The Lowoceratidae is a small family of discosorids, early nautiloid cephalopods, from the Middle

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 ...

in which the characteristic bullette is found only in early growth stages. Lowoceratids were first found in Southampton Island in the Canadian arctic. The Lowoceratidae consists so far of only two genera, ''Lowoceras'' and ''Tuylocras'', named by Foerste and Savage in 1927 and combined by Flower in 1940 as the Lowoceratidae. Both are strongly compressed, exogastric brevicones with the ventral side more narrowly rounded than the dorsal. Shells enlarge moderately to the middle of the mature body chamber, then contract gently toward the aperture. 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 ...

in each is slightly removed from the ventral margin. The siphuncle of ''Tuyloceras'', genotype ''T. percurvatum'', starts off with slender, subquadrate segments with thick rings and swollen bullettes, but later in life the segments become expanded, rings thinner, and bullettes simple and unswollen. ''Lowoceras'', genotype ''L. southamptonense'', is like Tuylocrdas, but with endocones in the siphuncle like the later ''Discosorus''. (Flower and Teichert 1957, Teichert 1964) The Lowoceratidae are derived from the

Westonoceratidae Westonoceratidae are exogastric, mostly compressed, Discosorida of moderate size from the Middle Ordovician to the Lower Silurian. The siphuncle is typically close to the convexly curved outer margin of the phragmocone – the chambered part of ...

(Teichert 1964), most likely from ''Faberoceras'', by the suppression of bullettes in the later growth stages. The

Discosoridae Discosoridae comprise a family of endogastric Discosorida, discosorids (Cephalopoda, Nautiloidea), with endocones in the siphuncle, ranging from the Middle Silurian to Middle Devonian. Shell morphology The shell in the Discosoridae is conical, e ...

are their likely descendants.

References

* Curt Teichert,C, 1964. Nautiloidea -Discosorida;

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 ...

, Vol.K,(K320-K325 & K334). * Flower,R.H.& Teichcert,C., 1957. The Cephalopod Order Discosorida;University of Kansas Paleontological Contributions,Mollusca, Article 6. {{Taxonbar, from=Q6694131 Discosorida Silurian first appearances Silurian extinctions