About 80 million years ago, human-sized sea creatures with tentacle-like arms and coiled shells measuring up to 1.8 meters wide slipped through the Atlantic Ocean, a new study reveals.
These creatures were the largest ammonites in the world, a group of ammonites cephalopods which died out about 66 million years ago. The largest ammonite fossil ever discovered belongs to the species Parapuzosia seppenradensis; the fossil, which was found in Germany in 1895, has a gargantuan shell measuring 5.7 feet (1.7 m) in diameter.
Although this famous fossil was discovered over a century ago, few ammonite fossils of similar size had been found until recently, leaving big questions about how and when. P. seppenradensis evolved to be such an impressive size.
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Now, in a new study, published Wednesday, November 10 in the journal PLOS A, researchers completed the evolutionary history of the massive cephalopod by examining 154 ammonite fossils, including a handful of historic specimens and more than 100 fossils found in England and Mexico. Based on this analysis, they found that P. seppenradensis appeared on both sides of the Atlantic Ocean about 80 million years ago and probably evolved from a smaller related species called Parapuzosia leptophyllawhich grew to be only 3.2 feet (1 m) wide.
“We have this world-famous fossil here in Germany, and now we can tell its story,” said first author Christina Ifrim, a researcher at the Bavarian Natural History Collections and chief scientist at the Jura-Museum, a museum in Bavaria. natural history of Eichstatt, Germany.
To piece together the history of the famous ammonite, which is now on display at the Natural History Museum in Münster, the team traveled to a site about 40 kilometers north of Piedras Negras in northern Mexico.
Huge deposits of Cretaceous marine sediments can be found in various locations in Mexico, including the field site visited by the team, said Ana Bertha Villaseñor Martinez, a researcher at the Institute of Geology at the Universidad Nacional Autónoma de México, which was not involved in the study. Although giant ammonites have been discovered in Mexico in the past, the study authors applied new techniques to understand how the creatures grew and developed throughout their lives, and how they evolved as than species over time, Villaseñor Martinez told Live Science in an email. In this way, the team “enhanced the understanding of ammonites, in general,” she said.
In a wide, dry river bed at the Mexican site, the team searched through layers of chalk, limestone, mud and clay and found 66 Parapuzosis specimens, including both the giant P. seppenradensis and the smallest P. leptophylla. The fossils were between 0.3 and 4.8 feet (0.1–1.48 m) wide and represented different stages of the ammonite growth cycle.
“A small specimen doesn’t look like a small version of this giant ammonite…they change as they grow,” Ifrim told Live Science. But these distinct growth stages were difficult to study in the past due to lack of specimens. With more specimens in hand, the team was able to see how P. seppenradensis and P. leptophylla each followed a distinct five-stage growth cycle, where their shells grew steadily and their morphology changed, she said.
By sorting out the different Parapuzosis specimens, the team also dated the sediment layers from which the specimens appeared. They found that the P. leptophylla dating from the Late Santonian Age (86.3 to 83.6 million years ago), a subdivision of Upper Cretaceous. By comparing, P. seppenradensis appeared in younger sediments, dating later in the Santonian and early in the following period, the Campanian (83.6 million to 72.1 million years ago). The oldest of these specimens reaches only 3.2 feet wide, as P. leptophyllabut in the early Middle Campanian ammonites of more formidable sizes appeared in the fossil record.
And it turned out that ammonites of comparable circumference could also be found on the other side of the Atlantic, and in the same period the team found it. “We did not expect to find (P.) seppenradensis and this ancestor on the other side of the Atlantic when we started the study,” said Ifrim.
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In the UK, the team discovered dozens of giant ammonite specimens at the base of a white chalk cliff in Sussex and closer to the chalk cliffs in east Kent. When analyzing the samples, the team noticed: “‘Huh, these giants are happening, apparently, more or less at the same time on both sides of the Atlantic,'” Ifrim said. “There had to be a connection between the people on both sides, because they show the same evolutionthe same time.”
And in both England and Mexico, the team found an unusually high concentration of adult-sized shells. They speculated that these areas may have served as mating or hatching sites where giant ammonites completed their reproductive cycles and died soon after, like some modern ones. squid and cuttlefish species do. But while this may explain the remarkable abundance of ammonite fossils at these sites, it doesn’t answer two big questions: why did the ammonites get so big in the first place, and how did they arise on both sides? of the Atlantic?
Regarding the first question, ammonites may have faced evolutionary pressure to develop because a major Cretaceous predator, marine reptiles called mosasaurs, also grew in size during this time, the authors noted. However, although there is evidence of mosasaurs feeding on ammonites, there is no direct evidence that they interacted with P. seppenradensis, specifically, Ifrim told Live Science. So, for now, this is just speculation.
We also don’t know how the distribution of giant ammonites spread across the Atlantic. Ammonites are thought to be rather slow swimmers, similar to modern nautiluses, but it’s possible that giant ammonites cover distance more efficiently, thanks to their size, Ifrim said. On the other hand, cephalopods may have crossed the ocean during their smaller juvenile growth stages, carried away by ocean currents.
So while the new study fills in some gaps in the giant cephalopods’ past, many mysteries remain.
Editor’s note: This article was updated on November 15 to include comments from Ana Bertha Villaseñor. The original story was published on November 10.
Originally posted on Live Science.