What Was the Mysterious Palaeospondylus Fossil?
For over a century, the Palaeospondylus fossil defied classification. Learn how advanced imaging resolved a long-standing paleontological debate about this early vertebrate.
Discovered in Scotland in 1890, the fossil of a small, eel-like creature from the Devonian period presented an immediate puzzle to science. This animal, named Palaeospondylus gunni, was abundant in the fossil record, yet its unique anatomy resisted easy classification. For over 130 years, its place within the evolutionary tree of vertebrates remained one of paleontology’s most persistent mysteries.
Anatomy of a Prehistoric Enigma
The fossils of Palaeospondylus were first unearthed at the Achanarras slate quarry in Caithness, Scotland. These remains revealed an animal that grew up to 6 centimeters (about 2.4 inches) long, with a distinctly eel-shaped body. The fossils are plentiful, but their small size and the way they were preserved made detailed analysis difficult for early paleontologists.
What made Palaeospondylus so perplexing was its unusual combination of anatomical features. The creature possessed a well-developed skeleton made of cartilage, not bone. Its skull was flattened and featured large openings for eyes, but it completely lacked any evidence of teeth or the dermal (external) bones that form the jaws and skulls of most fish. This absence of key features, coupled with no paired fins or limbs, made it difficult to compare to any known group of animals.
The animal’s skeleton, though simple in some respects, showed a complex gill apparatus situated below the back of its skull. It also had a vertebral column, which is what its name, meaning “ancient vertebra,” refers to. This strange mix of traits meant that any attempt to classify it was met with conflicting evidence, leaving its true nature shrouded in mystery.
A Century of Scientific Debate
The puzzling anatomy of Palaeospondylus fueled a scientific debate that spanned over a century. Researchers proposed numerous, and often contradictory, identities for the creature. Its placement on the tree of life was a moving target, as its anatomy seemed to share traits with both jawed and jawless vertebrates.
One prominent theory suggested it was a type of jawless fish, or agnathan, similar to modern lampreys, based on its eel-like body and lack of jaws. Another hypothesis proposed that the fossils were not of an adult animal, but the larval stage of a different creature. Scientists considered it might be the tadpole-like larva of an early amphibian or a juvenile lungfish.
Other researchers pointed to similarities with cartilaginous fishes, suggesting it could be a relative of sharks and rays. A 2017 study proposed it was a stem chondrichthyan, an early member of the cartilaginous fish lineage. Each of these theories was based on the limited morphological evidence available from the flattened fossils.
The Breakthrough Identification
The mystery of Palaeospondylus was clarified by a technological advancement that allowed scientists to see inside the fossil in unprecedented detail. Using synchrotron radiation X-ray micro-computed tomography, researchers created detailed 3D reconstructions of the fossil’s skull without destroying the specimen. For this analysis, scientists chose fossils where the head was still embedded in rock, ensuring delicate internal structures were better preserved.
These high-resolution scans revealed anatomical features that had been hidden for over a century. Inside the skull, the team identified three semicircular canals, a structure of the inner ear that is characteristic of jawed vertebrates. This finding ruled out the theory that Palaeospondylus was related to jawless vertebrates. The scans also revealed details of the neurocranium, or braincase.
The detailed 3D models of the skull showed its structure was not like that of a jawless fish or a shark, but instead resembled the skulls of early sarcopterygians, or lobe-finned fishes. The neurocranium of Palaeospondylus was similar to that of Panderichthys and Eusthenopteron, which are well-known tetrapodomorphs. This group of fish includes the ancestors of all four-limbed vertebrates, placing Palaeospondylus on the evolutionary branch leading to tetrapods.
This new evidence established that Palaeospondylus was not a direct ancestor but an early, specialized offshoot of our own distant lineage. Its lack of teeth and limbs are now understood as unique evolutionary modifications, possibly representing a form of arrested development. After 130 years of debate, technology provided the evidence needed to place this creature in its proper evolutionary context.