PHYLOGENETIC SUPPORT FOR A SPECIALIZED CLADE OF CRETACEOUS ENANTIORNITHINE BIRDS WITH INFORMATION FROM A NEW SPECIES

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1 Journal of Vertebrate Paleontology 29(1): , March 2009 # 2009 by the Society of Vertebrate Paleontology ARTICLE PHYLOGENETIC SUPPORT FOR A SPECIALIZED CLADE OF CRETACEOUS ENANTIORNITHINE BIRDS WITH INFORMATION FROM A NEW SPECIES JINGMAI K. O CONNOR, *,1,2 XURI WANG, 3 LUIS M. CHIAPPE, 1 CHUNLING GAO, 3 QINGJIN MENG, 3 XIAODONG CHENG, 3 and JINYUAN LIU 3 1 Dinosaur Institute, Natural History Museum of Los Angeles, 900 Exposition Boulevard, Los Angeles, CA 90007, U.S.A. 2 University of Southern California, 3651 Trousdale Parkway ZHS 117, Los Angeles, CA U.S.A., jingmai@usc.edu 3 Dalian Natural History Museum, No. 40 Xicun Street Heishijiao Shahekou, District Dalian, P.R. China ABSTRACT A new species of enantiornithine bird from the Lower Cretaceous Yixian Formation of northeastern China is reported. The new taxon, Shanweiniao cooperorum, possesses several enantiornithine synapomorphies as well as the elongate rostral morphology (rostrum equal to or exceeding 60% the total length of the skull) of the Chinese early Cretaceous enantiornithines, Longipteryx chaoyangensis and Longirostravis hani. The discovery of this new specimen highlights the existence of a diverse clade of trophically specialized enantiornithines, Longipterygidae, for which we present phylogenetic support in a new comprehensive cladistic analysis of Mesozoic birds. Shanweiniao provides new information on the anatomy of longipterygids, and preserves a rectricial morphology previously unknown to enantiornithines, with at least four tail feathers closely arranged. This supports the hypothesis that enantiornithines were strong fliers and adds to the diversity of known tail morphologies of these Cretaceous birds. INTRODUCTION Since their establishment as a clade less than three decades ago, Enantiornithes has become the most specieous group of Cretaceous birds known to science. A large portion of this diversity comes from the Lower Cretaceous (~ Ma) Jehol ( ) Group deposits of northeastern China (Fig. 1; Swisher et al., 2002; Zhu et al., 2007), a lithostratigraphic unit that has yielded a wealth of early birds including taxa such as Confuciusornis sanctus, Jeholornis prima, Liaoningornis longidigitrus, Yanornis martini, and Yixianornis grabaui (Zhang et al., 2003). Though highly specieous, enantiornithines are morphologically very similar, and in this respect, often compared to the modern passerines (Chiappe, 2007). Although their cranial morphology is not well known because the skull is missing, crushed, or fragmentary in most specimens, available data show that most of these birds possess relatively short rostra, approximately 50% of the total skull length. However, three enantiornithines from the Jehol biota are known to possess a relatively elongate rostrum, Longipteryx chaoyangensis (Zhang et al., 2001), Longirostravis hani (Hou et al., 2004) and a yet unnamed taxon (Morschhauser et al., 2006). These taxa share several characteristics (elongate rostrum [here defined as a preorbital length equal to or exceeding 60% of the total skull length]; upper dentition limited to the premaxilla; lower dentition restricted to the rostral tips of the dentaries; dentary long, slender, and ventrally concave) (Hou et al., 2004; Zhang et al., 2001), which suggests they form a monophyletic group of trophically specialized enantiornithine birds (Chiappe et al., 2006). Here we describe a new taxon, Shanweiniao cooperorum, and provide phylogenetic support for the formation of such a clade. Institutional Abbreviations: DNHM, Dalian Natural History Museum; PKUP, Peking University. Anatomical Abbreviations: See Appendix 1. * Corresponding author SYSTEMATIC PALEONTOLOGY Aves Linnaeus, 1758 Pygostylia Chiappe, 2002 Enantiornithes Walker, 1981 Longipterygidae, Zhang et al Phylogenetic Definition The most recent common ancestor of Longipteryx chaoyangensis and Longirostravis hani and all its descendents. Included Taxa Longipteryx chaoyangensis (Zhang et al., 2001), Longirostravis hani (Hou et al., 2004), Shanweiniao cooperorum, and a yet unnamed new taxon here referred to as DNHM D2522 (Morschhauser et al., 2006). Stratigraphic Distribution Yixian and Jiufotang Formations of the Jehol Group, Lower Cretaceous, Ma (Swisher et al, 2002; He et al., 2004; Zhu et al., 2007). Geographic Distribution Chaoyang, Lingyuan and Yixian, western Liaoning Province, northeastern China. Diagnosis Small to medium-sized enantiornithine birds with the rostral portion of the skull equal to or exceeding 60% the total skull length; dentition restricted to the premaxilla and rostral-most portion of the dentary; coracoid with nearly straight lateral margin. Shanweiniao cooperorum gen. et sp. nov. (Figs. 2, 3) Holotype A nearly complete and largely articulated adult individual preserved in a slab (Fig. 2) and counterslab (Fig. 3). The bones contained in the slab, DNHM D1878/1, are exposed primarily in ventral view, while those in the counterslab, DNHM D1878/2, are mainly exposed in dorsal view. Feathers are preserved as carbonized traces concentrated around the head, wings, and tail. Locality and Horizon Lingyuan, Liaoning Province, China. Dawangzhangzi Bed, middle Yixian Formation, Lower Cretaceous (Swisher et al., 2002). 188

2 O CONNOR ET AL. SPECIALIZED CLADE OF CRETACEOUS ENANTIORNITHINE BIRDS 189 FIGURE 1. Map of Liaoning, China showing longipterygid fossil localities. The new taxon comes from the Lingyuan locality; Longirostravis hani was collected near Yixian; and several Longipteryx specimens and an undescribed longipterygid (DNHM D2522) were found near Chaoyang. Etymology Shan wei niao ( ) meaning fan-tail bird in Chinese, refers to the fact that this specimen preserves the first known occurrence of an enantiornithine fan-shaped feathered tail. The species name is in honor of Carl and Lynn Cooper for their generous support in the study of Mesozoic birds from China. Diagnosis Shanweiniao cooperorum is a longipterygid enantiornithine that possesses the unique combination of the following characters: elongate cranium that is 62% rostrum (64% in Longipteryx; 60% 64% in Longirostravis; 65% in DNHM D2522); second phalanx of manual major digit reduced and wedge-shaped (as in Longirostravis and DNHM D2522; unreduced in Longipteryx); omal one-third of the clavicular rami dorsally curved; acute (40 ) interclavicular angle (70 in Longipteryx; 55 in Longirostravis and DNHM D2522); clavicular symphysis broad, exceeding the hypocleidium in length (short in Longipteryx, Longirostravis and DNHM D2522); sternum with simple distal expansions of the lateral trabeculae (similar to Longipteryx; forked in DNHM D2522; branching, moose-horn morphology in Longirostravis); intermembral index (humerus + ulna/femur + tibia) of 1.23 (Longipteryx = ; Longirostravis = 1.07; DNHM D2522 = 1.09); tarsometatarsus with metatarsal III longest and cranially convex, closely approached in length (in order) by metatarsals IV and II (as in Longirostravis and DNHM D2522; IV longest in Longiptyerx); longitudinal crest on central portion of pedal unguals; pedal unguals relatively unrecurved with large, curved keratinous sheaths; tail composed of at least four elongate rectrices. ANATOMY Anatomical nomenclature primarily follows Baumel and Witmer (1993) using the English equivalents of the Latin terms; certain structures not named therein follow Howard (1929). The skull is preserved in lateral view (Fig. 4). It is elongate and delicate. The rostrum (measured from the rostral margin of the orbit to the rostral margin of the premaxilla) constitutes 62% of the total skull length, slightly less than the percentage in Longipteryx and DNHM D2522 (estimated at 64% and 65% respectively), and within the range estimated for Longirostravis, 60% 64%. The rostralmost tip of the skull is obscured where it abuts wing bones, but the presence of alveoli and small teeth can be confirmed in the premaxilla, indicating that this bird shares with Longipteryx, Longirostravis and DNHM D2522 an edentulous maxilla but dentigerous premaxilla. A lone tooth can be identified near the rostral portion of the left dentary (Fig. 4), but there is no evidence of either teeth or alveoli along the majority of the length of the dentaries, indicating that mandibular teeth, if present, were also restricted to the rostral portion of the dentary, as in Longipteryx, Longirostravis and DNHM D2522. The proximally restricted teeth and overall delicate and elongate nature of the skull suggests that Shanweiniao may also have occupied the mudprobing niche of Longirostravis (Hou et al. 2004). Distal rhynchokinesis is often associated with this ecological adaptation, however preservation makes it difficult to determine if Shanweiniao (or Longirostravis) possessed this specialization. The mandibles are straight (Fig. 4), lacking the ventral curvature present in Longipteryx, Longirostravis and DNHM D2522. Both dentaries are visible in DNHM D1878/2; the left mandible is preserved in lateral view; a lateral groove paralleling the tomial edge is marked with small, oval foramina. The right mandible is preserved in medial view; although crushed, the articulation between the dentary and surangular is visible. Fragments of thin, curved bones interpreted as the angular bones are preserved below the mandible and above the right dentary (Fig. 4). In DNHM D1878/1, the distal end of the left dentary is visible; the caudal end shows the caudoventrally slanted and tapered ancestral condition (unforked), seen in Archaeopteryx lithographica (Elzanowski, 2001) as well as in Longipteryx, Vescornis hebeiensis and Eoenantiornis buhleri (Zhang et al., 2004; Zhou et al., 2005). The frontal processes of the premaxilla are long, approaching 50% the length of the skull, but the ends are not preserved and their caudal extent cannot be ascertained. The jugal bar is preserved but displaced dorsally, obscuring the morphology of the antorbital fenestra. The proximal end of the jugal is covered by the broken and displaced proximal end of the left dentary. The external nares appear to be retracted caudal to the rostralmost one-third of the skull. A curved bony margin preserved rostral to the orbit is interpreted as the displaced caudal margin of the nares. The exact size, shape, and relative position of the nares and antorbital fenestra cannot be determined. A sclerotic ring is preserved, but the number of individual ossicles is impossible to discern. Vertebral Column Approximately ten poorly preserved but articulated vertebrae are preserved in ventral (DNHM D1878/1) and dorsal (DNHM D1878/2) view extending from the base of the skull to a point near the interclavicular symphysis (visible in DNHM D1878/1). The last vertebra appears to be associated with a long rib and is thus interpreted as a thoracic vertebra. The exact position of the cervico-thoracic transition is difficult to determine due to the poor preservation where the vertebrae intersect the pectoral girdle. The cervical series (eight or nine preserved plus an atlas, for a minimum of nine total) appears to be at least partially heterocoelous: in the seventh cervical, the cranial articular surface is visible and transversely concave. Nevertheless, the exact degree of heterocoely present throughout the cervical series cannot be determined. A small tubercle present on the dorsal margin of cervical five is interpreted as a spinous process. The postzygapophyses of the cervical vertebrae are well developed and project caudally beyond the vertebral body by a distance that exceeds one-third the vertebral bodies total

3 190 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 29, NO. 1, 2009 FIGURE 2. Holotype (DNHM D1878/1) of Shanweiniao cooperorum. A, photo; B, camera lucida drawing. See Appendix 1 for anatomical abbreviations. length. The shorter prezygapophyses extend cranially half the lengths of their caudal counterparts. Cervicals five, six, and seven preserve thin, elongate caudally directed ribs. The longest (cervical five) approaches the length of the vertebra. Two poorly preserved thoracic vertebrae are preserved distal to the sternum in DNHM D1878/2. A portion of the synsacrum is displaced caudally between the two femora and exposed in ventral view in DNHM D1878/1 (Fig. 3). The piece consists of two or three fused vertebrae; the transverse processes are long, equal to the width of the vertebral body, and project perpendicular to the axis of the synsacrum. The distal ends of the transverse processes are wider than the proximal portion. A groove preserved along the midline is interpreted as a ventral sulcus, present also in DNHM D2522. The pygostyle is broken; proximally, the right side bears a process reminiscent of the dorsal fork present in enantiornithine taxa such as Longipteryx and Halimornis thompsoni (Chiappe et al., 2002). The distal margin is broken, although it was clearly constricted, as in Longipteryx, DNHM D2522, and Halimornis. In DNHM D1878/2, a pair of keel-like processes are visible running parallel down the midline of the pygostyle; preservation makes it difficult to ascertain in what view the pygostyle is preserved, so these processes cannot be definitively correlated to the paired ventral keels seen in the pygostyle of Halimornis (Chiappe et al., 2002). The number of vertebrae constituting the pygostyle likewise cannot be determined. Short, parallel sternal rib segments are preserved on either side of the sternum. Visible thoracic rib segments appear to lack ossified uncinate processes on both slabs. Gastralia, however, are present, preserved disarticulated between the sternum and the pelvis. Pectoral Girdle Both coracoids, preserved in articulation with the sternum, are strut-like. The sternal margin of each is concave as in Longipteryx, not straight as in Longirostravis and DNHM D2522. The lateral margin is straight to slightly concave, as in Longirostravis and Longipteryx, not strongly convex as in some enantiornithines (e.g., Concornis lacustris; Sanz et al., 1995). In DNHM D1878/2, a slight depression embays on the dorsal surface of the coracoid. This depression is weak, unlike the deep fossae present in some other enantiornithine taxa (Chiappe and Walker, 2002) and the basal ornithuromorph Apsaravis ukhaana (Clarke and Norell, 2002). The coracoid neck is simple, lacking a procoracoid process, as in other enantiornithines. No supracoracoideus nerve foramen is visible.

4 O CONNOR ET AL. SPECIALIZED CLADE OF CRETACEOUS ENANTIORNITHINE BIRDS 191 FIGURE 3. Holotype (DNHM D1878/2) of Shanweiniao cooperorum. A, photo; B, camera lucida drawing. See Appendix 1 for anatomical abbreviations. In ventral view (DNHM D1878/2), the right scapula is preserved in articulation with the right coracoid such that the blade is covered by the coracoid. It possesses a robust and elongate acromion as in the Spanish enantiornithine Eoalulavis hoyasi (Sanz et al., 2002). In DNHM D1878/1 the scapular blade is exposed in costal view; it is long and straight, with no visible groove like those reported in the enantiornithine material from El Brete, Argentina (Chiappe and Walker, 2002). The scapula exceeds the coracoid in length (Table 1; org/publications/jvpcontent.cfm); the distal end is blunt, tapering only very slightly. The furcula of Shanweiniao, preserved in DNHM D1878/1, is narrow and Y-shaped. The morphology and exact length of the long, poorly preserved hypocleidum relative to the rami cannot be determined, though it is clearly at least one-third the length of either ramus. The rami are each approximately as long as the coracoid. The omal one-third of each ramus is curved dorsally and the omal tip slightly expanded. The caudal end of the clavicles fuse over a broad area so that the symphysial portion of the furcula is nearly the same length as the hypocleideum unlike other longipterygids in which the symphysis is restricted. The interclavicular angle is acute, describing an angle of approximately 40, which is much smaller than those of Longirostravis (55 ) and Longipteryx (70 ). The sternum is preserved in dorsal view (DNHM D1878/2) but badly broken (Fig. 3). The proximal portion is displaced along a small crack that runs diagonally through the element, offsetting the proximal portion of the right humerus as well (DNHM D1878/2). The cranial margin of the sternum appears to be rounded. A large, well-preserved piece clearly shows the morphology of the right lateral and medial trabeculae, as well as the morphology of the midline, which allows a confident reconstruction of its overall morphology (Fig. 5). The distal end of the right lateral trabecula is slightly broken, but the morphology appears to have been relatively simple: straight with the distal end only slightly expanded. The cross-sectional profile of each lateral trabecula appears to change from dorsoventrally oval proximally to spatulate at the distal end, but this may be a preservational artifact. The branching, moose-antler morphology of Longirostravis remains an autapomorphy of that taxon. The medial trabeculae are smaller than the lateral trabeculae (onethird the length); their axes are slightly angled medially with

5 192 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 29, NO. 1, 2009 FIGURE 4. Skull of Shanweiniao cooperorum in left lateral view. A, photo; B, camera lucida drawing. See Appendix 1 for anatomical abbreviations. respect to the lateral trabeculae and sternal midline. The caudal margin of the sternum forms a relatively long xiphoid process, which projects slightly farther caudally than the lateral trabeculae, unlike Longirostravis and DNHM D2522 in which the lateral trabeculae project furthest. Distally as in the lateral trabecula, the xiphoid process expands slightly, and its distal margin is straight. The distal end of the sternum, like Longipteryx, Longirostravis and DNHM D2522, is imperforate, as opposed to the condition in several ornithuromorph taxa (e.g., Yixianornis, Yanornis, and Songlingornis linghensis; Clarke et al., 2006). Forelimb The humerus is only partially preserved on both sides. The proximal tip of the right humerus is preserved in DNHM D1878/2, broken and displaced; its head displays the typical enantiornithine condition: concave on the midline, rising dorsally and ventrally. The left humerus, preserved in pieces in both slabs, indicates that the deltopectoral crest was weak, transversely measuring less than the width of the shaft. The distal margin (right humerus DNHM D1878/1) is angled, but not as strongly as in some enantiornithines (e.g., Longipteryx, DNHM D2522, Alexornis antecedens; Brodkorb, 1976). The ulna and radius are nearly equal in length. The ulna is robust, approaching the width of the humeral shaft. The ulna is slightly bowed, creating a proximal interosseous space between it and the radius that closes distally. The dorsal and ventral condyles and the carpal tuberosity are visible on the distal end of the left ulna (DNHM D1878/2). The two condyles are weakly developed and not separated by a deep sulcus; the dorsal condyle appears to form a semilunate ridge. The carpal tuberosity (also visible on the left side of DNHM D1878/2) is prominent and not separated from the ventral condyle by an incisure as in some neornithines (e.g., Cathartes). The radius is straight and half the width of the ulna. The presence of a longitudinal groove cannot be determined. A cup-shaped structure preserved between the ulna, radius and underlying metacarpal bones is interpreted as the carpal trochlea of the carpometacarpus. An indeterminate fragment located dorsolateral to the right manus in DNHM D1878/2 may represent at least part of the ulnare or radiale. The manus of Shanweiniao is reduced, as in Longirostravis and DNHM D2522; based on the available specimen, it appears to be x-x but is likely to have been x-x, as in Longirostravis, which has two phalanges in the minor digit (personal observation), the second being extremely reduced. The alular digit is covered proximally by the ulna and radius. The distal half of the first phalanx of the alular digit is straight (DNHM D1878/2). A fragment in DNHM D1878/1 is interpreted as the second phalanx; it is reduced and wedge-shaped, approximately the same size as the lone preserved minor digit phalanx. The major digit possesses two phalanges, lacking the large claw present in Longipteryx. The first phalanx is cylindrical and not dorsoventrally expanded as in more advanced birds (e.g., Gansus yummenensis, Neornithes; You et al., 2006). The distal phalanx is wedge-shaped and tapers distally. The single preserved phalanx of the minor digit is cylindrical in shape and approximately the same size as the distal phalanx of the major digit. The relative lengths of the metacarpals and the degree of fusion present in the carpometacarpus cannot be determined. Pelvic Girdle The pelvic girdle is preserved primarily in DNHM D1878/2, including portions of both ilia and pubes (Fig. 3). Details of the preacetabular wings of the ilia are not clear. The right acetabulum contains the broken head of the right femur. The pubic peduncle is long, but it cannot be ascertained if the new taxon shares the same laterally compressed and hooked condition seen in Longirostravis and Longipteryx. The postacetabular wing of TABLE 1. Selected measurements of longipterygid taxa in millimeters. Longipteryx (holotype) Longirostravis Shanweiniao Right Left Right Left Right Left Skull, length (32.86) (32.69) Rostrum (35.45) (24.57) (19.59) Coracoid (11.86) (12.61) Furcula (10.17) (8.95) (8.44) Humerus (42.03) (25.71) (21.31) Ulna (47.1) (25.14) (24.11) (23.36) Radius (43.48) (21.43) (22.53) Femur (17.6) Tibiotarsus Tarsometatarsus Pygostyle (12.37) Measurements of Shanweiniao are a composite of the slab and counterslab; see Table 1S ( for a complete table of measurements. Parentheses denote estimated measurements; incomplete bones and measurements have been omitted.

6 O CONNOR ET AL. SPECIALIZED CLADE OF CRETACEOUS ENANTIORNITHINE BIRDS 193 FIGURE 5. Reconstruction of longipterygid sterni. A, Shanweiniao cooperorum; B, Longirostravis hani; C, Longipteryx chaoyangensis. the left ilium, preserved in lateral view, is directed slightly ventrally, rendering the ventral margin concave. The distal end tapers, as in Longipteryx, DNHM D2522 and other enantiornithines (e.g., Eoalulavis, Cathayornis yandica; Zhou, 1995; Sanz et al., 2002). The ventral margin of the ilium bears a large, rounded antitrochanter. The delicate, rod-like pubis is short, approximately equal to the femur in length, but more than twice the length of the postacetabular wing of the ilium. The pubes together form a caudally directed V, indicating that they were likely retroverted. The distal ends, not preserved, approach each other as if to form a short pubic symphysis. Hindlimb The hindlimb is shorter than the forelimb. The intermembral index (ImI, measured as the sum of the lengths of the humerus and ulna divided by the sum of the lengths of the femur and tibiotarsus) measures approximately 1.23, intermediate between Longipteryx (ImI = 1.5) and Longirostravis (ImI = 1.07). In DNHM D1878/2, the left femur is preserved in ventral view and the right femur in lateral view. The proximal portions of both femora are preserved in DNHM D1878/1. The femur is shorter than the tibiotarsus and longer than the tarsometatarsus. The femur is slightly bowed craniocaudally; the head is round and directed at an angle of 90 from the shaft. The trochanteric crest is separated from the femoral head by a distinct neck and projects farther proximally than the femoral head. Distally, both a fossa for the insertion of the capital ligament and a patellar groove are absent. In lateral view, the femur lacks the crest present in some enantiornithines (e.g., the El Brete material; Chiappe, 1996). A tibiofibular crest is also absent. Both tibiotarsi are preserved in DNHM D1878/2. Each is more than double the length of the tarsometatarsus. The right tibiotarsus is preserved in lateral view and the left in caudal view. The lateral condyle is visible on the distal end of the right tibiotarsus. A faint fibular crest is visible 4 mm from the proximal end of the right tibiotarsus. The fibula is preserved in articulation with the left tibiotarsus in DNHM D1878/2. Its proximal end is wedge shaped. The void of the left fibula in DNHM D1878/1 indicates the bone extended for at least half the length of the tibiotarsus. The feet are primarily preserved in dorsal view in DNHM D1878/1 and in plantar view in DNHM D1878/2, although voids of the dorsal view are also preserved in DNHM D1878/2. The tarsometatarsus is short and proximally fused, with suture lines visible distally. Proximally, an intercotylar eminence is absent. The metatarsals are subequal in mediolateral width. Distally, the third metatarsal extends farthest but is closely approached by the fourth. The second metatarsal is the shortest, but it approaches the metatarsal IV in length: the distal end of metatarsal II surpasses the proximal end of the trochlea of metatarsal IV. Metatarsal I, preserved in lateral view, is reversed and transversely compressed with a convex dorsal margin; whether it was straight or J-shaped cannot be determined. Metatarsal I articulates low on the tarsometatarsus, as in Longipteryx and DNHM D2522. A dorsal tubercle on metatarsal II lies approximately 3 mm from the proximal end of the tarsometatarsus. As in Longipteryx, it appears to be located more on the dorsal surface as opposed to the lateral surface of metatarsal II (e.g. Apsaravis; Clarke and Norell, 2002). Metatarsal III is transversely convex in dorsal view. In DNHM D1878/2, the left tarsometatarsus is preserved in caudoventral view, and it is clear that a hypotarsus is absent. The feet are visible on both sides of both slabs; the left foot in DNHM D1878/1 preserves the most information (Fig. 6). The pedal phalangeal formula is x. The first phalanx of the hallux is longer and more slender than the proximal phalanges of digits II IV. In each digit, the penultimate phalanx is longer than the preceding phalanges. The total length of each digit, except the hallux, exceeds the length of the tarsometatarsus. The first phalanx of the second digit is approximately two-thirds the length of the penultimate phalanx, which is the longest phalanx in the entire foot. The two proximal phalanges of digit III are approximately equal to each other and shorter than the proximal phalanx in digit II. The penultimate phalanx in digit III is shorter than that of digit II. The proximal three phalanges in digit IV are subequal and each is approximately half the length of the penultimate phalanx. The claws of all the pedal digits (including the hallux) are large, subequal in size and triangular in shape, lacking both strong degrees of curvature and strong flexor tubercles. The ungual of digit II, including its keratinous sheath, nearly exceeds the length of the preceding two phalanges combined. A longitudinal crest on the central portion of the claw is visible in the best-preserved unguals (left digits II and IV on DNHM D1878/1 and digit III in DNHM D1878/2) and is also present in DNHM D2522 and ornithomimid theropods (e.g., Struthiomimus altus, Harpymimus okladnikovi; Makovicky et al., 2004). Horny sheaths are preserved, giving the claws a sickle shape. The sheaths appear to lack the distal constrictions FIGURE 6. Left tarsometatarsus of Shanweiniao cooperorum in dorsal view. A, photo; B, camera lucida drawing. See Appendix 1 for anatomical abbreviations.

7 194 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 29, NO. 1, 2009 interpreted as wear facets seen in some climbing birds (e.g., Picoides; personal observation). Integument Carbonized traces of feathers are preserved throughout the specimen. Wing feathers are present, but the exact number of primaries and secondaries is difficult to determine. The outermost contour (presumed to be a primary) is estimated at 82 mm, measured from the impression of the right wing in DNHM D1878/1. Tail rectrices are preserved in both slabs and are most clearly visible in DNHM D1878/1 (Fig. 7). Four rachises are clearly discernable, indicating the presence of four vaned, long and slender rectrices. The feathers are incomplete, missing their proximal and distal ends. The vanes of these rectrices are parallel to one and another and directed toward the pygostyle. The outline of the right wing is complete in DNHM D1878/1 and does not overlap the retrices. The vanes of the wing feathers are also directed at a different angle from that of the tail feathers, and thus we feel confident that these feather impressions do not represent wing elements. Because the distal most ends are not preserved, it cannot be determined if the rectrices were graded, as in Yixianornis. Upper- and under-tail covert feathers are also preserved on the lateral margins of the pygostyle. DISCUSSION Phylogenetic Relationships of Shanweiniao The phylogenetic position of Shanweiniao cooperorum was determined using a modified version of the Chiappe (2002) dataset. The updated character list includes characters from Clarke (2002), enantiornithine characters from Chiappe and Walker (2002), as well as several new characters (Appendix 2). Twentynine taxa were scored for 242 characters. Thirty-one characters were considered ordered; twenty-three characters were removed as uninformative with the current taxonomic sample (Appendix 2). Neornithes was represented by Anas platyrhynchos and Gallus gallus, and Dromaeosauridae was used as the outgroup. The matrix (Appendix 3) was run using NONA (Goloboff, 1993); optimal trees were identified using five random addition sequence replications of taxa, each followed by Tree Bisection Reconnection (TBR) branch-swapping and 100 iterations of jackknife ratchet, collapsing the trees on TBR rearrangements. The result was 20 most parsimonious trees of 588 steps. The trees differ within the relative placement of taxa within the enantiornithine and ornithuromorph clades. The strict consensus tree (Fig. 8) places Shanweiniao within Enantiornithes; the results agree with previous analyses (Clarke et al., 2006; Zhou et al., 2005) in the placement of most taxa with a few significant differences. Zhongornis (Gao et al., 2008), Jeholornis, and Archaeopteryx form consecutive outgroups of Pygostylia with the confuciusornithid clade and Sapeornis as basalmost pygostylians. Previous analyses have differed in the placement of Sapeornis and Confuciusornis, placing the former in a more basal position (Clarke et al., 2006; Zhou and Zhang, 2002) and the latter as more derived, within the ornithothoracine clade (Zhou and Zhang, 2002). The results of this analysis place Sapeornis in a more derived position than Confuciusornithidae, as sister taxon to Ornithothoraces. A large ornithothoracine clade is formed by Enantiornithes and Ornithuromorpha. Enantiornithine taxa form two polytomies; the basal polytomy consists of Shanweiniao, Longirostravis, Longipteryx and DNHM D2522, a relationship supported by three unambiguous synapomorphies (see Supplementary Data 2 at: JVPContent.cfm): upper dentition restricted to the premaxilla FIGURE 7. Tail rectrices of Shanweiniao cooperorum in DNHM D1878/1. A, photo; B, camera lucida drawing.

8 O CONNOR ET AL. SPECIALIZED CLADE OF CRETACEOUS ENANTIORNITHINE BIRDS 195 FIGURE 8. Strict consensus cladogram illustrating the phylogenetic position of Shanweiniao cooperorum. Tree length: 588 steps; consistency index = 48, retention index = 68. (5:1) with maxillary teeth absent (8:1) and the presence of a distally constricted pygostyle (79:1; a character known to occur in other enantiornithines) (Fig. 9). Within the longipterygids, Longirostravis and DNHM D2522 form a more exclusive clade supported by two unambiguous synapomorphies: the presence of outer sternal trabecula that project distally farther than the sternal midline (111:2) and an ulna/radius shaft width ratio that is greater than 0.7 (144:0, a reversal). A longipterygid clade formed by Longipteryx and Longirostravis was previously supported (Chiappe et al., 2006) by two synapomorphies: teeth restricted to the premaxilla and the presence of an elongate rostrum. The current analysis expands the anatomical support for the close relationship of these taxa and provides evidence for the existence of a more diverse longipterygid clade (Table 2). The second enantiornithine polytomy groups the remaining taxa together, a result not surprising considering the results of previous Mesozoic bird analyses (Clarke et al., 2006; Chiappe, 2002) and even enantiornithine specific matrices (Chiappe and Walker, 2002; Chiappe, et al., 2006) have failed to bring much resolution to the clade. The relationships between enantiornithine taxa presented in Zhou et al., (2006) are not supported here, which may be due to the inclusion of a larger number of enantiornithine taxa in the current analysis. Ornithuromorpha, a clade previously well resolved (Clarke et al., 2006; Chiappe, 2002), has resulted in a polytomy, indicating the need for further work on this part of the tree. The consensus tree places Patagopteryx basal to a large polytomy with the Early Cretaceous Chinese hongshanornithids (Hongshanornis longicresta + PKUP V1069; O Connor et al., in review) and ornithurines nested within (Fig. 8). Within Ornithurae, Neornithes falls in a polytomy with Hesperornis regalis and Ichthyornis dispar; the Carinatae (Ichthyornis + Neornithes) clade is not supported here. Enantiornithine Tail Morphology Shanweiniao preserves an elongate tail composed of at least four closely aligned rectrices, a previously unrecorded morphology. The typical enantiornithine tail morphology consists of short coverts covering the pygostyle (Clarke et al., 2006), however, several species have been described possessing elongate streamer-like tail feathers; Protopteryx fengningensis (Zhang and Zhou, 2000) and Dapingfangornis sentisorhinus (Li et al., 2006) possess paired tail feathers, similar to those preserved in some specimens of Confuciusornis (Chiappe et al., 1999) and the recently named Paraprotopteryx gracilis has been described with four (Zheng et al., 2007). The elongate rectrices

9 196 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 29, NO. 1, 2009 preserved in Paraprotopteryx, Protopteryx, Dapingfangornis, and Confuciusornis have often been interpreted as display structures, a conclusion that is supported here due to their length, splay and morphology (Chiappe et al., 1999; Zhang and Zhou, 2000; Li et al., 2006; Zheng et al., 2007). The distal ends of the feathers in these taxa are preserved widely spaced, not closely aligned along their lengths in such a way as to create a surface capable of acting as an airfoil and thus generate any aerodynamic benefit. In contrast, the rachi of the feathers preserved in Shanweiniao are closely arranged forming a continuous surface indicating that this tail had the potential to act as an airfoil and generate lift. Whether the function of the tail in Shanweiniao was aerodynamic or for display is unclear given the incomplete preservation of the holotype, however the presence of this morphology among enantiornithines reveals the possibility that some members of the clade had evolved this aerodynamic specialization. Among Mesozoic birds, Shanweiniao represents the second known occurrence of a tail morphology capable of generating substantial lift. Fan-shaped tails in Mesozoic taxa have only been previously reported in a single specimen, the holotype of Yixianornis grabaui, a basal ornithurmorph (Clarke et al., 2006). Since the only previously known fan-shaped tail was associated with the anatomically modern, plough-shaped pygostyle of Yixianornis (also found in modern birds), it was suggested that the fan-shaped tail and the bulb rectricium, which controls the fanning motion of the tail rectrices in modern birds, coevolved with the ornithuromorph plough-shaped pygostyle morphology, and that the bulb rectricium was absent in the pygostyles of more basal birds, such as enantiornithines and confuciusornithids (Clarke et al., 2006). The presence of a fan-shaped tail outside the Yixianornis + Neornithes clade suggests that while it cannot be determined if a bulb rectricium was present in basal birds, it cannot be ruled out based on pygostyle or rectricial morphology. The retricial morphology of Shanweiniao suggests the taxon may have possessed as bulb rectricium, which would imply the structure was not restricted to Ornithuromorpha. Whether the bulb rectricium was present in other enantiornithines (or widespread among), or whether the structure, if present, was homologous to that of modern birds, cannot be determined with out additional specimens and a phylogenetic tree with higher resolution. Enantiornithines were first envisioned to be poor fliers (Walker, 1981) but the discovery of Neuquenornis (Chiappe, 1991) abolished this notion. The discovery of a specimen with an alula lent support to the latter view, that enantiornithines were able fliers, comparable to modern birds (Sanz et al., 1996). A fanshaped tail greatly increases aerodynamic performance through increased lift and maneuverability (Gatesy and Dial, 1996). The tail morphology preserved in Shanweiniao is the first among enantiornithines, and the second among Mesozoic avians, to suggest the evolution of enhanced flight capabilities in the caudal region. Enantiornithines developed flight specializations beyond the skeletal level, having developed integumentary aerodynamic specializations as well. This supports the hypothesis that enantiornithines possessed sophisticated aerodynamic abilities, which likely facilitated their rapid diversification within the Cretaceous (Chiappe, 2007). FIGURE 9. Stratigraphic column showing the longipterygid bearing formations of the Jehol Group (modified from Zhou and Zhang, 2002; ages in Ma) and the temporal distributions of these taxa. Each taxon is known only from point distributions; the elongate ranges indicate the lack of information regarding the exact bed from which the fossil was extracted. Longirostravis hani is known only to come from the Yixian Formation, while Shanweiniao cooperorum is known to come from the Dawangzhangzi bed of the Yixian. ACKNOWLEDGMENTS We thank S. Abramowicz for preparing the illustrations, G. Takeuchi and D. Goodreau for the preparation of the specimen, Zhou Z-H. and Zhang F-C. for kindly granting access to comparative material at the Institute of Vertebrate Paleontology and Paleoanthroplogy and D. Varricchio for providing access to unpublished material. We also thank J. Harris and two anonymous reviewers for reading the manuscript and providing useful

10 O CONNOR ET AL. SPECIALIZED CLADE OF CRETACEOUS ENANTIORNITHINE BIRDS 197 TABLE 2. Measurements of the % rostrum across Mesozoic avian taxa. Taxon Collection # Clade % Rostrum Archaeopteryx, Thermopolis WDC-CSG-100 Archaeopterygidae 55 Archaeoperyx, berlin HNM1880/81 Archaeopterygidae 50 Archaeopteryx, Eischtatt Archaeopterygidae 55 Confuciusornis sanctus GMV 2130 Confuciusornithidae 52 Confuciusornis sanctus GMV 2132 Confuciusornithidae 54 Sapeornis chaoyangensis DNHM D2523 Aves 54* Cathayornis indet. DNHM D9769 Enantiornithes 47 53* Protopteryx fengningensis IVPP V11665 Enantiornithes 47 55* Vescornis hebeiensis NIGP CAS Enantiornithes 55* Shanweiniao cooperorum DNHM D1878 Enantiornithes 62* Longipteryx chaoyangensis IVPP V12352 Enantiornithes 64 Longipteryx chaoyangensis IVPP V12552 Enantiornithes 64 Longipterygidae n sp. DNHM D2522 Enantiornithes 60* Hongshanornis longicresta IVPP V14533 Ornithuromorpha 53* Yanornis martini IVPP V13358 Ornithuromorpha 55 58* The number of taxa for which this measurement can be taken is limited; skulls must not only be reasonably well preserved (to determine the level of the lacrimal) but also preserve the lateral view and be articulated. An asterisk indicates that the specimens did not completely fulfill the above criteria and thus represent estimates. All specimens fall within 50% 70%, the mesorostrine range (with the exception being the lower end estimates of a few enantiornithines; the averages for these taxa however, do fall within the mesorostrine range) thus suggesting that the mesorostral condition is basal within Aves (Marugan-Lobon and Buscalioni, 2003). edits and comments. This research was funded by the National Science Foundation (DEB ) and support from Carl and Lynn Cooper, Ron and Judy Perlstein, and Richard and Eileen Garson. LITERATURE CITED Baumel, J. 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Anatomical abbreviations. al alular metacarpal nas nasal al I first phalanx, alular digit pmx premaxilla al II second phalanx alular digi pub pubis ang angular pyg pygostyle cau caudal vertebrae qd? possible quadrate cmc carpometacarpus rad radius cor coracoid rdn right dentary crv cervical vertebrae rib ribs den dentary sca scapula fem femur scl sclerotic ring fur furcula stn sternum hum humerus sur surangular ili ilium syn synsacrum jug jugal tbt tibiotarsus ldn left dentary thv thoracic vertebrae mac major metacarpal tmt tarsometatarsus ma I first phalanx, major digit tth teeth ma II second phalanx, major digit uln ulna max maxilla I - IV pedal digits mi I first phalanx, minor digit I-(1-5) pedal phalanges mt I metatarsal one APPENDIX 2. Description of characters used in phylogenetic analysis. The matrix was run using 242 characters. Of these, 17 represent original characters (5, 10, 41, 43, 46, 47, 67, 70, 71, 79, 101, 110, 111, 161, 176, 234, 242), 110 are from the Chiappe (2002) dataset (2, 3, 6, 11, 14, 17-23, 26-28, 30, 36, 38, 48-51, 53, 54, 56, 60-63, 69, 72, 73, 75, 76, 82-84, 86, 89-91, 96, 106, 109, 112, 113, 115, , 124, 126, 132, 134, 135, 139, 143, 144, , 153, 157, 163, , , 179, , 192, 194, 195, , 204, , , 223, 224, 227, 229, 230, 236, 239, 241), 77 are from the Clarke (2002) dataset (8, 12, 13, 15, 16, 24, 25, 29, 31-35, 37, 39, 40, 42, 44, 45, 52, 57, 58, 64, 65, 68, 74, 80, 81, 85, 87, 95, 97, 99, 100, 108, 114, 116, 119, 127, 128, 130, 133, , , , 151, 152, , , 162, 164, 169, 178, 180, 181, 193, 203, , 225, 228, 232, 233, 235), eight are from the Chiappe and Walker (2002) dataset (78, 88, 94, 98, 102, 222, 237, 238), two are from the Clarke et al. (2006) dataset (118, 129) and four are from the Chiappe et al. (2006) dataset (55, 77, 175, 196). An additional ten characters from the Chiappe (2002) dataset have been included in modified form (9, 66, , 107, 117, 177, 190, 205), as have seven from Clarke (2002) (7, 59, 123, 131, 191, 206, 231) and six that are modified versions of similar characters from both Clarke (2002) and Chiappe (2002) (1, 4, 92, 93, 125, 226). During the analysis, 23 characters (16, 31, 34, 39, 40, 63, 64, 66, 69, 72, 74, 78, 99, 100, 101, 102, 118, 179, 186, 193, 195, 206, and 222) were excluded on the basis they are uninformative in this analysis. Thirty-one characters are considered ordered (see below). Skull and Mandible 1. Premaxillae in adults: unfused (0); fused only rostrally (1); completely fused (2). (ORDERED) 2. Maxillary process of the premaxilla: restricted to its rostral portion (0); subequal or longer than the facial contribution of the maxilla 3. Frontal process of the premaxilla: short (0); relatively long, approaching the rostral border of the antorbital fenestra (1); very long, extending caudally near the level of lacrimals (2). (ORDERED) 4. Premaxillary teeth: present throughout (0); present but rostral tip edentulous (1); absent (2). 5. Upper dentition restricted to premaxilla: absent (0); present 6. Caudal margin of naris: far rostral than the rostral border of the antorbital fossa (0); nearly reaching or overlapping the rostral border of the antorbital fossa 7. Naris longitudinal axis: considerably shorter than the long axis of the antorbital fossa (0); subequal or longer We are using the longitudinal axis of these structures as a proxy for their relative size. The longitudinal axis is often easier to measure than the actual area enclosed by either the naris or the antorbital fossa. 8. Maxillary teeth: present (0); absent 9. Dorsal (ascending) ramus of the maxilla: present with two fenestra (the promaxilllary and maxillary fenestra) (0); present with one fenestra (1); absent (2). (ORDERED)