Figure 1. The evolution of birds as a consequence of miniaturization. Artist: Davide-Bonnadonna. I think the last taxon on the right has been enlarged somewhat for clarity. See Figure 2.
A new paper by Lee et al. (2014)
reported on their phylogenetic analysis of theropods employing 1549 characters. They found two drivers underlying the dinosaur-bird transition: 1) sustained miniaturization; and 2) the evolution of skeletal adaptations 4x faster than other dinosaurs.
They found that miniaturization facilitated the evolution of morphological novelties associated with small size: reorientation of the body mass; 2) increased aerial ability; and 3) paedomorphic skulls with enlarged eyes and brains along with a reduced snout and smaller teeth without serrations.
I heartily endorse this work.
It supports a paradigm that miniaturization that produces new clades. Cope’s Rule generally does not. We see similar miniaturization at the genesis of reptiles, amphibians, therapsids, mammals, dinosaurs, crocodylomorphs (together the archosaurs), pterosaurs, diapsids and several clades within each of these, like bats and pterodactyloid-grade pterosaurs. We knew for a long time that sustained miniaturization also produced birds. So that’s not news. It just has never been so well laid out before.
Lee et al. conclude:
“Because size reduction, feather elaboration, paedomorphism, and other anatomical novelties permitted by small size all evolved in concert along the bird stem, identifying the primary driver of this sustained trend is probably impossible. It is likely that all traits influenced and provided the context for the evolution of others.”
Actually not so impossible.
The authors make no mention of the fact that smaller taxa generally mature more quickly, breed more often and die sooner. In other words, generational turnover happens more quickly in smaller taxa, as everyone know. The rate of evolution over time is accelerated by the rate of reproduction over time (all other things being equal). That’s the primary driver.
In the family tree of amniotes,
you see this all the time, not just in bird origins. Good to see this get the press it deserves.
References
Lee MSY, Cau A, Naish D and Dyke GJ 2014. Sustained miniaturization and anatomical innovation in the dinosaurian ancestors of birds.
The Pterosaur Heresies 