Trails left in ancient rock have been attributed to fish getting the hang of the then-newfangled concept of crawling. If correct, this would push back the conquest of the land by vertebrates by 10 million years. That’s assuming these pioneers didn’t just nope it out back to the water (like whales later would), until braver souls (or soles) tried again.
The attraction of land for the first fish to explore seems obvious. Sure, the atmosphere might have been poisonous, but early plants offered plenty of food, and there was also the major attraction of not having anything there trying to eat you. It’s a lot less obvious why a modern fish would try to recreate the journey, but it’s just as well some do, because they allow us to see the process in action.
That includes modern lungfish leaving furrows in mud as they lever themselves with their heads, while controlling direction with their tail and fins.
In Poland’s Holy Cross Mountains, traces just over a centimeter (0.45 inches) wide and deep, and sometimes stretching for more than a meter (3.3 feet), resemble these closely. With more than 240 of these marks found in ancient sandstone, they’re certainly not a coincidence.
“Traces of crawling were found over a large area in four horizons in the profile. In our opinion, this indicates the widespread nature of the phenomenon of penetration into the nearest coastal areas,” Dr Piotr Szrek of the Polish Geological Institute, and lead author of the study analyzing the trace fossils, told IFLScience.
“A fish getting stuck on the shore and attempting to escape from such a trap would be an extremely interesting moment captured in the fossil record, but it would be a rare incident.”
What would have been the sea shore 400 million years ago showing fossil traces of crawling dipnoan fish.
Image credit: Photo by and courtesy of P. Szrek
Differences between the tracks indicate they were made by two species. Szrek and co-authors have called one Reptanichnus acutori and another Broomichnium ujazdensis. The latter resembles much more recent traces left by Broomichnium flirii, but the first type of track is considered novel enough that the makers have been designated a new genus, not just species.
The authors think these both were dipnoan fish, a class better known as lungfish that appear in the fossil record 415 million years ago, and survive today in inland waterways. Protopterus, one of these survivors, can crawl in search of water when its habitat dries up.
Nevertheless, the marks surprised scientists because of the age of the rocks. At 410-400 million years ago, they date to the lower Devonian, around 10 million years before the prints showing vertebrates moving relatively freely have been found as far apart as Ireland and Australia. Although some of the traces are new, some have been known for a while, but were previously thought to have been made by invertebrates.
Szrek and co-authors knew that extraordinary claims require extraordinary proof, and a visual similarity does not provide that. They used a 3D scanner to map the trails and compared them to those made by living Protopterus. The scans not only confirmed the similarity, but allowed the researchers to spot the marks of fins and tail as well as face in the rocks.
Resting trace of the dipnoan fish Broomichnium ujazdensis.
Image credit: Photo by and courtesy of P. Szrek
It’s easy to imagine modern lungfish as surviving descendants of the first piscine explorers, but that is not what genetics reveals. Instead, it seems they’re late arrivals to terrestrial conquest, but it is plausible that the first fish to take on the land used similar methods to get around until legs were invented.
The marks these fish left are so well-preserved that Szrek and co-authors think they can even tell which way the makers leaned. Back in 2016, when they published their first paper on traces of dipnoan fish that left mouth impressions in the mud, they noted that many of the imprints indicated the head was tilted to the left, which was hypothesized then as indicating “handedness”, Szrek told IFLScience. However, they only had 10 cases back then. Now they have 2.5 times the specimens analyzed.
In the new study, thirty-six of the marks show signs of being made with a tilted head, and in 35 of those cases, it was angled to the left. “In our opinion, this was not accidental and was a definite preference, which may be the first record of handedness,” Szrek said.
“The find from the Holy Cross Mountains shows that the process of acquiring left- or right-handedness was at least more complex than the once-and-for-all phenomenon of dominant right-handedness.”
The evolutionary basis for handedness has long been debated, but if this evidence is right, it was with us from the start.
The study is published in Scientific Reports.