A new Jurassic theropod from China documents a transitional step in the macrostructure of feathers
Genuine fossils with exquisitely preserved plumage from the Late Jurassic and Early Cretaceous of northeastern China have recently revealed that bird-like theropod dinosaurs had long pennaceous feathers along their hindlimbs and may have used their four wings to glide or fly. Thus, it has been postulated that early bird flight might initially have involved four wings (Xu et al. Nature 421:335–340, 2003; Hu et al. Nature 461:640–643, 2009; Han et al. Nat Commun 5:4382, 2014). Here, we describe Serikornis sungei gen. et sp. nov., a new feathered theropod from the Tiaojishan Fm (Late Jurassic) of Liaoning Province, China. Its skeletal morphology suggests a ground-dwelling ecology with no flying adaptations. Our phylogenetic analysis places Serikornis, together with other Late Jurassic paravians from China, as a basal paravians, outside the Eumaniraptora clade. The tail of Serikornis is covered proximally by filaments and distally by slender rectrices. Thin symmetrical remiges lacking barbules are attached along its forelimbs and elongate hindlimb feathers extend up to its toes, suggesting that hindlimb remiges evolved in ground-dwelling maniraptorans before being co-opted to an arboreal lifestyle or flight.
KeywordsParaves Birds Feathers Barbules Jurassic Flight evolution
Palaeontological Museum of Liaoning
Yizhou Fossil and Geology Park
The Late Jurassic-Early Cretaceous formations of northeastern China are well known for the extraordinary abundance and diversity, and the exceptional preservation of feathered dinosaurs that shed light on the origin and early diversification of birds (Han et al. 2014; Sullivan et al. 2014). Several small non-avian paravians (e.g. Microraptor, Pedopenna, Anchiornis, Changyuraptor, Xiaotingia, Jianianhualong) are characterized by long pennaceous feathers attached to both their tibia and metatarsus, suggesting that early bird flight might initially have involved four wings (Xu and Zhang 2005; Xu et al. 2003, 2017; Hu et al. 2009; Han et al. 2014). However, because deinonychosaurian theropods and earliest birds show a similar distribution of long pennaceous feathers along their forelimbs, hindlimbs and tail, and because different hypotheses of paravian phylogenies have recently been proposed (Hu et al. 2009; Xu et al. 2011; Godefroit et al. 2013a, b), the origin and early evolution of feather-based flight within Paraves remains controversial. Moreover, the morphology and internal structure of the feathers in the earliest paravians is poorly documented, so their real aerodynamical capacities remain conjectural. For example, the presence or absence of interlocked barbules, commonly considered as a criterion for determining whether a feather can produce useful aerodynamic lift (Zhang et al. 2006), remains unclear in the Late Jurassic basal paravians from China described so far.
Here we report a new paravian theropod, Serikornis sungei gen. et sp. nov., from the Late Jurassic Tiaojishan Formation of Linglongta (Jianchang County, Liaoning Province, China), based on a complete articulated skeleton, PMOL-AB00200, with associated integumentary structures. The plumage of this new specimen brings new information on the structure and function of the feathers in basal paravians and consequently on the early evolution of flight.
Materials and methods
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The life science identifiers for this publication are urn:lsid:zoobank.org:pub:65432F68-4DF9-4CC2-BE86-F366B858893A.
Theropoda Marsh, 1881
Maniraptora Gauthier, 1986
Paraves Sereno, 1997; Avialae Gauthier, 1986
Serikornis sungei gen. etsp. nov.
Serikos, ancient Greek for silk, because the body is almost covered with plumulaceous-like feathers; Ornis, ancient Greek for bird; named in honour of Sun Ge, for his contribution to our knowledge of Jurassic and Cretaceous ecosystems in Asia.
PMOL-AB00200 is a single complete articulated skeleton with associated integumentary structures preserved on a slab. The counterpart is missing, but most of the skeleton is preserved on the main slab.
Locality and horizon
PMOL-AB00200 was collected in the Tiaojishan Formation (Oxfordian, Upper Jurassic; Chu et al. 2016) from Daxishan village, Linglongta (Jianchang County, Liaoning Province, China).
Serikornis is characterized by the following combination of characters (autapomorphies are marked with an asterisk): four anterior maxillary teeth twice as long as the others regarding the crown height*; coracoid tuber well-developed and laterally projected from the lateral margin of the coracoid and forming a subglenoid shelf along the caudoventral margin of the bone; the distal end of the lateral process of the coracoid is thicker than the proximal part and forms a ventral rounded bump; ventrodistal process of ischium narrow, hook-like, strongly deflected caudodorsally and set at the distal end of the ischium*; smooth ventral side of coracoid devoid of small pits.
Serikornis differs from other paravians in the following osteological features: the first four maxillary teeth that are twice as large as the others (Fig. S3 and Fig. S4). This anisodonty does not reflect taphonomic factors (i.e. a partial extrusion of first teeth from maxillary sockets) as the maxillary rim recovering the tooth is erased, allowing to observe that the roots are not displaced. X-ray analysis of the skull shows that the posterior maxillary teeth were completely erupted at the moment of the dead (Fig. S4). The biceps tuber of the coracoid is strongly developed and well detached from the lateral margin of the coracoid; the lateral process of the coracoid forms a rounded bump (Fig. S5). The ventrodistal process of the ischium is narrow, hook-like, strongly deflected caudodorsally and placed at the distal end of the bone, a unique feature in basal paravians (Fig. 3). The shape of the ventrodistal process of the ischium is not the result of a local deformation during the diagenesis because other long bones (humerus, ulna, radius, femur and tibia) remain straight along their entire length.
Serikornis differs from Aurornis in the morphology of its ischium: the postacetabular process tapers distally and the dorsal margin is gently convex, whereas it is not markedly deflected ventrally with a horizontal dorsal margin in Aurornis. The ischium also lacks the ventrally expanded, hook-like ventral process on the distal end that delimits a prominent distal obturator notch in Aurornis (Godefroit et al. 2013a).
Serikornis differs from Eosinopteryx in the longer rostral plate and larger maxillary process of the premaxilla (although this latter difference may be biased by the different ontogenetic stages of the type of individuals). It also differs by the presence of an anterior process of the lacrimal (the latter is vestigial in Eosinopteryx). This feature is likely not an ontogenetic feature as Serikornis is a subadult specimen whereas Eosinopteryx is thought to be an earlier juvenile (Godefroit et al. 2013b).
Serikornis differs from Pedopenna in its relatively longer metatarsal I, its metatarsal III longer than metatarsals II and IV, and in the more robust first phalanx of pedal digit I (Xu and Zhang 2005).
Serikornis differs from Xiaotingia in the posterior process of its maxilla that does not exceed in depth the dentary at mid-length, in having a metacarpal III as long as and not more robust than metacarpal II, in the interclavicular angle of its furcula wider than 75° with a slender omal end, and in lacking a strongly laterally everted acromial process on its scapula that overhangs a groove along the lateral surface of the scapular blade (Xu et al. 2011).
Densely packed integumentary filaments that are joined together proximally are present along the posterior part of the hindlimbs—from the femur up to the penultimate pedal phalanges—contrasting with Eosinopteryx in which the lower leg is devoid of any integumentary structures (Godefroit et al. 2013b), although it cannot be excluded that this character reflects taphonomical conditions. Besides the down-like feathers, meticulous preparation of PMOL-AB00200 reveals the presence of tibial and of short metatarsal remiges (Fig. 7d and Fig. S11), as in Anchiornis, Archaeopteryx and Sapeornis (Foth et al. 2014; Xu et al. 2003; Xu and Zhang 2005; Zheng et al. 2013b). Serikornis can be viewed as a tetrapterygian paravian (= four-wing biplan made of elongated feathers), although its forewing is composed of undifferentiated contour and flight feathers, and its hindwing includes both pennaceous and plumulaceous-like feathers (both wings may lack barbules). As in Anchiornis, the presence of both plumulaceous-like feathers and pennaceous feathers on the hindwing of PMOL-AB00200 represents a transitional stage between the fully plumulaceous leg of basal coelurosaurians (e.g. Sinocalliopteryx and Yutyrannus) and a fully pennaceous hindlimb recovered in more derived Avialae (e.g. Archaeopteryx and Sapeornis) (Godefroit et al. 2013a; Hu et al. 2009; Zheng et al. 2013b).
The tail of Serikornis is proximally covered by numerous down-like feathers (Fig. 7e), while short pennaceous symmetric feathers with a slender rachis are inserted along the distal end of the tail (Fig. 7f). The distal tail feathers of Serikornis cannot be considered as true rectrices (that is, large-sized asymmetrical feathers of the tail) but more closely resemble tectrices (upper tail coverts) in Avialae (O’Connor et al. 2013). As in the hindlimb, the tail feathers of Serikornis therefore represent a transitional condition between the fully plumulaceous tail filaments of more basal coelurosaurs and the longer rectrices inserted all along the tail of Anchiornis (see YFGP-T5199 specimen in Lindgren et al. 2015) and basal Pennaraptorans (Foth et al. 2014; Hu et al. 2009).
Our phylogenetic analysis places Serikornis among basal paravians, outside the Avialae-Deinonychosaurian node (Eumaniraptora) (Fig. 8 and Fig. S11), as the sister taxon of Eosinopteryx, and closely related to Aurornis and Pedopenna (Foth et al. 2014; Godefroit et al. 2013a; Hu et al. 2009; Lefèvre et al. 2014). Our analysis also recovers Anchiornis as the sister taxon of Eumaniraptora.
Our results are consistent with the presence of four wings as the primitive condition for Eumaniraptora (Fig. 8) and inherited by basal birds or, in other words, that the flapping flight of modern birds was preceded by a four-winged gliding stage (Godefroit et al. 2013a, b; Longrich et al. 2012; Xu et al. 2003, 2011; Xu and Zhang 2005; Zheng et al. 2013b). Fully developed hindlimb wings, implying the presence of elongated remiges along both the tibia and the metatarsus, are present in Microraptor (Xu et al. 2003), Pedopenna (Xu and Zhang 2005), Anchiornis (Xu and Zhang 2005), Changyuraptor (Han et al. 2014) and Sapeornis (Zheng et al. 2013b). Although the hindlimbs of Serikornis are covered both by bundles of filaments joined proximally and by fully developed pennaceous feathers, this pattern remains consistent with the tetrapterygian condition of basal birds. Eosinopteryx seems to be devoid of hindlimb wings: pennaceous feathers are only present along the posterior part of the thigh and crus, but this absence can be the result of a taphonomic bias. This apparent reduction of the hindlimb plumage may be regarded as a secondary loss in Eosinopteryx (Godefroit et al. 2013b) or as an ontogenetically controlled feature.
The symmetrical vanes on the hindwing feathers of Serikornis, Anchiornis and Pedopenna (Xu and Zhang 2005) seem to be less efficient from an aerodynamic perspective than those of more derived paravians, so these taxa may have used their hindwing feathers for other functions, such as visual display or mate recognition. The high plasticity in the development of metatarsal plumage in paravians can still be observed in modern birds, with the recurrent development of feathered feet in birds of prey (e.g. Aquila chrysaetos, Asio flammeus, Bubo scandiacus) or in chicken breeds such as Silkies (Barrows 1981; Bartels 2003). Although their small size suggests that these animals were probably not top predators, they would have needed fast movements to escape predation. In this way, the development of hindwing feathers remains disadvantageous and should be regarded as a sign of sexual selection (Chiappe et al. 1999; O’Connor and Chang 2015).
Although elongated rectrices are present along the distal half of the tail in Oviraptorosauria (Ji et al. 1998; Xu et al. 2010) and Microraptor (Li et al. 2012), they seem to be absent in Serikornis, Aurornis and Eosinopteryx, which may suggest that the tail of those basal paravians had no aerodynamic function in increasing the total lift of the animal while gliding as in Archaeopteryx (Longrich et al. 2012), nor any display function as well. The long bony tail was completely covered by elongated pennaceous rectrices in both Anchiornis (Hu et al. 2009) and Archaeopteryx (Foth et al. 2014), whereas Jeholornis had both proximal and distal tail fans (O’Connor et al. 2013). The tail plumage is highly variable in pygostylian birds: a pair of elongate rectrices in Confuciusornis male specimens (Chinsamy et al. 2013) and, in some Enantiornithes (Wang et al. 2014), a graded fan of pennaceous feathers in Sapeornis, a forked tail of pennaceous feathers in Schizooura and a fan-shaped tail of pennaceous feathers in Hongshanornis and in most modern birds (Lucas and Stettenheim 1972; Wang et al. 2014). Rectrices are secondarily absent in Confuciusornis females (Chinsamy et al. 2013) and some enantiornithine female birds (Foth et al. 2014; Zheng et al. 2013a).
Although they extensively covered both arms and hands, the forelimb feathers of the Middle-Late Jurassic basal paravians Anchiornis, Eosinopteryx and Serikornis remained unspecialized and undifferentiated into elongated remiges and shortened coverts, unlike in Archaeopteryx, Microraptor and modern birds (Foth et al. 2014; Longrich et al. 2012): all were rather short, slender, symmetrical and, at least in Serikornis and Eosinopteryx, devoid of barbules, contrasting with the more elongated and asymmetrical wing feathers with well-developed barbules in Archaeopteryx (Carney et al. 2012; Foth et al. 2014) and modern birds (Lucas and Stettenheim 1972). Based on these results, the forelimb feathers in Late Jurassic basalmost paravians were obviously not adapted for active flight (although glide flight cannot be excluded) and were therefore more likely related to other biological phenomena, including visual display and sexual selection (Foth et al. 2014; Ji et al. 1998; Li et al. 2010; O’Connor et al. 2013; Prum and Brush 2012; Xu and Guo 2009).
The supposed limited flight capabilities of Aurornis, Eosinopteryx and Serikornis, as evidenced by the study of their preserved plumage, is also reflected in their osteology. Their forelimbs are proportionally shorter and more gracile than in Anchiornis and Archaeopteryx (Table S1-S2), resulting in a reduced wing surface. All these specimens also have a relatively straight ulna and radius (thus limiting the pronation and supination movements necessary for producing a wing beat), lack a bony sternum for attachment of powerful pectoral muscles and have proximodistally decreasing pedal phalanges, together with small pedal unguals that are poorly recurved (Pike and Maitland 2004). Moreover, the relatively high ratio of tibiotarsus length to femur length can be regarded as a good evidence of cursoriality (Boles 1997). All these characters suggest that basal paravians were primarily ground-dwelling animals with good cursorial abilities (Foth et al. 2014; Hu et al. 2009). However, the manual digits of Serikornis are long and slender with strongly curved unguals I and III. This supposed that they could have been effective for climbing trees as in Archaeopteryx (Feduccia 1993; Manning et al. 2009; Wellnhofer 2009). In this way, the hindlimbs can be regarded as less specialized than the forelimbs for grasping.
The recent discovery of a patagium in scansoriopterygids (a lineage found among basalmost paravians in our phylogenetic analysis, and lacking evidence of remiges) (Xu et al. 2015) suggests that the earliest adaptation to an arboreal/gliding lifestyle among paravians did not involve exaptation of the plumage as an aerodynamic surface. This is particularly true as the insertion of several rows of forelimb feathers requires a large propatagium. The absence of true flying adaptation in the ‘tetrapterygian’ Serikornis and the gliding membrane of scansoriopterygids both challenge an aerodynamic function as earliest driver of plumage elaboration in basalmost Paraves.
Birds and, by extension, some other archosaurs are characterized by a pneumatic postcranial skeleton with invasion of bones by the pulmonary air-sac system (Benson et al. 2012; Britt et al. 1998; O’Connor and Claessens 2005). This system allows a flow-through ventilation and exceptionally efficient gas exchanges (Duncker 1971), and has two evident additional functions: weight reduction in large-bodied or flying taxa and density reduction by energetic savings during foraging and locomotion (Benson et al. 2012; Bramwell and Whitfield 1974; Britt 1993; Cope 1877; Currey and Alexander 1985). The latter function is widely accepted as the main reason for skeletal pneumatization because body size has no significant influence on the proportion of pneumatized skeletal compartments (O’Connor 2004).
Pneumatic foramina (that is, the opening that allows an air sac to enter bone) are proportionally much larger than the primitive nutriment foramina in non-pneumatic vertebrae (including apneumatic bird vertebrae) (Britt et al. 1998). Pneumatic foramina are present in most tetanuran theropods (Benson et al. 2012; Britt et al. 1998), dromaeosaurs (Makovicky et al. 2005; Ostrom 1969), oviraptorosaurs (Osmolska et al. 2004), birds (Apostolaki et al. 2015; Baumel 1993; O’Connor 2004), sauropods (Cope 1877; Marsh 1877; Upchurch et al. 2004; Wedel 2003) and pterosaurs (Bonde and Christiansen 2003; Butler et al. 2009; Claessens et al. 2009; Eaton 1910; Seeley 1870). The character distribution of pneumaticity shows that although axial pneumaticity may lighten the skeleton, its evolution cannot be considered to be an adaptation for flight (Britt et al. 1998). The pneumaticity of cervical and anterior dorsal vertebrae occurred early in theropod evolution (Benson et al. 2012), and the presence of pneumatic foramina in vertebrae of non-avialan and avialan theropods indicates that some components of the avian air-sac lung system was already, to some degree, in place (Britt et al. 1998).
Britt (1993) proposed several osteological correlates of vertebral pneumaticity, based on osteological study of extant ratites (Struthio camelus and Dromaius novaehollandiae) (e.g. large external foramina, external fossae with a crenulate surface texture, thin outer bone walls) (Benson et al. 2012). However, O’Connor (2006) noted that several of these features are present in crocodilians, which lack postcranial pneumaticity. Thus, the presence of internal chambers (called camerate or camellate based on the number and size of internal chambers) opening externally via large (and thus not simply vascular) foramina is the only unambiguous evidence of skeletal pneumaticity (Britt et al. 1998; O’Connor 2006; Wedel 2007). The presence of a high pneumaticity of the anteriormost cervical vertebrae and the limited flight capacities of Serikornis suggest that high pneumatization in small maniraptorans reflects the demands of an increasingly high-performance metabolic regime rather than a prerequisite for flight (Benson et al. 2012; Britt et al. 1998; Cubo and Casinos 2000; Currey and Alexander 1985; Fajardo et al. 2007; O’Connor 2004). Further analyses are required to explore the pneumaticity of the whole specimen. The laminography technique is promising as it allows to investigate the proportion of pneumaticity without external traces of foramina or pneumaticity.
This work was supported by a grant (BL/36/62) to P.G. from the SPP Politique scientifique (Belgium), by FRIA Grants to U.L. and A.Ci. from the F.R.S.-FNRS and by grants to H.D. from the National Natural Science Foundation of China (41172026) and the Natural Science Foundation (201102199). Photographs were taken by Thierry Hubin (RBINS). The use of TNT was kindly permitted by the Willi Hennig Society. The genus name was found after the preliminary expertise of the specimen by Danielle Dhouailly (Université Joseph Fourier). We thank each reviewer that spent time and made efforts in order to improve the final version of our paper. We would also like to thank Emily Willoughby who painted the life reconstruction of Serikornis sungei.
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