The belemnoids appear to be the oldest and most primitive of the coleoid
cephalopods. Their earliest known occurrence is in rocks of
Mississippian age, and they were particularly abundant during the
Mesozoic. They became extinct at the end of Cretaceous time but have
left considerable evidence of their existence in the Mesozoic strata of
many parts of the world. Certain forms, because of their abundance and
relatively short geologic range, are excellent guide fossils. Belemnoids
have been found in the Upper Cretaceous of Texas (Pl. 33) but in general
are rare or unknown in most Texas formations.