The extinction of dinosaurs at the Cretaceous/Paleogene (K/Pg) boundary was the seminal event that opened the door for the subsequent diversification of terrestrial mammals. Our compilation of maximum body size at the ordinal level by sub-epoch shows a near-exponential increase after the K/Pg. On each continent, the maximum size of mammals leveled off after 40 million years ago and thereafter remained approximately constant. There was remarkable congruence in the rate, trajectory, and upper limit across continents, orders, and trophic guilds, despite differences in geological and climatic history, turnover of lineages, and ecological variation. Our analysis suggests that although the primary driver for the evolution of giant mammals was diversification to fill ecological niches, environmental temperature and land area may have ultimately constrained the maximum size achieved.

Toothed whales (order Cetacea: suborder Odontoceti) are highly encephalized, possessing brains that are significantly larger than expected for their body sizes. In particular, the odontocete superfamily Delphinoidea (dolphins, porpoises, belugas, and narwhals) comprises numerous species with encephalization levels second only to modern humans and greater than all other mammals. Odontocetes have also demonstrated behavioral faculties previously only ascribed to humans and, to some extent, other great apes. How did the large brains of odontocetes evolve? To begin to investigate this question, we quantified and averaged estimates of brain and body size for 36 fossil cetacean species using computed tomography and analyzed these data along with those for modern odontocetes. We provide the first description and statistical tests of the pattern of change in brain size relative to body size in cetaceans over 47 million years. We show that brain size increased significantly in two critical phases in the evolution of odontocetes. The first increase occurred with the origin of odontocetes from the ancestral group Archaeoceti near the Eocene-Oligocene boundary and was accompanied by a decrease in body size. The second occurred in the origin of Delphinoidea only by 15 million years ago.

A group of eminent cetacean researchers respond to headlines charging that dolphins might be "flippin' idiots". They examine behavioural, anatomical and evolutionary data to conclude that the large brain of cetaceans evolved to support complex cognitive abilities.

From Big Fish to Big Whales Whales are the largest animals today, and many feed on the abundant plankton, particularly diatoms, in the oceans. Whales arose and diversified in the Cenozoic, about 30 to 40 million years ago (see the Perspective by Cavin ). Marx and Uhen (p. 993 ) show that their diversity parallels the diversity of diatoms and changes in ocean temperature. Whether there were large predators of plankton before whales has been enigmatic, because the fossil record during the Mesozoic (245 to 65 million years ago) is sparse. Friedman et al. (p. 990 ) now show that a group of large fish filled this role for nearly 100 million years in the Mesozoic. Although not as large as whales, these globally distributed fish were still several meters long. Their extinction at the Cretaceous-Paleogene boundary 65.5 million years ago may have cleared the seas for the evolution of whales.

The end of the Pliocene marked the beginning of a period of great climatic variability and sea-level oscillations. Here, based on a new analysis of the fossil record, we identify a previously unrecognized extinction event among marine megafauna (mammals, seabirds, turtles and sharks) during this time, with extinction rates three times higher than in the rest of the Cenozoic, and with 36% of Pliocene genera failing to survive into the Pleistocene. To gauge the potential consequences of this event for ecosystem functioning, we evaluate its impacts on functional diversity, focusing on the 86% of the megafauna genera that are associated with coastal habitats. Seven (14%) coastal functional entities (unique trait combinations) disappeared, along with 17% of functional richness (volume of the functional space). The origination of new genera during the Pleistocene created new functional entities and contributed to a functional shift of 21%, but minimally compensated for the functional space lost. Reconstructions show that from the late Pliocene onwards, the global area of the neritic zone significantly diminished and exhibited amplified fluctuations. We hypothesize that the abrupt loss of productive coastal habitats, potentially acting alongside oceanographic alterations, was a key extinction driver. The importance of area loss is supported by model analyses showing that animals with high energy requirements (homeotherms) were more susceptible to extinction. The extinction event we uncover here demonstrates that marine megafauna were more vulnerable to global environmental changes in the recent geological past than previously thought.