About 80 million years ago, human-sized sea creatures with tentacle-shaped arms and coiled shells up to 1.8 meters wide slid into the Atlantic Ocean, a new study reveals.
These creatures were the largest ammonites in the world, a group of shells cephalopods which went extinct 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 great questions about how and when P. seppenradensis evolved into such an impressive size.
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Now, in a new study, published Wednesday (November 10) in the journal PLOS A, the researchers completed the evolutionary history of the massive cephalopod by examining 154 ammonite fossils, including a handful of historical specimens and more than 100 new fossils found in England and Mexico. Based on this analysis, they found that P. seppenradensis arose on both sides of the Atlantic Ocean around 80 million years ago and likely evolved from a smaller related species called Parapuzosia leptophylla, which reached a width of only 3.2 feet (1 m).
“We have this world-famous fossil here in Germany, and now we can tell its story,” said first author Christina Ifrim, researcher for the Bavarian Natural History Collections and head of science at the Jura-Museum, a museum history of Eichstätt, Germany.
To piece together the history of the famous ammonite, which is now on display at the Natural History Museum in Münster, the team visited a site about 40 kilometers north of Piedras Negras in northern Mexico.
Huge deposits of Cretaceous marine sediment can be found in various locations in Mexico, including the field site the team visited, said Ana Bertha Villaseñor Martinez, a researcher at the Institute of Geology at the Universidad Nacional Autónoma de México, who was not involved in the study. Although giant ammonites were discovered in Mexico in the past, the study’s authors applied new techniques to understand how creatures grew and developed throughout their lives, and how they evolved as that species over time, Villaseñor Martinez told Live Science in an email. In this way, the team “improved the understanding of ammonites, in general,” she said.
In a large, dry riverbed at the Mexican site, the team excavated layers of chalk, limestone, mud and clay and found 66 Parapuzosia specimens, including both the giant P. seppenradensis and the smallest P. leptophylla. The fossils were 0.3 to 4.8 feet (0.1 to 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 it grows,” Ifrim told Live Science. But these distinct growth stages were difficult to study in the past due to the shortage of specimens. With more specimens in hand, the team was able to see how P. seppenradensis and P. leptophylla each went through a separate five-stage growth cycle, during which their shells grew steadily and their morphology changed, she said.
By sorting the different Parapuzosia specimens, the team also dated the layers of sediment from which the specimens appeared. They found that the P. leptophylla samples dated to the late Santonian Age (86.3 million to 83.6 million years ago), a subdivision of the Upper Cretaceous. By comparing, P. seppenradensis appeared in younger sediments, dating later to the Santonian and the beginning of 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. leptophylla, but at the beginning of the 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 across the Atlantic, and around the same time, the team discovered. “We didn’t expect to find (P.) seppenradensis and that ancestor across 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 of east Kent. As they analyzed the samples, the team noticed, “Um, these giants appear, apparently, around the same time on both sides of the Atlantic,” Ifrim said. “There must have been a connection between the populations on both sides, because they show the same evolution, the same timing. ”
And in both England and Mexico, the team found an unusually high concentration of adult-sized seashells. They speculated that these areas may have served as mating or hatching sites where giant ammonites complete their breeding cycles and die soon after, as some squid and cuttlefish species do. But while that may explain the remarkable abundance of ammonite fossils at these sites, it doesn’t answer two big questions: why did ammonites get so big in the first place, and how they manifested on both sides. from the Atlantic?
Regarding the first question, ammonites may have faced evolutionary pressure to develop because a major predator of the Cretaceous, marine reptiles called mosasaurs, also increased during this period, the authors noted. However, while there is evidence of mosasaurs attacking ammonites, there is no direct evidence that they interacted with it. P. seppenradensis, more specifically, Ifrim told Live Science. So, for now, this is just speculation.
We also do not know how the distribution of giant ammonites spread across the Atlantic. Ammonites are believed to be rather slow swimmers, similar to modern nautiluses, but giant ammonites may cover the 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, driven by ocean currents.
So while the new study fills some gaps in the past of giant cephalopods, 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.