Endless skulls most beautiful.

The amazing disparity of living birds is self-apparent, yet immensely challenging to fully quantify. After all, birds are represented by nearly 11,000 living species, comprising a mind-boggling spectrum of shapes, sizes, and colors (1). This incredible variability manifests in an incalculable number of ways (from habitat type to diet to life history), but adequately characterizing any of these axes of variation presents distinct challenges with respect to analytical complexity and the requisite scale of data collection. In PNAS, Felice and Goswami (2) exemplify the vanguard of comparative vertebrate morphology by taking up the challenge of characterizing and analyzing avian phenotypic disparity on a scale that was, until quite recently, unimaginable. The authors focus on the bird skull, a structure whose extreme evolutionary potential has rendered it a frequent topic of study among those interested in the tempo and mode of avian adaptive radiation (3–6) (Fig. 1). Whereas other recent studies have focused on estimating rates of evolutionary change in the shape of that most ecologically adaptable avian feature, the beak (e.g., ref. 4), Felice and Goswami (2) treat the avian skull as a cohesive whole, devising a methodology flexible enough to gather data from nearly all living bird families, yet detailed enough to (almost) completely characterize the external morphology of the skull. They accomplish this impressive feat using laser-surface scanning and high-resolution computed tomography, similar to the techniques employed by Cooney et al. in their work on the avian bill (4), and Bright et al. on raptor skulls (5). This approach yielded a vast amount of anatomical data that Felice and Goswami (2) sought to marshal for quantifying cranial shape, and additional downstream parameters like rates of shape change. This demanded an approach to quantify cranial geometry in a way that would facilitate meaningful comparisons across species. To accomplish this goal, Felice and Goswami began by identifying homologous “key landmarks” on each skull, as well as a hemispherical template with additional densely packed landmarks. Using an innovative shape-morphing approach (7), Felice and Goswami (2) then “morphed” the template into the shape of each skull, using the key landmarks as anchor points. The degree to which the position of the key landmarks and additional landmarks were thereby digitally “stretched” from the hemispherical starting shape allowed Felice and Goswami to quantify the universe of avian cranial shapes in unprecedented detail. From there, Felice and Goswami (2) employed a likelihood-based approach to identify regions of the avian skull that appear to evolve as reasonably autonomous entities, or modules (8). The authors (2) identified seven such modules, which together compose the entire skull. These include the well-studied rostrum, as well as the top of the skull, back of the skull, and palate. The recognition of substantial modularity in the avian skull challenges conflicting results from previous studies that employed more idiosyncratic taxon-sampling schemes and approaches to data collection (9, 10). This modularity is the basis for Felice and Goswami’s (2) assessment of the avian skull as a classic example of “mosaic evolution,” whereby different modules exhibit differing rates and modes of evolutionary change. Felice and Goswami (2) were able to tackle another major analytical challenge: discerning the tempo and mode by which rates of cranial shape change evolved throughout the phylogenetic history of living birds. They employed a recent time-scaled evolutionary tree for birds (11) to determine how quickly shape evolved among the seven cranial modules along the branches of the tree, yielding some interesting insights. For example, rates of evolutionary change in the avian rostrum were especially high along the lineage leading to (long-billed) hummingbirds following the divergence from their extant sister taxon, (short-billed) swifts. Additionally, elevated rates of change were inferred for virtually every cranial module in the immediate aftermath of the Cretaceous–Paleogene (K–Pg) mass extinction event, 66 million years ago, an event that profoundly influenced avian evolutionary history (12–15). The K–Pg transition has been posited to have