DOI: 10.5061/dryad.jsxksn0ng
title: Data from: Enlargement of sternum traits facilitated the evolution of powered flight in birds

description: An enlarged sternum with a prominent keel is a central feature of the
 flight apparatus of modern birds. However, sterna of near-bird dinosaurs
 (Pennaraptora) and early avialans are either significantly different from
 those of living birds or absent altogether, raising questions on how
 specialized sternal structures evolved in birds and how they are related
 to function. This remains poorly understood due to the fragmentary nature
 of the fossil record, and the challenges in inferring form and function
 from crushed fossils. We employ ancestral character estimations to trace
 sternal trait acquisition through the bird stem group, and multivariate
 phylogenetic regressions to analyze relationships between sternum
 morphology, body mass, and flight capabilities. We find that sternum
 evolution was episodic: basal members of Pennaraptora had proportionally
 small sterna, which became larger and more craniocaudally elongated in
 Avialae. This enlargement precedes the appearance of a midline ridge, a
 possible precursor of the sternal keel, in Pygostylia. Sternum size
 increased again in crownward Ornithuromorpha, alongside a fully formed
 sternal keel and enlarged caudal projections, both critical areas of
 flight muscle attachment. Sternal experimentation in relation to flight
 characteristics occurs multiple times throughout Pennaraptora, including
 within Paraves and Enantiornithes, indicating that powered flight may have
 evolved multiple times before proliferating in crown-group birds.
description: # Data from: Enlargement of sternum traits facilitated the evolution of
 powered flight in birds Associated publication: Lowi-Merri, T.M., Benson,
 R., Hu, H., et al. Enlargement of sternum traits facilitated the evolution
 of powered flight in birds. Nat. Ecol. Evol. ## Dryad dataset Dataset DOI:
 [10.5061/dryad.jsxksn0ng](10.5061/dryad.jsxksn0ng) ## Description of the
 data and file structure Comparative analysis of fossil stem avian sternum
 size and dimensions to determine the evolutionary trajectory of the
 sternum across the dinosaur-bird transition and during the evolution of
 flight. ### Files and variables Most variables are self-explanatory and
 are defined when deemed necessary. All units of measurement are in
 millimetres (mm) unless specified otherwise. #### File: Fossils25.csv
 **Description:** Morphological data on fossil specimens for which sternum
 size and metrics were collected. ##### Variables * Taxon * Specimen number
 * Higher Classification: larger clade that the taxon belongs in * Sternum
 Footprint Area (mm^2): area of the basal plate of the sternum * Femur
 circumference * Sternum Width * Sternum Min. Length: length of sternum
 from base of craniolateral process to dorsalmost point of lateral process
 * Sternum Total Length: length of sternum including full length of
 craniolateral and all lateral and/or intermediate processes * Keel state:
 0  = sternum absent; 1 = sternum without keel; 2 = medial ridge; 3 =
 developed keel * Fused or unfused: 0 = sternum absent; 1 = sternum
 unfused; 2 = sternum fused * First appearance data (FAD): earliest
 occurrence date of taxon in MYA (million years ago) * Last appearance data
 (LAD): last occurrence date of taxon in MYA (million years ago) #### File:
 SimpFullTree.tre **Description:** Phylogenetic tree of all fossils for
 which sterna were analyzed, in which extant birds are collapsed into one
 tree branch called Neornithes #### File: FullTree.tre
 **Description:** Phylogenetic tree of all fossils and extant birds for
 which sterna were analyzed #### File: FosSkelProp.csv
 **Description:** Data on skeletal proportions of fossil taxa, in mm #####
 Variables * Taxon * Specimen * Higher Classification * Humerus Length *
 Humerus proximal width  * Ulna length * Ulna width * Radius length *
 Carpometacarpus length * Carpometacarpus proximal width * Femur length *
 Femur midshaft diameter * Tibiotarsus length * Tarsometatarsus length *
 Synsacrum Length fused vertebrae * Pubis length * Ilium length * Ischium
 length #### File: ExtantSternum_metadata.csv **Description:** Metadata
 sheet containing body mass and flight data used in sternum analysis from
 Lowi-Merri et al. (2023), as well as links to all Morphosource pages and
 objects. ##### Variables * Tree_name: name of taxon in phylogenetic tree *
 Genus * Species * Specimen * Specimen page URL * Sternum model URL * CT
 data URL * Filename * Body mass (g) * Keel area: area of keel in mm *
 Total FP (mm): footprint area (sternum basal plate area)  * Flight_style:
 primary locomotory style. CF = continuous flapping; BF = burst flight; FSD
 = foot-propelled swimming or diving; FL = flightless; SO = soaring; TER =
 terrestrial * Forelimb-driven lift-based propulsion: This and the
 remaining variables are coded as present/absent in each taxon. * Aerial
 flight  * Foot-propelled swimming/diving * Wing-propelled diving * Burst
 flight * Sustained flight * Soaring * Flap-gliding * Continuous flapping *
 Scansorial climbing * Cursorial * Terrestrial locomotion * Intermittent
 bounding * Maneuverability #### File: FosSternCode2025.R **Description:** 
 R script for running sternum analysis #### File: SkelPropData.csv
 **Description:** Data on skeletal proportions of extant taxa, in mm #####
 Variables * Name * BODY MASS (g) * Simple.groups: primary locomotory
 style; same as 'Flight_style' above in
 'ExtantSternum_metadata.csv' * Aerial flight * Forelimb-driven
 lift-based propulsion * Underwater locomotion * Foot-propelled
 swimming/diving * Wing-propelled diving * Burst flight * Sustained flight
 * Soaring * Flap-gliding * Continuous flapping * Scansorial climbing *
 Cursorial * Terrestrial locomotion * Intermittent bounding *
 Maneuverability * Humerus.Length * Humerus.proximal.width *
 Humerus.distal.width * Ulna.length * Ulna.proximal.width * Radius.length *
 Carpometacarpus.length * Carpometacarpus.proximal.width * Coracoid.length
 * Coracoid.mediolateral.sternal.facet.length *
 Coracoid.humeral.articulation.facet * Scapula.length * Sternum.length *
 Keel.height * Femur.length * Tibiotarsus.length * Tarsometatarsus.length *
 Synsacrum.Length.fused.vertebrae * Pubis.length * Ilium.length *
 Ischium.length #### File: Cooney_extensive_sampling.tree
 **Description:** phylogenetic tree used to control for evolutionary
 history in comparative analyses. Sourced from Cooney et al. (2017)
 Mega-evolutionary dynamics of the adaptive radiation of birds
 (doi:10.1038/nature21074). #### File: GPAFitextant.RData
 **Description:** R workspace & aligned Procrustes coordinates (shape
 data) for running sternum morphometric analysis ## Code/software Data is
 viewed in Excel and a text editor. Code is run in R and/or RStudio. ##
 Access information Data was derived from the following sources: *Prior
 publication by Lowi-Merri et al (2023):
 [https://doi.org/10.5061/dryad.zw3r228c5](https://doi.org/10.5061/dryad.zw3r228c5).