The valvular intestine in fishes and a reassessment of the formation of the spiral coprolites

Abstract

Living vertebrates with valvular intestines include agnathans, chondrichthyans, actinopterygians, dipnoans, and the actinistians. Fossils which have valvular intestines preserved, are included in the placoderms, chondrichthyans, possibly the acanthodians, and the actinopterygians. The most common intestine is Type D, found in all groups (except the agnathans), and is considered as the primitive condition. The actinopterygians progressively reduce the valvular intestine until it is lost.

The Type D valvular intestine in Scyliorhinus canicula is demonstrated to produce spirally coiled fecal masses. The fecal masses are similar in morphology to heteropolar coprolites, previously described as "enterospirae" (fossilized valvular intestines). The "enterospirae" are reinterpreted as true coprolites.