These ancient jawless fish possessed stomachs long before jaws ever evolved in vertebrates

Nature · Published 3 October 2026 Swipe more like this →

In short: Thelodonti were once thought to be flattened, armor-plated bottom dwellers. New fossil evidence reveals they were actually three-dimensional, shark-like swimmers with complex guts. These mysterious creatures, covered in tiny, resilient scales rather than heavy plates, challenge our understanding of how early vertebrate anatomy and ecological roles first emerged.

Thelodonti, or nipple-toothed fish, represent a class of extinct jawless vertebrates that thrived from the Late Ordovician through the Late Devonian. Unlike their contemporaries, which were often encased in heavy, inflexible armor, thelodonts were covered in thousands of small, non-overlapping scales. These scales, typically 0.5 to 1.5mm in size, were composed of dentine and aspidine, providing protection against abrasion while allowing for a more flexible, streamlined body shape. This agility likely enabled them to occupy diverse ecological niches, including reef environments, much like modern sharks.

Because thelodonts lacked an internal bony skeleton, complete fossils are exceptionally rare. Most of our knowledge stems from their abundant, resilient scales, which detached easily after death. However, 1993 marked a turning point when three-dimensional specimens were discovered. These fossils overturned long-held assumptions that thelodonts were flattened, revealing instead a body plan with a developed braincase and fork-shaped tails. Most surprisingly, these specimens contained visible stomachs, a feature previously thought to have emerged only after the evolution of jaws.

The taxonomy of the group remains a subject of intense debate. Researchers are divided on whether thelodonts form a single, monophyletic group or a collection of disparate stem groups leading to later jawed and jawless fish. Current classifications, such as those proposed by Wilson and Märss in 2009, rely heavily on scale histology and morphology to organize these fish into orders like Archipelepidiformes and Furcacaudiformes. While scale morphology is the primary tool for identification, it is inherently limited by the fact that scale shapes vary significantly across different body regions of a single fish, making it difficult to distinguish between species with absolute certainty.

Ultimately, thelodonts serve as a vital link in understanding vertebrate evolution. Their internal anatomy, particularly the presence of a stomach and heterocercal tails, suggests a closer relationship to jawed gnathostomes than previously suspected. As we continue to analyze their unique scale growth patterns, thelodonts remain a critical, if elusive, piece of the puzzle in the history of early vertebrate life.

General information, not professional advice. Spotted an error? Flag the card or see corrections and complaints.

Intellect Plus is a visual educational discovery platform for exploring fascinating facts and ideas. Brain Food, Not Brain Rot. What is Intellect Plus?

Related

• The earliest vertebrates were jawless, armored filter-feeders that thrived in ancient, mysterious waters.
• Before modern sharks ruled the oceans, hump-backed predators dominated the ancient seas.
• Jaws remade vertebrates and now define nearly every living one
• Ancient jawless fish reveal the hidden origins of our paired limbs
• The prehistoric shark with a buzzsaw jaw that baffled scientists for over a century
• The scythe lizard: a giant theropod that traded meat for a plant-based diet
• The improbable oceanic voyages that built Madagascar's unique wildlife
• Therocephalians were the ancient, beast-headed cousins that almost became our mammalian ancestors.
• The cartoonist who traded a music store job for surrealist absurdity
• Did this overlooked dinosaur spend its final days digging underground to survive?
• Why did these bizarre marine reptiles disappear while their competitors thrived for millions of years?
• The Australian thorny devil drinks water through its skin using a complex capillary system.

More in Vertebrates · More in Nature · Latest · A–Z · All topics