From The Urantia Book:
(60:2.14) One hundred million years ago the reptilian age was drawing to a close. The dinosaurs, for all their enormous mass, were all but brainless animals, lacking the intelligence to provide sufficient food to nourish such enormous bodies. And so did these sluggish land reptiles perish in ever-increasing numbers. Henceforth, evolution will follow the growth of brains, not physical bulk, and the development of brains will characterize each succeeding epoch of animal evolution and planetary progress.
(60:3.20) 60,000,000 years ago, though the land reptiles were on the decline, the dinosaurs continued as monarchs of the land, the lead now being taken by the more agile and active types of the smaller leaping kangaroo varieties of the carnivorous dinosaurs. But sometime previously there had appeared new types of the herbivorous dinosaurs, whose rapid increase was due to the appearance of the grass family of land plants. One of these new grass-eating dinosaurs was a true quadruped having two horns and a capelike shoulder flange. The land type of turtle, twenty feet across, appeared as did also the modern crocodile and true snakes of the modern type. Great changes were also occurring among the fishes and other forms of marine life.
(60:3.21) The wading and swimming prebirds of earlier ages had not been a success in the air, nor had the flying dinosaurs. They were a short-lived species, soon becoming extinct. They, too, were subject to the dinosaur doom, destruction, because of having too little brain substance in comparison with body size. This second attempt to produce animals that could navigate the atmosphere failed, as did the abortive attempt to produce mammals during this and a preceding age.
(60:3.22) 55,000,000 years ago the evolutionary march was marked by the sudden appearance of the first of the true birds, a small pigeonlike creature which was the ancestor of all bird life. This was the third type of flying creature to appear on earth, and it sprang directly from the reptilian group, not from the contemporary flying dinosaurs nor from the earlier types of toothed land birds. And so this becomes known as the age of birds as well as the declining age of reptiles.
ScienceDaily reported from June 2021, out of the University of Bristol:
The death of the dinosaurs 66 million years ago was caused by the impact of a huge asteroid on the Earth. However, palaeontologists have continued to debate whether they were already in decline or not before the impact.
In a new study, published today in the journal Nature Communications, an international team of scientists, which includes the University of Bristol, show that they were already in decline for as much as ten million years before the final death blow. …
“Then, the loss of herbivores made the ecosystems unstable and prone to extinction cascade. We also found that the longer-lived dinosaur species were more liable to extinction, perhaps reflecting that they could not adapt to the new conditions on Earth.”
Fabien Condamine added: “This was a key moment in the evolution of life. The world had been dominated by dinosaurs for over 160 million years, and as they declined other groups began their rise to dominance, including the mammals.
“The dinosaurs were mostly so huge they probably hardly knew that the furry little mammals were there in the undergrowth. But the mammals began to increase in numbers of species before the dinosaurs had gone, and then after the impact they had their chance to build new kinds of ecosystems which we see today.”
Science Daily article from April 2017 on fossil evidence confirming that dinosaurs did not die out from a massive astroid strike before the birds evolved.
Dr Wolfgang Stinnesbeck of Heidelberg University and Prof. Dr Eberhard Frey of the State Museum of Natural History Karlsruhe also indicate that bird species spread and diversified at the same time the dinosaurs disappeared. Their results were published in the journal Geological Society of America Bulletin.
While conducting paleontological research in northeastern Mexico, the scientists came upon sedimentary rock deposited toward the end of the Cretaceous Period that evidenced an enormous diversity of fossils, including the tracks of birds, dinosaurs and pterosaurs. “Most of the imprints come from at least five different species of birds; dinosaur tracks, however, are rare. Only a single footprint comes from a predatory dinosaur,” explains Prof. Stinnesbeck. The finds therefore indicate a gradual decline of the dinosaurs with a simultaneous increase in the diversity of birds even before the end of the Cretaceous Period. “Until now, it was generally assumed that the dinosaurs died out first and bird species diversified afterward,” states the researcher. “Our data, however, substantiate the theory that birds ascended before dinosaurs became extinct.”