Bactritida are a small and poorly studied order of more or less straight-shelled ("orthocone") nautiloids which first appeared during the Emsian Stage of the Devonian Period (390 my ago) and persisted until the Carnian Stage of the Triassic Period (235 my ago). They are considered ancestors of the ammonites as well as of the coleoids (octopus, squid, cuttlefish, and the extinct belemnites). Scientific classification or biological classification is how biologists group and categorize extinct and living species of organisms. ... Phyla Porifera (sponges) Ctenophora (comb jellies) Cnidaria (coral, jellyfish, anemones) Placozoa (trichoplax) Subregnum Bilateria (bilateral symmetry) Acoelomorpha (basal) Orthonectida (parasitic to flatworms, echinoderms, etc. ... Classes Caudofoveata Aplacophora Polyplacophora - Chitons Monoplacophora Bivalvia - Bivalves Scaphopoda - Tusk shells Gastropoda - Snails and Slugs Cephalopoda - Squids, Octopuses, etc. ... Orders Sepiida Sepiolida Spirulida Teuthida Octopoda Vampyromorphida Nautilida The Cephalopods (head-foot) are the mollusk class Cephalopoda characterized by bilateral body symmetry, a prominent head, and a modification of the mollusk foot, a muscular hydrostat, into the form of arms or tentacles. ... Orders Nautilida Bactrida Nautiloids are a group of marine animals which all possess an external shell, the most well known example being the modern nautiluses. ... Orders Palcephalopoda Plectronocerida (extinct) Ellesmerocerida (extinct) Actinocerida (extinct) Endocerida (extinct) Tarphycerida (extinct) Oncocerida (extinct) Discosorida (extinct) Nautilida Neocephalopoda Orthocerida (extinct) Ascocerida (extinct) Bactritida (extinct) Nautiloids are a group of marine mollusks which all possess an external shell, the most well known example being the modern nautiluses. ... Disambiguation: Devonian is also an adjective relating to the English county of Devon or the people there. ... The Carnian is a stage on the geologic time scale occuring from 228 +/- 2 to 216. ... The Triassic is a geologic period that extends from about 245 to 202 Ma (million years ago). ... Orders and Suborders Order Ammonitida Ammonitina Acanthoceratina Ancyloceratina Phylloceratina Lytoceratina Order Goniatitida Goniatitina Anarcestina Clymeniina Order Ceratitida Ceratitina Prolecanitina Ammonites are an extinct group of marine animals (subclass Ammonoidea) in the phylum Mollusca and class Cephalopoda. ... Orders Sepiida Sepiolida Spirulida Teuthida Octopoda Vampyromorphida Subclass Coleoidea is the grouping of cephalopods containing all the primarily soft_bodied creatures. ... Families 14 in two suborders, see text. ... Suborders Myopsina Oegopsina Squids are the large, diverse group of marine cephalopods popular as food in cuisines as widely separated as Korean and Italian. ... Families Sepiadariidae Sepiidae Cuttlefish are animals of the order Sepiida, and are marine cephalopods, small relatives of squids and nautilus. ... Extinct Orders Aulacocerida Phragmoteuthida Belemnitida Diplobelida Belemnoteuthina Belemnites (or belemnoids) are an extinct group of marine cephalopod, very similar in many ways to the modern squid and closely related to the modern cuttlefish. ...

The bactritids are distinguished from the other, more primitive nautiloids by the globular shape and small size of the shell's first chambers, the so-called embryonic shell. All nautiloids have relatively large embryonic shells, and living species lay a few large eggs. In contrast, bactritids produced large numbers of small eggs, each housing a small embryonic shell -- just like the ammonites. Orders Palcephalopoda Plectronocerida (extinct) Ellesmerocerida (extinct) Actinocerida (extinct) Endocerida (extinct) Tarphycerida (extinct) Oncocerida (extinct) Discosorida (extinct) Nautilida Neocephalopoda Orthocerida (extinct) Ascocerida (extinct) Bactritida (extinct) Nautiloids are a group of marine mollusks which all possess an external shell, the most well known example being the modern nautiluses. ...

The adult bactritid shell consists of a linear series of chambers, each having been successively occupied by the body of the animal over the course of the its life. Bactritids likely fed on animals caught in the tentacles. Although horizontal movements was probably slow, acceleration directly forward and backward was likely swift and unencumbered.

During early Emsian evolution, there was a trend toward increasing curvature; simultaneously, the orientation of the aperture moved from oblique upward to horizontal (40–70°). In more derived bactritids with curved shells, the aperture reached an oblique downward orientation (Cyrtobactrites: 20-30°). Evolution of the coiled ammonoid shell from the uncoiled bactritid shell was probably coupled with changes in manoeuvrability and swimming velocity.