Cameroceras trentonense

Cameroceras ("chambered horn") is a genus of extinct, giant orthoconic cephalopod that lived mainly during the Ordovician
 period. It first appears during the middle Ordovician, around 470
million years ago, and was a fairly common component of the fauna in
some places during the period, inhabiting the shallow seas of Laurentia, Baltica and Siberia. Its diversity and abundance became severely reduced following the Ordovician–Silurian extinction events, and the last remnants of the genus went extinct sometime during the Wenlock.

Cameroceras is a cephalopod,
 a taxon of molluscs that includes the octopuses, squids and cuttlefish.
 From comparison with living cephalopods, particularly the shelled chambered nautilus,
 some inferences about the biology of Cameroceras can be made. The head
of the animal would have been soft muscular tissue situated at the
opening of the hard cone-like shell, with the mantle (main body) lying
within the shell for protection. Tentacles would have grown from the
base of the head like in a modern nautilus, and these tentacles would
have been used to seize and manipulate prey. At the base of these
tentacles within the buccal mass (analogous to the mouth) a hard
keratinous beak would have bitten into the bodies of its prey, and is
assumed to have been strong enough to breach the prey's exoskeleton or
shell. Within the beaks of modern cephalopods a radula, or 'toothed'
tongue is used to rasp out soft tissue from within the prey's shell.
Size
The
 partial shell of one giant Cameroceras yielded a total length estimated
 at the time at nearly 9 m (30 feet). This estimate has since been
revised downward quite a bit; Frey (1995) gives a length of up to 6 m
(20 feet). Regardless of this estimate's degree of accuracy, this
gargantuan cephalopod is thought to be among the largest known Paleozoic
 molluscs.

Cameroceras has become a "wastebasket taxon" in which large orthoconic endocerids such as Endoceras, Vaginoceras, and Meniscoceras
 were originally placed. This makes it extremely difficult to describe
Cameroceras as a distinct genus. Although the type species Cameroceras
trentonense was first described by Conrad in 1842, since then the
generic term has had variable meaning.
Hall, who named and described
 Endoceras in 1847 recognized C. trentonense specifically, but used
Endoceras for other specimens of large endocerids from the Trenton
Limestone of western New York state. Cameroceras and Endoceras have even
 been applied to different stages of the same species. Although
Cameroceras takes precedence where the two refer to the same species,
its vague application leaves Endoceras or other better-described genus
the term of choice.

The lifestyle and habits of Cameroceras can only be surmised. They were
almost certainly stalkers and ambush predators that moved across the sea
 floor or lay in wait. The large rigid shell would have made maneuvering
 difficult. The larger ones especially were probably not active
swimmers. The very largest may have remained on the bottom without ever
changing position.
As with all endoceratids Cameroceras was
weighted so as to be horizontally stable. This would have been achieved
by the endocones in the apical portion of the siphuncle that would have
compensated for the visceral mass forward in the body chamber. The
endocones would also have sealed off the more apical chambers from the
siphuncle limiting changes in ballast to the more forward, centrally
located, chambers. The result is that changes in buoyancy would have had
 little effect on horizontal stability.