In the eastern desert of Egypt, where today rock, heat and dust remain, there was a sea alive enough to almost seem crowded. Hundreds of people tell it fish fossils remained closed in the sediments for 62.2 million years, in a point called Qreiya 3. Looking at them now, immobile in the stone, they seem to arrive from a frozen time. Instead they talk about movement: of ecosystems that reorganize themselves, of species that occupy spaces left empty, of oceans that begin to resemble the ones we know today much sooner than expected.
The site was described in a study published in Science Advances. The research group documented a marine deposit from the Danian, the first phase of the Paleocene, that is, that stretch of terrestrial history that arrived just after the great Cretaceous-Paleogene extinction. We are talking about the crisis that, about 66 million years ago, wiped out the avian dinosaurs except for and redesigned entire food chains. At Qreiya 3, the researchers found hundreds of specimens, with at least 21 types of ray-finned fish distributed across nine large groups – more diversity than all other known Danian skeletal assemblages so far combined.
The interesting thing lies precisely in the date. These animals were swimming about four million years after the impact that closed the Cretaceous. Four million years, in geology, can be the blink of an eye with the patience of a stone. On the mainland, scholars had already reconstructed some stages of the recovery: the expansion of ferns after the catastrophe, then the return of forests, mammals, reptiles and birds to new balances. The sea, however, had left more voids. There were no bodies, skeletons, entire communities to observe. There was a lack of clear enough evidence to understand who remained, who had lost ground and who had taken advantage of the disaster to become protagonists.
A sea enclosed in rock
Qreiya 3 is a Lagerstätte, a German word used by paleontologists to indicate a deposit with exceptionally preserved fossils. In simpler words: one of those places where the stone has done the work of a meticulous archivist. The fish preserved at the Egyptian site allow very precise comparisons with still living groups, almost bone for bone, and show an already surprisingly modern marine community. Inside that deposit appear ancient relatives of tuna and mackerel, scabbardfish, sunfish, trevally, pipefish and other branches that we now recognize in contemporary oceans.
The most present group is that of percomorphsa large evolutionary line that today includes over 17 thousand species, from tuna to seahorses, from perches to anglerfish. Before the mass extinction these fish were present, but they occupied a much smaller space in the Cretaceous seas. After the crisis, their weight increases. Qreiya 3 photographs precisely this passage: a marine fauna in which percomorphs already appear central, with different shapes and different ecological roles.
The scene, reconstructed from fossils, has little of the postcard. There are predators, small armored fish, tapered shapes, organisms that lived in an offshore environment, at a depth estimated between 150 and 250 meters. The conservation conditions also seem to be linked to oxygen-poor seabeds, capable of slowing down decomposition and protecting the remains. The study also links the assemblage to the Latest Danian Event, a brief interval of warming that occurred during the Danian. A detail that adds another layer: those fossils tell of the recovery after an extinction, within a world that is still unstable, with a climate and oceans in full adjustment.
The void of ten million years
To understand why these fish fossils they matter so much we must also look at what was missing. In the fossil record of bony fishes there was a gap about ten million years long, known as the Patterson Gap, named after paleontologist Colin Patterson. A kind of gray area precisely in the most delicate moment: the one between the end of the Cretaceous and the first million years of the Paleocene, when marine ecosystems were looking for a new structure.
Qreiya 3 fills a major part of that gap. Fossils indicate that some lines that are now fundamental in the oceans were already present very early. Among the most notable finds are also the oldest known skeletal remains of several living groups, including relatives of tuna and mackerel. A similar discovery shifts attention: modern seas have older and more rapid roots than the fossil record has so far allowed us to see.
Trevally, the group that includes modern trevally, and armored forms linked to the evolutionary lineage of pipefish and seahorses also appear on the site. There is also a specimen referring to the Lampris genus, close to the sunfish, indicated as one of the most abundant finds in the deposit. Variety matters as much as quantity: around 500 specimens allow us to observe a community, rather than a single lucky fragment. It’s the difference between finding a sentence and opening a chapter.
After the disaster, free space
Mass extinctions leave huge holes in ecosystems. Predators, prey, competitors, entire eating habits disappear. In the post-asteroid seas, many food chains had been broken. In that void, some groups have had room to expand and change. Percomorphs seem to have exploited precisely this window, diversifying into forms capable of occupying different niches: fast predators, small inhabitants of complex environments, fish with new or more efficient feeding strategies.
The study also highlights a significant absence. Qreiya 3 lacks several predatory groups common in Cretaceous seas, despite very good conservation and a high number of specimens. This suggests that some older lineages have been reduced in size or have disappeared since the great crisis, while other groups, closer to the current marine fauna, have taken their place in the roles that were left vacant.
The comparison with the mainland comes almost naturally. After the extinction, placental mammals and many birds expanded their presence, transforming an emptied world into an evolutionary laboratory. In the seas, it seems, something similar was happening to fish. Qreiya 3 offers one of the clearest images of this restart, with the caution needed before every fossil window: a site tells a lot about its environment, then it must be placed next to other sites, other latitudes, other depths.
The tropical track
The Egyptian deposit was located in a tropical belt during the Paleocene. This detail opens up an interesting hypothesis: the areas closest to the equator may have seen the formation of modern-looking fish communities before others. In other regions, in the same period, some marine faunas may have maintained an archaic imprint for longer. Recovery after a global catastrophe rarely proceeds smoothly. Climate, depth, currents, availability of food, oxygen and basin geography change.
The researchers themselves urge caution. Qreiya 3 is already one of the most informative sites for understanding the reorganization of the seas in the first million years after the extinction, however many specimens are still in the preparation and study phase. The story, therefore, has just been opened. Other fossils may confirm, correct or complicate this tropical track. And it is precisely here that paleontology becomes less dusty than it seems: each plate can change the chronology, move an origin, remove a group from the darkness.
In the Egyptian desert, hundreds of fish left in the rock tell a simple thing: after the asteroid, the oceans had already started to populate again. Some groups had disappeared, others were taking up space. Qreiya 3 preserves that very passage, with fins, teeth, armor and skeletons remaining still for 62 million years.
©Science Advances
