Described here is a small acritarch with a fine reticulation on the internal surface of the cyst wall which provides a new criterion for future acritarch taxonomy. Sixty-six specimens of Introvertocystis rangiaotea gen. et sp. nov. have been identified from sediments of Ngaterian to Arowhanan age (Cenomanian to early Turonian; c. 99–92 Ma) within the Tupuangi Formation, Chatham Islands, New Zealand. The diagnostic feature of this fossil cyst is internal reticulation. This requires the emendment of the genus Palaeostomocystis. Some specimens may show waist constriction; this constriction is more common in larger specimens where it is suggestive of a paracingulum. However, the absence of a definite paracingulum, parasulcus or definable archeopyle precludes its assignment within the dinoflagellates. The wall structure is unusual in that the sculptural ornamentation is present on the internal surface, a feature that distinguishes it from all known acritarch genera.

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