taxonID	type	description	language	source
0389AA11FFBDFFF5FCF3F934FE7FF901.taxon	description	Morphology. All the morphological variation that has been previously used to either support the existence of two species, or the synonymization of C. yandica under S. santensis, is reviewed here. Some previously published data are questioned and alternative interpretations of the preserved features are provided. Note that, although a size difference of approximately 10 % is present between the two holotype specimens (Sereno et al., 2002; Zhou & Hou, 2002), because the ontogenetic stage of each specimen is unknown (this aspect of enantiornithine biology also remains unclear) this difference should not be considered diagnostic until further explored through histological analysis. Furthermore, this size differential falls well within the typical size range for a comparably-sized species of living bird (Dunning, 1993). Skull. Despite the very poor preservation of the skull in the S. santensis holotype, from which little or no information can actually be gained (Figs 1, 3 B), Sereno et al. (2002) compared the morphology of the nasals and maxilla of the two specimens (Fig. 3). Both were described as possessing broad nasals which expand caudal to the external nares and have a triangular caudal margin (Sereno et al., 2002). We concur that the nasal of the holotype of C. yandica is wellpreserved; the nasals articulate medially for approximately half their length, separated rostrally by the nasal (frontal) processes of the premaxilla and caudally by the frontals (Fig. 3 A). The rostrally directed maxillary and premaxillary processes are sharply tapered; of these, the maxillary process is fairly short, although contributing to the lateral border of the external nares (in contrast with taxa in which this process is absent and the nasal contribution to the external nares is restricted to the caudal and dorsal margins, e. g., DNHM D 2950 / 1, Rapaxavis pani and Longipteryx chaoyangensis; Zhang et al., 2000; Morschhauser et al., 2009; Wang et al., 2010). In S. santensis, however, no nasal can be definitively identified because the skull is comprised of numerous incomplete bone fragments, none of which preserve any morphology that allows unequivocal identification. Even if Sereno & Rao (1992) are correct and some bone fragments that are in articulation with what has been interpreted as the frontal (Sereno & Rao, 1992) are indeed nasals (Fig. 3 B), with only the caudal half of this region preserved we cannot determine if they were expanded (relative to the rostral half). The caudal half of the maxilla of S. santensis was described as differing from that of Archaeopteryx because its dorsal and ventral margins are parallel and the jugal ramus does not taper significantly caudally (Sereno et al., 2002). These details are impossible to verify, however, because neither holotype specimen, S. santensis or C. yandica, has a maxilla that clearly preserves this region (Fig. 3). The left maxilla in the C. yandica holotype is visible nearly in articulation with the premaxilla; the premaxillary and jugal rami appear subequal (as opposed to the elongate premaxillary ramus in longipterygids; O’Connor et al., 2009). The maxilla of C. yandica has a long and thin caudodorsally directed nasal process; the caudal articulation with the jugal is not completely preserved so we cannot rule out that the jugal ramus was tapered, although, in its preserved portions, this ramus does have parallel rostral and dorsal margins. In S. santensis, the maxilla is even less clear; one element may represent the maxilla based on the presence of what appears to be a nasal process, as in C. yandica (absent in Gobipteryx minuta; Chiappe et al., 2001) (Fig. 3 B). If correct, the premaxillary ramus is not preserved; the dorsal and ventral margins of the jugal ramus are parallel except for the distalmost articulation with the jugal where this process tapers abruptly (as opposed to the slow tapering of this process in Archaeopteryx; Elzanowski, 2002), morphologically similar to Hebeiornis fengningensis (= Vescornis hebeiensis; Xu et al., 1999; Zhang et al., 2004). We conclude that, given the skull preservation in the holotype of S. santensis, no cranial morphologies are useful for either distinguishing, or aligning, the two taxa. Vertebral column. While on the one hand we agree with Sereno et al. (2002) that the thoracic vertebral morphology preserved in the holotypes of S. santensis and C. yandica are comparable, this region of the skeleton is generally conserved among Early Cretaceous enantiornithines (Chiappe & Walker, 2002) and neither specimen preserves any deviant morphologies (O’Connor, 2009). Note however that while Sereno et al. (2002) recorded the number of sacral vertebrae as eight in S. santensis, it is impossible to count in this region of the skeleton because the articulated pelvic girdle is preserved in lateral view, obscuring the sacrals from view (Fig. 1). The pygostyles of S. santensis and C. yandica, as noted by Sereno et al. (2002), are also morphologically very similar, with dorsal and ventral processes that reach beyond the articular facet and ventrolaterally directed processes that rapidly diminish distally, causing a marked taper (Sereno et al., 2002; Zhou & Hou, 2002) (Fig. 4). Since the discovery of these specimens, however, several new species have been described that possess the same morphology, now regarded as typical of most enantiornithines (Zhang et al., 2000; Chiappe & Walker, 2002; Chiappe et al., 2002; Li et al., 2006; O’Connor, 2009). As noted by Zhou & Hou (2002), the pygostyles of the two taxa do differ in length with respect to their overall body size, which can only be considered a true morphological difference and thus a diagnostic character: the pygostyle of C. yandica is nearly 25 % longer than that of S. santensis (Fig. 4). Thoracic limb. Zhou & Hou (2002) listed differences in the morphology of the ulnare as a diagnostic distinction between the two taxa: the ulnare of C. yandica is described as having a larger metacarpal incision than that of S. santensis (Zhou & Hou, 2002). The ulnares certainly differ in morphology between the two specimens (Fig. 5); the ulnare in the holotype specimen of S. santensis (Fig. 5 B) is much more U-shaped, with a deeper and narrower incision relative to that of C. yandica, which is much shallower and wide (sensu Zhou & Hou, 2002). The fairly deep incisure preserved in the S. santensis holotype was noted by Sereno & Rao (1992) and compared to the ulnare morphology of more derived birds such as Ichthyornis. The narrow and deep metacarpal incisure on the ulnare of S. santensis differentiates this taxon from C. yandica. The morphology of the manus is also similar in both species (Fig. 5): S. santensis and C. yandica have small claws on both their alular and major digits and have short alular digits that do not distally surpass the distal end of the major metacarpal. The proximal carpometacarpus was considered to be unfused in S. santensis as opposed to fused in C. yandica (Sereno & Rao, 1992; Zhou et al., 1992), however this character is unclear and cannot be unambiguously determined in either taxon owing to preservation (Fig. 5). The absence of an intermetacarpal space in the holotype of C. yandica has also been used to distinguish the two taxa but this morphological difference was dismissed as diagenetic by Sereno et al. (2002). After reviewing the morphology preserved in IVPP V 9769, we concur with the latter interpretation. The preserved morphology is very distinctive and observed among other enantiornithines (e. g., Rapaxavis, Dapingfangornis sentisorhinus, Hebeiornis), with the minor metacarpal having a rectangular crosssection, dorsoventrally wider than craniocaudally thick. In fully articulated enantiornithines, the cranial surface of the minor metacarpal is concave so that the size of the intermetacarpal space does not appear the same from dorsal and ventral views. While no space appears present in either carpometacarpus in dorsal view (IVPP V 9769 a), a small space is clearly visible in ventral view (IVPP V 9769 b; Fig. 5 A). As in other enantiornithines, the minor metacarpal is proximally contiguous with the pisiform process (IVPP V 9769 b), slightly wrapping around the major metacarpal distally (e. g., Eoenantiornis buhleri, Hebeiornis, Rapaxavis; Hou et al., 1999). This morphology is also present in the holotype of S. santensis (Fig. 5 B). The intermetacarpal space still appears larger in S. santensis however this may be due to the better preservation of the manus (in articulation) in the holotype specimen (Fig. 5 B). Sereno et al. (2002) also noted a difference in the relative proportions of the manual claws between the holotypes of S. santensis and C. yandica, with the major ungual larger than the alular ungual in the former taxon and the two claws subequal in the latter. Zhou & Hou (2002) considered that the alular claw is larger than the major claw in C. yandica, with claws of subequal size in S. santensis. These different interpretations based on the same specimens result partially from the poor preservation of the manus in C. yandica and the very small difference in size that is being considered here. Based on our observations, the manual claws are nearly subequal in both taxa, although the major claw is proportionately larger in S. santensis while the alular claw is larger in C. yandica (Fig. 5). The two specimens therefore preserve the opposite condition, and this morphological difference, although slight, can be used to distinguish the two taxa. The morphology of the first phalanx of the minor digit also differs between the taxa. In the holotype of C. yandica this phalanx is clearly straight, rectangular to trapezoidal, tapering distally (Fig. 5 A) while in S. santensis it is curved with a concave ventral margin (Fig. 5 B) (Zhou & Hou, 2002). The curved phalanx of S. santensis is also proportionately longer than that of the hand of C. yandica (Zhou & Hou, 2002). The morphological disparity of this phalanx between the two specimens was noted by Sereno et al. (2002) but was dismissed as intraspecific variability. The curvature present in the S. santensis holotype is clearly distinct from the straight morphology preserved in the C. yandica holotype. Given the differences already noted between the specimens, however, additional specimens of S. santensis are required to argue that this curvature is indeed intraspecific variation. Until then, this morphology is considered diagnostic of S. santensis. Pelvic girdle. The pelvic girdle is well-preserved in the holotype of S. santensis, nearly complete, fully articulated and missing only the preacetabular wing of the ilium (Fig. 6 A). This contrasts with the almost completely disarticulated and incomplete pelvic girdle of the holotype of C. yandica. Although both specimens were reported by Sereno et al. (2002) to have ischia of similar morphology, this cannot be verified given the poor preservation of this region in the holotype of C. yandica. This element was reconstructed by Zhou & Hou (2002); however this information comes from a referred specimen first published by Hou (1997 — no collection number). The specimen is an isolated fully articulated pelvis; Zhou & Hou (2002, fig. 7.7 H) illustrated the pubis as bent in lateral view, a morphology that cannot be confirmed in C. yandica IVPP V 9769. With insufficient justification for the assignment of this pelvis to C. yandica, information from this specimen is not utilized here pending further investigation. The ischium of S. santensis is consistent with that of other enantiornithines in that it possesses a large proximally located dorsal process but lacks an obturator process (Chiappe & Walker, 2002). The dorsal curvature of the caudal ischium present in the holotype specimen of S. santensis is also present in Eoenantiornis and clearly distinct from the straplike ischia present in some other Early Cretaceous Chinese enantiornithines (e. g., Longipteryx, DNHM D 2950 / 1). Visible in the cast of the IVPP V 9769 b is what appears to be the corpus of the ischium preserved overlapping the ilium. While detailed morphologies remain unclear, this bone shows the same dorsal and medial curvature that is present in the S. santensis holotype (contra Sereno & Rao, 1992). Given the poor preservation of the ischium in the holotype of C. yandica, this morphology is subject to interpretation and cannot be used to align the two species; medial curvature of the ischia is also known in several other enantiornithines (e. g., Concornis lacustris, Dapingfangornis, Rapaxavis; Sanz & Buscalioni, 1992).	en	O'Connor, Jingmai, Dyke, Gareth J. (2010): A Reassessment of Sinornis santensis and Cathayornis yandica (Aves: Enantiornithes). Records of the Australian Museum 62 (1): 7-20, DOI: 10.3853/j.0067-1975.62.2010.1540, URL: http://dx.doi.org/10.3853/j.0067-1975.62.2010.1540
0389AA11FFBDFFF5FCF3F934FE7FF901.taxon	description	Although the ilium in both species is very similar in terms of its general shape and proportions, close comparison of the two specimens reveals additional minor differences in the morphology of the postacetabular wing (Fig. 6). The dorsal margin of this wing of the ilium in S. santensis is dorsally convex while the ventral margin is slightly concave so that the entire postacetabular wing is slightly curved in a caudoventral direction (Fig. 6 A). The postacetabular blade is thus strongly triangular. In contrast, the left postacetabular process of C. yandica has a straight dorsal margin with a ventrally concave ventral margin and is directed caudally (Fig. 6 C, D). The process tapers caudally in both specimens but much less in the C. yandica holotype, which terminates in a blunt distal margin. In the holotype of S. santensis this termination forms a sharper point (Fig. 6 A). Overall, the morphology of C. yandica is more strap-like than that of S. santensis (Fig. 6 D). A three-dimensionally preserved enantiornithine (CAGS-IG- 05 – CM- 06) from the slightly younger Xiagou Formation, Gansu, China, possesses a nearly complete and fully articulated pelvic girdle, and displays the same straight morphology as C. yandica (JOC pers. obs.). This results in a difference in the shape of the ‘ ilioschiadic fenestra’ between S. santensis and the Xiagou enantiornithine, which suggests that the morphology observed in C. yandica is not a preservational artifact but a true difference that can be used to differentiate the two taxa.	en	O'Connor, Jingmai, Dyke, Gareth J. (2010): A Reassessment of Sinornis santensis and Cathayornis yandica (Aves: Enantiornithes). Records of the Australian Museum 62 (1): 7-20, DOI: 10.3853/j.0067-1975.62.2010.1540, URL: http://dx.doi.org/10.3853/j.0067-1975.62.2010.1540
0389AA11FFB0FFF7FF22F8E1FEADFA7D.taxon	description	Sinornis santensis (BNHM BPV 538) differs from C. yandica (IVPP V 9769) in that: (1) the first phalanx of the minor digit is curved (cf. straight in C. yandica) and proportionately longer relative to the first phalanx of the major digit; (2) the claw of the major digit is proportionately larger relative to that of the alular digit; (3) the postacetabular wing of the ilium is proximally broad, curved and tapered distally (cf. ‘ strap-like’ in C. yandica); (4) the ulnare is U-shaped with a relatively deep narrow incisure (wide and shallow incisure in C. yandica) and; (5) the pygostyle is shorter (¾ the length of that of C. yandica).	en	O'Connor, Jingmai, Dyke, Gareth J. (2010): A Reassessment of Sinornis santensis and Cathayornis yandica (Aves: Enantiornithes). Records of the Australian Museum 62 (1): 7-20, DOI: 10.3853/j.0067-1975.62.2010.1540, URL: http://dx.doi.org/10.3853/j.0067-1975.62.2010.1540
0389AA11FFB0FFF7FF22F8E1FEADFA7D.taxon	materials_examined	Referred specimens of Cathayornis yandica. A number of specimens have been referred to C. yandica both in publications and informally in small museums around China (Martin & Zhou, 1997; Hou, 1997; Hou et al., 2002; Zhou & Hou, 2002; JOC, pers. obs.). Indeed, many of these specimens preserve regions of the skeleton poorly known in the holotype, such as an articulated pelvic girdle (Hou, 1997, fig. 54, p. 136, no collection number) and hindlimbs (IVPP V 9936, Zhou & Hou, 2002; IVPP V 10533, V 10904, Hou, 1997) that offer the possibility of further comparison with S. santensis and additional morphological differentiation if correctly assigned. For example, both C. yandica and S. santensis have a plantarly excavated tarsometatarsus formed by keel-like medioplantar and lateroplantar margins of metatarsals II and IV respectively, a morphology observed in a number of enantiornithines (e. g., avisaurids). Although the distal ends of the metatarsals in C. yandica are not preserved in the holotype specimen, a well preserved referred specimen IVPP V 9936 reveals that the metatarsals of these birds are arranged in a single horizontal plane while the trochlea of metatarsals II and IV of S. santensis (visible in the holotype specimen) are displaced plantarly. However, because the morphological information comes from an unconfirmed referred specimen, this cannot yet be consider- ed a true diagnostic character of C. yandica. The small overall size and subtly of the morphological differences between these birds suggest that only a complete and detailed inspection of all referred specimens will determine their taxonomic assignment. Pending such a revisionary study, it is recommended that referred specimens should not be used to supplement data from the holotype or used with extreme caution.	en	O'Connor, Jingmai, Dyke, Gareth J. (2010): A Reassessment of Sinornis santensis and Cathayornis yandica (Aves: Enantiornithes). Records of the Australian Museum 62 (1): 7-20, DOI: 10.3853/j.0067-1975.62.2010.1540, URL: http://dx.doi.org/10.3853/j.0067-1975.62.2010.1540
