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'Platypus fish' discovery changes what we know about one type of evolution

This mystery is only just getting started.

Jason art, Shenzhen

When an Australian scientist uncovered an ancient-looking placoderm skull in the 1960s, he thought he'd cracked the code on an evolutionary mystery. This so-called 'platypus fish,' scientists had supposed, was just another one of a kind of ancient, jawless, armored fish.

But more than 60-years later, researchers armed with new technology — namely digital X-ray scanners — are being forced to rethink this fish's place in the evolutionary tree. By peering inside the fish's skull, they discovered an even bigger mystery.

By reconstructing the skull using virtual anatomy, the team recreated the bony infrastructure of this fish's brain cavity without disturbing the specimen itself. In doing so, they reveal this ancient fish may actually have had much closer evolutionary links to humans and sharks than previously supposed.

Why it matters — Answering this evolutionary mystery could help scientists better understand the evolution of vertebrates' jaw bones over time — and how we humans have gotten stuck with the relatively weak jaws we have today.

Sam Giles is co-first author on the research and a senior research fellow at the University of Birmingham. She tells Inverse the new fossil discoveries "can help us understand how the living groups of jawed vertebrates, including the lineage that eventually led to humans, evolved."

The study was published Wednesday in the journal Current Biology.

Here's the background — You're probably familiar with the platypus — a taxon-defying animal that is at once mammalian, egg-laying, and aquatic. Yet it has an ancient, perhaps even stranger, fish-y counterpart known as the 'platypus fish,' or Brindabellaspis stensioi.

An artist's interpretation of the fossilized platypus fish.Jason art, Shenzhen

The nickname 'platypus fish' stems from the fish's long beak, which resembles that of its mammal namesake, but the comparisons stop there.

When paleontologist Gavin Young pulled several fossil samples of this long-beaked fish from Australian limestone in 1969, it was immediately thought to be a placoderm — an ancient type of fish which thrived over the course of 60 million years during the early Paleozoic era, between 420 and 360 million years ago. These fish were wiped out during Earth's second mass extinction.

Placoderm are jawless and identified internally by their characteristically compact inner ear sac — these sacs are a kind of pressure valve, and in humans they regulate balance. Modern, jawed, vertebrate fish, like sharks or bony fish, have comparatively massive sacs to the 'platypus fish' — as a result, scientists had assumed large sacs went hand-in-hand with the evolution of strong jaws.

Young and colleagues initially made this classification of the Brindabellaspis based on an analysis of its internal brain cavity, but scientists from institutions across Europe and Asia now believe they might've missed something big: these fossils appear to have secret brain cavity passageways never discovered before.

Thanks to modern advances in paleontology tech since 1969, researchers can now take a closer look inside Brindabellaspis' brain cavity.

Fossil specimen of Brindabellaspis stensioi as shown by MicroCT scanning.Image by IVPP [Institute of Vertebrate Palaeontology and Palaeonthropology]

What they did — The researchers used microCT scans, which Giles tells Inverse are essentially 3D X-ray scanners that digitally recover a sample's internal anatomy.

"This lets us digitally cut into the objects without damaging them," Giles says. "It’s an amazing technique that has the power to radically change our understanding of how some of our deepest ancestors evolved."

With these digital anatomy scans in hand, the researchers created colorful, interactive models of the fish's skull on a computer, enabling them to poke and prod the fossil in virtual reality. In the future, a similar method could be used to 3D print fossils, too.

Artistic rendering of Brindablleaspis (foreground) with diversified placoderms and other fishes. The white shark and human diver in the upper right corner represent modern jawed vertebrates.Yang Hongyu and Zheng Qiuyang

What they discovered — When poking around this digital skull, team discovered Brindabellaspis' inner ear much more closely resembled those of humans and sharks than the tiny sacs of other placoderms. In fact, if held side by side, the authors write it would be challenging to tell the difference between a Brindabellaspis' inner ear and a human's own.

Seemingly neither modern nor ancient, the authors write in the study this fish appears to be more of a placoderm-bony-fish-shark chimera — one which could have transformative impact on scientists' understanding of vertebrate evolution.

What we still don't know — These seemingly contradicting traits have created an even greater mystery for scientists. Giles says microCT scans could play a big role in answering these new questions, however.

"This fossil has revealed a really intriguing mosaic of primitive features and a surprisingly modern inner ear," Giles said in a statement.

"We don't yet know for certain what this means in terms of our understanding of how modern jawed vertebrates evolved, but it's likely that virtual anatomy techniques are going to be a critical tool for piecing together this fascinating jigsaw puzzle."

Abstract: Our understanding of the earliest evolution of jawed vertebrates depends on a credible phylogenetic framework for the jawed stem gnathostomes collectively known as ‘‘placoderms’’. However, their relationships, and whether placoderms represent a single radiation or a paraphyletic array, remain contentious. This uncertainty is compounded by an uneven understanding of anatomy across the group, particularly of the phylogenetically informative braincase and brain cavity—endocast. Based on new tomographic data, we here describe the endocast and bony labyrinth of Brindabellaspis stensioi from the Early Devonian of New South Wales. The taxon was commonly recovered as branching near the base of placoderms. Previous studies of Brindabellaspis emphasized its resemblances with fossil jawless fishes in the braincase anatomy and endocast proportions and its distinctive features interpreted as autapomorphies, such as the elongated premedian. Although our three-dimensional data confirmed the resemblance of its endocast to those of jawless vertebrates, we discovered that the inner ear and endolymphatic complex display a repertoire of previously unrecognized characters close to modern or crown-group jawed vertebrates, including a pronounced sinus superior and a vertical duct that connects the endolymphatic and the labyrinth cavity. Both parsimony and Bayesian analyses suggest that prevailing hypotheses of placoderm relationships are unstable, with newly revealed anatomy pointing to a radical revision of early gnathostome evolution. Our results call into question the appropriateness of arthrodire-like placoderms as models of primitive gnathostome anatomy and raise questions of homology relating to key cranial features.
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