The Ordovician Period (485-443 million years ago) marked the emergence of the first complete vertebrate fossils, featuring jawless armored fish known as ostracoderms. These primitive fish, more correctly classified as Pteraspidomorphi, had distinctive armor consisting of large bony plates covering their heads and smaller scales protecting their tails.1 One prominent group, the Arandaspids, included species like Sacabambaspis, which resembled "a watermelon with a tail" rather than modern fish.23 Their armor was likely composed of aspidin (a primitive bone-like tissue), dentine, and enameloid—materials remarkably similar to those in human teeth.2

These ancient armored fish served as evolutionary pioneers, though their protective coverings may have had dual purposes. While offering some defense against predators, the bony armor also likely functioned as mineral storage for calcium and phosphorus—essential elements for muscle movement and energy transfer.2 This adaptation potentially allowed these fish to explore diverse aquatic environments, including both saltwater and freshwater habitats.24 Modern descendants of these armored jawless fish include lampreys and hagfish, connecting us to these fascinating Ordovician creatures that first developed the sensory dentine tissue linked to today's tooth sensitivity.4

The case of Anatolepis heintzi exemplifies how scientific understanding evolves through improved technology and careful analysis. Once thought to represent the earliest vertebrate teeth containing dentine, these 500-million-year-old Cambrian fossils have now been reclassified as arthropod remains.12 Using advanced synchrotron X-ray tomography at Argonne National Laboratory, researchers discovered that what appeared to be dentine tubules were actually sensory structures similar to those found in modern crabs and other arthropods—a case of convergent evolution where unrelated organisms developed similar features.2

This misidentification clarified the timeline of vertebrate dental evolution, pushing back the origin of true vertebrate dental tissue by 40 million years to the Middle Ordovician period (approximately 470 million years ago).2 The reclassification of Anatolepis as an invertebrate rather than an early vertebrate "effectively clarifies a persistent confusion in the fossil record about early vertebrate presence" and establishes a clearer boundary between vertebrate and invertebrate sensory adaptations.1 True vertebrate dental structures were subsequently identified in Middle Ordovician fossils of ancient fish species like Eriptychius and Astraspis, whose tooth-like odontodes showed clear evidence of sensory function rather than primarily feeding purposes.2

The research team, led by Dr. Yara Haridy, discovered that dentine first functioned as sensory tissue in odontodes—bumpy structures on the exoskeletons of early vertebrate fish from the Ordovician period.12 These structures contained microscopic tubules that likely helped the fish detect pressure, temperature changes, and potentially even pain through their armor.3 Remarkably, similar sensory adaptations evolved independently in both vertebrates and invertebrates, as the study found comparable structures in ancient arthropods like crabs and shrimp, suggesting convergent evolution toward similar solutions for environmental sensing.45

This evolutionary connection explains why modern teeth remain so sensitive—they retain their ancient sensory function despite now serving primarily for feeding. The discovery supports the "outside-in" hypothesis of tooth evolution, where external sensory structures predated teeth and were later co-opted for use in the mouth.3 Modern examples of this evolutionary link can be seen in miniature suckermouth catfish, which possess skin denticles (tiny tooth-like scales) that connect to nerve fibers, allowing them to sense mechanical stimuli through their skin—a living parallel to the ancient odontodes.3