Here we present biomechanical models to test when and if a flight stroke may have contributed to flap running, WAIR, or leaping takeoff along the phylogenetic lineage from Coelurosauria to birds and if these models coincide with the evolution of pennaceous feathers and musculoskeletal adaptations for flight. Our goal is to take evolutionary narratives about pathways to flight origins and evaluate them using quantitative, mechanical models derived from living birds. Although feathery integument is likely to have been a synapomorphy for all dinosaurs and perhaps even all ornithodirans ( Godefroit et al., 2014 but see Barrett, Evans & Campione, 2015 ), the evolution of pennaceous forelimb and hindlimb feathers has been hypothesized to have been driven by selection for locomotion ( Burgers & Chiappe, 1999 ; Xu et al., 2003 ; Dial, Randall & Dial, 2006 ; Heers, Tobalske & Dial, 2011 ). Thus we set up a testing regime to determine if non-avian theropods could produce biomechanical values that fit within the realms of those measured in modern animals exhibiting these behaviors, and if is there a decoupling of the timing of the success in these behaviours from the origin of previous proposed flight related traits.

Of all the models for the origin of the flight stroke from a terrestrial life history two major categories exist: those that have locomotory functional aspect are flap running ( Burgers & Chiappe, 1999 ), wing assisted incline running or WAIR ( Dial, 2003 ), and vertical leaping ( Caple, Balda & Willis, 1983 ). Behaviors in the second category are non-locomotory behaviors, such as balancing during prey capture ( Fowler et al., 2011 ) and braking during high-speed turns ( Schaller, 2008 ). The three stringent locomotory behaviours (WAIR, flap running and vertical leaping) are variations on a proto-flight stroke assisting in force generation to increase ground and launch velocities ( Burgers & Chiappe, 1999 ) or to assist in ascending steep inclines to facilitate escape to elevated refuges such as into trees or up inclined rock faces ( Dial, 2003 ). All three are present throughout much of extant bird diversity and have been areas of research into the possible pathways for the origins of powered flight.

Three additional estimates for wing contributions to vertical leaping were made. The first estimates the percentage increase possible to the maximum leap through the addition of thrust generated by flapping. This calculation assumed the maximum wing output occurred at the top of the leap arch, and that the forces generated were directed vertically. This was done through a modification of the terrestrial launch methodology of Witton & Habib (2010 , see Data S3 ) to accommodate bipedal theropod models with and without wing generated thrust. The difference between the maximum heights gained with wing generated thrust was presented as a percentage increase (see Datas S3 and S4 for more detailed description of the equations used and a sample calculation spreadsheet). The second evaluates the horizontal distance extension to a leap through the addition of flapping generated thrust. This was calculated by using the speed at takeoff generated by the equations for bipedal launch (see Datas S3 and S4 ) at both 30 and 45° launch angle. The later corresponds to the theoretical best angle for a projectile while the former more closely resembles the angle of takeoff measured in human and lizard leapers ( Toro, Herrel & Irschick, 2004 ; Linthorne, Guzman & Bridgett, 2005 ; Wakai & Linthorne, 2005 ). In both cases our models were treated as if there was no difference in takeoff and landing height, thus making the calculation of jump distance D jump = ( v 2 sin 2 Θ ) / g

We excluded the potential drag and damage caused by hindlimb feathers of some paravians through contact with the substrate. At low hindlimb angles used during the ascent of inclined surfaces (see the metatarsus during WAIR in Fig. 1 from Jackson, Segre & Dial, 2009 ) the distal limb feathers would have contacted the surface and caused frictional drag, which would have reduced performance and damaged the feathers ( Dececchi & Larsson, 2011 ). Although these variables may have evolved throughout the transition from theropods into early birds, treating them as constants provided a “best case scenario” for non-avian theropods constraining the upper limits for when these behaviours were possible.

Given the abduction limitations of the non-avian theropod glenoid, we chose flap angles of 50, 70 and 90° to encapsulate the range of values expected across Theropoda and ran them for all taxa. An angle of 90° is likely unattainable for all non-avian theropods due to the constraints of reducing contact with the substrate on the latter part of the downstroke and shoulder morphology since the humerus cannot exceed the dorsal rim of the glenoid which is aligned with the vertebral axis (or vertebral frame of reference per Dial, Jackson & Segre, 2008 ). It was included to create an upper bracket on possible support values.

During low advance ratio wing-driven behaviors (launch, landing, WAIR, etc.), the coefficient of drag can be quite large. In young Chukars, the coefficient of drag can be near the coefficient of lift, thereby potentially providing a significant component of weight support during controlled descent or significantly affecting reaction forces during WAIR ( Heers, Tobalske & Dial, 2011 ). To confirm that using pure Cl as our specific fluid force coefficient was an accurate approach (instead of the total fluid resultant with both Cl and Cd), we compared predicted reaction forces and weight support to values measured in vivo and reported in the literature ( Tobalske & Dial, 2007 ; Heers, Dial & Tobalske, 2014 ). Because a close match was found across multiple size classes, we assume for the remainder of the calculations that reaction forces during WAIR are not greatly affected by a high coefficient of drag (though we note that for controlled descent or burst climb out, behaviors we did not investigate, high Cd is likely a critical component).

As WAIR ability is not uniform across ontogeny and seems to be linked to force production ( Jackson, Segre & Dial, 2009 ), we created two-benchmarks of proportion of body mass supported for taxa to reach. Values between 0.06–0.49 body weight (bw) are classified as level 1 WAIR, which corresponds to the earliest stages of ontogeny and sub vertical ascents (late stage I and early stage II per Jackson, Segre & Dial, 2009 ) with greater than 50% contribution to external vertical work generated by the hindlimbs ( Bundle & Dial, 2003 ). 0.5 bw and greater denote level 2 WAIR, equivalent to more mature Stage II and III individuals (per Jackson, Segre & Dial, 2009 ) which are capable of high angle to vertical ascents and whose forelimbs become more prominent in force production ( Bundle & Dial, 2003 ). Although we understand the transition between stages during WAIR is semi-artificial, we wished to create a classification scheme that corresponds to the different levels of WAIR capabilities seen in extant systems ( Jackson, Segre & Dial, 2009 ). The selection of 0.06 bw for achieving stage I was chosen to represent real world recorded minima for this behavior and thus should be considered minimal levels achieved before reconstructions of WAIR are accepted.

Where bw denotes the proportion of body weight supported by the lift generated by the wings (see Supplemental Materials Section S4 for more complete description of all formula and calculations). This relatively simple model was chosen as it is easier to update with new paleobiological information and allowed us to see directly the result of varying the input data to see how varying models of theropod functional limitations shape the results. To test the accuracy of our model, we compared our body weight support results to published data for Chukar partridges during WAIR across the three ontogenetic stages, Pigeon data during WAIR, and birds during takeoff ( Table 2 ). Our values are within the range seen in published data for all three stages of WAIR development and show values greater than 1.0 for all birds undertaking leaping takeoff. As our simple model accurately matches real world experimentally derived values of extant taxa, we believe it a suitable starting point to derive comparative force production data for fossil avian and non-avian theropods.

Citipati osmolskea and MPC-D100/1018 Oviraptor incertae sedis per Lü et al. (2013) were included in this analysis. These specimens are incomplete, but forelimb lengths could be estimated based on the fact that the humerus/forelimb ratio in non-avian and basal avian theropods does not change significantly across ontogeny ( Table S2 ). We used the value of ∼43% MOR 246-1 based on the ratios seen in other Troodontids (range between 39–45%) based on Mei , Jinfengopteryx , Anchiornis , Aurornis , Sinovenator , Sinornithoides and Xiaotingia . For MPC-D100/971 and MPC-D100/1018 we used 41% based on Citipati . For all late stage embryos we reconstructed wing area as if they possessed wings with pennaceous feathering proportional to that seen in adults. This is likely an overestimation, as hatchling and young juveniles in other non-avian theropods do not show pennaceous development to the extent of adults ( Xu et al., 2009 , Zelenitsky et al., 2012 ).

Forty-five specimens representing twenty-four non-avian theropod taxa and five avian taxa were examined. Non-avian theropod specimens ranged in mass from approximately 60 g to 18 kg ( Tables 1 and S1 ). Of these, twenty-eight are from specimens accounting for twelve non-avian theropod taxa with preserved feather material, the rest are from closely related taxa that are inferred to be feathered and were included to broaden the scope of the maniraptorans represented. We a priori excluded the tyrannosaurids Yutyrannus , because of its large size (estimated mass ∼1,400 kg), and Dilong , due to its incompletely preserved forelimb. Multiple individuals were included for Anchiornis , Similicaudipteryx , Caudipteryx , Microraptor , Sinosauropteryx , Mei , Archaeopteryx , Jeholornis , and Sapeornis to represent different size classes and ontogenetic stages as different stages in ontogeny may have different life history strategies ( Parsons & Parsons, 2015 ). To address the possibility of WAIR in juvenile but not adult members of Pennaraptora, three late stage embryos: MOR 246-1 Troodon formosus per Varricchio, Horner & Jackson (2002) , MPC-D100/971.

Yet power and muscle mass may not be the main determinant for the use of wings as locomotory structures. Jackson, Tobalske & Dial (2011) estimated that pigeons, with approximately 20% of their body mass as pectoralis muscles, only used approximately 10% of their mass-specific power for low angle WAIR. Further, it has been suggested that power output itself may not determine flight ability, but lift to power ratio ( Marden, 1987 ). For this analysis we have assumed extant bird power productions and metabolic capacities for short “burst” activities for non-avian theropods and early birds. Although paravian metabolism was not at the levels seen in extant birds, it was sufficient to perform short burst activities ( Erickson et al., 2009 ). Regardless, as our methodology uses wing-beat frequency in conjunction with body size and wing arc measures to generate a lift production value, we are not dependent on either theory (power or lift force) to produce meaningful results.

A greater source of uncertainty and debate is fraction of forelimb muscle mass that is due to the M. pectoralis and its potential power output. Extant birds have extremely large wing muscles, as a proportion to their bodyweight ( Marden, 1987 ). The mass of M. pectoralis for birds’ ranges between 10–20% of total body mass ( Greenewalt, 1975 ; Askew, Marsh & Ellington, 2001 ), and total flight muscle fractions for birds can reach 40% ( Hartman, 1961 ; Greenewalt, 1962 ). This is significantly larger than that estimated in non-avian theropods or early birds. For example, Archaeopteryx ’s pectoral muscles are estimated at only 0.5% of its body mass ( Bock, 2013 ) with the entire forelimb (including bone and all other tissues) at 11–14% ( Allen et al., 2013 ). For our analysis, we calculated values for power available from the forelimb and hindlimb based on the assumption that non-avian theropods had forelimb muscle mass fractions of 10% their total mass and that hindlimb muscle mass fractions were 30% of total mass. These values are likely significant overestimations for non-paravians pectoral regions, but the pelvic region values are within the range previous estimated for non-avian maniraptorans ( Allen et al., 2013 ), whose estimates do not include the M. caudofemoralis. The pectoral muscle values we assigned are similar to estimates of pectoral region mass in Microraptor and Archaeopteryx , though those estimates are based on the entire pectoral region tissues (except feathers) and thus the relative mass of the pectoral musculature is likely smaller.

Due to uncertainty regarding soft tissues in fossil organisms, some variables were treated as constants in the taxa modeled and based on values for extant birds. These include feather material properties, arrangement and muscle power. Using these values provided conservative estimates in the sense that they would yield more capable performances for taxa that may lie near biomechanical thresholds. Wing feather arrangements for some fossils appear to be similar to modern birds ( Elżanowski, 2002 ; Xu et al., 2003 ; Foth, Tischlinger & Rauhut, 2014 ) though for some taxa this has been disputed ( Xu, Zheng & You, 2010 ; Longrich et al., 2012 ).

Among non-avians, only Microraptor gui achieved body weight supports greater than 1 under any flap angle or flapping frequency permutation under the two avian derived take off speeds assessed. No non-paravian showed values greater than 0.15 bw under these conditions ( Tables S11 – S13 ). Outside of Microraptor, Changyuraptor and the smaller specimen of Anchiornis , deinonychosaurians did not have values beyond 0.5 bw under either speed or any flap frequency permutation. In avians at the lower body weight estimate, all taxa showed values greater than 1.0 bw at the high end of their flapping angle range. At the higher mass estimates, multiple specimens of Archaeopteryx showed levels below 1.0 bw, with the lowest values seen in the Eichstatt and London specimens ( Tables S11 – S13 ). Many extant avians use launch speeds between 1.5 m/s and 3.8 m/s ( Earls, 2000 ; Berg & Biewener, 2010 ; Heers, Dial & Tobalske, 2014 ). At these takeoff speeds avians more derived than Archaeopteryx achieved values in excess of 1.0 bw, with the exception of the larger mass estimates of Sapeornis under the ALL and GF flapping estimates ( Tables S4 – S6 and S11 – S13 ). At the higher speed of 5.1 m/s, achievable by strong leapers, beyond Microraptor the only other non-avian theropods to achieve greater than 1.0 bw support was the smaller specimen of Anchiornis under a single flap rate permutation at 90° flap angle.

Similar to vertical leaping, there was a marked disparity between distance gained in the “four winged” paravian taxa and all others ( Table S10 ). Only one non-paravian Similicaudipteryx STM-22, under the highest setting and at a 45° takeoff angle, showed distance increases of 5% or greater. Among paravians Microraptor , Changyuraptor , the smaller Anchiornis and all species of Archaeopteryx show leaping values greater than 20% non-flapping horizontal distance at the 45° take off, though this drops to 15% at 30°.

No non-paravian gained more than 8% additional height with flapping using the highest flap angles, and most gained less than 3% ( Fig. 3 , Table S9 ). Using more reasonable flap angles of 50°, none exceeded 4%. Within paravians, several taxa generated greater than 10% height increases, including Anchiornis , Microraptor , Eosinopteryx , Changyuraptor , Aurornis and all Archaeopteryx specimens ( Table S9 ). Despite this most troodontids, both the “short armed” Jehol Dromaeosaurs, Mahakala and Sinornithosaurus showed values more similar to non-paravians, between 1–8.5% increase in height. Of interest, the “four winged” taxa used here ( Anchiornis , Microraptor , and Changyuraptor) yielded increased height gains on the order of 16–64%, with Microraptor gui specimens showing values in excess of 50% ( Fig. 3 , Table S9 ). Even under the lowest flap angle settings, both specimens of M. gui showed leaping height increases of greater than 30%, almost four times the value for the non-paravians under any setting, and Changyuraptor and Microraptor hanqingi showed values of approximately 20%, which is greater than twice the highest value seen in any non-paravian. All Archaeopteryx specimens showed height gains greater than 30% under all mass permutations, with the lighter estimates for the Berlin, Thermopolis and 11 th specimen exceeding 190% non-flapping height values. Interestingly the only specimen that did not reach the 50% height gain under any permutation is the Eichstatt specimen, the smallest in our analysis, whose range between 34–48% gains is similar to what is seen in the larger microraptorine specimens (excluding Sinornithosaurus ).

The use of forelimbs during jumping was divided into three discrete analyses, one examining the potential of the wings to increase maximum jump height, one to examine distance gained horizontally, and finally to see if the wings could generate enough force to take off from a standing start as seen in most extant birds.

Among non-avian theropods, flap running peaked in effectiveness within small-bodied paravians ( Fig. 3 ; Table S8 ). With a 90° flap angle, the smaller Anchiornis specimen and Microraptor gui were the only non-avian taxa to show increases greater than 1.0 m/s under all permutations (71–79 and 75–208% performance increases, respectively), although only Microraptor achieved speeds capable of flight. More realistic 50° flap angles yielded only a 23–27 and 26–65% performance increase for these taxa. Among non-paravians, even under the highest flap angle and flap frequency permutations no taxon exceeded an increase of 17% in running speed with the highest values found in the larger specimen of Similicaudipteryx . At flap angles below 90° only the larger Similicaudipteryx and the lighter mass estimated Yixianosaurus specimens among non-paravians yielded velocity increases approaching 10%. Although some paravians had high levels of increased speed, Mahakala, Mei, Jinfengopteryx, Xiaotingia, Tianyuraptor , and Sinovenator showed increases of less 17% under all permutations, with many showing values in the single digits. At 50° only Microraptor sp., Changyuraptor , Eosinopteryx and Anchiornis showed a greater than 10% increase in running velocity. All specimens of Archaeopteryx showed speed increases similar to or greater than those seen in Microraptor and Anchiornis though there is no clear pattern relating body size to speed, as the largest (London) and smallest (Eichstatt) specimens yielded similar values ( Table S8 ). Only Microraptor and all specimens of Archaeopteryx showed the ability to achieve takeoff velocities by this method alone ( Table S8 ).

Among Mesozoic birds, the different mass estimation methods produced significantly different body weight support values and are more prominent in the most basal birds in our analysis Sapeornis and Jeholornis ( Fig. 2 ; Tables S4 – S6 ). All basal avians show the capability of level 1 WAIR (bw support values of 0.06 or greater) under all flap frequencies estimates, mass estimates or flap angles used here and no avians showing values below 0.1 bw under any permutation. In Archaeopteryx , there is no clear trend in WAIR capability and allometry as all specimens besides the Eichstatt individual show a similar range of body weight support values ( Table 3 ). At the higher flap angle and lower mass, all avians show the capability for level 2 WAIR (> 0.5 bw). All birds more derived than Archaeopteryx yield a body weight support values in excess of 1.0 bw at their lower mass estimate at 1.5 m/s 90° flap angle under all 3 flap frequencies, except for Sapeornis where the smaller specimen exceeds 1.0 bw only under the MOD permutation. Of note, the values recovered for more derived avians are significantly higher than those observed in experimental data ( Tobalske & Dial, 2007 ) or calculated using extant measurements ( Tables 2 and S7 ) and well above the 1.0 threshold for takeoff. This suggests that these taxa could have performed this behavior at lower wing beat frequencies, body velocities and flap angles than the values used here, as seen in some extant birds ( Jackson, Tobalske & Dial, 2011 ), or that physiology and power production differed between extant and basal birds ( Erickson et al., 2009 ; O’Connor & Zhou, 2015 ), or a combination of both. If the latter is correct, it suggests our measurements for non-avian theropods overestimate the power production potential in these taxa, and thus overestimate their WAIR capabilities.

At a CoM velocity of 1.5 m/s nine of thirty-four specimens of non-avian theropods reached the minimal benchmark for level 1 WAIR (0.06 bw) under at least one of the three flapping speed and flap angle permutations ( Fig. 2 ; Tables 3 and S4 – S6 ). When the velocity was decreased to 0.6 m/s number that succeed decreased to eight as the Sinornithosaurus specimen based on the measurements of Sullivan et al. (2010) failed to achieve the 0.06 bw benchmark ( Fig. 2 ; Table 3 ). All are deinonychosaurs. Three specimens (the larger Similicaudipteryx specimen, and the smaller mass estimates for Yixianosaurus and Yulong ) approach the WAIR level 1 criteria, but none yield values higher than 0.05 bw, and this only under the MOD reconstruction at the highest abduction angle. All specimens of Microraptor and the smaller specimens of Anchiornis and Eosinopteryx yielded bodyweight support values above 0.06 bw across all permutations at 1.5 m/s whereas at 0.6 m/s only the smaller Anchiornis and Microraptor gui specimens achieve this. Within non-avian theropods using a 90° flap angle at 1.5 m/s, only a single specimen of Microraptor gui (BMNHC PH881) has body weight support values reaching the 0.5 bw cutoffs for WAIR level 2, though the larger specimen (IVPP V 13352) comes close under the MOD reconstruction ( Tables 3 and S4 – S6 ). At 50° only the smaller Anchiornis , Changyuraptor , Eosinopteryx and all 3 Microraptor specimens, achieve the 0.06 bw benchmark at 1.5 m/s and this decreases to only the smaller Anchiornis and Microraptor at 0.6 m/s. No non-avians or Archaeopteryx achieved bw support values higher than 0.33 under the 50° at 1.5 m/s and only Microraptor gui, Archaeopteryx specimens and the smaller Anchiornis reaching a minimal of 0.1 bw under this permutation.

Increase in WAIR ability broadly corresponds to decreased wing loading in Chukars ( Heers & Dial, 2015 ), something noted in other galliform birds ( Dial & Jackson, 2011 ). Thus wing loading values may offer a rough comparison between non-avian theropod specimens and Chukars of a similar body mass. Among non-avian theropods, wing loading values ranged from 46 N/m 2 ( Microraptor ) to over 11,000 N/m 2 ( Sinosauropteryx ). Of the thirty-four non-avian specimens included, only eight, representing five genera (all are deinonychosaurs) showed loading values less than that seen in 1-day-old Chukars (170 N/m 2 ), the highest values recorded across ontogeny. 1-day-old Chukar chicks do not WAIR, can only surmount inclines of less than 48° still performed asynchronous wing beats and their wings make prolonged contacts with the substrate in a crawling fashion ( Jackson, Segre & Dial, 2009 ; Heers & Dial, 2015 ). No non-paravian showed values less than the 160 N/m 2 measured at 3 dph Chukars, with most pennaraptorans at values 2–8 times that seen at even the highest Chukar chick loadings ( Table 1 ; Fig. 1 ). Focusing on the embryonic and early ontogenetic stage specimens in our analysis, to test whether WAIR was possible at early ages and lost through ontogeny, we recovered loading values again significantly higher than the highest values seen during Chukar ontogeny, with values 126–234% those of 1-day-old chicks which were also significantly smaller. For comparison, the hatchling size Similicaudipteryx specimen (STM 4-1) had a body mass estimated at approximately 63 g, similar to a 17 dph Chukar chick (stage II), but wing loading values of 372 N/m 2 , 5.8 times higher than seen in the 17 dph chick and over twice that seen in 3 dph Chukars due to Similicaudipteryx having a wing area only the size of a 6 dph chick which weight approximately 16 g. This suggests that none of the non-paravian theropods could perform the lowest levels of WAIR, even disregarding their limited range of motion and flapping frequency compared to juvenile extant avians. None of the Mesozoic avian taxa, under either mass reconstruction, showed loading values above 74 N/m 2 , which corresponds to approximately 11 dph (stage II) Chukar chicks, which is approximately the time where fledgling begins ( Harper, Harry & Bailey, 1958 ; Christensen, 1996 ).

Discussion

A major challenge of attempting to create models that examine evolutionary transitions is that of efficiency versus effectiveness. Evolved traits may need to only function at some basic level, rather than contribute high degrees of functional adaptation. Thus, an argument against our use of thresholds, such as a 6% body weight support as the minimum for WAIR, is that smaller values, such as 5% or even 1%, may still provide selective advantages for individuals. Although this line of thought is defensible, we suggest a challenge to this. The first is that these low values are not testable in the sense that there are not physically defined thresholds to demarcate when a behaviour may or may not function. Without these parameters to test, any discussion becomes a story-telling scenario. In addition, we have used liberal parameters in reconstructing extinct taxa based on output values measured in modern, derived avians. This optimistic reconstruction of the possible ignores that non-avian theropods have additional functional restrictions based in their musculoskeletal, neuromuscular and integumentary systems not present in extant birds. The minimal age of origin for powered flight in avian theropods where is 130 million years ago (Wang et al., 2015) and this behavior and all its functional and morphological components have been under refinement through selection ever since. Thus, we postulate that the claim that non-avian theropod would be able to perform functions at output levels below the threshold minimums seen in extant avian taxa difficult to defend. For example, flapping frequency and flap angle have large effects on the resulting body weight support values and using avian take off values are likely significant over estimations for values obtainable in most if not all the taxa sampled here. Our use of a velocity of 1.5 m/s is based on the speed of adult Chukars, whose WAIR ability is much greater than proposed of any non-avian taxa examined here. Using juvenile values (0.6 m/s of stage I) reduces the bw support values by approximately one third. Additionally, by using coefficient of lift values of 1, which is higher than is seen in a 20 dph Chukar at 45° angle of attack (stage II per Jackson, Segre & Dial, 2009), we are likely highly positively biasing the results. Thus, we argue that due to our relaxed constraints and the significantly higher wing loadings to that seen in any stage of Chukar development (even the asymmetrical crawling stage of 1–3 dph from Jackson, Segre & Dial, 2009), the taxa sampled here that did not reach the 0.06 bw threshold derived from in vivo experiments or meet the wing loading values seen in the earliest stages of ontogeny should not be considered WAIR capable. Although we do not have in vivo derived values to compare with leaping and flap running estimates, it is not parsimonious to propose that small incremental increases measured only under unnaturally lenient conditions support a behavior.

For all behaviours tested here there is a sharp contrast in performance levels between a small number of paravian taxa (Microraptor, Anchiornis, Changyuraptor, Aurornis and Eosinopteryx) and all other non-avian taxa. This discrepancy is marked not only because it does not correlate to the origin of pennaceous feathers at pennaraptora but it also does not include all members of Paraves within the high performing category. Multiple small bodied and basal members of both deinonychosaurian subgroups, such as Mahakala, Xiaotingia, Jinfengopteryx, Mei, Sinovenator and Sinornithosaurus, show little evidence of benefit from flapping assisted locomotion. As these taxa are similar in size to the paravians that do show potential benefits, the argument that this loss is a byproduct of allometry is not possible. Allometric loss of performance is possible though in the larger, feathered dromaeosaurs like Velociraptor (∼15 kg, Turner et al., 2007) or Dakotaraptor (∼350 kg, Depalma et al., 2015), but our data from embryonic maniraptorans does not support this postulate. As our measurements for the small paravian wing areas are based either on preserved feather length (Sinornithosaurus) or on long feathered close relatives (Anchiornis for Xiaotingia, Jinfengopteryx, Mei, Sinovenator and Microraptor for Mahakala) our values for them are likely overestimates and suggests that locomotion was not a major driver for forelimb evolution, even among small sized paravians.