Over 300 million years ago, a minnow-sized fish died and fell to the bottom of the prehistoric swamp near the village of Trawden, Lancashire, in northwest England. The remains of this tiny fish—known as Trawdenia planti—became fossilized, embedding proof of its existence in a layer of soapstone sandwiched between coal seams in the Burnley coal fields. By some combination of marine chemistry, mineral composition of the seafloor, timing, and luck, not only was the bony skeleton of this fish preserved, but also the soft neural tissues of its brain.
A new study published in the Proceedings of the National Academy of Sciences (PNAS) by paleontologists at the University of Chicago describes this extraordinary preservation of an early ray-finned fish brain. Using CT scans of the skull and preserved soft tissues, the researchers show how the brain fit snugly inside, unlike other fossil brains that appear too small for the interior brain case. This means that the inside of fossilized skulls can serve as a reliable proxy for brain size and shape of such fishes even when brain tissues are not well preserved.
"Soft tissue preservation, in general, is not common in the fossil record, and usually what gets preserved are things like skin or muscles. It's quite rare for neural tissues to be preserved at all because they decay so quickly," said Abigail Caron, PhD, lead author of the study, who recently received her doctorate from the Committee on Evolutionary Biology at UChicago. "So, the importance of this specimen is that we can now study brain evolution in similar fossils where we only have the bony parts or the infill."
The bush at the bottom of the evolutionary tree
Ray-finned fishes, so called for their thin fins stretched between bony spines, make up nearly 99% of the more than 30,000 living species of fish, and about half of all modern vertebrates. Scientists understand the origins of birds and modern tetrapods like mammals, reptiles, and amphibians fairly well, but the early days of ray-finned fishes are "like a bush at the bottom of the evolutionary tree," says Michael Coates, PhD , Professor and Chair of Organismal Biology and Anatomy at UChicago and senior author of the new study.
The dense thicket of fish species that proliferated after the end of the Devonian period have been difficult to relate to modern groups of fish. But, the well-preserved brain of Trawdenia has distinct characteristics that suggest a deep-time relationship to modern sturgeons and paddlefish, including part of the cerebellum that wraps around the middle of the brain.
"What we're learning by looking at the shapes of the soft tissues is that it's not their relative sizes that matter; it's how they're packed together inside the skull," Coates said. "We might be seeing the earliest radiation of fishes that nowadays are represented by paddlefish and sturgeons."
From coal mine to CT scanner
This particular Trawdenia specimen was first discovered in 1888 and has a long, adventurous history. Coal miners used to collect fossils dug from mines around Lancashire as novelties, but geologists also studied them because the presence of plant fossils provided a means of mapping productive coal seams. The rock nodule containing Trawdenia was split in two and ended up at a natural history museum in London as separate specimens, until Coates realized they belonged together.
Coates started studying the fossil in the 1990s, first describing its skeleton in detail in a 1999 paper , and later CT scanning it for a 2018 publication with Kristen Tietjen, a scientific illustrator who worked in Coates' lab and is now at the University of Kansas, and a co-author on the new PNAS study. Caron continued that work for her PhD thesis, applying more sophisticated imaging techniques and computational analyses to learn more about these ancient fish.
The new technology helped the researchers detect signatures of the outer and inner membranes of neural tissues filling the interior cavities of the brain case, including ventricle structures that helped circulate cerebrospinal fluid. 3D models of the skull created from the CT scans show that the brain filled the interior of the skull's brain case.
Caron said that as imaging technology improves, it's likely that researchers will be able to learn more about brains and nervous systems in these early fish, even when they aren't as well preserved as Trawdenia's—especially now that they know what they're looking for.
"It's possible that we just didn't have the technology before to look for that kind of signature, but this kind of preservation also would only happen in very special circumstances," she said. "There are definitely a lot more specimens out there that have reasonably good brain case morphology than there are specimens with good soft tissue preservation. So, that really expands the data set that you can use to study brain evolution across these different fossils."
