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‘This is the peer reviewed version of the following article: Serrano, F. J., Chiappe, L. M., Palmqvist, P., Figueirido, B., Long, J., & Sanz, J. L. (2019). The effect of long-term atmospheric changes on the macroevolution of birds. Gondwana Research, 65, 86–96. https://doi.org/10.1016/ j.gr.2018.09.002 which has been published in final form at https://doi.org/10.1016/j.gr.2018.09.002

© 2018 Elsevier Ltd. This manuscript version is made available under the CC-BY-NC-ND 4.0 license: http://creativecommons.org/licenses/by-nc-nd/4.0/ Accepted Manuscript

The effect of long-term atmospheric changes on the macroevolution of birds

Francisco José Serrano, Luis María Chiappe, Paul Palmqvist, Borja Figueirido, John Long, José Luis Sanz

PII: S1342-937X(18)30229-6 DOI: doi:10.1016/j.gr.2018.09.002 Reference: GR 2018 To appear in: Gondwana Research Received date: 18 March 2018 Revised date: 12 September 2018 Accepted date: 12 September 2018

Please cite this article as: Francisco José Serrano, Luis María Chiappe, Paul Palmqvist, Borja Figueirido, John Long, José Luis Sanz , The effect of long-term atmospheric changes on the macroevolution of birds. Gr (2018), doi:10.1016/j.gr.2018.09.002

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Journal: GONDWANA RESEARCH

Title: The effect of long-term atmospheric changes on the macroevolution of birds

Short title: Atmospheric effect on avian evolution

Author names and affiliations:

Francisco José Serranoa,b,c*, Luis María Chiappea*, Paul Palmqvistc, Borja Figueiridoc, John

Longd, and José Luis Sanze a The Dinosaur Institute, Natural History Museum of Los Angeles County. 900 Exposition

Boulevard, Los Angeles, CA 90007 (USA). b Fundación Sierra Elvira. Avenida de Andalucía nº 139, Atarfe, 18230 - Granada (Spain). c Departamento de Ecología y Geología, Facultad de Ciencias, Universidad de Málaga.

Campus Universitario de Teatinos s/n, 29071 - Málaga (Spain). d School of Biological Sciences, Flinders University, POB 2100, Adelaide, South Australia,

Australia 5001. e Unidad de Paleontología, Departamento de Biología, Facultad de Ciencias, Universidad

Autónoma de Madrid. C/Darwin 2, Cantoblanco, 28049 - Madrid (Spain)

*Corresponding authors

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Abstract

Atmospheric conditions are critical for a range of biological functions—locomotion among others—and long-term changes in these conditions have been identified as causal for different macroevolutionary patterns. Here we examine the influence of variations in atmospheric O2 concentration (AOC), temperature (Tair), and air density (ρair) on the power efficiency, as it relates to locomotion, during the evolutionary history of birds. Specifically, our study centers on four key evolutionary events: (1) the body mass reduction of non-avian theropods prior to the rise of birds; (2) the emergence of flapping flight in the earliest birds; (3) the divergence of basal pygostylians; and (4) the diversification of modern birds. Our results suggest that a marked increase in AOC and ρair during the Middle Jurassic—coeval with a trend in miniaturization—improved the power efficiency of the dinosaurian predecessors of birds.

Likewise, an increase in these conditions is hypothesized as having played a major role in the diversification of early pygostylians during the Early Cretaceous. However, our analyses do not identify any significant paleoatmospheric effects on either the emergence of flapping flight or the early cladogenesis of modern birds. Extinct birds flew within the range of atmospheric conditions in which modern birds fly but varying past conditions influenced their flight performance. Our study thus highlights the importance of considering paleoatmospheric conditions when reconstructing the flight efficiency of the forerunners of modern birds.

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Keywords: Macroevolution; birds; theropods; climate change; locomotion efficiency ACCEPTED MANUSCRIPT

1. INTRODUCTION

Evolutionary transformations of ecologically-related features in animals, including body size, morphology, physiology, and locomotor modes, are closely tuned to the physical factors of the environments they inhabit (Rayner, 2003; Shubin, 2008; Clack, 2012). In consequence, past changes in environmental conditions—atmospheric concentrations of O2 and CO2, temperature, and others—can explain (and potentially drive) a number of macroevolutionary patterns, including adaptive radiations (Graham et al. 1995; Thomas, 1997; Cornette et al.,

2002, Falkowski et al., 2005, Saarinen et al. 2014), mass extinctions (Huey and Ward, 2005;

Long et al., 2015), trends in body size (Braddy et al., 2008; Choo et al., 2014), and major habitat transitions (e.g., water-to-land, land-to-air) that are intimately connected to key genetic, morphological, biomechanical, and physiological transformations (Dudley 2000; Chiappe,

2007; Shubin, 2008; Clack, 2012; Bottjer, 2017).

The evolution of animals with energetically costly modes of locomotion, such as powered flight (insects, pterosaurs, birds and bats), is especially constrained by atmospheric factors (Dudley, 2000; Rayner, 2001; Pennycuick, 2008). Physiologically, air O2 concentration

(AOC) and temperature (Tair) impact energy availability for oxidative metabolism and water loss, respectively, during respiration (Bishop and Butler 2015). AOC and Tair also influence aerodynamics because these factors determine air density (ρair), which in turn correlates with the power required to fly and other kinetic variables (Videler, 2005; Tobalske, 2007;

Pennycuick, 2008). Several studies have identified the potential role played by varying ACCEPTED MANUSCRIPT atmospheric factors in the evolution of animal flight. Significant increments in AOC might have driven the diversification of late Paleozoic aerial insects (and in some cases, gigantism) and the origin of active flight in vertebrates during both Mesozoic (pterosaurs and aves) and

Cenozoic (bats) (Harle and Harle, 1911; Graham et al., 1995; Dudley, 1998, 2000; Rayner,

2003; Ward 2006). ACCEPTED MANUSCRIPT

Regarding birds (=Aves), there is an overwhelming consensus that avian evolutionary history was profoundly influenced by a series of key events related to the attainment of powered flight (Sereno and Rao, 1992; Gatesy and Dial, 1996; Padian and

Chiappe, 1998; Dial et al. 2003; Chiappe, 2007; Heers and Dial 2012; Zhang et al., 2014; Feo et al., 2015; Brusatte, 2015). However, in spite of the potential impact of past atmospheric conditions on flight performance, inferences about the aerial locomotion of extinct birds have largely ignored these aspects, with only a few recent exceptions (e.g., Serrano and Chiappe

2017, Serrano et al. 2018).

The present study examines the role of long-term atmospheric changes on two variables related to the attainment and fine-tuning of powered flight in birds: the available power output from aerobic flight muscles (Pav) and the mechanical power required for flapping flight (Pmec), which ratio (Pav/ Pmec) is a proxy for power efficiency (E) (Pennycuick, 1968;

Tucker, 1973). Quantifying Pav and/or Pmec for a broad sample of non-avian theropods, stem, and modern birds, we assess the influence of these variables on the evolution of these organisms

(Fig. 1). Particularly, we focus on four evolutionary milestones: (1) the prolonged trend of miniaturization and forelimb elongation (i.e., decreasing wing loading) of non-avian theropods prior to the origin of birds (Chiappe 2007, Turner et al., 2007; Dececchi and Larsson, 2013;

Lee et al., 2014; Puttick et al., 2014; Dececchi et al. 2016; see Benson et al. 2017 for a different opinion); (2) the emergence of flapping flight, a morphofunctional innovation straddling the transition from non-avian theropods to birds (Ostrom, 1976; Padian and Chiappe, 1998; ACCEPTED MANUSCRIPT Brusatte, 2017); (3) the evolutionary radiation of basal pygostylians, the earliest diversification of birds (Chiappe, 2007; O´Connor et al., 2011; Benson and Choniere, 2013; Wang and Lloyd,

2016); and (4) the massive cladogenesis of neoavian neornithines (i.e., the main clade of modern birds) after the K/P mass extinction (Pacheco et al., 2011; Ericson et al., 2014; Jarvis et al., 2014; Mayr, 2014; Claramunt and Cracraft, 2015; Prum et al., 2015). ACCEPTED MANUSCRIPT

2. MATERIAL AND METHODS

All abbreviations identifying the variables used in the study are shown in Table 1.

TABLE 1. Summary of variables and corresponding abbreviations used in the present study.

AOC Atmospheric O2 concentration

Tair Air temperature

ρair Air density BM Body mass B Wing span

SL Lift surface or wing area

VO2 Oxygen consumption rate

Pav Maximum available power for flapping flight from aerobic metabolism

Pmec Mechanical power output required for flapping flight

Pind Induced power

Ppar Parasite power

Ppro Profile power k Induced power factor

Sb Frontal area of the body

CDb Body drag coefficient

Cpro Profile power constant

Vt Forward speed

Vmp Minimum power speed

Vmr Maximum range speed or cruising speed

Pmin Mechanical power output at Vmp

Popt MechanicalACCEPTED power output at Vmr MANUSCRIPT E Efficiency or power margin

Emin Efficiency flying at Vmp

Eopt Efficiency flying at Vmr

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2.1. Reconstruction of atmospheric O2 concentration (AOC), mean global temperature (Tair), and air density (ρair) values

Past AOC values were taken from three different models. The models of Falkowski et al. (2005)

(hereafter Fk), and Ward and Berner (2011) (hereafter WB) calculated AOC values from different sources of carbon and sulfur isotopes, while the model of Glasspool and Scott (2010)

(hereafter GS) used data from charcoal sediments. Values for Tair over the last 180 Myr were taken from Retallack (2009), based on chemical salinization of paleosols.

We used the equation of ideal gases (ρair = Patm / R Tair g, where Patm is the atmospheric pressure, g is gravity´s acceleration, and R is the universal gas constant) to calculate ρair values at 10-Myr-intervals, which provided an air density curve spanning the last

180 Myr (Table 2). Values of past Patm were calculated using molecular masses and concentration of O2 and N2 relative to modern values (the contribution of other gases to present atmospheric pressure is negligible; Supplementary Table 1). While we obtained the concentration of O2 from the three mentioned models (i.e., Fk, WB, and GS), we assumed that

N2 concentration remained constant throughout the Phanerozoic (Berner, 2006). Gravity´s acceleration was also assumed to remain unchanged (g = 9.81 m s-2), as documented by direct measures of paleogravity (Stewart, 1978) and Earth’s thermal evolution (Tsuchiya et al., 2013).

TABLE 2. Values of air density (ρair), temperature (Tair), and O2 concentration (AOC) during the last 180 Myr. Temperature values from Retallack (2009); AOC percent from Falkowski ACCEPTED MANUSCRIPT et al. 2005 (Fk), Ward and Berner 2011 (WB), and Glasspool and Scott 2010 (GS); ρair was derived from these two previous variables.

Mesozoic-Cenozoic O2 models Age Fk WB GS (oC) (Myr) Tair AOC (%) ρair AOC (%) ρair AOC (%) ρair (Kg/m3) (Kg/m3) (Kg/m3) 0 15 20.9 1.225 20.9 1.225 20.9 1.225 10 11.6 21 1.241 21.3 1.245 21.6 1.249 ACCEPTED MANUSCRIPT

20 11.2 22.6 1.265 22.3 1.260 21.1 1.244 30 14.1 23 1.257 22.3 1.247 22.2 1.247 40 11.6 22 1.255 21.8 1.253 21.2 1.244 50 12.5 19 1.211 21.1 1.240 23.7 1.275 60 12.7 17.7 1.192 21.1 1.239 26 1.305 70 11.5 18 1.201 21.3 1.246 25.5 1.304 80 8.6 18 1.214 21.7 1.265 25.6 1.319 90 14.5 16.4 1.168 21.3 1.234 25.5 1.291 100 10.8 16.5 1.185 21.2 1.250 29.3 1.360 110 10.8 17.3 1.196 21.0 1.247 28.1 1.344 120 11.4 16.4 1.181 18.5 1.209 28.7 1.349 130 10.9 16.4 1.183 15.9 1.175 - - 140 8.6 15.8 1.185 14.5 1.167 22.6 1.280 150 10 16.4 1.188 15.7 1.178 27.4 1.339 160 10.9 18.4 1.212 15.4 1.171 25.7 1.312 170 7.7 15.5 1.185 14.5 1.172 28.2 1.362 180 8.2 12.3 1.138 13.9 1.160 22.7 1.282

2.2. Body mass, wingspan, and lift surface in extinct and extant birds

We estimated the body mass (BM), wingspan (B), and lift (wing) surface (SL) for five specimens of Archaeopteryx (Late Jurassic) and 33 specimens of 25 stem pygostylian taxa from the Jehol biota (Lower Cretaceous) (Table 3) using the multivariate approach of Serrano et al. (2015;

2017). BM, B and SL values from 58 extant species of flying Neognathae were measured directly

(Table 3); this sample of modern birds covers a broad range of phylogenetic representation (45 families within 20 orders), size (from 5 to 11900 g), aerial locomotion (thermal and dynamic soaring, continuous flapping, bounding, flap-gliding, short burst), and ecological aspects (diet, migration, non-aerACCEPTEDial locomotion). MANUSCRIPT

TABLE 3. Dataset of extinct and extant birds used in this study. Values of body mass (BM), wingspan (B), and wing area (SL) for extinct birds were obtained from multiple regressions using anatomical measures from modern birds (see Serrano et al. 2015, 2017). Superscript numbers indicate the references corresponding to the age of the fossils: 1Wellnhofer (2008), ACCEPTED MANUSCRIPT

2Pan et al. (2013), 3Chang et al. (2009). Abbreviations: BMMS, Bürgermeister Müller Museum

(Solnhofen); BMNH, British Museum of Natural History (London); BMNHC, Beijing

Museum of Natural History (Beijing); BR, Bruderer et al. (2010); DNHM, Dalian Natural

History Museum (Dalian); FS, Flight software’s dataset; HGM, Henan Geological Museum

(Zhengzhou); IVPP, Institute of Vertebrate Paleontology and Paleoanthropology (Beijing); JM,

Jura Museum (Eichstätt); MCFO, Cosmocaixa (Madrid); NIGPAS, Nanjing Institute of

Geology and Paleontology (Nanjing); PKUP, Peking University, Paleontological Collection

(Beijing); UWBM, Burke Museum, University of Washington (Seattle); VF, Viscor and Fuster

(1987); WDC, Wyoming Dinosaur Center (Thermopolis).

2 Species Specimen code BM (kg) B (m) SL (m ) Age (Myr)

Archaeopteryx lithographica BMNH 37001 0.450 0.766 0.0740 150.8 1

A. lithographica HMN 1880/1881 0.343 0.598 0.0601

A. lithographica JM 2257 0.176 0.400 0.0232

A. lithographica BMMS 500 1.088 0.784 0.1238

A. lithographica WDC-CSG-100 0.264 0.549 0.0401

Eoconfuciusornis zhenghi IVPP V 11977 0.217 0.450 0.0244 130.7 2

Protopteryx fengningensis IVPP V 11665 0.033 0.317 0.0189

P. fengningensis BMNHC Ph1060 0.049 0.352 0.0237

P. fengningensis BMNHC Ph1158 0.046 0.351 0.0230

Confuciusornis sanctus IVPP V 11374 0.520 0.668 0.0818 124.1-122.9 3

C. sanctus ACCEPTEDIVPP V 11640 MANUSCRIPT0.762 0.786 0.0938

C. sanctus MCFO-0374 0.550 0.674 0.0669

Sapeornis chaoyangensis DNHM 3078 0.491 0.912 0.1305

Vescornis hebeiensis NIGPAS 130722 0.050 0.272 0.0126

Eoenantiornis buhleri IVPP V 11537 0.080 0.317 0.0148

Longirostravis hani IVPP V 11309 0.039 0.318 0.0164

Archaeorhynchus spathula IVPP V 14287 0.272 0.550 0.0479 ACCEPTED MANUSCRIPT

Hongshanornis longicresta IVPP V 14533 0.088 0.350 0.0163

Longicrusavis houi PKUP V1069 0.089 0.299 0.0176

Sapeornis chaoyangensis IVPP V 12698 0.900 1.248 0.2611 120.0 2

S. chaoyangensis IVPP V 13275 1.041 1.147 0.2158

S. chaoyangensis IVPP V 13276 1.038 1.141 0.2237

S. chaoyangensis HGM 41HIII0405 0.663 0.943 0.1335

S. chaoyangensis BMNHC Ph1067 1.168 1.287 0.2573

Pengornis houi IVPP V 15336 0.437 0.695 0.1008

Cathayornis yandica IVPP V 9169 0.063 0.309 0.0199

Cuspirostrisornis houi IVPP V 10897 0.070 0.330 0.0169

Eocathayornis walkeri IVPP V 10916 0.056 0.286 0.0193

Longchengornis sanyanensis IVPP V 10530 0.086 0.335 0.0167

Otogornis genghisi IVPP V 9607 0.171 0.432 0.0291

Longipteryx chaoyangensis IVPP V 12325 0.193 0.463 0.0373

Rapaxavis pani DNHM D2522 0.047 0.238 0.0141

Yixianornis grabaui IVPP V 12631 0.321 0.522 0.0440

Zhongjianornis yangi IVPP V 15900 0.570 0.724 0.0825

Yanornis martini IVPP V 12558 0.772 0.773 0.1079

Y. martini IVPP V 10996 0.618 0.820 0.1154

Jianchangornis microdonta IVPP V 16708 0.820 0.822 0.0980

Junornis houi BMNHC-PH 919 0.031 0.303 0.0168

Aegypius monachus FS 9.9 3.04 1.4 Present

Alauda arvensis BR 0.031 0.36 0.0233

Alcedo atthis UWBM 66027 0.041 0.278 0.0128 Anas crecca ACCEPTEDFS MANUSCRIPT0.235 0.582 0.0458 Anas platyrhynchos BR 1.094 0.89 0.1054

Anser anser FS 3.77 1.6 0.331

Apus apus BR 0.04 0.4 0.015

Ardea cinerea BR 1.21 1.6 0.358

Ardeotis kori FS 11.9 2.47 1.06

Asio otus UWBM 59694 0.34 0.975 0.1458 ACCEPTED MANUSCRIPT

Buteo jamaicensis UWBM 69545 1.14 1.026 0.2064

Calidris canutus FS 0.127 0.538 0.0332

Caprimulgus carolinensis UWBM 87389 0.099 0.61 0.0533

Carduelis spinus FS 0.011 0.212 0.0079

Chalcopsitta cardinalis UWBM 58701 0.235 0.555 0.051

Charadrius dubius UWBM 56904 0.036 0.36 0.0149

Columba livia UWBM 66264 0.245 0.638 0.0718

Coracias garrulus UWBM 56886 0.146 0.67 0.0711

Corvus corone UWBM 56471 0.575 0.966 0.1603

Corvus monedula FS 0.181 0.6 0.0618

Cotournix cotournix UWBM 64806 0.096 0.363 0.019

Cuculus canorus UWBM 56781 0.118 0.609 0.0563

Diomedea exulans FS 9.57 3.06 0.644

Diomedea exulans VF 8.502 3.408 0.6211

Emberiza hortulana BR 0.022 0.26 0.0138

Falco peregrinus UWBM 78842 0.699 1 0.1248

Fregata magnificens FS 1.67 2.14 0.372

Fringilla coelebs FS 0.023 0.264 0.0131

Gavia arctica VF 1.495 1.2 0.1196

Hirundo rustica FS 0.019 0.318 0.0132

Hydrobates pelagicus FS 0.026 0.355 0.0161

Jynx torquilla BR 0.03 0.29 0.015

Lanius collurio BR 0.03 0.3 0.0145

Larus argentatus UWBM 56577 1.175 1.585 0.2794 Limosa lapponica ACCEPTEDFS MANUSCRIPT0.367 0.748 0.0568 Luscinia luscinia FS 0.027 0.263 0.013

Merops apiaster UWBM 56659 0.057 0.445 0.027

Morus serrator UWBM 63012 1.33 1.705 0.2064

Motacilla flava BR 0.017 0.26 0.0103

Muscicapa striata BR 0.015 0.24 0.011

Oenanthe oenanthe BR 0.025 0.31 0.0157 ACCEPTED MANUSCRIPT

Pandion haliaetus UWBM 79102 1.05 1.56 0.282

Parus ater BR 0.009 0.19 0.0061

Perdix perdix UWBM 58115 0.372 0.53 0.0527

Phalacrocorax carbo FS 2.56 1.35 0.224

Podargus strigoides UWBM 62802 0.298 0.935 0.1798

Podiceps cristatus UWBM 56440 0.985 0.75 0.0637

Pterocles coronatus BR 0.3 0.57 0.0432

Puffinus pacificus UWBM 68952 0.389 1 0.0912

Regulus regulus FS 0.005 0.156 0.0053

Scolopax minor UWBM 57803 0.162 0.501 0.0451

Sturnus vulgaris FS 0.082 0.384 0.0253

Sylvia borin FS 0.022 0.24 0.011

Tringa glareola UWBM 56947 0.059 0.41 0.022

Turdus philomelos FS 0.072 0.361 0.0225

Tyto alba UWBM 66223 0.324 1.046 0.1433

Uria aalge UWBM 79468 0.925 0.722 0.0613

Vanellus vanellus UWBM 58086 0.21 0.742 0.092

2.3. Available power output, required mechanical power and efficiency

In modern birds, the maximum available power output (aerobic metabolism) for attaining active flight (Pav) scales negatively with body size (Butler 1991; Bishop and Butler 1995;

Templin 2000; Pennycuick 2008). Pav was calculated using an oxygen consumption rate (VO2, mL/sec) in whichACCEPTED 1 mL O2/sec generates 20.1 MANUSCRIPT W during aerobic activity (Schmidt-Nielsen

1997). Values of VO2 were estimated for the dataset of Aves (both extinct and extant) using the equation [1], which was derived from birds flying at modern O2 concentration (i.e., 20.9 mL

O2/100 mL air) (Bishop and Butler 2015).

0.74 [1] VO2 = 160 BM ACCEPTED MANUSCRIPT

In the case of non-avian theropods, we estimated their VO2 values from the eq. [2], which was derived from running cursorial birds (Butler 1991), using the BM of the nodes recovered by

Lee et al. (2014) for the 180-160 Myr interval (Supplementary Table 2).

0.74 [2] VO2 = 145 BM

By assuming a conversion efficiency of 0.2 from the total metabolic power input to Pav (Tucker

1973, 1975, Pennycuick 2008), and translating minutes into seconds—as eq. [1] and [2] were

derived from VO2 measured in mL/min—the Pav at modern conditions was calculated using the equation [3].

[3] Pav = 20.1 · 0.2 / 60 VO2

Pav = 0.067 VO2

Past values of Pav for birds (or non-avian theropods) were adjusted to the percentage of variation between today’s concentration and past AOC values (%Δ AOC; eq. [4]). Thus Pav values obtained in eq. [3] were recalculated as in eq. [5].

[4] %Δ AOC = [AOC (past) - AOC (today)] · 100 / AOC (today)

[5] Pav = (0.067 VO2 · %Δ AOC) + 0.067 VO2

We use the software Flight v.1.24 (http://www.bio.bristol.ac.uk/people/pennycuick.htm)

(Pennycuick, 2008) to calculate the power curve of birds, which relates the mechanical power output required to fly (Pmec) and the forward speed (Vt). Following Pennycuick (2008), the values of Pmec for a range of velocities (Vt) were calculated, according eq. [6], as the summation of the induced powerACCEPTED (Pind; eq. [7], where k MANUSCRIPTis the induced power factor and g is the gravity acceleration), the parasite power (Ppar; eq. [8], where Sb is the frontal area of the body and CDb is the body drag coefficient), and the profile power (Ppro; eq. [9], where Vmp is the minimum power speed and Cpro is the profile power constant).

[6] Pmec = Pind + Ppar + Ppro

2 2 [7] Pind = 2k [BM g] / Vt π B ρair, ACCEPTED MANUSCRIPT

3 [8] Ppar = ρair Vt Sb CDb / 2

2 2 3 2 [9] Ppro = [2k (BM g) / Vmp π B ρair + ρair Vmp Sb CDb /2] Cpro /[B /SL]

Some components of these calculations are difficult to measure experimentally in flying birds while flapping (i.e., CDb, k, Cpro, Sb). In fossils, calculation of these components requires a number of assumptions (see Serrano et al., 2018). Body drag coefficient (CDb), the most influential of these components, was calculated from the equation Sb CDb = 0.01 SL (Taylor et al., 2016) instead of using the constant value (0.1) given by Pennycuick (2008); Taylor et al.

(2016) equation results in higher Pmec (i.e., more costly).

Following Pennycuick (1968) and Tucker (1973) we used the ratio between Pav (from eq. [5]) and Pmec (from eq. [9]) as the power margin or efficiency (E). Because this ratio constrains the range of flight performance (Rayner 1988; Norberg 2002; Pennycuick, 2008), it is often used for inferring the flight capabilities of extinct taxa (see Templin 2000; Chatterjee et al. 2007; Ksepka 2014). Pmec values were taken at the minimum power speed (Vmp; Vt at the lowest point of the power curve) and the maximum range speed (Vmr; Vt where the tangent to the power curve intersects with the origin of the speed–power plot; see Rayner 1999) (Fig. 2).

Values of Pmec at Vmp (i.e., Pmin) and at Vmr (i.e., Popt) were used to obtain Emin and Eopt values for birds flying at these speeds, as shown in eq. [10] and [11].

[10] Emin = Pav / Pmin

[11] Eopt = Pav / Popt

ACCEPTED MANUSCRIPT 2.4. Statistical analyses

For cursorial theropods, we tested the differences concerning mass-specific available power

(i.e., Pav/BM) between the nodes included in the range 180-170 Myr (N=10) and those included in 170-160 Myr interval (N=24) (see Supplementary Table 2). This was performed for the three

AOC models included in this study (i.e., Fk, WB and GS). In the case of Aves, we calculated ACCEPTED MANUSCRIPT

values of Emin and Eopt for the same individual at different times. We evaluated the variation in

E between different times by comparing values of mean or median subsample corresponding to each particular time. At first, we tested the parametric assumptions of normality and homogeneity of variances through the Shapiro-Wilk test and Levene’s statistic, respectively. If accomplished, signification of differences between sample means was tested through one-way

ANOVA using the F statistic to approach p-values. In the case in which these assumptions were unaccomplished, we performed the non-parametric Kruskal-Wallis test to compare sample medians, using the H statistic to approximate p-values (according to a chi-squared distribution).

3. RESULTS AND DISCUSSION

3.1. Paleoatmospheric conditions over the last 180 Myr

While the absolute values of the three AOC models show significant discrepancies that are most likely the result of methodological differences (see Methods), several long-term trends

(increment or decrement) are congruent among these models (Fig. 1; Table 2). All models document an increase in AOC from 180 to 160 Myr, although the net values were more pronounced in the Fk model (+49.6%) than in either the GS (+13.2%) or WB (+10.8%) ones.

Such an increase is consistent with newer analyses that also indicate a rise in AOC between

180 and 160 Myr ago (Mills et al., 2016; Baker et al., 2017). Based on these values and on Tair estimates provided by Retallack (2009), our calculations show that ρ increased between 1 ACCEPTED MANUSCRIPTair and 6 % (depending on the model) during the Early-Middle Jurassic (Table 2). During the Late

Jurassic and at the beginning of the Cretaceous (i.e., 160 to 140 Myr), the three models show a decrease in AOC (net values: Fk, -14.1%; GS, -12.1%; WB, -5.8%), which implies a reduction of ρair ranging from 0.3% to 2.5% depending on the model (Table 2). After 140 Myr, the Fk model predicts a subtle increment in AOC until about 45-50 Myr ago. However, the other two ACCEPTED MANUSCRIPT

models record a sharp increase during the Early Cretaceous: GS predicts a 27.0% increase of

AOC from 140 to 120 Myr ago, while WB documents a 44.2% increase by 110 Myr ago (Fig.

1). Consequently, our calculations of ρair for the Early Cretaceous indicate an increase of 5.5%

(GS), 6.9% (WB), or negligible (Fk) depending on the model (Table 2). During the Late

Cretaceous, the three models show relatively stable AOC, despite that the actual concentrations differ from one another (Fig. 1); due to significant swings in Tair, values of ρair during this period vary significantly, with a marked peak at around 80 Myr ago (Fig. 1; Table 2). Following the K-P mass extinction (~65 Myr), the WB model predicts AOC values that remain close to present atmospheric levels throughout the Cenozoic; however, there is a noticeable increase in values between 50 and 30 Myr ago (5.7% net increase). Due to the combined effect of AOC and Tair, estimates of ρair based on this model remain higher than today, starting a gradual descend at around 20 Myr. The Fk model also predicts a steep rise in AOC during the early

Cenozoic (21% increase between 50 and 30 Myr); consequently, density estimates under this model show a major increase, moving from below today’s levels to above these levels. In contrast to these two models, the GS model shows a significant drop in AOC (i.e., -18.5%) during the early-mid Paleogene (i.e., 60 to 40 Myr), oscillating slightly below modern values until present time (Fig. 1; Table 2).

3.2. Role of paleoatmospheric conditions in the evolution of birds

While the three AOC models (the most influential factor) disagree between them (mostly in ACCEPTED MANUSCRIPT absolute values), they all show important differences when compared to modern values (Fig.

1). Such differences highlight the importance of considering past AOC conditions (as well as

Tair and ρair) when reconstructing the efficiency of non-avian theropods and extinct birds, because it is clear that deviations from the present standard atmosphere have significant effects on the calculations of this property in both flightless and flighted species. This aside, the ACCEPTED MANUSCRIPT

prevailing paleoatmospheric conditions (i.e., AOC, Tair and ρair) of the late Jurassic and onwards indicate that all extinct flying birds flew within a range of conditions similar to those that characterize the ‘comfort zone’ of modern birds (i.e., the range of atmospheric conditions within which modern birds fly; Liechti 2006, Altshuler and Dudley 2006) (Supplementary

Table 3). We note that adaptations related to enhance oxygenation, which our methods are unable to detect, could have evolved in some of these extinct lineages (e.g., adaptations similar to those known for bar-headed goose; Butler 2016), thus allowing them to fly in less favorable atmospheric conditions (e.g., high altitude). Below, we discuss how efficiency was influenced by past atmospheric conditions during time periods encompassing four key evolutionary events in the history of birds.

3.2.1. Miniaturization of non-avian theropods

Evolotuionary trends are long-term directional changes that affect to the mean value of a whole clade (see McShea, 1994). Most researchers agree that the evolution of flying birds from cursorial non-avian theropods entailed a prolonged trend of miniaturization and forelimb elongation [Sanz et al., 2002; Chiappe, 2007; Turner et al., 2007; Dececchi and Larsson, 2013;

Lee et al., 2014; Puttick et al., 2014; see Benson et al. (2017) for a different opinion], which led to important adaptive changes related to wing loading and the origin of flight. A considerable portion of this trend towards body size reduction took place between 180 and 160

Myr ago (Lee et al., 2014; Fig. 3A), when AOC increased by at least 10.8% (under WB´s model) ACCEPTED MANUSCRIPT and perhaps by as much as 49.6% (under Fk’s model) (Fig. 1). During this period, the reduction in body mass would have allowed a large number of non-avian theropod lineages to increase their mass-specific Pav by 3.6 times [using Lee et al.’s (2014) data, Pav mean value for clade nodes at 160 Myr is 3.6 times greater than the value at 179-180 Myr; Supplementary Table 2].

If the effects of the increase in AOC values are also accounted, the mass-specific Pav of these ACCEPTED MANUSCRIPT

animals would have risen even further, between 4.7 times (according to the WB model) and

6.4 times (according to the Fk model) (Fig. 3B). Such increase in the mass-specific Pav of middle Jurassic non-avian theropods is statistically significant for the values obtained from any of the three AOC models analyzed (Table 4). Bishop and Butler (2015) reported that on average, the mass-specific Pav of flying non-passerine birds is 2.2 times the one for similarly- sized mammals running at maximum sustainable speed. This observation provides a quantitative context to the increment of Pav in the predecessors of birds as O2 levels increased and body masses decreased. Recent discoveries have highlighted a significant degree of aerial experimentation among the closest relatives of birds (Xu et al., 2003; Chiappe, 2007; Han et al., 2014; Xu et al., 2015; Dececchi et al., 2016; Brusatte, 2017); the escalation in Pav documented by our results most likely played an important role in the transition from land to air and the attainment of aerial locomotion in dinosaurs.

TABLE 4. Tests comparing differences in power benefit (i.e. Pav/BM, Pav/Pmin and Pav/Popt) due to the effect of atmospheric change in particular time intervals. When variable increment, the median at the start of time interval is compared with the median at interval end. ANOVA test (F-statistic) or Kruskal-Wallis non-parametric test (H-statistic) were performed according to compliance of parametric assumptions (i.e., Shapiro-Wilk: p > 0.05; and Levene: p > 0.05).

ACCEPTED MANUSCRIPTAOC model Fk WB GS Interval Variable N (Myr) Median Median Median Median Median Median at at at at at at sig. sig. sig. interv. interv. interv. interv. interv. interv. start end start end start end

180-160 Pav/BM 34 2.72 5.02 ** 2.72 4.48 ** 4.98 7.93 **

Emin 1.01 0.89 - 0.83 0.86 ns 1.47 1.48 ns 160-130 18 Eopt 0.80 0.71 - 0.66 0.68 ns 1.17 1.17 ns ACCEPTED MANUSCRIPT

Emin 0.80 0.83 ns 0.73 0.95 * 1.19 1.56 * 140-120 66 Eopt 0.62 0.64 ns 0.56 0.73 * 0.91 1.19 *

Emin 1.49 1.84 ns 1.67 1.75 ns 1.82 1.68 - 50-30 116 Eopt 1.15 1.41 ns 1.29 1.37 ns 1.41 1.34 -

** significant at p < 0.01; * significant at p < 0.05; ns – non significant; - no increment

3.2.2. Emergence of flapping flight

Notwithstanding the aerial innovations documented by putative non-avian dinosaur fliers as early as the middle-late Jurassic (Hu et al., 2009; Dyke et al., 2013; Xu et al., 2015; Dececchi et al., 2016), the 150-million-year-old Archaeopteryx is broadly regarded as the earliest example of flapping flight capacity (Burgers and Chiappe, 1999; Wellnhofer, 2008), albeit limited to short bursts (Serrano 2015; Voeten et al., 2018). While such an aerodynamic breakthrough might have evolved earlier, compelling evidence for the attainment of sustained flapping flight first appears at about 131 Myr, as it is documented by several avian lineages

(e.g., confuciusornithids,ACCEPTED enantiornithines, andMANUSCRIPT early ornithuromorphs) from the Huayijing

Formation of northeastern China (Zhang and Zhou, 2000; Zhang et al., 2008; Wang et al.,

2015).

During a time when flapping flight is expected to have evolved, our results show that the atmospheric conditions would have had no significant effect on the aerial performance of the earliest birds. Between 160 and 130 Myr ago, the values of AOC and ρair either decreased ACCEPTED MANUSCRIPT

slightly (Fk model) or remained largely invariant (GS and WB model); our analysis does not indicate any improvement in the values of Emin and Eopt (calculated for five specimens of

Archaeopteryx, one of Eoconfuciusornis and three of Protopteryx) during this time interval

(Table 4). Therefore, regardless of the possible drivers behind the evolution of flapping flight, it appears clear that the aerodynamic efficiency of the earliest birds was not favored by atmospheric conditions. These conditions could have delayed the appearance of more sophisticated flight performances, which are first evidenced in the enantiornithines and basal ornithuromorphs of the 131-million-year-old Huajiying Formation.

3.2.3. Earliest diversification of birds and the attainment of sustained flapping flight

Adaptive radiation is a rapid speciation that results in a large number of descendant species that occupy a broader range of ecological niches (see Ridley, 2004). In the Early Cretaceous, between 131 and 120 Myr ago, the Jehol Biota documents the highest diversity of avian stem taxa for the entire Mesozoic (O’Connor et al., 2011; Pan et al., 2013; Zhou, 2014; Chiappe and

Meng, 2016). Cretaceous avian fossils prior to 131 Myr ago are lacking but the diversity recorded in the Jehol Biota underscores an earliest Cretaceous (or latest Jurassic) adaptive radiation of pygostylians (short-tailed birds), when birds expanded their ecological range and developed traits that improved their flight performance (e.g. triosseal foramen, bony tail reduction, alula, patagium, and others) (Sanz et al., 1996; Chiappe, 2007; O´Connor et al. 2011;

Benson and Choniere, 2013; Navalón et al., 2015; Chiappe and Meng, 2016) (Fig. 1). ACCEPTED MANUSCRIPT Our analysis of 25 early birds contained within the 140-120 Myr period shows an increment in the power margin under all models; such increment is statistically significant under WB (30.3%) and GS (31.0%) models (Fig. 4, Table 4). This indicates that the aerobic efficiency for sustained flapping flight would have improved with the rise of AOC and ρair documented prior to, and throughout, the time period represented by the Jehol Biota. Given a ACCEPTED MANUSCRIPT

recognizable increase in ecological and taxonomic diversity during the Jehol Biota, from the oldest (Huayijing Formation) to the youngest avifauna (Jiufotang Formation) (Fig. 4A;

Chiappe and Meng 2016), we propose that flying benefits derived from the rise of AOC and

ρair might have played a major role in the radiation of early pygostylians (e.g., confuciusornithids, sapeornithids, enantiornithines and stem ornithuromorph lineages) and the attainment of sustained flapping flight. This hypothesis is consistent with an ecological expansion of the entire Jehol Biota that correlates to an increase in plant productivity and the onset of the angiosperm radiation—resulting from a greenhouse climate—through the middle and late Early Cretaceous (Zhou, 2006; Zhang et al., 2016).

3.2.4. Neoavian cladogenesis

Paleobiological and molecular evidence indicate that the bulk of the diversification of Neoaves

(the clade including 95% of all living species of birds) took place after the Cretaceous-

Paleogene boundary (Pacheco et al., 2011; Jarvis et al., 2014; Mayr, 2014; Claramunt and

Cracraft, 2015; Prum et al., 2015). Following this diversification, Jetz et al (2012) has proposed an increase in speciation rates between 50 and 15 Myr ago, which were particularly faster during the first 20 million years of this interval. In this time interval, trends observed in AOC models are incongruent (Fig. 1); consequently, our analysis shows that Emin and Eopt decreased under the GS model but increased according with WB and particularly, the Fk model. However, the increases observed under WB and Fk models are not statistically significant (Tabla 3). ACCEPTED MANUSCRIPT Throughout their radiation, modern birds evolved the full range of aerial strategies observed today, some of which also evolved among stem birds (e.g., thermal soaring, continuous flapping and bounding, burst flight) (Serrano and Chiappe 2017, Liu et al. 2017, Serrano et al

2017, 2018, Voeten et al., 2018). Our results indicate that the extensive diversification of modern birds—likely driven by the occupation of ecological niches vacant after K-P mass ACCEPTED MANUSCRIPT

extinction (Penny and Phillips, 2004; Lindow, 2011)—and the development of their aerial strategies were not supported by atmospheric variation.

3.3. Conclusions

Our study documents that all extinct birds flew under atmospheric conditions corresponding to the ‘comfort range’ of modern avian fliers (i.e., the conditions at altitudes where most living birds fly), even if we cannot ruled out the evolution of adaptations enhancing oxygenation such as those known for a few specialized modern birds (e.g. bar-headed goose; Butler, 2016).

Nonetheless, atmospheric conditions varied significantly during the last 180 million years, and these oscillations variably impacted the efficiency and/or the aerodynamic performance of non- avian theropods and their extinct avian relatives. Specifically, we found that a marked increase in oxygen concentration and air density—combined with a marked reduction in body size— during the Middle-Late Jurassic improved the efficiency of non-avian theropods, which likely played a major role in the subsequent development of aerial locomotion among dinosaurs. The small body size attained by the avian predecessors, together with the development of new traits that improved aerial performance (e.g., larger wing surface, reduced bony-tail, asymmetric vanes in flight feathers), allowed the earliest birds to evolve active flight during the Late

Jurassic-Early Cretaceous. Our results indicate that while flapping flight emerged in non- beneficial atmospheric conditions, those prevailing between 160 and 130 Myr ago, the ACCEPTED MANUSCRIPT subsequent refinement of active flight (i.e., sustained flapping flight) was benefited by an increase in oxygen concentration and air density from 140 to 120 Myr. Such atmospheric changes likely played an important role in the first radiation of early pygostylians. In contrast, our results suggest that changes in atmospheric conditions did not play part in the evolutionary drivers behind the diversification of most modern avian lineages in the early Cenozoic. In all, ACCEPTED MANUSCRIPT

our study underscores the influence of paleoenvironmental settings in a range of key events during the long evolutionary history of birds, emphasizing the importance of accounting for the varying paleoatmospheric conditions when interpreting the macroevolutionary patterns of these animals and their ancestors.

REFERENCES

Altshuler, D. L., Dudley, R., 2006. The physiology and biomechanics of avian flight at high

altitude. Integrative and Comparative Biology 46:62-71.

Baker, S. J., Hesselbo, S. P., Lenton, T. M., Duarte, L. V., Belcher, C. M., 2017. Charcoal

evidence that rising atmospheric oxygen terminated Early Jurassic ocean anoxia. Nature

Communications 8:15018.

Benson, R. B., Choiniere, J. N., 2013. Rates of dinosaur limb evolution provide evidence for

exceptional radiation in Mesozoic birds. Proceedings of the Royal Society of London B

B: Biological Sciences 280:20131780.

Benson, R. B., Hunt, G., Carrano, M. T., Campione, N., 2017. Cope's rule and the adaptive

landscape of dinosaur body size evolution. Palaeontology 61:13-48.

Berner, R. A., 2006. Geological nitrogen cycle and atmospheric N2 over Phanerozoic

time. Geology 34:413-415.

Bishop, C., Butler, P., 1995. Physiological modelling of oxygen consumption in birds during

flight. Journal of Experimental Biology 198:2153-2163. ACCEPTED MANUSCRIPT Bishop, C. M., Butler, P. J., 2015. Flight. In: Scanes, C.G. (Ed.), Sturkie's avian physiology.

Academic Press, 919–974.

Bottjer, D. J., 2017. Geobiology and palaeogenomics. Earth-science reviews 164:182-192.

Braddy, S. J., Poschmann, M., Tetlie, O. E., 2008. Giant claw reveals the largest ever

arthropod. Biology Letters 4:106-109. ACCEPTED MANUSCRIPT

Bruderer, B., Peter, D., Boldt, A., Liechti, F. Wing-beat characteristics of birds recorded with

tracking radar and cine camera. Ibis 152:272-291.

Brusatte, S. L., O’Connor, J. K., Jarvis, E. D., 2015. The origin and diversification of birds.

Current Biology 25:888-898.

Brusatte, S. L., 2017. A mesozoic aviary. Science 355:792-794.

Burgers P., Chiappe L. M. 1999. The wing of Archaeopteryx as a primary thrust generator.

Nature 399:60-62.

Butler, P. J., 1991. Exercise in birds. Journal of Experimental Biology 160:233-262.

Chang, S. C., Zhang, H., Renne, P. R., Fang, Y., 2009. High-precision 40 Ar/39 Ar age for

the Jehol biota. Palaeogeography, Palaeoclimatology and Palaeoecology 280:94-104.

Chatterjee, S., Templin, R. J., 2003. The flight of Archaeopteryx. Naturwissenschaften 90:27-

32.

Chatterjee, S., Templin, R. J., Campbell, K. E. 2007. The aerodynamics of Argentavis, the

world's largest flying bird from the Miocene of Argentina. Proceedings of the National

Academy of Sciences. U.S.A. 104:12398-12403.

Chiappe, L. M., Meng, Q., 2016. Birds of stone. Johns Hopkins University Press, Baltimore,

Md.

Chiappe, L. M., 2007. Glorified dinosaurs: the origin and early evolution of birds. John Wiley

& Sons. Inc., Hoboken, New Jersey.

Choo, B., Zhu, M., Zhao, W., Jia, L., Zhu, Y. A., 2014. The largest Silurian vertebrate and its ACCEPTED MANUSCRIPT palaeoecological implications. Scientific Reports 4:4252.

Clack, J. A., 2012. Gaining ground: the origin and evolution of tetrapods. Indiana University

Press.

Claramunt, S., Cracraft, J., 2015. A new time tree reveals Earth history’s imprint on the

evolution of modern birds. Science Advances 1:e1501005. ACCEPTED MANUSCRIPT

Cornette, J. L., Lieberman, B. S., Goldstein, R. H., 2002. Documenting a significant

relationship between macroevolutionary origination rates and Phanerozoic pCO2

levels. Proceedings of the National Academy of Sciences. U.S.A. 99:7832-7835.

Dececchi, T. A., Larsson, H. C., Habib, M. B. 2016. The wings before the bird: an evaluation

of flapping-based locomotory hypotheses in bird antecedents. PeerJ 4:e2159.

Dececchi, T. A., Larsson, H. C. 2013. Body and limb size dissociation at the origin of birds:

uncoupling allometric constraints across a macroevolutionary

transition. Evolution 67:2741-2752.

Dial K.P., 2003. Wing-assisted incline running and the evolution of flight. Science 299:402-

404.

Dudley, R. 1998. Atmospheric oxygen, giant Paleozoic insects and the evolution of aerial

locomotor performance. Journal of Experimental Biology 201:1043-1050.

Dudley, R. 2000. The evolutionary physiology of animal flight: paleobiological and present

perspectives. Annual Review of Physiology 62:135-155).

Dyke, G., De Kat, R., Palmer, C., Van Der Kindere, J., Naish, D., Ganapathisubramani, B.

2013. Aerodynamic performance of the feathered dinosaur Microraptor and the

evolution of feathered flight. Nature Communications 4:2489.

Ericson, P. G., Klopfstein, S., Irestedt, M., Nguyen, J. M., Nylander, J. A., 2014. Dating the

diversification of the major lineages of Passeriformes (Aves). BMC Evolutionary

Biology 14:8. ACCEPTED MANUSCRIPT Falkowski, P. G., Katz, M. E., Milligan, A. J., Fennel, K., Cramer, B. S., Aubry, M. P.,

Zapol, W. M. 2005. The rise of oxygen over the past 205 million years and the

evolution of large placental mammals. Science 309:2202-2204. ACCEPTED MANUSCRIPT

Feo, T. J., Field, D. J., Prum, R. O., 2015. Barb geometry of asymmetrical feathers reveals a

transitional morphology in the evolution of avian flight. Proceedings of the Royal

Society of London B: Biological Sciences, 282:20142864.

Gatesy, S. M., Dial, K. P. 1996. Locomotor modules and the evolution of avian flight.

Evolution 50:331-340.

Glasspool, I. J., Scott, A. C., 2010. Phanerozoic concentrations of atmospheric oxygen

reconstructed from sedimentary charcoal. Nature Geoscience 3:627.

Graham J.B., Dudley R., Aguilar N., Gans C. 1995. Implications of the late Paleozoic oxygen

pulse for physiology and evolution. Nature 375:117-120.

Han, G., Chiappe, L. M., Ji, S. A., Habib, M., Turner, A. H., Chinsamy, A., Liu, X., Han, L.

2014. A new raptorial dinosaur with exceptionally long feathering provides insights

into dromaeosaurid flight performance. Nature Communications 5:4382.

Harle, E. and Harle, A., 1911. Le vol des grands reptiles et insectes disparus semble indiquer

une pression atmospherique elevee. Bulletin de la Société géologique de France 4:117-

121.

Hawkes, L. A., Balachandran, S., Batbayar, N., Butler, P. J., Chua, B., Douglas, D. C.,

Prosser, D. J., 2012. The paradox of extreme high-altitude migration in bar-headed

geese Anser indicus. Proceedings of the Royal Society of London B: Biological

Sciences, 20122114.

Heers, A. M., & Dial, K. P., 2012. From extant to extinct: locomotor ontogeny and the ACCEPTED MANUSCRIPT evolution of avian flight. Trends in Ecology and Evolution 27:296-305.

Hu, D., Hou, L., Zhang, L., Xu, X. 2009. A pre-Archaeopteryx troodontid theropod from

China with long feathers on the metatarsus. Nature 461:640.Huey, R. B., Ward, P. D.,

2005. Hypoxia, global warming, and terrestrial Late Permian extinctions. Science

308:398-401. ACCEPTED MANUSCRIPT

Jarvis, E. D., et al. 2014. Whole-genome analyses resolve early branches in the tree of life of

modern birds. Science 346:1320-1331.

Jetz, W., Thomas, G. H., Joy, J. B., Hartmann, K., Mooers, A. O., 2012. The global diversity

of birds in space and time. Nature 491:444-448.

Ksepka, D. T. Flight performance of the largest volant bird. Proceedings of the National

Academy of Sciences. U.S.A. 111:10624-10629.

Lee, M. S., Cau, A., Naish, D., Dyke, G. J. 2014. Sustained miniaturization and anatomical

innovation in the dinosaurian ancestors of birds. Science 345:562-566.

Liechti, F., 2006. Birds: blowin’by the wind? Journal of Ornithology 147:202-211.

Lindow, B.E., 2011. Bird evolution across the K–Pg boundary and the basal neornithine

diversification. In: Dyke, G., Kaiser, G. (Eds.), Living Dinosaurs. Wiley-Blackwell,

Oxford.

Liu, D., Chiappe, L. M., Serrano, F., Habib, M., Zhang, Y., & Meng, Q., 2017. Flight

aerodynamics in enantiornithines: Information from a new Chinese Early Cretaceous

bird. Plos One 12:e0184637.

Long, J. A., Large, R., Lee, M., Benton, M. J., Danyushevsky, L., Chiappe, L. M., Halpin, J.

A., Cantrill, D., Lottermoser, B, 2015. Severe selenium depletion in the Phanerozoic

oceans as a factor in three global mass extinction events. Gondwana Research 36, 209-

218.

Mayr, G., 2014. The origins of crown group birds: molecules and fossils. ACCEPTED MANUSCRIPT Palaeontology 57:231-242.

McShea, D. W. 1994. Mechanisms of large‐scale evolutionary trends. Evolution, 48:1747-

1763. ACCEPTED MANUSCRIPT

Mills, B. J., Belcher, C. M., Lenton, T. M., Newton, R. J., 2016. A modeling case for high

atmospheric oxygen concentrations during the Mesozoic and Cenozoic. Geology

44:1023-1026.

Navalón, G., Marugán-Lobón, J., Chiappe, L. M., Sanz, J. L., Buscalioni, A. D., 2015. Soft-

tissue and dermal arrangement in the wing of an Early Cretaceous bird: Implications for

the evolution of avian flight. Scientific Reports 5:14864.

Lindhe Norberg, U. M., 2002. Structure, form, and function of flight in engineering and the

living world. Journal of Morphology 252:52-81.

O’Connor, J. K., Chiappe, L. M., Bell, A. 2011. Pre-modern Birds: Avian Divergences in the

Mesozoic. In: Dyke, G., Kaiser, G. (Eds.), Living Dinosaurs. Wiley-Blackwell, Oxford,

39-114.

Ostrom, J. H. 1976. Archaeopteryx and the origin of birds. Biological Journal of the Linnean

Society 8:91-182.

Pacheco, M. A., et al. 2011. Evolution of modern birds revealed by mitogenomics: timing the

radiation and origin of major orders. Molecular Biology and Evolution 28:1927-1942.

Padian, K., Chiappe, L. M., 1998. The origin and early evolution of birds. Biological

Reviews 73:1-42.

Pan, Y., Sha, J., Zhou, Z., Fürsich, F. T., 2013. The Jehol Biota: definition and distribution of

exceptionally preserved relicts of a continental Early Cretaceous ecosystem. Cretaceous

Research 44:30-38. ACCEPTED MANUSCRIPT Penny, D., & Phillips, M. J., 2004. The rise of birds and mammals: are microevolutionary

processes sufficient for macroevolution? Trends in Ecology and& Evolution 19:516-

522.

Pennycuick, C. J., 1968. Power requirements for horizontal flight in the pigeon Columba

livia. Journal of Experimental Biology 49:527-555. ACCEPTED MANUSCRIPT

Pennycuick, C. J., 1989. Bird flight performance. Oxford University Press.

Pennycuick, C. J., 2008. Modelling the Flying Bird. AP Theoretical Ecology Series Elsevier.

Academic Press, Oxford.

Prum, R. O., J. S. Berv, A. Dornburg, D. J. Field, J. P. Townsend, E. M. Lemmon, A. R.

Lemmon, 2015. A comprehensive phylogeny of birds (Aves) using targeted next-

generation DNA sequencing. Nature 526:569-577.

Puttick, M. N., Thomas, G. H., Benton, M. J., 2014. High rates of evolution preceded the

origin of birds. Evolution 68:1497-1510.

Rayner, J. M., 1988. Form and function in avian flight. Current Ornithology. Springer,

Boston, MA, 1-66.

Rayner, J. M., 1999. Estimating power curves of flying vertebrates. Journal of Experimental

Biology 202:3449-3461.

Rayner, J. M., 2001. On the origin and evolution of flapping flight aerodynamics in birds.

New perspectives on the origin and early evolution of birds: Proceedings of the

International Symposium in Honor of John H. Ostrom, 363-381.

Rayner, J. M., 2003. Gravity, the atmosphere and the evolution of animal locomotion. In:

Rothschild, L.J., Lister, A.M. (Eds.), Evolution on planet Earth: the impact of the

physical environment, 161-183.

Retallack, G. J., 2009. Greenhouse crises of the past 300 million years. Geological Society of

America Bulletin 121:1441-1455. ACCEPTED MANUSCRIPT Ridley, M. 2004. Evolution. Wiley-Blackwell Science Ltd. Malden, USA. 751 pp.

Tobalske, B. W., 2007. Biomechanics of bird flight. Journal of Experimental

Biology 210:3135-3146.

Saarinen, J. J., Boyer, A. G., Brown, J. H., Costa, D. P., Ernest, S. M., Evans, A. R.,

Lintulaakso, K., 2014. Patterns of maximum body size evolution in Cenozoic land ACCEPTED MANUSCRIPT

mammals: eco-evolutionary processes and abiotic forcing. Proceedings of the Royal

Society of London B: Biological Sciences 281:20132049.

Sanz JL, Álvarez JC, Soriano C, Hernández-Carrasquilla F, Pérez-Moreno B, Meseguer J.,

2002. Wing loading in Primitive Birds. In: Z. Zhou, F. Zhang (Eds.), Proceedings of the

5th Symposium of the Society of Avian Paleontology and Evolution. Science Press,

Beijing, 253-258

Sanz, J. L., Chiappe, L. M., Pérez-Moreno, B. P., Buscalioni, A. D., Moratalla, J. J., Ortega,

F., Poyato-Ariza, F. J., 1996. An Early Cretaceous bird from Spain and its implications

for the evolution of avian flight. Nature 382:442-445.

Schmidt-Nielsen, K., 1997. Animal physiology: adaptation and environment. Cambridge

University Press.

Sereno, P. C., Rao, C., 1992. Early evolution of avian flight and perching: new evidence from

the Lower Cretaceous of China. Science 255:845-848.

Serrano, F.J. 2015. Ecomorphology and evolution of the avian flying system: aerodynamic

implications for the flight of stem birds. PhD Thesis, University of Málaga, Spain.

Serrano, F.J., Palmqvist, P., Sanz, J.L., 2015. Multivariate analysis of neognath skeletal

measurements: implications for body mass estimation in Mesozoic birds. Zoological

Journal of Linnean Society 173:929-955.

Serrano, F. J., Chiappe, L. M., 2017. Aerodynamic modelling of a Cretaceous bird reveals

thermal soaring capabilities during early avian evolution. Journal of The Royal Society ACCEPTED MANUSCRIPT Interface 14:20170182.

Serrano, F. J., Palmqvist, P., Chiappe, L. M., Sanz, J. L., 2017. Inferring flight parameters of

Mesozoic avians through multivariate analyses of forelimb elements in their living

relatives. Paleobiology 43:144-169. ACCEPTED MANUSCRIPT

Serrano, F.J., Chiappe, L.M., Palmqvist, P., Figueirido, B., Marugán‐Lobón, J., Sanz, J.L.,

2018. Flight reconstruction of two European enantiornithines (Aves, Pygostylia) and

the achievement of bounding flight in Early Cretaceous birds. Palaeontology. DOI:

10.1111/pala.12351.

Shubin, N., 2008. Your inner fish: a journey into the 3.5-billion-year history of the human

body. Penguin Books Ltd., London.

Speakman, J. R., 1993. Flight capabilities in Archaeopteryx. Evolution 47:336-340.

Stewart, A.D., 1978. Limits to palaeogravity since the late Precambrian. Nature 271:153-155.

Taylor, G.K., Reynolds, K.V., Thomas, A.L., 2016. Soaring energetics and glide performance

in a moving atmosphere. Philosophical Transactions of the Royal Society London:

Biological Sciences 371:20150398.

Templin, R. J., 2000. The spectrum of animal flight: insects to pterosaurs. Progress in

Aerospace Sciences 36, 393-436.

Thomas, A. L., 1997. The breath of life—did increased oxygen levels trigger the Cambrian

explosion? Trends in Ecology & Evolution 12:44-45.

Tobalske, B. W., 2007. Biomechanics of bird flight. Journal of Experimental Biology

210:3135-3146.

Tsuchiya, T., Kawai, K., Maruyama, S, 2013. Expanding-contracting earth. Geosciences

Frontiers 4:341-347. Tucker, V. A., 1973.ACCEPTED Bird metabolism during MANUSCRIPT flight: evaluation of a theory. Journal of Experimental Biology 58:689-709.

Tucker, V. A., 1975. The energetic cost of moving about: walking and running are extremely

inefficient forms of locomotion. Much greater efficiency is achieved by birds, fish—

and bicyclists. American Scientist 63:413-419. ACCEPTED MANUSCRIPT

Turner A.H., Pol D., Clarke J.A., Erickson G., Norell M., 2007. A basal dromaeosaurid and

size evolution preceding avian flight. Science 317:1378-1381.

Videler, J. J., 2005. Avian Flight. Oxford Ornithology Series. Oxford University Press,

Oxford.

Viscor, G., Fuster, J. F., 1987. Relationships between morphological parameters in birds with

different flying habits. Comparative Biochemistry and Physiology Part A: Physiology,

87:231-249.

Voeten, D., Cubo, J., de Margerie, E., Roper, M., Beyrand, V., Bures, S., Tafforeau, P.,

Sanchez, S. 2018. Wing bone geometry reveals active flight in Archaeopteryx. Nature

Communications. DOI: 10.1038/s41467-018-03296-8

Wang, M., Lloyd, G. T., 2016. Rates of morphological evolution are heterogeneous in Early

Cretaceous birds. Proceedings of the Royal Society London: Biological Sciences

283:20160214.

Wang, M., Zheng, X., O’Connor, J. K., Lloyd, G. T., Wang, X., Wang, Y. Zhang, X., Zhou,

Z., 2015. The oldest record of ornithuromorpha from the early cretaceous of China.

Nature Communications: 6:6987.

Ward, P., 2006. Out of Thin Air: Dinosaurs, Birds, and Earth's Ancient Atmosphere. Joseph

Henry Press, Washington DC.

Ward, P., Berner, R. 2011. Why Were There Dinosaurs? Why Are There Birds? In: Dyke, G.,

Kaiser, G. (Eds.), Living Dinosaurs. Wiley-Blackwell, Oxford (2011). ACCEPTED MANUSCRIPT Wellnhofer, P., 2008. Archaeopteryx, der urvogel von Solnhofen. Verlag Dr. Friedrich Pfeil,

München.

Xu, X., Zheng, X., Sullivan, C., Wang, X., Xing, L., Wang, Y., Zhang, X., O’Connor, J.K.,

Zhang, F., Pan, Y., 2015. A bizarre Jurassic maniraptoran theropod with preserved

evidence of membranous wings. Nature 521:70-73. ACCEPTED MANUSCRIPT

Zhang, X., Zhang, G., Sha, J. 2016. Lacustrine sedimentary record of early Aptian carbon

cycle perturbation in western Liaoning, China. Cretaceous Research 62:122-129.

Zhang, Z., Chen, D., Zhang, H., Hou, L., 2014. A large enantiornithine bird from the Lower

Cretaceous of China and its implication for lung ventilation. Biological Journal of the

Linnean Society 113:820-827.

Zhang, F., Zhou, Z., 2000. A primitive enantiornithine bird and the origin of feathers. Science

290:1955-1959.

Zhang, F., Zhou, Z., Benton, M. J., 2008. A primitive confuciusornithid bird from China and

its implications for early avian flight. Science in China Series D: Earth Sciences

51:625-639.

Zhou, Z., 2006. Evolutionary radiation of the Jehol Biota: chronological and ecological

perspectives. Geological Journal 41:377-393.

Zhou, Z., 2014. The Jehol Biota, an Early Cretaceous terrestrial Lagerstätte: new discoveries

and implications. National Sciences Reviews 1:543-559.

ACKNOWLEDGEMENTS

We thank Tayler Hayden and Stephanie Abramowicz for editorial and graphic assistance, respectively. R. FaucettACCEPTED (UWBM, Seattle), J.MANUSCRIPT Barreiro (MNCN, Madrid), and Z. Zhou (IVPP, Beijing) provided access to specimens under their care. We also thank to Jorge Cubo and one anonymous reviewer for their comments that greatly improve the earlier versions of this study.

Funding was provided by the Spanish Ministry of Economy and Competitiveness (Research

Project CGL2015-68300-P and CGL2016-78577-P). F.J. Serrano was supported by a

Postdoctoral Fellowship Agreement between the Natural History Museum of Los Angeles ACCEPTED MANUSCRIPT

County (California, USA) and the Sierra Elvira Foundation (Granada, Spain). Mrs. Gretchen

Augustyn and family provided support to the Dinosaur Institute (NHM).

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**FIGURE CAPTIONS**

Figure 1. Atmospheric conditions and evolution of theropods during the last 180 Myr. Values of mean annual temperature (Tair) were taken from Retallack (2009). Values of AOC were taken from three different models (Fk, GS, and WB). The dashed grey line indicates the present standard AOC (20.9 %). The stratigraphic record of the theropod lineages (thick, black bars) is indicated over a simplified, calibrated phylogeny. Abbreviations: OTH – Ornithothoraces;

PAR – Paraves; PYG – Pygostylia. Several key flight-correlated synapomorphies are indicated in the cladogram. Photographs correspond to some of the main avian clades shown in the cladogram: Archaeopteryx lithographica (1; HMN 1880/1881), Confuciusornithidae (2;

Eoconfuciusornis zhenghi IVPP V 11977), Sapeornithidae (3; Sapeornis chaoyangensis

DNHM D3078), Enantiornithes (4; Junornis houi BMNHC-PH-919), Hongshanornithidae (5;

Hongshanornis longicresta DNHM D2945).

Figure 2. Power curves calculated for a herring gull, Larus argentatus (UWBM 56577), at different conditions of AOC and ρair. Dotted line represents the estimated maximum available power (Pav). Graphical calculation of minimum power speed (Vmp), maximum range speed

(Vmr), minimum Pmec (Pmin) and optimum Pmec (Popt) are shown through grey arrows. The lowest point of the power curves indicates the minimum power speed (Vmp), which supports flapping flight with the least P . The tangent between the origin and its contact point with the power ACCEPTEDmec MANUSCRIPT curve indicates the maximum range speed (Vmr)—the speed at which transport cost, is minimum (Rayner 1999, Pennycuick 2008). A. Calculations under standard sea level

3 conditions (AOC = 20.9%, ρair = 1.225 Kg/m ). As Pav > Pmec, sustained flapping flight using aerobic metabolism is efficient. B. Calculations under the most extreme conditions (AOC at

3000 m and ρair at 500 m of altitude) found in any of the three models, from 150 Myr onwards ACCEPTED MANUSCRIPT

3 (AOC = 14.5%, and ρair = 1.167 Kg/m ). As Pav < Pmec, sustained flapping flight using aerobic metabolism is inefficient.

Figure 3. Mass-specific power variation in theropods during the 180-160 Myr interval. A.

Upper graph - Nodal values of body mass were taken from Lee et al. (2014). Middle graph –

AOC variation (each model) from 180 to 160 Myr. Lower graph –Lines represent trends of the mean values of mass-specific Pav at 2.5 Myr intervals, according to the three AOC models (WB,

, Fk, and GS). B. Range of mass-specific Pav for theropod nodes during the 180-170 Myr and

170-160 Myr intervals. Each graph represents mass-specific Pav according to each AOC model.

Box length shows the interquartile range (25th and 75th percentiles), and whisker lengths indicate the 5–95% confidence limits. Middle thick line represents the median value. Note that the power efficiency increases from 180 to 160 Myr as the body mass decreases and AOC rises

(A). A comparison of efficiency between nodes contained in the 180-170 Myr interval with those contained in the 170-160 Myr interval shows a statistically significant increase (see Table

4) for the latter interval in each of the three models (B).

Figure 4. Aerial efficiency variation in early birds. A. Upper graph - Number of new avian species described from each of the three formations of the Jehol Biota (see Supplementary

Table 4). Middle graph - AOC (solid lines) and air density (dashed lines) variation from 140 to

120 Myr according to each AOC model. Lower graph - Percentile variation of aerial efficiency ACCEPTED MANUSCRIPT at Vmr (i.e. Eopt) with respect to atmospheric conditions at 140 Myr. Values of Eopt were obtained for the 24 Jehol birds analyzed and mean values were calculated at 10 Myr intervals according to each AOC model. B. Range of Eopt values for early birds. Each graph represents Eopt according to its corresponding AOC model. Each box represents the full dataset of Jehol birds analyzed under atmospheric conditions at 140 and 120 Myr. Box length shows the interquartile ACCEPTED MANUSCRIPT

range (25th and 75th percentiles), and whisker lengths indicate the 5–95% confidence limits.

Middle thick line represents the median value. Note that taxonomic diversity (i.e., number of new species) grows as the aerial efficiency at cruising speed increases (together with AOC and air density) from 140 to 120 Myr (A). The increment in efficiency is statistically significant

(see Table 4) in two of the three models (B).

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HIGHLIGHT REMARKS

. Extinct birds flew in atmospheric conditions within the modern birds’ range . Atmospheric changes favor efficiency prior to powered flight and during refinement . Paleoatmospheric conditions played an important role in avian macroevolution

ACCEPTED MANUSCRIPT Figure 1 Figure 2 Figure 3 Figure 4