A fish fossil that is around 455 million years old provides new insights into how the skull of vertebrates developed. Using computer tomography, researchers have reconstructed the fossilized head of the prehistoric fish Eriptychius americanus in three dimensions. Accordingly, his brain was not protected by a closed skull capsule, but was only surrounded by cartilage plates that had not grown together. This means that the head of the prehistoric fish differs from all previously known skull structures in living or extinct vertebrates.
An important feature of vertebrates is their skull, which protects the delicate brain. But how did this protective cap made of bone or cartilage develop? This question has so far been difficult to answer. In almost all modern vertebrates - including us humans - the individual parts of the skull are fused together. The so-called neurocranium, the structure that surrounds the brain, consists of a single unit. Only in roundmouths, a primitive group of vertebrates that include lampreys and hagfishes, does the neurocranium consist of an open cartilage framework.
Museum specimen re-examined
However, how the simple cartilaginous structure of round mouths changed into a closed skull during early vertebrate evolution remains largely unknown. “The fossil record offers little bridge between the two states, as the cranial anatomy of early vertebrates shows large phylogenetic and temporal gaps,” explain Richard Dearden of the University of Birmingham in the United Kingdom and his colleagues. To fill this gap, they have now examined a museum specimen of one of the earliest known vertebrates in more detail.
“Using computer tomography, we reconstructed the enigmatic jawless fish Eriptychius americanus in three dimensions,” report the researchers. The specimen comes from the Harding Sandstone in Colorado in the USA and is dated to be 455 million years old. “At first glance, Eriptychius is not the prettiest of fossils,” says Dearden. “However, by using modern imaging techniques, we were able to show that it preserves something unique: the oldest three-dimensionally preserved vertebrate head in the fossil record.”
Unlike any known vertebrate skull
The research shows that the fossil fish's skull consisted of a symmetrical set of cartilage plates. “These surround the front of the lateral eye sockets, the terminal mouth, the olfactory bulbs and the pineal gland,” writes the team. “However, the cartilage is not fused into a single neurocranial unit. This suggests that this feature only developed later in the evolution of vertebrates.” At the same time, the skull of the fossil differs significantly from the open cartilaginous framework of today’s round mouths. “The neurocranium thus reveals an anatomy that differs from that of all previously described vertebrates,” write the researchers.
In terms of its skull development, the prehistoric fish Eriptychius americanus occupies an evolutionary intermediate position between roundmouths and modern vertebrates. The fossil fills a 100-million-year gap in the vertebrate skull fossil record. “These are extremely exciting results that may shed light on how primitive vertebrates protected their brains in early evolutionary history,” says Dearden’s colleague Ivan Sansom. “Eriptychius americanus appears to be the first evidence of a series of cartilages separating the brain from the rest of the head. This study highlights the importance of museum collections and the use of new techniques to explore them.”
Source: Richard Dearden (University of Birmingham, UK) et al., Nature, doi: 10.1038/s41586-023-06538-y