The Estonioceratidae are a family of loosely coiled tarphycerids in which the inner side of the whorls, which forms the dorsum, is rounded or flat with no impression, and in which the siphuncle, composed of thick tubular segments, is located ventrally. The Estonioceratidae seem to form a link between the ancestral Bassleroceratidae and the more tightly coiled Tarphyceratidae.^[1][2]

Estonioceratidae Temporal range: L Ordovician
Estonioceras ariense
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
Subclass:	Nautiloidea
Order:	†Tarphycerida
Family:	†EstonioceratidaeHyatt in Zittel, 1900 References edit ^ Flower, R. H. 1976. Ordovician Cephalopod Faunas and Their Role in Correlation in The Ordovician System: proceedings of a Palaeontological Association symposium, Sept 1974. ^ Furnish, W. M, and Glenister, Brian F. 1964. Nautiloidea – Tarphycerida; Treatise on Invertebrate Paleontology, Part K, Geol. Soc. of America, Teichert and Moore (eds.)