Paleontologists Find Ancient Fish With Nostrils: May Have Been First Animal to Breathe Air - Science - Haaretz
When and why did fish leave the sea, and how did they do it? A growing body of research and horse sense suggest that the ability to breathe air had to emerge before the first fishapod even dreamed of a stroll. Lungs before legs.
Now a recent report in the Journal of Vertebrate Paleontology reveals nostrils on the face of the lovely Harajicadectes zhumini, a fish that lived in what is today Australia's Northern Territory 380 million years ago – the time of the great piscine transition to land.
Harajicadectes had a big flattish head and was roughly 40 to 50 centimeters (16 to 20 inches) long when alive, the team happily reports after half a century of collecting odd fragments of the creature, and finally finding goods ones. It had fins, not legs, and had a frog-like toothy smile – it was a predator, paleontologists Prof. Brian Choo and colleagues from Adelaide's Flinders University deduce. And it had large spiracles atop its head.
Spiracles are apertures that in this case apparently served as air holes. They were where you would expect nostrils to be: between its mouth and eyes.
Harajicadectes is one of many – let's call them – transitional fish that lived in the Devonian. The team calls it a "basal tetrapodomorph" or "tetrapodomorph fish," which means it's still a fish, still possessing fishy fins rather than limbs with digits, Choo explains to Haaretz by email. But it or its ilk are ancestral to all land animals.
It had been known before, but the first specimens had been in terrible condition. The new analysis of its features and its position in the evolutionary tree is based on the discovery of several almost-complete skulls, as well as articulated post-cranial skeletons.
Spiracles have been observed in other fishapods living at the same time, including the famous Tiktaalik – which already had feet! Why is this one special? "The majority of Devonian tetrapodomorph fishes either lacked spiracles or only had minute spiracles that couldn't have served a significant respiratory function," Choo clarifies. "Before the discovery of Harajicadectes, large air-breathing spiracles were only known in two tetrapodomorph groups."
Large spiracles were known in the northern hemisphere's limbed elpistostegalians, which look a lot like Tiktaalik, and in Gogonasus – an enigmatic fossil species from Australia. Now they're found in a third branch, Harajicadectes.
Is it case closed that Harajicadectes breathed air? The spiracles may have evolved for purposes other than breathing.
"Outside of bony fishes, modern rays also possess large spiracles to draw in and expel water while lying on the seafloor (with their mouths pressed against the sediment). However, Harajicadectes shows no anatomical indicators for a similar benthic lifestyle," Choo says.
So, air. He believes these three lineages – the elpistostegalians, Gogonasus and now Harajicadectes – developed their large air-breathing spiracles independently at roughly the same time, he says.
"We're not certain when the first fish was capable of air-breathing, as such a capacity has probably evolved dozens of times (well over 30) within the Osteichthyes [bony fishes], and many modern fish capable of air-breathing have no obvious skeletal adaptations for doing so. However, Harajicadectes is among the earliest bony fishes to show a distinct skeletal adaptation for surface air-breathing – the enlarged spiracles on the top of the skull – and is potentially the earliest," he says.
Yes, the team thinks they may have happened on the first fish to take a deep breath of air.
Have lung, will travel
Fish began to emerge over 530 million years ago, in the Ordovician era. The earliest were simple bug-eyed, jawless fish such as Haikouichthys, which had gills.
Later fish would develop bony protective plates and jaws. And in time, possibly in the early Devonian 400 million years ago, the ability to breathe air emerged in lobe-finned fish.
As a 2017 paper notes, the ability to breathe air may have emerged independently multiple times; Choo suggests dozens of times. The ability exists in different forms, suggesting diverse origins. Also, clearly the first air-breathing animals were aquatic.
Asked about the significance of the spiracles' size, Choo explains: "The size and position of the spiracles are strongly correlated with surface air-breathing. The only modern bony fishes with similar structures are the bichirs (Polypterus) of Africa, which have been demonstrated to use their spiracles in this manner."
Bichir are an African freshwater fish that breathes air when coming to the lake surface. Bichir aren't lobe-finned fish like Harajicadectes. They are ray-finned fish that may have diverged from other ray-finned fish over 300 million years ago. To ichthyologists, bichir are a sister group to all other ray fins.
Point being, they have spiracles they use to breathe air, supporting the team's theory about Harajicadectes' head holes.
A separate 2014 paper published in Nature on spiracles in bichir and ropefish demonstrated once and for all that the apertures serve for breathing. "Spiracle-mediated aspiration accounts for up to 93 percent of all air breaths in four species of Polypterus," that paper states. "Similarity in the size and position of polypterid spiracles with those of some stem tetrapods suggests that spiracular air-breathing may have been an important respiratory strategy during the fish-tetrapod transition from water to land," it adds.
To be clear, most fish can't breathe air and don't have lungs. You need not fear your goldfish will take a walk. Fish take water into their mouths and pass it over their gills, which have a vast surface area (as do our lungs). As the water passes through the gills, they extract its oxygen and transfer it to the fish's blood.
Note that baby amphibia go through an embryonic stage in which they have gills; some amphibians retain those into adulthood.
Ahead of the Devonian transition, the ability to breathe air as well, not instead, arose. A fish that could extract oxygen from both water and air would plausibly have had an evolutionary advantage, especially under onerous conditions where the water becomes hypoxic, for example.
"The synchronized appearance of this air-breathing adaptation may have coincided with a time of decreased atmospheric oxygen during the mid-Devonian," co-author John Long of Flinders told Sci News.
That decreased oxygen is postulated to have followed the explosive evolution of plants, leading to a riot of decay and to anoxic conditions that would seem likely to encourage the emergence of fishapods that could gulp in air – leading to the rise of animalia, and ultimately resulting in we.
It is also possible that the ability emerged time and again simply because during dry seasons, when lakes and waterways vanish, if one is a fish one may wish to relocate, which will require crossing land.
As for the lungs, paleontologists suspect they may have begun as a single organ that later became paired, as in our case, but that specifically is a mystery.
Is he The One?
In any case, separate work has shown that the ability to breathe air arose by at least 390 million years ago, based on extraordinary tetrapod footprints unearthed in the Zachelmie Quarry in Poland from that time. That is well before this specific Harajicadectes poked its spiracles out of the sea and took a deep breath. But according to Choo, its predecessors could still have been pioneers in spiracular gasping.
The newly named sleek fishiform was named for the Harajica Sandstone region where it was found, the Greek for "biter" (dektes) and paleontologist Prof. Min Zhu, currently at the Chinese Academy of Sciences, Flinders University explains.
So spiracles appeared in multiple tetradomorph lineages during the Middle to Late Devonian, which means we can't say Harajicadectes is our ancestor. But what are the odds that we found The One? Well. In 2022, paleontologists identified a fishapod from exactly the same time, about 380 million years ago, whose ancestors exited the sea, conquered land – and went back to the sea.
Today's lungfish have both gills and lungs. That is nice for them. The "living fossil" coelacanth – a deep-water fish thought to have been extinct since the Cretaceous but in fact still alive – also has gills and a vestigial, nonfunctional single lung. Fish are interesting. They say coelecanth tastes terrible.
In a final note, perhaps "transitional" forms between finned fish and terrestrial quadrupeds, such as Tiktaalik, which lived 18 million years after Harajicadectes, weren't "transitional" forms but "relics" of the transitional forms, a separate paper posits. If walking on land began almost 400 million years ago, then Tiktaalik certainly was no pioneer.
So what have we? Breathing – good. Spiracles for breathing emerged in at least three separate lineages of tetrapodomorphs (elpistostegalians, Gogonasus and now Harajicadectes) at roughly the same time and in widely differing habitats: Harajicadectes lived in inland lakes, elpistostegalians liked freshwater and estuarine habitats, and Gogonasus lived in oceanic reefs, Choo explains. We see that Devonian Australia was a hotbed of vertebrate evolutionary innovation.
Come the Carboniferous, from about 350 to 300 million years ago, the remaining transitional finny tetrapodomorphs died out. Fish with fins stayed fish with fins, and on land tetrapods with feet really took off. You know what happened next.
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