Weird and diverse: A hadrosaur family tree conveying the diversity of this group of duck-billed dinosaurs. Many hadrosaur fossils date to the period of the so-called 'Cretaceous Terrestrial Revolution'.
Credit: Wikimedia
PARIS: The belief that dinosaurs underwent explosive species diversification shortly before they were wiped out is an illusion, for the beasts' main evolutionary shifts took place millions of years before.
The mysterious demise of the dinosaurs at the end of the Cretaceous era some 65 million years ago has given rise to a popular view that almost has the tinge of Greek tragedy.
Just as the rulers of the Earth had reached their evolutionary zenith, the fable goes, a catastrophic event – likely a comet that slammed into Earth – brought the curtain down on their long reign.
Terrestrial revolution
Scientific support for this view comes from the number of dinosaur fossils dating from a period dubbed the Cretaceous Terrestrial Revolution, between 125 million to 80 million years ago, when Earth's book of life was changed forever.
During this epoch of riotous biodiversity, flowering plants, social insects, butterflies, modern groups of lizards, mammals, and possibly birds too, all emerged.
Some experts have suggested that dinosaurs were also part of the show, as so many weird fossils, such as duckbilled hadrosaurs, horned ceratopsians, pachycephalosaurs and other wonders, date from this time.
But a new study, published today in the British journal Proceedings of the Royal Society B, says that dinosaurs were less than a sideshow in the DNA spectacular.
Researchers led by Graeme Lloyd of the University of Bristol, in western England, devised a "supertree" of dinosaur evolution, patiently analysing how more than 450 species – about 70 per cent of the known finds – developed.
Big evolutionary splurge
They conclude that the big evolutionary splurge for dinosaurs occurred in the Late Triassic, some 225 million to 200 million years ago. This was about 15 to 40 million years after dinosaurs first emerged.
A second, but smaller, diversification occurred in the mid-Jurassic, some 170 to 160 million years ago.
By that time, all the main dinosaur lineages that were hoofing around in the Cretaceous Terrestrial Revolution had been established.
"Our new evidence confirms that the [Cretaceous Terrestrial Revolution] was a key in the origination of modern continental ecosystems, but that the dinosaurs were not a part of it," the authors write.
"Sample bias" – the availability of many fossils from this period – could be to blame for the distorted picture, it suggests.
A leading theory for the end of the dinosaurs is that a large asteroid or comet whacked into the modern-day Yucatan peninsula in Mexico, unleashing firestorms and dust clouds that obscured the Sun, inflicting climate change that ravaged vegetation.
