The straight-shelled cephalopod that fooled scientists for over a century

Nature · Published 3 October 2026 Swipe more like this →

In short: For years, the name Orthoceras was a catch-all term for any straight-shelled fossil found in the ground. Modern research has since dismantled this wastebasket taxonomy, revealing that these ancient marine creatures are far more exclusive than once believed. Discover the true identity of this iconic, long-extinct nautiloid.

The name Orthoceras, derived from the Ancient Greek words for straight and horn, was once applied broadly to almost any conical-shelled nautiloid from the Paleozoic or Triassic periods. This broad usage turned the genus into a wastebasket taxon, a scientific dumping ground where unrelated species were lumped together based solely on their superficial, tubular appearance. It was not until rigorous re-cataloging and detailed analysis of internal structures—such as the siphuncle and cameral deposits—that paleontologists realized these fossils actually belonged to a diverse array of distinct groups and even different orders.

Today, the authoritative Treatise on Invertebrate Paleontology restricts the genus Orthoceras to a much narrower definition. It now refers specifically to the type species, Orthoceras regulare, which dates back to the Middle Ordovician period. These creatures were native to marine environments across a region spanning modern-day Sweden, Russia, Ukraine, Belarus, Estonia, and Lithuania. While they are characterized by slender, elongate shells with a transversely constricted body chamber and a network of fine surface lirae, many similar-looking fossils previously labeled as Orthoceras are now classified under the genus Michelinoceras.

A common point of confusion persists in the commercial fossil trade, where Orthoceras is frequently mistaken for Baculites. While both are straight-shelled cephalopods, they are separated by millions of years of evolutionary history. Orthoceras thrived during the Middle Ordovician, whereas Baculites appeared much later in the Late Cretaceous. The two can be reliably distinguished by their suture lines: the lines on Orthoceras are simple, while those on Baculites are intricately foliated. This distinction is vital for collectors and researchers alike, as these two lineages evolved their tubular forms independently of one another at different stages of Earth's history.

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