This is the conclusion of a team led from the University of Manchester who studied deposits at the Tanis fossil site in North Dakota that formed at the time of the impact.
To narrow down the time of the impact, the team performed multiple different analyses of the annual growth lines in fossil fish bones preserved at the site.
They coupled this with evidence of certain behaviours of insects — such as the mining of leaves and the spawning of mayflies — that have a seasonal component.
The mass extinction marks the boundary between the Cretaceous and Palaeogene periods, and led to the demise of 75 per cent of species alive at the time.
The 6.2-mile-wide asteroid slammed into the Earth in what we know today as Mexico's Yucatán Peninsula, leaving behind the 93-mile-wide Chicxulub crater.
HOW AN ASTEROID ENDED THE REIGN OF THE DINOSAURS
Around 66 million years ago non-avian dinosaurs were wiped out along with more than half the world's species .
This mass extinction paved the way for the rise of mammals.
The Chicxulub asteroid is often cited as a potential cause of the Cretaceous-Paleogene extinction event.
The asteroid slammed into a shallow sea in what is now the Gulf of Mexico.
The collision released a huge dust and soot cloud that triggered global climate change, wiping out 75 per cent of all animal and plant species.
Researchers claim that the soot necessary for such a global catastrophe could only have come from a direct impact on rocks in shallow water around Mexico, which are especially rich in hydrocarbons.
Within 10 hours of the impact, a massive tsunami waved ripped through the Gulf coast.
The research was undertaken by palaeontologist Robert DePalma of the University of Manchester and his colleagues.
'The end-Cretaceous Chicxulub impact triggered Earth’s last mass-extinction, extinguishing around 75 per cent of species diversity and facilitating a global ecological shift to mammal-dominated biomes,' the team wrote in their paper.
'Temporal details of the impact event on a fine scale (hour-to-day) […] have largely eluded previous studies.'
Pinning down the exact timing of the impact event, they added, is crucial to gaining a better understanding of the early course of the mass extinction that followed.
This is because time plays a vital role in many biological functions, such as when to reproduce and hibernation, which feeding strategies to pick and even the nature of host–parasite interactions.
Given this, the timing of a global-scale hazard can affect how harshly the disaster impacts life, which species end up going extinct and exactly how well the remainder of the biota can recover in the wake of the event.
Understanding when the Chicxulub impactor hit Earth could therefore refine our predictions of how life today might respond should a similar catastrophe befall the Earth in the future.
'The hindsight that the fossil record provides can yield critical data, which can be applied today, so that we might plan for tomorrow,' noted paper author and palaeontologist Phil Manning of the University of Manchester.
In their study, the team present evidence from the first-ever recorded 'vertebrate mass-death assemblage' from within hours of the impact — preserved in a rock layer corresponding in time to the boundary between the Cretaceous and the Palaeogene.
This 'assemblage' comprises a densely-packed tangle of animal and plant fossils, alongside material from the impact itself, that are thought to have been rapidly buried thanks to a surge of water caused by impact-associated earthquakes.
The deposit also features a cap of clay that is rich in iridium, an element that is rare on Earth yet abundant in many asteroids, formed when the Chicxulub impactor was vaporised and dispersed around the atmosphere to latter settle to the ground.
It was this global iridium layer — with concentrations hundreds of times greater than normal for the Earth — that cemented the hypothesis of father-and-son team Luis and Walter Alvarez that an asteroid was the cause of the end-Cretaceous extinction.
Analysis of the fossil remains enabled the team to narrow down the times of year in which the mass-death assemblage could have been deposited and, by extension, when the concurrent asteroid impact took place.
First, they looked at fossilised fish bones from the site, focussing on growth lines which — like the rings round in tree trunks — provide an record of each animal's life history and can be used to determine in which season they stopped growing.
Inspection of the growth lines indicated that all the fish examined died during the Spring–Summer growth season, a conclusion backed up by an isotopic analysis of the lines, which confirmed that they formed following a distinct annual pattern.
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