''Forresteria'' is an extinct genus of

cephalopod A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...

belonging to the subclass

Ammonoidea Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttle ...

. They flourished during the late Turonian and early

Coniacian The Coniacian is an age or stage in the geologic timescale. It is a subdivision of the Late Cretaceous Epoch or Upper Cretaceous Series and spans the time between 89.8 ± 1 Ma and 86.3 ± 0.7 Ma (million years ago). The Coniacian is preceded by t ...

of the Late Cretaceous,Cobban, William A. ''et al'' (2006) "A USGS Zonal Table for the Upper Cretaceous Middle Cenomanian-Maastrichtian of the Western Interior of the United States Based on Ammonites, Inoceramids, and Radiometric Ages

USGS (Open-File Report 2006–1250) and were global in extent. ''Forresteria alluaudi'' and ''Forresteria hobsoni'' are considered marker fossils for the lower Coniacian in the American West.

Description

Although the whorl section and ornament of ''Forresteria'' are variable, it is easily distinguished from ''

Barroisiceras ''Barroisiceras'' is an acanthoceratacean ammonite from the Upper Cretaceous, Coniacian, included in the family Collignoniceratidae. Diagnosis The shell of ''Barroisiceras'' is rather involute, coiled such that the outer whorl embraces much of ...

'' by the presence of mid-lateral tubercles on the inner whorls, which later disappear or fuse with either umbilical or ventrolateral tubercles. Four subgenera are recognized W.J Arkell ''et al.'', 1957 Mesozoic Ammonidea. Treatise on Invertebrate Paleontology, Part L Ammonoidea. * ''F. (Forresteria)'': Whorl section moderately to very inflated. Mid-lateral tubercles fuse with ventrolateral. * ''F. (Reesideoceras)'': Whorl section less inflated than with ''F. (Forresteria)''. Mid-lateral tubercles fuse with the umbilical. Keel disappears on outer whorl leaving venter flat or concave, bordered by ventrolateral clavi. * ''F. (Harleites)'': Shell compressed, with high keel and steep umbilical wall. Early whorls have weak umbilical, strong mid-lateral, and fine, feeble ventrolateral tubercles. * ''F. (Zumpangoceras)'': Inclusion doubtful. Known only from crushed specimens from Mexico. Mid lateral tubercle strengthens with age.

Species

species in ''Forresteria'' include: * ''Forresteria alluaudi'' (Boule, Lemoine and Thévenin, 1907) * ''Forresteria brancoi'' * ''Forresteria hobsoni'' * ''Forresteria neo-mexica'' * ''Forresteria petrocoriensis'' (Coquand, 1859) * ''Forresteria peruana'' ''Forresteria'' was named for Robert Forrester of Salt Lake City, Utah.

References

Ammonitida genera Collignoniceratidae Cretaceous ammonites Ammonites of North America Turonian genus first appearances Coniacian genus extinctions {{Ammonitina-stub