Ascocerida

From Palaeos.org

The Ascocerida comprises an order of nautiloid cephalopods included in the Orthoceratoidea characterized by a shell with a deciduous juvenile portion that at maturity becomes detached.

Morphology

The juvenile, or deciduous, portion of an ascocerid shell consists typically of a narrow, circular cyrtocone that undergoes periodic truncation. Cumulative length, included broken off segments, may reach about 20cm. (~8in.) The siphuncle is located half way between the center of the shell and the venter, and is thin walled and tubular with short, orthochoanitic septal necks and segments that are only slightly inflated.

The adult, or mature, ascocerid shell is typically an expanded exogastric brevicone with special unique features. The apical end is formed by the septum of truncation which is about three times s thick as the 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 provide a concentrated buoyancy above the visceral mass.

Derivation and Phylogeny

The Ascocerida first appeared early in the Middle Ordovician and are most likely derived from the michelinocerid family Clinocertidae, possibly from Clinoceras beginning with such slender forms as Montyoceras and Hebetoceras. The connection between these slender deciduous forms and the more typical ascocerid Probillingsites from the early Upper Ordovician can be made through the slightly inflated Redpathoceras.

A phylogenetic sequence can be seem 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 was 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 possibly upper Ordovician ancestral forms within the Ascocerida 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

• Rousseau H Flower; Development of the Mixochoanites, Journal of Paleontology, v.15, n.5, pp523-548, Sept 1941
• Rousseau H Flower; Ordovician Cephalopod Faunas and Their Role in Correlation; The Ordovician System: proceedings of a Palaeontological Association symposium, University of Wales Press and the National Museum of Wales, Cardiff, 1976
• W.M. Furnish and Brian F. Glenister. Nautiloidea-Ascocerida; Treatise on Invertebrate Paleontology, part K, (...Nautiloidea); Geological Society of America and University of Kansas Press, 1964.
• Charles H Holland; The Nautiloid Order Ascocerida in the British Silurian; Palaeontology v.42, pt 4,, pp683-689. 1999