Andrewsarchus was the largest carnivorous land mammal that ever lived.
It lived about 32-60 million years ago. (Image: BBC Walking With Beasts)
Contrary to popular belief, a new study shows that the rise of mammals was not connected to the extinction of dinosaurs that occurred 65 million years ago. The evidence challenging this connection comes from the most complete family tree ever compiled for mammals. This supertree, comprised of genetic and fossil data reveals the relationships between mammals such as primates, rodents and hoofed mammals, including when they evolved into separate groups.
According to the most popular hypothesis, throughout the Cretaceous Period, mammals were relatively few in number, and were prevented from diversifying and evolving into major taxonomic subgroups, known as orders, in ecosystems that were dominated by dinosaurs. However, the mass extinction of the dinosaurs — probably caused by an asteroid or comet striking Earth — removed this constraint. This allowed mammals to diversify and flourish, and allowed them to evolve into their present position of dominance on Earth. This bolide impact was recorded as shock waves in rock and is referred to by geologists as the boundary between the Cretaceous and Tertiary periods, or the K/T boundary.
However, contrary to this hypothesis, the new mammal “supertree” shows that placental mammals had already diverged into several separate orders by 93 million years ago — long before the bolide impact and at a time when dinosaurs still ruled the planet. This supertree was constructed from both fossil and genetic data (se supertree below). It shows most of today’s mammalian lineages appeared between 100 and 85 million years ago. It also rebeals that the rate of mammalian evolution barely changed after the dinosaurs had gone extinct. Despite this, it was not until 10-35 million years after the K/T boundary when these mammalian lineages began to thrive and further radiate into more modern sub-groups.
“The [supertree] is a new way of showing all the mammal species on the planet, starting with a common ancestor. Species relationships can be inferred from morphological characteristics and genetic sequences,” explained the study’s co-author, Kate Jones. “If we had done this from scratch, we would have had to get molecular and morphological data for 4,000 different species.”
The team relied on “data mining” from already-published data sets to construct their supertree.
“What we did instead was use already published information from hundreds of researchers around the world. We used a new technique called supertree construction which allows us to get all the information that’s out there, re-code it and re-analyse it as if it’s all part of one dataset,” said Jones.
However, because most researchers now recognize that birds descended from dinosaurs, at least one lineage of dinosaurs is still with us (the birds) and there are more than 40 taxonomic orders of birds living today.
“There was a period of several million years at the end of this period which witnessed several extinctions of non-avian dinosaurs,” explained Jones. “So the old textbook idea that at the K/T boundary dinosaurs disappeared and mammals appeared is a bit of a straw man.”
Mammalian supertree. All orders are labelled and major lineages are coloured as follows: black, Monotremata; orange, Marsupialia; blue, Afrotheria; yellow, Xenarthra; green, Laurasiatheria; and red, Euarchontoglires. Families that were reconstructed as non-monophyletic are represented multiple times and numbered accordingly. Branch lengths are proportional to time, with the K/T boundary indicated by a black, dashed circle. The scale indicates Myr. Nature. vol 446 (29 March 2007):507-512. [bigger size].
Original Paper: The delayed rise of present-day mammals, by Olaf R. P. Bininda-Emonds, Marcel Cardillo, Kate E. Jones, Ross D. E. MacPhee, Robin M. D. Beck, Richard Grenyer, Samantha A. Price, Rutger A. Vos, John L. Gittleman & Andy Purvis. Nature. vol 446 (29 March 2007):507-512.
BBC News (quotes)