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Fish fossils help paint picture of the last days of dinosaurs

The extraordinarily well-preserved fossils may give scientists insight as to why the dinosaurs went extinct while other species survived.

(CN) — Researchers have determined that the asteroid that killed most of the dinosaurs occurred during the springtime.

In a study published Wednesday in the journal Nature, scientists explain how they used powerful X-ray scans and carbon isotope records to examine the fossilized bones of fishes that died less than an hour after the asteroid’s impact. The international team of scientists studied fossilized paddlefishes and sturgeons from a site in North Dakota called Tanis. The fishes were extraordinarily well-preserved, with the soft tissue of some preserved along with the bones. The fossils proved key to helping researchers determine the season when the asteroid struck.

“These bones registered seasonal growth very much like trees do,” said Sophie Sanchez, one of the study’s authors and a lecturer at Sweden’s Uppsala University, in a statement. 

Comparable to how a tree’s rings can indicate its age and history, the bones indicated at what point in the fishes’ lives they died. The fossils showed the fishes had not yet reached their peak bone density at the time of their death.

Once the meteorite hit the planet’s surface, massive standing waves surged through bodies of water. Spherules — the remnants of the shattered asteroid — showered down from the sky. The fishes were buried alive in the massive amount of sediment and the spherules remained lodged in their gills approximately 65 million years later.

The carbon isotope analysis revealed that one of the study’s paddlefish ate different prey depending on the season, with zooplankton making up much of its diet throughout the spring and summer. The consumption of zooplankton would bolster the fish’s skeleton with a particular carbon isotope, giving scientists a metric to track how long the fish had been feasted on the zooplankton before it died.

“The carbon isotope signal across the growth record of the unfortunate paddlefish confirms that the feeding season had not yet climaxed — death came in spring,” said Melanie During, the study’s lead author from Uppsala University, in a statement.

The mass extinction that closed out the Mesozoic Era marked the end of a plethora of species, including the non-avian dinosaurs, pterosaurs, certain marine mollusks and many marine reptiles. But the researchers hope their work will aid scientists in answering why species including many mammals, birds, crocodiles and turtles survived.

According to a statement accompanying the research, the study’s finding that the extinction likely “abruptly started during northern-hemisphere spring” suggests that the asteroid struck when many of the organisms in the Northern Hemisphere were at the onset of their reproduction cycles. Organisms in the Southern Hemisphere preparing for winter may have been more protected from total extinction.

“This crucial finding will help to uncover why most of the dinosaurs died out while birds and early mammals managed to evade extinction,” said During.

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