Bactritida
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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 million years ago) and persisted until the Carnian Stage of the Triassic Period (235 million years ago). They are considered ancestors of the ammonites as well as of the coleoids (octopus, squid, cuttlefish, and the extinct belemnites).
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.
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 its life. Bactritids likely fed on animals caught in the tentacles. Although horizontal movements were 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 maneuverability and swimming velocity.
