balanced symmetry, and sculptural beauty make it a centrepiece fossil for any collection.

All of our Fossils are 100% Genuine Specimens and come with a Certificate of Authenticity.
Scale cube = 1 cm – for full sizing, please refer to the photos.