These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) than they are to shelled nautiloids such as the living Nautilus species. So, for now, this is just speculation.Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. seppenradensis, specifically, Ifrim told Live Science. However, although there's evidence of mosasaurs preying on ammonites, there's no direct evidence that they interacted with P. Regarding the former question, the ammonites may have faced an evolutionary pressure to grow because a major predator of the Cretaceous, marine reptiles called mosasaurs, also grew larger during this time, the authors noted. But while this might 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 show up on both sides of the Atlantic? They theorized that perhaps these areas served as mating or hatching sites where the giant ammonites completed their reproductive cycles and died shortly afterward, like some modern squid and cuttlefish species do. (Image credit: Courtesy of Christina Ifrim)Īnd both in England and Mexico, the team found an unusually high concentration of adult-size shells. "There must have been a connection between the populations of both sides, because they show the same evolution, the same timing."Īn ammonite fossil uncovered at the team's field site in Mexico. Upon analyzing the samples, the team noticed, "'Huh, these giants occur, apparently, at more or less the same time on both sides of the Atlantic,'" Ifrim said. In the U.K., the team uncovered dozens of giant ammonite specimens at the base of a white chalk cliff in Sussex and more near the chalk cliffs of east Kent. ![]() "We did not expect to find ( P.) seppenradensis and this ancestor on the other side of the Atlantic when we started the study," Ifrim said. leptophylla, but by the middle early Campanian, ammonites of more formidable sizes cropped up in the fossil record.Īnd it turned out, ammonites of comparable girth also could be found across the Atlantic, and at the same time period, the team found. The oldest of these specimens reached only 3.2 feet wide, like P. ![]() seppenradensis appeared in younger sediments, dating later in the Santonian and early in the following time period, the Campanian (83.6 million to 72.1 million years ago). leptophylla samples dated to the late Santonian age (86.3 million to 83.6 million years ago), a subdivision of the Upper Cretaceous. In sorting through the various Parapuzosia specimens, the team also dated the layers of sediment from where the specimens appeared. (Image credit: Ifrim et al., 2021, PLOS ONE, CC-BY 4.0 ()) The largest ammonite fossil ever found measures about six feet across. leptophylla each followed a distinct, five-stage growth cycle, where their shells grew steadily and their morphology changed, she said. With more specimens in hand, the team could see how P. ![]() But these distinct growth stages were difficult to study in the past because of the dearth of specimens. "A small specimen doesn't look like a small version of this giant ammonite … they change during growth," Ifrim told Live Science. The fossils measured between 0.3 and 4.8 feet (0.1 to 1.48 m) wide and represented different stages in the ammonite growth cycle. In a broad, dry riverbed at the Mexican field site, the team searched through layers of chalk, limestone, mud and clay and found 66 Parapuzosia specimens, including both the giant P. In this way, the team has "improved the understanding of ammonites, in general," she said. Although giant ammonites have been uncovered in Mexico in the past, the study authors applied new techniques to understand how the creatures grew and developed throughout their lifespans, and how they evolved as a species over time, Villaseñor Martinez told Live Science in an email. Huge deposits of Cretaceous marine sediment can be found at various locations in Mexico, including the field site the team visited, said Ana Bertha Villaseñor Martinez, a researcher in the Institute of Geology at the Universidad Nacional Autónoma de México, who was not involved in the study. To piece together the history of the famed ammonite, which now stands on display at the Museum of Natural History in Münster, the team traveled to a field site about 25 miles (40 kilometers) north of Piedras Negras in northern Mexico. "We have this world-famous fossil here in Germany, and now we can tell its story," said first author Christina Ifrim, a researcher for the Bavarian Natural History Collections and head of science at the Jura-Museum, a natural history museum in Eichstätt, Germany.
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