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A new taxon of birds (Aves) from the Early Cretaceousof Western Siberia, RussiaEvgeny N. Kurochkin a , Nikita V. Zelenkov a , Alexandr O. Averianov b & Sergei V.Leshchinskiy ca Borissiak Palaeontological Institute, Profsojuznaya 123, Moscow, 117997, Russiab Zoological Institute, Saint-Petersburg, Russiac Tomsk State University, Tomsk, RussiaVersion of record first published: 22 Dec 2010.

To cite this article: Evgeny N. Kurochkin , Nikita V. Zelenkov , Alexandr O. Averianov & Sergei V. Leshchinskiy (2011): A newtaxon of birds (Aves) from the Early Cretaceous of Western Siberia, Russia, Journal of Systematic Palaeontology, 9:1, 109-117

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Journal of Systematic Palaeontology, Vol. 9, Issue 1, March 2011, 109–117

A new taxon of birds (Aves) from the Early Cretaceous of Western Siberia, RussiaEvgeny N. Kurochkina∗, Nikita V. Zelenkova, Alexandr O. Averianovb and Sergei V. Leshchinskiyc

aProfsojuznaya 123, Borissiak Palaeontological Institute, Moscow 117997, Russia; bZoological Institute, Saint-Petersburg, Russia;cTomsk State University, Tomsk, Russia

(Received 29 January 2010; accepted 6 July 2010)

In recent decades numerous findings, mostly from the Early Cretaceous of China, have changed traditional conceptionsabout the diversity and evolution of the most ancient Aves. Findings of Mesozoic birds in Russia are extremely rare. Herewe describe a new bird from the Lower Cretaceous (Barremian–Aptian, Ilekskaya Svita) Shestakovo-1 locality (southernWestern Siberia, Russia), that has also yielded dinosaurs, mammals, crocodiles, pterosaurs and lizards. Mystiornis cyrili gen.et sp. nov. is based on an isolated metatarsus which displays a mosaic of morphological features allowing us to create a neworder, Mystiornithiformes. Mystiornis has a fully consolidated (ornithurine-like) gracile metatarsus with a primitive coplanararrangement of the metatarsals, three separate proximal articular facets, and a uniquely located distal interosseal canal. Italso displays diving adaptations previously documented only in Ornithurae.

Keywords: Aves; Mystiornithiformes ordo nov.; Mystiornis gen. n.; Early Cretaceous; Russia; metatarsus

Introduction

Numerous findings from the Cretaceous of China havegreatly improved our knowledge of the early history ofbirds, resulting in essential changes in traditional conceptsof the evolution of Mesozoic Aves (Chiappe & Witmer2002; Kurochkin 2006; Zhou & Zhang 2007). By theEarly Cretaceous, enantiornithines were well establishedand diverse; Ornithurae were present, as well as an aber-rant primitive clade, the Confuciusornithidae. Simultane-ously, the systematic position of some Cretaceous birds (e.g.Jeholornis, Sapeornis, Zhongornis, Piksi, Zhongjianornis,Hollanda, Vorona) has remained obscure: some are simplyidentified as Aves incertae sedis, with unknown affinities tothe main lineages of Avialae (Varricchio 2002; Gao et al.2008; Zhou & Zhang 2006b, Zhou, Zhang & Li 2010; Bellet al. 2010). Plumage has been shown to be present inseveral families of Theropoda (Xu & Guo 2009).

Outside China, Mesozoic birds have been described fromother regions of Asia, Europe, North and South America,Africa, Australia and even Antarctica. These discoveriestestify to the degree of evolutionary experimentation thatcharacterized these birds, including numerous attempts tomaster the air, parallel evolution of different groups, andthe absence of a direct connection between the origin offlight and the occurrence of plumage (Zhou et al. 2003;Kurochkin & Bogdanovich 2008a, b; Xu & Guo 2009).

Findings of Mesozoic birds in Russia are very rare(Kurochkin 2000; Kurochkin et al. 2006). Here we describea new order of birds (Aves) from the Early Cretaceous

∗Corresponding author. Email: enkur@paleo.ru

(Barremian-Aptian, Ilekskaya Svita) Shestakovo-1 local-ity in Western Siberia (Fig. 1). Dinosaur remains fromthis locality have been known since 1953. Since the mid1990s, employees of the Tomsk State University and thePalaeontological and Zoological Institutes of the RussianAcademy of Sciences have discovered numerous mammals,new crocodiles, pterosaurs and lizards in addition to diversedinosaurs (Alifanov et al. 1999; Lopatin et al. 2005). Theonly avian find to date is an isolated metatarsus, collectedin 2000 by a joint team of Tomsk and Moscow palaeon-tologists, showing a peculiar mosaic of ornithurine andenatiornithine characters. This specimen is housed in thePaleontological Museum of Tomsk State University (PMTSU). Despite the isolated nature of the find, morphologicalfeatures of this metatarsus are considered both taxonomi-cally significant and diagnostic, thus making it possible toprovide a taxonomic description. As the metatarsus in ques-tion differs considerably from those of the main clades ofbirds (see below), we believe it to represent a new orderwithin class Aves.

Systematic palaeontology

Class: Aves Linnaeus, 1758Order: Mystiornithiformes ordo nov.

Diagnosis. Metatarsals II–IV completely ossified, copla-nar throughout their entire length, except for the separate

ISSN 1477-2019 print / 1478-0941 onlineCopyright C© 2011 The Natural History MuseumDOI: 10.1080/14772019.2010.522202http://www.informaworld.com

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Figure 1. Map showing the location of the Early Cretaceous Shestakovo-1 locality in Western Siberia.

proximal ends of metatarsals III and IV. Metatarsals II–IVeach have isolated proximal concave articulate facets.Dorsal surfaces of metatarsals II–IV tapered in the form oflongitudinal ridges. Fossa infracotylaris dorsalis not devel-oped. Canalis interosseus distalis originates in the distalpart of sulcus extensorius and terminates on the proximalwall of foramen vasculare distale.

Remarks. Mystiornithiformes differs from all otherknown avians by the unique combination of apomor-phies (coplanar ossification of metatarsals, developmentof the longitudinal ridges on the dorsal surface, andabsence of fossa infracotylaris dorsalis) with autapo-morphies (distinctive canalis interosseus distalis, threeconcave proximal articulated facets). The coplanar arrange-ment of three metatarsals is considered a primitive aviancondition (Chiappe 1996), since it is present in mostbasal Avialae: Archaeornithes, Enantiornithes, Vorona,

Confuciusornithes, Sapeornis (Forster et al. 1996, 2002;Zhou & Zhang 2002), and also in advanced theropods:Alvarezsauridae, Rahonavis and non-avian maniraptors(Chiappe et al. 2002; Clark et al. 2002). But in almost all ofthese, the metatarsals are fused just proximally, with theirdistal ends remaining either completely separate or tightlyattached. Nearly complete fusion of metatarsals II and IIIis observed only in the Upper Cretaceous enantiornithinesLectavis and Yungavolucris from Argentina, and Voronafrom Madagascar (Chiappe 1993; Forster et al. 1996,2002). In contrast, Mystiornithiformes has three metatarsalscompletely coalesced for almost all their lengths, exceptfor the divided proximal ends of metatarsals III andIV and an oval, fissure-like foramen between the proxi-mal parts of their shafts. Ornithurine birds (Neornithes,Ichthyornithes, Hesperornithes, Early Cretaceous ChineseYixianornis, Yanornis, Hongshanornis and Gansus, andalso the Late Cretaceous Apsaravis and Vegavis) differ

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in that the metatarsals are fully consolidated throughouttheir entire lengths. However, in Ornithurae the proximalpart of metatarsal III submerges plantad, i.e. below thelevel of metatarsal II and IV. Only the ornithuromorphArchaeorhynchus from the Lower Cretaceous of Chinaand Patagopteryx from the Upper Cretaceous of Argentinapossess coplanarly fused metatarsals (Alvarenga &Bonaparte 1992; Zhou & Zhang 2006a). The state of fusionof the three metatarsals in Mystiornithiformes is close tothe condition seen in Ornithurae. Another peculiar featureof the described bird is that its metatarsus is very gracile,which is also characteristic of Ornithurae in contrast toEnantiornithes.

The presence of three isolated concave articulate facets(cotylae medialis, centralis and lateralis) on the proximalsurface of the metatarsals is not known for any group withinOrnithuromorpha. Normally only lateral and medial facetsseparated by eminentia intercotylaris are present. BasalAvialae with a coplanar arrangement of their metatarsals(Enantiornithes, Confuciusornithes) lack eminentia inter-cotylaris, but also only have two facets. In non-aviantheropods, each metatarsal bears a separate but convex artic-ular facet (Barsbold 1983). However, in Dromeosauridaethese facets are concave.

The tapered dorsal surfaces of the metatarsals are alsocharacteristic of Avisauridae. In the Shestakovo specimen,a longitudinal ridge is especially strongly developed onthe proximal half of metatarsal III, with a characteristicexpansion to the edge of the articular surface, consistentwith other avisaurids (Chiappe & Walker 2002). In all otherfeathered and other Theropoda, these surfaces are usuallyflattened (Weishampel et al. 2004).

Also characteristic of Mystiornithiformes is the absenceof an excavation (fossa infracotylaris dorsalis) in the proxi-mal part of the dorsal surface of the metatarsus. This fossa,with vascular foramens and tubercles for attachment of m.tibialis cranialis, is well developed in the Ornithurae, butabsent in Patagopteryx, in which a submerged metatarsalIII is absent. The absence of this fossa and placement of thetubercle for the m tibialis cranialis on the dorsal surface ofmetatarsal II is consistent with enantiornithines.

The canalis interosseus distalis begins in Mystiornithi-formes in the distal part of the sulcus extensorius and opensinside the foramen vasculare distale, on its proximal wall.Similar localization of the canalis interosseus distalis hasno analogue among known feathered theropods. In Ornithu-rae, if such an interosseus canal is present, it begins on thedistal wall of the vascular foramen, perforates a bony bridgebetween the distal ends of metatarsals III and IV, and opensbetween the corresponding metatarsal trochleae. A foramenvasculare distale is absent in the majority of the Enantior-nithes, but is present in Avisauridae.

Family Mystiornithidae fam. nov.Genus Mystiornis gen. nov.

Type species. Mystiornis cyrili sp. nov.

Etymology. From mysterion (Gr.) – mysterious, and ornis(Gr.) – bird. The gender is masculine.

Diagnosis. Proximal articular surface of metatarsal II posi-tioned more distally than the proximal articular surfaces ofthe other two metatarsals, but its plantar part protrudes prox-imally beyond the level of the proximal articular surface.Middle parts of metatarsal shafts and metatarsal trochleaecurved plantad. Plantar part of metatarsal II flattened medi-olaterally and noticeably projected plantad. Metatarsal IIshortest of the three, tr. metatarsi II not reaching the levelof distal vascular foramen and strongly flattened dorsoplan-tarly. Trochleae of all three metatarsals lying in differentplanes, so their sagittal planes meet plantarly with sharpangles.

Occurrence. Lower Cretaceous of Western Siberia,Russia.

Remarks. Proximal articular surfaces of metatarsals II–IVin all Enantiornithes and Vorona are positioned at thesame level. The condition in adult Ornithurae is differ-ent since their proximal tarsometatarsus is formed by fuseddistal tarsals, completely covering the proximal ends of themetatarsals. However, in juvenile ornithurine specimens,before fusion of the distal tarsals with the metatarsals, theproximal ends of the metatarsals are at the same level (visi-ble as well in the osteology of juvenile Ornithurae).

Plantar convexity of the metatarsal shafts is also char-acteristic of some avisaurids (Soroavisaurus) and Vorona(Chiappe 1993; Forster et al. 1996). Soroavisaurus alsoshares with Mystiornis a flat metatarsal II with a taperedplantar surface of its shaft. A strongly shortened metatarsalII and similar mutual orientation of trochleae are charac-teristic of the Early Cretaceous Gansus and the Late Creta-ceous gaviiform Neogaeornis (Lambrecht 1929; Hou & Liu1984; Olson 1992). In these birds, such structure is obvi-ously connected with diving adaptations.

Mystiornis cyrili gen. et sp. nov.(Fig. 2)

Holotype. PM TSU,16/5–45, left metatarsus.

Diagnosis. As for the genus.

Etymology. This species is dedicated to Cyril Walker.Slim-legged, from leptos (Gr.) – slim, and kolon (Gr.) –leg.

Material. Holotype only.

Locality and horizon. Shestakovo-1 locality (55◦ 54.6′ N,87◦ 56.9′ E), Tchebulinski District, Kemerovskaya Oblast,

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Figure 2. Mystiornis cyrili gen. et sp. nov., holotype, PM TSUNo. 16/5-45; left metatarsus; Kemerovskaya Oblast, Shestakovo-1 locality; Lower Cretaceous, Barremian-Aptian, Ilekskaya Svita.A, dorsal view; B, lateral view; C, plantar view; D, medial view; E,distal view; F, proximal view. Abbreviations: cc, cotyla centrale;cid, canalis interosseus distalis; cl, cotyla laterale; cm, cotyla medi-ale; cmIII, crista metatarsi III; ddtIII, depressio dorsalis trochleaIII; dptII, depressio plantaris trochlea II; emII, eminentia metatarsiII; fmI, fossa metatarsi I; fvd, foramen vasculare distale; fvp, fora-men vasculare proximale; tII, trochlea metatarsi II; tIII, trochleametatarsi III; tIV, trochlea metatarsi IV.

Russia; Lower Cretaceous, Barremian-Aptian, IlekskayaSvita.

Measurements. Length of metatarsus 26.4 mm; length ofmetatarsal II 21.4 mm, length of metatarsal III 26.1 mm;length of metatarsal IV 25.8 mm; maximal mediolateralwidth of proximal end 3.6 mm; depth of medial cotyle2.0 mm, mediolateral width of metatarsus at the level offossa metatarsi I 2.0 mm; maximal depth of metatarsusin the middle 1.3 mm; mediolateral width of tr. metatarsiII 1.9 mm; depth of tr. metatarsi II 1.2 mm; mediolateral

width tr. metatarsi III 1.6 mm dorsoplantar diameter of tr.metatarsi III 1.9 mm, depth of tr. metatarsi IV 1.5 mm.

Description and comparisons. The holotype is repre-sented by a graceful, elongated and practically completemetatarsus with well preserved metatarsals II, III and IV,as well as proximal cotylae and distal trochleae. Slightlydestroyed are the surfaces of the dorsolateral angle of theproximal articular surface of metatarsal II, lateroplantarside of the proximal end of metatarsal IV, the lateral sideof metatarsal III at the level of the proximal foramen,and the top point of a tuberculum on the medioplantarangle of trochlea II. Metatarsal V and signs of its artic-ulation are missing. The metatarsals are tightly co-ossifiedwith each other practically throughout their entire lengths,except for the most proximal ends of metatarsals III and IV,visible in proximal and dorsal views. A similar condition,partial non-fusion of the shafts, is known in the ornithurineLiaoningornis and in enantiornithine avisaurids (Chiappe1993; Hou 1997). Besides the lack of fusion of the prox-imal metatarsals III and IV, there is a narrow oval crackbetween metatarsals III and IV in the proximal quarterof their shafts. This crack is most likely to be of natu-ral origin (proximal vascular foramen?), although it mightbe the result of postmortal destruction of the thin edgesof the metatarsals. The distal tarsals and a cap coveringthe metatarsus are clearly absent. All three metatarsals liein coplanar (transversal) plane. The proximal surfaces ofmetatarsals II–IV have concave separated articular facets:medial, largest central and smallest lateral cotylae, accord-ingly. There is a small longitudinal prominence betweenthe medial and central cotylae that confirms their separa-tion. An elevation (eminentia metatarsi II) on the plantarpart of the medial cotyle protrudes proximally beyond thesurfaces of the central and lateral cotylae. The hypotarsusis not developed. The dorsal surfaces of the metatarsalsare tapered in the form of longitudinal crests, especiallydistinctive on metatarsals III and IV. Therefore, the shaft ofthe metatarsus looks three-crested in dorsal view. The crestof metatarsal III is the highest, while that of metatarsal IIis the least pronounced. The most proximal area of cristametatarsi III abruptly extends on the dorsal edge of thecentral cotyle.

In lateral aspect, the middle portion of the metatarsus isslightly curved plantad, the whole proximal end is slightlyinclined dorsally, and the entire distal part with trochleaeis noticeably deflected plantad. Metatarsal II is the shortestand completely co-ossified with metatarsal III throughoutits entire length. The shaft of metatarsal II is mediolater-ally flattened; its plantar surface forms a ridge (crista plan-taris medialis) which circumscribes a deep plantar concav-ity (sulcus flexorius), as is present in a number of taxa ofEnantiornithes. In lateral view, this ridge noticeably projectsplantad beyond the plantar edge of metatarsal IV. There isa small but conspicuous tubercle (tuberculum metatarsi II)

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with a roughness (possibly a place for the attachment ofthe tendon for m. tibialis cranialis) on the dorsal surfaceof metatarsal II on the proximal fifth of its length. Thischaracter is considered a synapomorphy of Enantiornithes(Chiappe & Witmer 2002).

The distal portion of the shaft of metatarsal II, beforeits transition into the trochlea, is flattened dorsoplantarlyand extends considerably lateromediad. Metatarsal II endsmuch more proximally than metatarsals III and IV. Fossametatarsi I faces medioplantad, is positioned on the shaftof metatarsal II, and closely approaches the correspond-ing trochlea. Such an orientation of this fossa indicates acompletely reversed position of the hallux and thus the pres-ence of an anisodactyl foot. The shaft of metatarsal III isflattened dorsoplantarly and is approximately twice as wideas the shafts of metatarsals II and IV; it has a constantwidth almost throughout its entire length. The shaft ofmetatarsal IV is flattened lateromedially and is the narrow-est of the three, as in Enantiornithes. There is a narrowextensor groove running alongside the dorsal surface ofthe bone between the distal third of metatarsals III andIV. A small entry foramen of a canal (?canalis interosseusdistalis) lies within an excavation in the distal end of thisgroove. This canal opens on the proximal wall of a ratherlarge distal vascular foramen. The latter has an oval shapeand is circumscribed distally by a bony bridge betweenmetatarsals III and IV; this bridge is not perforated by anyforamen. The plantar outlet of the distal vascular foramenis wide; a low tubercle, circumscribing a shallow groove(possibly for the tendon of m. extensor brevis dig. IV ) onthe bony bridge, is present on the lateral edge of the baseof metatarsal III.

The plantar surface of the metatarsus is trough-shaped(sulcus flexorius), with a wide flat floor formed bymetatarsal III, and the lateral and medial sides of the sulcuscircumscribed by the keeled plantar edges of metatarsals IIand IV, as in representatives of Strigidae and Accipitridaeas well as numerous enantiornithines and other Mesozoicbirds. The sulcus continues distally practically to the baseof metatarsal II, but in its distal part is bounded just by anarrow plantar edge of metatarsal IV.

The sagittal planes of the metatarsal trochleae meet thesagittal plane of the metatarsus at different angles. Thesagittal plane of trochlea II is inclined with an angle of about15◦ with respect to the sagittal plane of metatarsus. Trochleametatarsi II is very strongly flattened dorsoplantarly; itsdistal edge does not reach the level of the distal vascularforamen. This trochlea is the largest of the three and isdorsoplantarly compressed, as in Enantiornithes (Chiappe& Witmer 2002). On the dorsal and plantar surfaces ofthe base of trochlea metatarsi II are shallow dorsal andplantar depressions. The lateral condyle of trochlea II isslightly larger than the medial one, and a ginglymoid notchbetween them is almost not developed. The medial marginof trochlea II appears to be flattened, with a low rugosity at

the probable point of origin of the collateral ligament. Onthe lateral margin of this trochlea there is a rather deep pitwhich probably marked the place of origin of another collat-eral ligament. A small plantarly directed pointed tubercle isalso developed on the medioplantar angle of this trochlea.Trochlea metatarsi II is positioned very close to the shaft ofmetatarsal III, from which it is separated only by a narrowfissure on the dorsal side; on the plantar side they arecompletely fused to each other. Trochlea metatarsi III isinclined at an angle about 5◦ to the longitudinal axis ofthe metatarsus; it is the longest of the three trochleae. Thistrochlea is practically rounded in lateral and medial views.Its medial condyle is somewhat larger than the lateral one.The lateral and medial surfaces of the trochlea have deeppits for collateral ligaments. There is a conspicuous depres-sion (depressio dorsalis trochlea III) on the dorsal side ofthe base of the trochlea. Distally this continues into a smallginglymoid notch between condyles which have a quitesimilar depth. Trochlea metatarsus IV is the smallest of thethree and is slightly abducted. It is rounded, the condylesand intercondyle notch are almost not developed, and itsplantar margin is pointed and shifted somewhat laterally.Small pits are present on the medial and lateral surfaces, atpossible places of origin of collateral ligaments. The sagit-tal plane of this trochlea is inclined to the sagittal plane ofthe metatarsus at an angle of about 10◦.

Cladistic analysis

Since the mosaic pattern of characters of Mystiornisprevents unambiguous placement of this fossil within anyknown higher taxa of Aves using the comparative morphol-ogy approach, we performed a cladistic analysis to resolveits phylogenetic position. For this purpose we used one ofthe most recent and complete matrices for Mesozoic birds,that of O’Connor et al. (2009). Along with Mystiornis wecoded several other Mesozoic taxa, such as the avialansVorona and Avisaurus archibaldi, as well as the troodontidsMei and Anchiornis. Allosaurus was used as an outgroup.The modified matrix includes 242 characters across 35 taxa(see Online Supplementary Material for coding of the newlyincluded taxa). PAUP 4.0 (Swofford 2002), WinClade, andTNT (Goloboff et al. 2008) were used to analyse the matrix.

The strict consensus of 576 MPTs, each 626 steps inlength (CI: 0.49; RI: 0.69), that resulted from the analysiswith PAUP is shown in Fig. 3. On this tree, Mystiornis isoptimized as the sister taxon of Avisaurus, and both areincluded in Clade A that also includes Mei and Vorona.This clade is basal with respect to all other Avialae apartfrom Archaeopteryx. In contrast to PAUP, the strict consen-sus tree resulting from a traditional search in TNT does notsupport Clade A. In this tree, the phylogenetic position ofmost of the basal avian taxa, except Longipterygiformes,Ornithuromorpha and clades herein, were unresolved.

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Figure 3. Strict consensus tree recovered from analysis of data matrix modified after O’Connor et al. (2009) using PAUP (tree drawnusing WinClade). Clade A (see text) is indicated.

Heuristic search with WinClade yielded both of the afore-mentioned topologies with few variations (Fig. 4).

The clade Mystiornis + Avisaurus is supported by theapomorphic condition of the following characters: 236(trochlea metatarsi II broader than trochlea metatarsi III,as also found in the Enantiornithes excluding Concor-nis and Longipterygiformes); 230 (distal vascular fora-men completely enclosed, as in Ornithuromorpha); and221 (metatarsals at least nearly completely fused, asin Ornithuromorpha and Eoalulavis). Nevertheless, thegeneral shape of the metatarsus in Mystiornis andAvisauridae is different (the former has completely fusedgracile metatarsals), so we propose a new family forMystiornis.

There are no unique synapomorphies for Clade A,although it is supported by the advanced conditions of char-acters 229 (excavated plantar surface of metatarsus, alsopresent in Confuciusornis, Patagopteryx and Nequenor-nis) and 233 (developed fossa metatarsi I, as in advancedOrnithurae). In our opinion, both of these characters do notjustify establishing a new taxon since they are present in avariety of fossil Avialae. The putative troodontid Mei is alsoincluded in Clade A. Troodontidae are similar to Mystiornisin that metatarsal II is markedly shorter than metatarsals IIIand IV (Makovicky & Norell 2004). However, all troodon-tids have unfused metatarsals (firmly fused in Mystiornis);furthermore, in all taxa except the basal Sinovenator (Xuet al. 2002) and Anchiornis (Hu et al. 2009) the third

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Figure 4. Strict consensus tree recovered with a heuristic search in TNT (tree drawn with WinClade).

metatarsal is strongly constricted for most of its length,being excluded from the plantar view (Makovicky & Norell2004). Thus the morphology of the metatarsus is consid-erably different in advanced Troodontidae and Mystiornis,but the most basal troodontid taxa show some similaritiesto the new taxon. In this case the PAUP cladogram may beaccepted as a working hypothesis, pending more data onMystiornis. The systematic position of Mystiornis is thuspresently considered as Avialae incertae sedis.

Conclusion

Mystiornis cyrili gen. et sp. nov. represents a small birdthe size of a thrush, with probable adaptation to underwa-ter swimming reflected in the characteristic shortening of

metatarsal II and widening of the corresponding trochlea,similar to Gansus. However, the mosaic of characters doesnot permit assignment to any known major taxon of Avialae,either via evolutionary morphology or by cladistic analy-sis. Mystiornis differs from all known taxa of featheredvertebrates in the presence of separated concave articu-lation facets on the proximal extremities of metatarsalsII–IV and localization of a distal interosseal canal in theproximal part of the distal vascular foramen. Mystiornisdiffers from Confuciusornithes and Dromeosauridae in themorphology of the distal extremity of the metatarsus, i.e.complete coalescence of distal metatarsals and the pres-ence of a distal vascular foramen. However, Mystiornisresembles these taxa in the coplanar arrangement of theproximal ends of the metatarsals. The new taxon shareswith Enantiornithes the wide trochlea of metatarsal II and

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the presence of a tuberculum on the dorsal surface in theproximal part of metatarsal II. It differs from this taxon,however, in the degree of fusion of the metatarsals and theoverall consolidation of metatarsus. It differs from Ornithu-rae in the morphology of the proximal extremity, i.e. thecoplanar arrangement of proximal metatarsals, absence ofthe hypotarsus and dorsal intercotylar fossa, and separa-tion of the most proximal extremities of metatarsals IIIand IV. However, it resembles Ornithurae in having almostcomplete fusion of the metatarsals and in its fully developeddistal vascular foramen.

Cladistic analysis using PAUP places Mystiornis withina clade which also includes Mei, Vorona and Avisaurus,and occupies a basal position relative to all other Avialaeexcept for Archaeopteryx. MacClade and TNT leave thephylogenetic position of Mystiornis unresolved withinbasal Avialae. Lack of resolution and collapse of well-documented relationships may reflect the incompletenessof taxa such as the new species, and Vorona, as well asthe mosaic distribution of avian characters within closelyrelated maniraptorans such as Mei and Anchiornis.

The discovery of Mystiornis shows that in the Creta-ceous, swimming adaptations were achieved not only byOrnithurae (such as Gansus, Hesperornithiformes, Ichthy-ornis), but independently by other groups of birds. Thistestifies to the variety of morphologies seen in the earlyevolution of birds, and to the parallel evolution of differentgroups of feathered vertebrates.

Acknowledgements

The authors thank Zhou Zhonghe, Xu Xing and CorwinSullivan for data on Cretaceous birds and dinosaurs fromChina, Evgenia Baikina for help in making the map, Jing-mai O’Connor for useful comments and correction of theEnglish, and Gareth Dyke for correction of the Englishand for the invitation to submit our paper to the meetingand volume dedicated to the memory of Cyril Walker. Thisresearch was supported by Grant No. 10-04-00575 of theRussian Fund for Basic Research.

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