Discovery of 319-million-year-old fossilized fish brain provides insights into ancient neural anatomy and evolution

A groundbreaking discovery has been made involving the fossilized remains of a fish dating back 319 million years. This remarkable find, extracted from an English coal mine over a century ago, unveils the oldest known instance of a remarkably well-preserved vertebrate brain. The brain, along with its cranial nerves, measures approximately one inch in length and originates from an extinct fish comparable in size to a bluegill. Scientists from the University of Michigan, leading a study soon to be published in Nature, assert that this revelation grants us a glimpse into the neural anatomy and early evolution of ray-finned fishes, the predominant group of fish in existence today.

The unexpected discovery also sheds light on the conservation of soft tissues within fossils of vertebrate animals. Museums predominantly house fossils composed of hard structures like bones, teeth, and shells, but this finding presents a rare opportunity to examine the preservation of delicate organs. The CT scanning of the brain specimen from the fossilized fish, known as Coccocephalus wildi, provides a detailed analysis of its three-dimensional structure. This ancient fish, belonging to the ray-finned fish category, likely inhabited estuaries and subsisted on small crustaceans, aquatic insects, and cephalopods (including modern-day creatures like squid, octopuses, and cuttlefish). Ray-finned fishes possess bony rods called rays that support their fins.

During the fossilization process, the fish’s brain and cranial nerves underwent a remarkable transformation. The original soft tissues were replaced by a dense mineral, which flawlessly preserved the intricate details of their form and structure. This exceptional finding offers valuable insights into the evolutionary history of ray-finned fishes, as well as the overall understanding of how soft tissues can endure over vast periods of time within fossilized remains.

Animation showing CT-scanned images of the Coccocephalus wildi fossilized skull (light yellow), followed by the fossilized fish brain and cranial nerves (pink) and otolith ear-stones (white) inside the skull. Credit: University of Michigan Museum of Paleontology.

“This discovery highlights a significant finding: soft parts such as these can indeed endure and remain preserved within fossils that have been in our possession for extensive periods of time. Remarkably, this particular fossil has been known for over a century,” stated Matt Friedman, a paleontologist from the University of Michigan (U-M) and senior author of the study, as well as the director of the Museum of Paleontology.

The lead author of the study is Rodrigo Figueroa, a U-M doctoral student who conducted the research as part of his dissertation, supervised by Friedman within the Department of Earth and Environmental Sciences.

Figueroa expressed the importance of this unassuming and diminutive fossil, stating, “Not only does this seemingly modest specimen provide us with the earliest evidence of a fossilized vertebrate brain, but it also necessitates a reevaluation of our understanding of brain evolution based solely on observations from present-day species.”

Given the widespread availability of advanced imaging techniques, it would not be surprising if we discover that fossilized brains and other soft tissues are far more prevalent than previously believed. This realization has prompted our research group, as well as others, to approach the examination of fossil fish heads with a fresh and distinct perspective.

The skull fossil from England, being the sole known specimen of its species, necessitated the use of nondestructive methods throughout the University of Michigan-led study.

The study conducted by Friedman, Figueroa, and their colleagues focuses on employing computed tomography (CT) scanning to investigate the internal structures of early ray-finned fishes. By obtaining intricate details of their internal anatomy, the researchers aim to glean insights into evolutionary relationships.

In the case of Coccocephalus wildi, Friedman initially did not set out to examine the brain when he initiated the micro-CT scanning process on the skull fossil.

“I conducted the scan and loaded the resulting data into our visualization software. That’s when I noticed the presence of an unusual and distinct object inside the skull,” Friedman explained.

The unidentified object exhibited higher brightness on the CT image, indicating a greater density compared to the surrounding rock or the skull’s bones.

“While it is not uncommon to observe shapeless mineral formations in fossils, this particular object possessed a well-defined structure,” Friedman remarked.

The mysterious object displayed several characteristics reminiscent of vertebrate brains: bilateral symmetry, hollow spaces resembling ventricles, and multiple filament-like structures extending towards openings in the braincase, akin to cranial nerves that pass through such canals in extant species.

The fossilized skull of Coccocephalus wildi, an early ray-finned fish that swam in an estuary 319 million years ago. The fish is facing to the right, with the jaws visible in the lower right portion of the fossil. The eye socket is the circular, bumpy feature above the jaws. This fish would have been 6 to 8 inches long, about the size of a bluegill. Credit: Jeremy Marble, University of Michigan News.

“I couldn’t believe my eyes,” Friedman exclaimed. “All the features were there, and I found myself asking, ‘Could this really be a preserved brain?'” To investigate further, Friedman conducted a second, higher-resolution scan focused on that specific region of the skull. The results left no doubt—they were indeed examining a well-preserved brain. The unequivocal nature of this discovery prompted the researchers to delve deeper into its study.

While the preservation of brain tissue in vertebrate fossils is rare, scientists have had greater success in finding preserved brains in invertebrates. For instance, the intact brain of a horseshoe crab dating back 310 million years was reported in 2021, and brain structures have been observed through scans of insects preserved in amber. Even flattened specimens that are over 500 million years old have provided evidence of brains and other components of the nervous system.

In 2009, the preserved brain of a shark relative from 300 million years ago was reported. However, sharks, rays, and skates belong to the cartilaginous fish group, which has fewer species compared to the ray-finned fish lineage to which Coccocephalus belongs. The study of early ray-finned fishes like Coccocephalus provides valuable insights into the initial stages of evolution in the diverse group of fish species we see today, ranging from trout to tuna and seahorses to flounder.

Ray-finned fishes comprise approximately 30,000 species, representing about half of all species within the vertebrate animal kingdom. The other half is divided among land-dwelling vertebrates such as birds, mammals, reptiles, and amphibians, as well as less diverse fish groups like jawless fishes and cartilaginous fishes.

On loan to Friedman from England’s Manchester Museum, the Coccocephalus skull fossil was recovered from the Mountain Fourfoot coal mine in Lancashire. First described scientifically in 1925, the fossil was found in a soapstone layer adjacent to a coal seam within the mine.

While only the skull was unearthed, scientists estimate that C. wildi would have measured between 6 to 8 inches in length. Based on the shape of its jaw and teeth, it is likely that the fish was a carnivore, according to Figueroa.

Scientists suspect that when the fish died, it was rapidly buried in sediment, creating an environment low in oxygen that hindered the decomposition of its soft tissues.

Furthermore, Figueroa suggests that a specific chemical micro-environment within the braincase of the skull may have contributed to the preservation of the delicate brain tissues. This micro-environment could have facilitated the replacement of the brain tissues with a dense mineral, possibly pyrite.

CT scan of the Coccocephalus wildi skull, showing the fossilized brain and associated structures. The brain and cranial nerves appear bright white in the center of the images. The fish is facing to the left. The eye sockets are the large, black oval areas surrounded by bone, and the jaws are below the eye sockets. Credit: Figueroa et al. in Nature, February 2023.

Support for this notion arises from the examination of the cranial nerves, responsible for transmitting electrical signals between the brain and sensory organs. Within the Coccocephalus fossil, the cranial nerves remain intact within the braincase but disappear as they extend beyond the confines of the skull.

“There appears to be a micro-environment within this tightly enclosed space in the skull that facilitates the replacement of soft tissues with a mineral phase, thereby preserving the shape of these tissues that would otherwise decay,” explained Friedman.

Through a meticulous analysis of the fossil, along with comparisons to the brains of present-day fish specimens from the U-M Museum of Zoology, the researchers discovered that the brain of Coccocephalus consists of a central body about the size of a raisin, which can be divided into three main regions that roughly correspond to the forebrain, midbrain, and hindbrain found in extant fish species.

The cranial nerves extend from both sides of the central body, forming a unified structure that resembles a miniature crustacean such as a lobster or crab, complete with projecting arms, legs, and claws.

Significantly, the brain structure of Coccocephalus reveals a more complex pattern of fish brain evolution than what is observed in living species alone, as noted by the authors.

“These characteristics provide the fossil with genuine value in comprehending the patterns of brain evolution, rather than merely being an intriguing example of unexpected preservation,” Figueroa emphasized.

For instance, all present-day ray-finned fishes possess an “everted” brain, meaning that during embryonic development, brain tissues fold outward from the inside of the embryo, similar to turning a sock inside out. On the other hand, all other vertebrates possess “evaginated” brains, where neural tissues in developing brains fold inward.

“In contrast to all extant ray-finned fishes, the brain of Coccocephalus exhibits an inward folding,” stated Friedman. “Therefore, this fossil captures a period prior to the evolution of this distinctive characteristic of ray-finned fish brains. This provides us with valuable insights into the timing of when this trait emerged—a question that was previously uncertain without the data provided by Coccocephalus.”

By comparing the fossil to living fish species, the researchers determined that the brain of Coccocephalus bears the closest resemblance to the brains of sturgeons and paddlefish. These fishes are often referred to as “primitive” due to their divergence from all other living ray-finned fishes over 300 million years ago.

Friedman and Figueroa continue their work by CT scanning the skulls of additional ray-finned fish fossils, including specimens loaned from institutions in Brazil, Figueroa’s home country. Due to delays caused by the COVID-19 pandemic, Figueroa anticipates completing his doctoral dissertation in the summer of 2024.

The study published in Nature incorporates data generated at the University of Michigan’s Computed Tomography in Earth and Environmental Science facility, which receives support from the Department of Earth and Environmental Sciences and the College of Literature, Science, and the Arts.

The other co-authors of the paper include Sam Giles from London’s Natural History Museum and the University of Birmingham, Danielle Goodvin and Matthew Kolmann from the U-M Museum of Paleontology, and Michael Coates and Abigail Caron from the University of Chicago.

Friedman and Figueroa emphasized the significance of preserving specimens in paleontology and zoology museums. Friedman remarked, “In this case, we have discovered remarkable preservation in a fossil that has been examined multiple times by different researchers over the past century. However, with the advent of new tools for studying fossils’ internal structures, it reveals a new layer of information to us. This underscores the importance of maintaining physical specimens because, who knows, in the next 100 years, the fossils we have in our collections now might enable even more remarkable discoveries.”

Source: University of Michigan

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