Bird neurocranial and body mass evolution across the end-Cretaceous mass extinction: The avian brain shape left other dinosaurs behind

Abstract

Birds today are the most diverse clade of terrestrial vertebrates, and understanding why extant birds (Aves) alone among dinosaurs survived the Cretaceous-Paleogene mass extinction is crucial to reconstructing the history of life. Hypotheses proposed to explain this pattern demand identification of traits unique to Aves. However, this identification is complicated by a lack of data from non-avian birds. Here, we interrogate survivorship hypotheses using data from a new, nearly complete skull of Late Cretaceous (~70 million years) bird Ichthyornis and reassess shifts in bird body size across the Cretaceous-Paleogene boundary. Ichthyornis exhibited a wulst and segmented palate, previously proposed to have arisen within extant birds. The origin of Aves is marked by larger, reshaped brains indicating selection for relatively large telencephala and eyes but not by uniquely small body size. Sensory system differences, potentially linked to these shifts, may help explain avian survivorship relative to other dinosaurs.

Fig. 1. Photographs and digital renderings of new Ichthyornis (AMNH FARB 32773) craniofacial anatomy showing evidence of a segmented palate.

(A) Photograph of skull block including the neurocranium and partial upper and lower jaws. (B) Rendering of the skull block showing the position of the endocast (pink) relative to the cranium and nearby preserved facial elements (translucent white). (C) Photograph of the block including the premaxilla, left maxilla, and left palatine. Digital reconstructions of the facial skeleton in (D) lateral, (E) dorsal (mandible removed), and (F) ventral (mandible removed) views. Preserved elements are colored; symmetrical elements mirrored from the opposite side are in gray. Numbers correspond to novel insights from this specimen: 1, complete orbital process of the quadrate; 2, complete nasomaxillary contact; 3, segmented hemipterygoid. den, dentary; fr, frontals; hpt, hemipterygoid; ju, jugal; la, lacrimal; max, maxilla; mes, mesethmoid; na, nasal; nc, neurocranium; pa, palatine; pd, predentary; pman, posterior mandible; pre, premaxilla; qj, quadratojugal; qu, quadrate; vo?, tentative vomer. Photo credit: Christopher R. Torres, University of Texas at Austin.

Fig. 2. Evidence of a wulst in Ichthyornis.

(A and B) Reconstruction of the Ichthyornis endocast in (A) right lateral and (B) dorsal views. (C to H) CT slice data showing the endocranial surface in the area of the vallecula (arrowheads) in (C to E) coronal and (F to H) sagittal planes. Yellow, telencephalon; red, optic lobe; blue, cerebellum.

Fig. 3. Multiphase acquisition of the avian brain shape.

Fig. 4. Evidence of the presence of a neognath-like palate in Ichthyornis.

(A to F) Orange, pterygoid; purple, palatine; teal, hemipterygoid.

Fig. 5. The impact of including stem data on estimating body size evolution in birds.

(A) Body mass evolution across the avian stem. Ancestral body masses were estimated including 26 stem birds with well-supported phylogenetic affinities and mapped across the strict consensus tree from our primary phylogenetic analysis. Gray branches indicate taxa that were used to estimate phylogeny but not ancestral body mass reconstructions. Circles correspond to ancestral body masses estimated for major basal divergences within Aves including data from the total stem bird sample (pink), only stem birds with well-supported phylogenetic affinities (dark gray), and no stem birds except Archaeopteryx (light gray) and are to scale. (B) Comparison of body mass estimates of ¹Cretaceous birds known from within 300 ka (thousand years) of the K-Pg boundary (19) to our body mass estimates for divergences within the crown clade and taxa from the Late Cretaceous and Early Paleogene, as well as ²model-based ancestral body masses for Aves, Neognathae, and Neoaves estimated previously (11) without inclusion of outgroup taxa. Cretaceous crown birds: 1, Conflicto antarcticus; 2, Asteriornis maastrichtensis; 3, Polarornis gregorii; 4, Vegavis iaai.

Fig. 6. The impact of outgroup sampling on inferred ancestral relative brain size in Aves and major subclades.

(A) Brain size versus body size for extant birds (gray) and select non-avian dinosaurs (triangles) and a regression representing the ancestral avian and near-crown scaling relationship from (2). Increasingly dense outgroup body mass sampling (dark and pink) recovers ancestral avian relative brain size more consistent with extant birds than non-avian dinosaurs. (B) The same data depicted in (A) showing that increased outgroup body mass sampling recovers an increasingly marked shift in relative brain size from the divergences of total birds (Avialae) to major extant clades. ¹Data for extant birds and non-avian dinosaurs and regression from (2).