The Piloceratidae are a compressed, rapidly expanding, cyrtoconic brevicones with holochoanitic ventral

siphuncle The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and ...

s and simple endocones. Most likely evolved from ''

Clitendoceras ''Clitendoceras'' is a genus of cephalopods in the order Endocerida from the Lower Ordovician (m-u Canadian) with an elongate shell with a slight downward, endogastric, curvature and a siphuncle that lies along the ventral margin. Common for end ...

'', a narrow, slightly endogastric genus intermediate in form between straight shelled ''Proendoceras'' and the bulkier Piloceratidae. Found in shallow carbonate marine sediments of Demingian through the Cassinian age, (essentially Arenigian, = early Middle -Upper Canadian) . Pilocerids split off from the

Proterocameroceratidae The ''Proterocameroceratidae'' were the first of the Endocerida. They began early in the Ordovician with ''Proendoceras'' or similar genus which had developed endocones, replacing the diaphragms of the ellesmerocerid ancestor. Proterocamerocerat ...

very shortly after their inception and are the first family which the proterocamerocerids gave rise to. With the exception of Humeoceras, found in the middle

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...

, pilocerid genera are limited to the Lower

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...

(Canadian in North America). The general shape of the pilocerid shell precludes an ambush predator lying in wait on the sea floor, or a stealthful hunter drifting through the water. Rather, they probably crawled over the sea floor, head down with the shell off the bottom, searching for prey. Nothing is known of their soft part anatomy, other than what can be surmised from the fact they are cephalopods. They likely possessed tentacles of some sort, but were they numerous like those of modern

Nautilus The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in t ...

, limited to eight or ten as with octopus or squid, or of some other arrangement. Pilocerid genera include the strongly curved ''Piloceras'', ''Bisonoceras'' with its hooked bison horn shape, nearly straight ''Allopiloceras'', ''Dartonoceras'', and internally complex ''Cassinoceras''. Ecologically the Piloceratidae were replaced in the Middle Ordovician by the Cyrtendoceradae, which may have evolved early from the Endoceratidae rather than being direct pilocerid descendants.

References

*Flower 1950 in Flower and Kümmel; A Classification of the Nautiloidea; Jour Paleon V.24, n 5, pp 604–616, 1 text-fig, Sept 1950. *– – 1955, Status of Endoceroid Classification; Jour. Paleon. V 29. n.3, pp 327–370, May 1955 *– – 1976, Ordovician Cephalopod Faunas and Their Role in Correlation, pp 531–537 in The Ordovician System: Proceedings of a Paleontological Association Symposium; Univ of Wales and Welsh Nat’l Mus Press. *Teichert, Curt 1964, Piloceratidae, in the Treatise on Invertebrate Paleontology, Vol K pp K171-172; Geol Soc of America and University of Kansas Press; {{Taxonbar, from=Q7194247 Nautiloids Ordovician first appearances Early Ordovician extinctions