Bony fishes (Osteichthyes) are a diverse group marked by two main extant lineages: ray-finned fishes (Actinopterygii) and lobe-finned fishes (Sarcopterygii), the latter lineage including the ancestors of all tetrapods, which eventually led to terrestrial vertebrates including humans. Despite their evolutionary importance, the origin of bony fishes has been enigmatic due to a paucity of well-preserved fossil evidence predating the Devonian period, a time when many early fish fossils already appear as highly specialized forms.

The team’s discoveries stem from meticulous fieldwork spanning over a decade, involving fossil specimens from the Early and Late Silurian deposits of southern China. This region, encompassing Chongqing and Yunnan provinces, has proven to be a vital repository of early vertebrate fossils that unravel the complex evolutionary history preceding the Devonian radiation of jawed fishes.

The first key specimen, Eosteus chongqingensis, hails from the Early Silurian strata approximately 436 million years old. Measuring a mere 3 centimeters in length, the fossil is remarkably complete, preserving the entire organism from head to tail. This diminutive fish exhibits an intriguing mosaic of features, combining traits recognized in early ray-finned fishes with ancestral characteristics, such as the absence of lepidotrichia (the bony fin rays typical of more derived bony fishes), and the presence of an anal fin spine, a feature previously observed only in cartilaginous fishes and placoderms, which are more primitive jawed vertebrates.

Such a combination of characteristics in Eosteus chongqingensis challenges pre-existing assumptions about the timing of core bony fish traits’ emergence. Notably, morphological features thought to be exclusive to later ray-finned fishes were already present in this early bony fish, suggesting a considerably earlier advent of defining osteichthyan features than was hitherto recognized.

Complementing this discovery is the unearthing and detailed study of Megamastax amblyodus from the Late Silurian Kuanti Formation in Qujing, Yunnan, dating to approximately 423 million years ago. Megamastax stands out as the largest known vertebrate from the Silurian, surpassing one meter in length, which is extraordinary given the generally small size of contemporaneous vertebrates.

Prior to this study, efforts to reconstruct Megamastax’s anatomy were hampered by the fragmentary nature of fossil material and the complexity of internal structures. However, the application of advanced high-resolution computed tomography (HRCT) and three-dimensional digital reconstruction enabled the researchers to visualize the entirety of its cranial anatomy and internal morphology non-destructively. This breakthrough revealed intricate details of the cranial bones, braincase, and musculature, shedding light on its feeding adaptations and internal structures.

The dentition of Megamastax amblyodus is particularly revealing. Unlike modern bony fishes that generally have a single row of teeth, Megamastax’s jaws display a primitive condition with both inner and outer dental arcades. The inner row features tooth cushions resting on broad bases, a configuration that resolves long-standing debates about the taxonomic placement of similarly isolated Silurian tooth remains from the Baltic region. This anatomical evidence confirms that such dental structures are basal osteichthyan features rather than belonging to other groups.

Comprehensive phylogenetic analyses place both Eosteus and Megamastax firmly within the bony fish stem group, representing taxa that predate the divergence of ray-finned and lobe-finned fishes. This placement provides a crucial window into the ancestral condition of osteichthyans and informs reconstructions of their last common ancestor’s morphology. Their existence before the evolutionary split challenges earlier hypotheses suggesting that ancestral bony fishes resembled lobe-finned fishes more than ray-finned types.

These findings carry profound implications for understanding the early evolution of jawed vertebrates. By illuminating the morphological trajectory of key skeletal features, including jaw structure, scales, and fin architecture, the research advances our knowledge of how complex anatomical innovations arose and diversified within early bony fishes. This also refines models of how vertebrates transitioned from aquatic to terrestrial environments, through the lobe-finned lineage.

The study reinforces the significance of southern China as a pivotal paleontological region rich in vertebrate fossils that document critical episodes of early vertebrate evolution. The geological context and exceptional preservation conditions in this area have allowed scientists to capture an evolutionary snapshot that was previously inaccessible, filling a major gap in the fossil record.

Funding for this research was provided by the Key Program of the National Natural Science Foundation of China, the International Research Center of Big Data for Sustainable Development Goals, among other sources, underscoring the collaborative and interdisciplinary nature of this endeavor. The use of cutting-edge imaging technologies and fossil analysis techniques exemplifies the integration of modern scientific tools in unraveling deep-time biological mysteries.

Altogether, the discoveries of Eosteus chongqingensis and Megamastax amblyodus mark a paradigmatic shift in vertebrate paleontology by presenting the earliest clear anatomical evidence for the origin of bony fishes and elucidating critical evolutionary developments that laid the foundation for the diversity of vertebrates seen today. This work not only enhances the intricate tapestry of life’s history but also inspires continued exploration of early fossils to further decode the origins of vertebrate complexity.

Subject of Research: Origin and early evolution of bony fishes (Osteichthyes) based on new Silurian fossil discoveries.

Article Title: (Not provided in the content.)

News Publication Date: March 4 (year inferred as 2026 from DOI).

Web References:
DOI link to the article

Image Credits: Video by NICE PaleoVislab, IVPP

Keywords: Paleontology, Fossils, Bony fishes, Osteichthyes, Jawed vertebrates, Silurian period, Evolutionary biology, Vertebrate paleontology, Ray-finned fishes, Lobe-finned fishes, Early vertebrate evolution, Computed tomography (CT).

For decades, the evolutionary narrative has centered on the rise of jaws as the defining innovation enabling vertebrates to transition from simple bottom-dwelling creatures into agile, predatory swimmers dominating the water column. The paradigm held that structural changes supporting predatory habits, such as enhanced sensory organs and improved locomotion, followed the advent of jaws and teeth. However, by leveraging state-of-the-art imaging technologies, this new research overturns the “jaws-first” hypothesis, demonstrating that sophisticated sensory and circulatory systems developed well beforehand in jawless ancestors.

The exceptional preservation of the Norselaspis fossil lies at the core of this discovery. Retrieved from sandstone blocks collected in Spitsbergen, Norway, during a 1969 paleontological expedition, the fossil’s cranium is barely half an inch long yet holds a wealth of anatomical data. Using powerful high-energy X-ray beams at the Paul Scherrer Institute in Switzerland, researchers scanned the fossil slice by slice. These scans revealed delicate, tissue-thin layers of bone that cast ghostly shadows of soft organs such as the brain, inner ears, heart, and even the minute muscles controlling eyeball movement. This unprecedented level of detail presents Norselaspis in anatomical resolution rivaling that of many extant fish.

Dr. Tetsuto Miyashita, lead author and research scientist at the Canadian Museum of Nature, emphasizes the significance of these findings: Norselaspis possessed a sensory suite and circulatory system indicative of an active, awareness-driven lifestyle long before jaws appeared. The fish had seven small muscles orchestrating eye movements—a contrast to the six found in humans—and oversized inner ears facilitating precise detection of vibration, orientation, and acceleration. Most strikingly, its heart was proportionally enormous, framed by a complex vascular system configured to shuttle blood efficiently and power a high-energy mode of life.

These physiological adaptations are reminiscent of modern apex predators, offering what Miyashita describes as a “heart of a shark beneath the skin of a lamprey.” Complementing this extraordinary cardiovascular capacity were paddle-like fins positioned behind the gills, enabling sudden directional changes and bursts of speed. Such agility likely evolved as an essential survival strategy to evade predators, not necessarily to pursue prey actively.

Michael Coates, Professor and Chair of Organismal Biology and Anatomy at UChicago and senior author, elaborates on the evolutionary context: the innovations seen in Norselaspis provided a functional and ecological framework that would later allow jawed fishes to exploit prey capture more effectively. Instead of jaws catalyzing all changes, this research paints a picture of sensory, circulatory, and locomotor upgrades setting the stage for the subsequent emergence of jaws.

Of particular note is the study’s challenge to a long-standing anatomical hypothesis regarding the evolution of tetrapod shoulders and arms. By tracing the nerve supplying the shoulder in Norselaspis, the team found that its neural connections were independent of those innervating the gills. This directly contradicts the idea that these structures derived from modified gill arches. Instead, the shoulder appears to have evolved as a novel anatomical domain, distinct from gill architecture, signaling an early development of a neck region separating head and torso.

This structural separation is pivotal; in primitive jawless fishes, the head is continuous with the trunk, whereas jawed vertebrates possess a neck and throat to decouple the two regions. Norselaspis represents an intermediate stage where the head is still attached without a distinct neck, akin to arms protruding from behind the cheeks in a hypothetical human analogy. This configuration underscores a transitional anatomical design, integrating enhanced sensory organs and cardiovascular features at the head-trunk interface to optimize environmental navigation.

This evolutionary snapshot also sheds light on the broader “Nekton Revolution,” a transformative epoch when marine organisms began exploiting midwater habitats, driving a shift toward increased speed, sensory acuity, and maneuverability. Christian Klug of the University of Zurich, who was not involved in the study, suggests that Norselaspis belongs to this lineage thriving in such dynamic ecological niches, supporting the idea that evolutionary pressures favored enhanced swimming and sensory mechanisms before the acquisition of jaws.

In totality, the study reframes vertebrate evolution as a complex, multi-faceted process rather than a series of sudden innovations centered solely around jaws. The intricate anatomy uncovered in Norselaspis accentuates the importance of gradual acquisitions—advanced sensory capabilities, cardiovascular enhancements, and locomotor refinements—that predated and likely facilitated the advent of jawed predators.

Dr. Miyashita aptly cautions against oversimplifying these evolutionary milestones. “The evolution of jaws has often been likened to a singular trigger event, like the gunshot in Sarajevo that started World War I,” he says. “But to truly grasp the magnitude of this evolutionary chapter, one must understand the nuanced context—the environmental, physiological, and anatomical shifts embodied in creatures like Norselaspis.”

This study exemplifies how technological advances in imaging and fossil analysis are peeling back layers of deep evolutionary history, revealing the complexity and sophistication of early vertebrates. It sets a new standard for future investigations into the origin of fundamental vertebrate features, demonstrating that pivotal evolutionary transformations involve an interplay of multiple, concurrent adaptations rather than isolated innovations.

Subject of Research:
Animal tissue samples

Article Title:
Novel assembly of a head–trunk interface in sister group of jawed vertebrates

News Publication Date:
6-Aug-2025

Web References:
DOI: 10.1038/s41586-025-09329-9

Image Credits:
Kristen Tietjin

Keywords:
Norselaspis glacialis, jawless fish, vertebrate evolution, Devonian period, fossil reconstruction, brain anatomy, heart morphology, inner ear, high-energy X-ray imaging, head-trunk interface, jaw evolution, sensory adaptation, Nekton Revolution