heteromorph

ammonoid

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June 29, 2026

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Heteromorphs were aberrant ammonoids that appeared in the Late Triassic period. Ammonoids were extinct marine cephalopods related to modern nautiluses, found as fossils from the Devonian to the Cretaceous periods. These shelled creatures were likely predators, consuming zooplankton, crustaceans, and even other ammonoids. Their shells, which could be straight or coiled, provided protection and aided in buoyancy control.1

Ammonoids are distinguished from nautiloids by their complex suture patterns, where internal walls meet the outer shell. They are important index fossils due to their wide distribution, rapid evolution, and distinct features. Three main groups of ammonoids existed, each with increasingly complex sutures: goniatites (Paleozoic), ceratites (Triassic), and ammonites (Jurassic-Cretaceous).1

The heteromorphs, such as Choristoceras with loosely coiled chambers or Cochloceras with helical whorls, were short-lived, unusual forms that emerged during the Late Triassic. While most ammonoids nearly went extinct at the end of the Triassic, a few survived and diversified into true ammonites.2

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Triassic Period
Invertebrates
…the Late Triassic evolved bizarre heteromorphs with loosely coiled body chambers, such as Choristoceras, or with helically coiled whorls, such as Cochloceras. These aberrant forms were short-lived, however. A small group of smooth-shelled forms with more complex suture lines, the phylloceratids, also arose in the Early Triassic. They are regarded…
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…the Late Triassic evolved bizarre heteromorphs with loosely coiled body chambers, such as Choristoceras, or with helically coiled whorls, such as Cochloceras. These aberrant forms were short-lived, however. A small group of smooth-shelled forms with more complex suture lines, the phylloceratids, also arose in the Early Triassic. They are regarded…
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Cretaceous Period
Marine life
…of the more-unusual ammonites, called heteromorphs, were shaped like fat corkscrews and hairpins. Such aberrant forms most certainly had difficulty moving about. Ammonites preyed on other free-swimming or benthic invertebrates and were themselves prey to many larger animals, including the marine reptiles called mosasaurs.
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…of the more-unusual ammonites, called heteromorphs, were shaped like fat corkscrews and hairpins. Such aberrant forms most certainly had difficulty moving about. Ammonites preyed on other free-swimming or benthic invertebrates and were themselves prey to many larger animals, including the marine reptiles called mosasaurs.
Read More