The Ascocerida are comparatively small, bizarre Orthoceratoids known only from

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

and

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 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 third and shortest period of t ...

sediments in Europe and North America, uniquely characterized by a

deciduous In the fields of horticulture and botany, the term deciduous () means "falling off at maturity" and "tending to fall off", in reference to trees and shrubs that seasonally shed Leaf, leaves, usually in the autumn; to the shedding of petals, aft ...

conch consisting of a longer juvenile portion and an inflated short adult portion that separate sometime in maturity.W.M . Furnish and Brian F Glenister; Nautiloidea-Ascocerida, K261-K277;

Treatise on Invertebrate Paleontology The ''Treatise on Invertebrate Paleontology,'' published from 1953–2007 by the Geological Society of America and the University of Kansas, then 2009–present by the University of Kansas Paleontological Institute, is a definitive multi-authore ...

, part K, Nautiloidea; Geological Society of America and the University of Kansas Press, 1964

Morphology

The juvenile portion of an ascocerid consists typically of a narrow, circular cyrtocone that underwent periodic truncation. Cumulative length, including broken off segments, may have reached about in the largest individuals. The

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

is located halfway between the shell axis and the venter, is thin walled and tubular with short, orthochoanitic septal necks and segments that are only slightly inflated. The juvenile portion is known as the deciduous conch, as it is shed in a similar manner to

trees losing their leaves. The adult, or mature ascoceroid conch, as it is referred to, typically consists of an expanded exogastric brevicone with unique features. The apical end is formed by the septum of truncation which is about three times as thick as normal internal septa and about as thick as the external shell itself. The septa become confined to the dorsal side of the shell resulting in a series of dorsal chambers, or camerae, that provided stabilizing buoyancy. The ascocerid shell, or conch, is the thinnest and most fragile of any orthocone or cyrtocone of comparable size,Rousseau H Flower; p 542, Ordovician Cephalopod Faunas and Their Role in Correlation; in The Ordovician System: proceedings of a Palaeontological Association symposium, University of Wales Press and the National Museum of Wales, Cardiff, 1976 which accounts for their rarity. Often it is only the mature, ascoceroid portion that is found.

Derivation and phylogeny

The Ascocerida are derived from the michelinocerid family Clinocertidae in the early Middle Ordovician, possibly from '' Clinoceras'' through such slender forms as '' Montyoceras'' and '' Hebetoceras''. The connection between these slender deciduous forms and the typically ascoceroid '' Probillingsites'' from the early Upper Ordovician can be made through the slightly inflated '' Redpathoceras''. A phylogenetic sequence can be seen in the Ascoceratidae, from '' Probillingsites'' through '' Schucertoceras'', then '' Billingsites'', in the Upper Ordovician, and finally to the middle and upper Silurian '' Ascoceras''. Changes involve the arrangement and complexity of septa in the mature conch and in its shape, ending with the more elongate and compressed mature '' Ascoceras''. The Choanoceratidae is established for the derived Upper Silurian '' Choanoceras'', characterized by septa that form deep symmetrical cones and a subcentral expanded siphuncle with cyrtochoanitic and recumbent septal necks at maturity. The middle and possible upper Ordovician ancestral forms are placed in the Hebetoceratidae, which includes the orthoconic '' Hebetoceras'' and cyrtoconic '' Montyoceras''.

Paleoecology

The fully mature breviconic ascocerid, with the juvenile longiconic portion discarded was no doubt a facile swimmer. The dorsal chambers would have provided a stable center of buoyancy directly above the center of gravity with the shell in a horizontal orientation. The hyponomic sinus, observed in some, indicates active, directionally controlled hydro-jet propulsion. Just how maneuverable these creatures were is another matter as is how high in the water column they spent their time. How juvenile forms spent their time is less obvious. Juvenile shells are found in the same location as adult, indicating they lived in the same area. Juvenile ascocerids were probably more benthic than their adult counterparts, perhaps spending their time at or on the sea floor.

References

Additional sources

* * {{Taxonbar, from=Q3624873 Prehistoric cephalopod orders Orthoceratoidea Ordovician first appearances Silurian extinctions