Palaeos Invertebrates: Mollusca: Cephalopoda: Ascocerida (original) (raw)

Glossoceras, Silurian of Sweden

image from Curt Teichert, "Main Features of Cephalopod Evolution"

Introduction

In the early Paleozoic world, in which the majority of cephalopods were massive slow-moving forms, the ascocerids were unique. They solved the ever-present buoyancy problem not by counterweighting the shell, but by dropping off the early portions at maturity. The remaining bulbous ovoid shell is known as the "ascocerid" type. These were small, agile animals, comparable to Mesozoic belemnites and Cenozoic squid.

The ascocerids were not only cephalopods to evolve deciduous shells, although they were the first, beginning in the Middle Ordovician. An identical growth pattern was developed independently in the Silurian by the Sphooceratidae and in the Late Carboniferous by the Brachycycloceratidae.

Technical Diagnosis

Primitively orthoceracones and cyrtocones with a subcentral planoconvex siphuncle in which natural truncation of the shell appears. Adoral segments of the siphuncle become broadly expanded, the mature part of the shell becomes strongly inflated, the septa rise adorally on the dorsum in this part, finally developing sigmoidal septa.

Flower and Kummel, p.610

Septa

The juvenile shell (and in primitive forms) is a slightly curved, slender cone, subdivided by simple, transverse septa. With the ascocerid stage, the shell expands and the septa are extended forward on the upper (dorsal) side, so as to place the gas chambers above, rather than behind, the living chamber, thus avoiding the need for counterweights.

Siphuncle

The immature shell posses a simple orthochoanitic siphuncle near the ventral (bottom) margin. In the mature ascocerid stage the siphuncle becomes cyrtochoanitic, and near-ventral in position

Ontogenic Growth

Primitive ascocerids did not change much from young to adult. But in more advanced forms, the early, simple straight or slightly curved part of the shell is shed at maturity. In this process, the aperture becomes contracted and the early part of the shell is broken off behind a certain septum, termed the septum of truncation. The break in the siphuncle is sealed by a callus.

In the remaining mature portion of the shell the gas chambers are located partly or entirely above the living chamber. This was a very streamlined configuration, and the mature ascocerids were probably among the most agile and graceful of the early Paleozoic nautiloids, comparable to modern-day squid and cuttlefish.

The biological processes that led to the truncation of the early ("nautiloid") part of the shell are not well understood and Dzik (1981, 1984) has denied that truncation occurred, but nevertheless the phenomenon seems to be well documented as has been shown by Turek and Marek (1986). Furnish and Glenister (in Teichert et al., 1964) suggested that it was caused by the plugging of the siphuncle.

Phylogenetic Relationships

These Ascocerids seem to have branched off from the Orthoceratids in the very early part of the Middle Ordovician, presumably from orthoceridshaving slightly curved suborthochoanitic shells. Derivation of Ascoceras and its relatives from more normal types of nautiloids can be traced, various stages in the sequence being represented by Middle and Late Ordovician genera.

Evolutionary History

An evolutionary sequence of ascocerid nautiloids (left) leads from simple, straight or gently curved forms to highly modified shells of Ordovician and Silurian age, belonging to the family Ascoceratidae. In these advanced forms, the early shell is shed at maturity, leaving a compact and streamlined living chamber.

1. Centroonoceras Kobayashi, Early Ordovician, a slightly curved, suborthochoanitic form, probably near or in the line of ascocerid ancestry.
2. Montyoceras Flower, early Middle Ordovician of Vermont; retains normal septa, but truncates the early part of the shell at maturity.
3. Schuchertoceras Miller, Middle and Late Ordovician; a true ascoceroid, with mature cyrtochoanitic siphuncle, and its last two septa extended forward over the living chamber
4. Lindstroemoceras Miller, Silurian of Scandinavia; contains three ascoceroid septa.
5. Ascoceras Barrande, Silurian of Bohemia; contains as many as seven ascoceroid septa.

There is a gap in the geologic record of that group, because no Early Silurian representatives are known. In the Middle and Late Silurian odd specializations, like lacunose septa and complex septal formations in the body chamber, develop. The order died out at the end of the Silurian and left no descendants. It is apparently unrelated to the two orthocerid families in which deciduous shells developed: the Middle Silurian Sphooceratidae and the Upper Carboniferous Brachycycloceratidae.

image left/above from Moore, Lalicker and Fischer, Invertebrate Fossils

Lifestyle and ecology

Ascocerids went though a more radical metamorphosis than any other nautiloid type. Young forms lived a lifestyle like that of orthocerids, probably slow-moving bentho-nektonic predators on small invertebrates. The mature animal, with its light streamlined ovoid shell, was an agile coleoid-like animal [Holland, p.156], perhaps with a lifestyle much like a Mesozoic belemnite or Cenozoic squid.

Yet curiously, despite their apparent advantage, throughout their history the Ascocerida only seemed to have formed small and geographically isolated populations that were probably vulnerable to ecological competitors [Teichert 1988, p. 40].

Families and Genera

The following list is in no way definitive, up to date, or comprehensive. It is from on Flower and Kummel, p.609. The dates are from The Fossil Record II

Family HEBETOCERATIDAE Flower

Shamattawaceras ascoceroides Foerste & Savage
Late Ordovician, Manitoba
Length :
image from Moore, Lalicker and Fischer, Invertebrate Fossils, p.357

Shells slender or inflated adorally, adoral sutures swing forward on dorsum in advanced forms, but sutures are not sigmoid. Possibly derived fromClinoceratidae, gave rise to Choanoceratidae and Ascoceratidae
Middle to Late Ordovician (Llandeilo to Hirnantian).
Montyoceras, Hebetoceras, Ecdyceras, Pachecdyceras, Probillingsites, Shamattawaceras

Family CHOANOCERATIDAE Miller
Shells slender, slightly curved, Septa strongly conical, fused adorally ventral of the siphuncle.
Silurian (Wenlock).
Choanoceras

Family ASCOCERATIDAE Barrande

Billingsites candensis (Billings)
Late Ordovician, Anticosti Island
Length :
image from Moore, Lalicker and Fischer, Invertebrate Fossils, p.357

Mature shell inflated, retaining at most one normal septum and several ascoceroid septa with sigmoid sutures.
Late Ordovician to Late Silurian (Caradoc to Ludlow)
Schuchertoceras, Billingsites, Lindstromoceras Parascoceras, Pseudascoceras, Ascoceras, Aphragmites, Giossoceras.

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page uploaded 24 September 2002
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(originally uploaded on Kheper Site 28 December 2000)
text © M. Alan Kazlev 1999-2002
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