The dinosaurs — the so-called tyrants of the Mesozoic era — weren't exactly thriving during their last few million years on Earth, a new study finds.

The new analysis of the dinosaur family tree reveals that dinosaurs were disappearing even before the asteroid hit about 65.5 million years ago. Roughly 24 million years before that impact, dinosaur extinction rates passed speciation rates, meaning that the animals were losing the ability to replace extinct species with new ones, the researchers said.

The findings suggest that these striking extinction rates made the dinosaurs vulnerable to drastic environmental changes, such as the asteroid collision, the researchers said. [Wipe Out: History's Most Mysterious Extinctions]

"This implies that any group of animals that is under prolonged periods of high extinction rate can undergo mass extinction should there be a catastrophic event," said study lead researcher Manabu Sakamoto, a postdoctoral research assistant of biological sciences at the University of Reading in the United Kingdom.

The study isn't the first to suggest that dinosaurs were in a major decline before the asteroid event. In previous studies, scientists have recorded the number of species in each geological age and compared those levels to the subsequent ages (each age lasts for millions of years) to get a sense of how diverse the dinosaurs were, Sakamoto said.

But that method focuses on snapshots in time and doesn't take into account the extinction and speciation rate within each branch of the dinosaur family tree. So the researchers of the new study looked at the dinosaur fossil record and the family tree to get a robust picture of when new dinosaur species came onto the scene, Sakamoto said.

"Our study is the first to incorporate such phylogenetic [family tree] information when studying speciation and extinction in dinosaurs," Sakamoto told Live Science. "This is what has allowed us to build a more nuanced and certain picture of dinosaur speciation than has ever before been possible."

Dinosaur detectives

The researchers separately analyzed the three major groups of dinosaurs: the ornithischians (such as Stegosaurus), sauropodomorphs (the long-necked, long-tailed herbivores) and theropods (bipedal, mostly carnivorous dinosaurs, such as T. rex and Albertosaurus).

The sauropodomorphs had the most prominent downturn, the scientists found. The research showed spikes in new species of this type of dinosaur emerging during the Triassic and early Jurassic periods, until about 195 million years ago, when that speciation rate began to slow down. At 114 million years ago, during the early Cretaceous period, species of sauropodomorphs were going extinct faster than new species were emerging, the researchers found.

"The subsequent originations of [the] titanosaurian [group] were not nearly enough to compensate for the continuous loss of sauropods throughout the remainder of the Cretaceous," the scientists wrote in the study.

Theropods had an "early burst" of speciation followed by a speciation slowdown from the late Triassic to the early Cretaceous (about 215 million years ago to about 120 million years ago), when extinction rate exceeded speciation rate, the researchers found.

Likewise, ornithischians show an early increase followed by a speciation slowdown at about 114 million years ago, when extinction rate surpassed speciation rate. But there were a few success stories within this group. The hadrosauriforms (duck-billed dinosaurs) and ceratopsids (the horned dinosaurs, such as Triceratops) did well, likely because they had developed jaws that helped them munch on new food, possibly flowering plants, the researchers said. [Dinosaur Detective: Find Out What You Really Know]

When the researchers considered the three dinosaur groups separately, "We found unequivocal evidence that dinosaurs were in decline up to 50 million years prior to the mass extinction event 66 million years ago," Sakamoto said.

Extinction lessons

It's unclear why the dinosaurs started going extinct so early, but there are clues as to why speciation increased during certain periods, the scientists said. One idea is that rising sea levels cut into the land, fragmenting dinosaur habitats and nudging the beasts to evolve separately into new species in different areas, the researchers said.

However, after dinosaur extinction rates began to rise, another group of animals started thriving: mammals.

"The decline of the dinosaurs would have left plenty of room for mammals, the group of species which humans are a member of, to flourish before the impact, priming them to replace dinosaurs as the dominant animals on Earth [after the impact]," study co-author Chris Venditti, an evolutionary biologist at the University of Reading, said in a statement.

The new findings may help scientists understand the possible consequences of today's extinctions, the researchers added.

"We live in a time when species are undergoing unprecedented levels of extinctions," Sakamoto told Live Science. "This means that if some major catastrophe hits, then it is highly possible that whole groups of animals [will] be completely wiped out off the face of the Earth."

The study is an interesting one, said Alan Turner, an associate professor of anatomical sciences at Stony Brook School of Medicine in New York, who was not involved in the research.

"Through modeling of speciation dynamics, it appears that dinosaur diversity was declining well in advance of the end-Cretaceous extinction event that killed off all nonavian dinosaurs, as well as numerous other vertebrate groups," Turner told Live Science in an email.

The study was published online April 18 in the journal Proceedings of the National Academy of Sciences.

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