Westonoceratidae are exogastric, mostly compressed, Discosorida

Discosorida

Discosorida is a unique order of cephalopods that lived from the beginning of the Middle Ordovician, through the Silurian, and into the Devonian. Discosorids are unique in the structure and formation of the siphuncle, the tube that runs through and connects the chambers in cephalopods, which unlike...

 of moderate size from the Middle Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 to the Lower Silurian

Silurian

The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

.

The siphuncle

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 Spirula...

 is typically close to the convexly curved outer margin of the phragmocone -the chambered part of the shell- taken to be ventral but may be more central in some genera. Connecting rings are thin to moderately thick with inflated bullettes grasping the preveous septal foremina. Parietal deposits within the siphuncle from endocones in advanced genera and cameral deposits are found in some.

The Westonoceratidae are derived from the Cyrtogomphoceratidae

Cyrtogomphoceratidae

The Cyrtogomphoceratidae is a family in the cephalopod order Discosorida that comprises genera commonly with compressed, endogastrically curved shells. Siphuncles lie close to the venral side, segments are broadly inflated, connecting rings thick and apically expanded thick bullettes. Chambers are...

 through the cyrtogomphoceratid, Ulrichoceras

Ulrichoceras

Ulrichoceras is recognised as the basal cyrtogomphoceratid genus, which is the source for the rest of the Cyrtogomphoceratidae as well as for the Westonoceratidae...

. In tern, the Westonoceratidae gave rise to a small group of discosorids, the Lowoceratidae

Lowoceratidae

The Lowoceratidae is a small family of discosorids, early nautiloid cephalopods, from the Middle Silurian in which the characteristic bullette is found only in early growth stages...

, which form an intermediary step before the Discosoridae.

Two principal lineages have been identified, both stemming from the genus Teichertoceras

Teichertoceras

Teichertoceras is a discosorid genus in the family Westonoceratidae characterized by an endogastric curvature to the early portion of the phragmocone....

, a derivative of Ulrichoceras. One goes simply from Westonoceras

Westonoceras

Westonoceras is an extinct nautiloid genus from the Discosorida that lived during the Middle and Late Ordovician that has been found in North America, Greenland, and Northern Europe...

to Winnipegoceras

Winnipegoceras

Winnipegoceras is an extinct nautiloid genus from the Ordovician belonging to the Order Discosorida.-Phylogeny:Winnipegoceras is included in the discosorid family Westonoceratidae and is derived from Westonoceras...

. The other goes from Faberoceras

Faberoceras

Faberoceras is an extinct genus of prehistoric nautiloid. The nautiloids are a subclass of shelled cephalopods that were once diverse and numerous but are now represented by only a handful of species.-References:...

to Clarkesvillia

Clarkesvillia

Clarkesvillia is an upper Ordovician westonocerid genus the differs from Faberoceras in having a more flattened venter and from the later Glyptodendron in having its siphuncle more ventrally located.-references:...

to Glyptodendron

Glyptodendron

Glyptodendron is a Lower Silurian westonocerid characterized by compressed cyrtocones with a narrowly rounded dorsum and greatest width in the ventrolateral region. Sutures slope forward from the dorsum which is on the longitudinally concave side. The siphuncle is slightly ventral from the center....

. Faberoceras may also have given rise to Hecatoceras

Hecatoceras

Hectoceras is a genus in the nautiloid cephalopod order Discosorida from the Upper Ordovician of Australia , known from a few isolated siphuncle specimens....

and Reedsoceras

Reedsoceras

Reedsoceras is a large member of the discosorid family Westonoceratidae from the middle and upper Ordovician of North America....

and is the likely source for the Lowoceratidae. Two other genera are included, but are separately derived from Ulrichoceras. These are Sinclairoceras

Sinclairoceras

Sinclairoceras is a nautiloid cephalopod in the discosorid family Westonoceratidae from the Middle Ordovician Simard Limestone of Quebec. The shell is a compressed exogastric brevicone with venter narrowly and dorsum broadly rounded. The dorsum, at the upper surface, is only slightly concave...

and Simardoceras

Simardoceras

Simardoceras is a genus in the discosorid family Westonoceratidae from the Middle Ordovician of Quebec.The shell of Simardoceras is a moderately expanding exogastric cyrtocone, said to resemble a rather large Oncoceras, but with a siphuncle that is definitely discosorid. The venter is almost...

.

References

• Teichert, Curt 1964. Nautiloidea - Discosorida; Treatise on Invertebrate Paleontology
  Treatise on Invertebrate Paleontology
  The Treatise on Invertebrate Paleontology published by the Geological Society of America and the University of Kansas Press, is a definitive multi-authored work of some 50 volumes, written by more than 300 paleontologists, and covering every phylum, class, order, family, and genus of fossil and...
  
  , Part K ; Geol Soc of America and University of Kansas Press
• Flower, R H and Teichert, C; 1957. The Cephalopod Order Discosorida. University of Kansas Paleontological Contributions. Mollusca, Article 6, pp 1-144.