Sinornis santensis (Zhou & Hou, 2002)

O'Connor, Jingmai & Dyke, Gareth J., 2010, A Reassessment of Sinornis santensis and Cathayornis yandica (Aves: Enantiornithes), Records of the Australian Museum 62 (1), pp. 7-20 : 16-18

Differentiating S. santensis and C. yandica

After examining the morphologies proposed to either align, or separate, the two specimens, we find significant morphological variation between the two holotypes that we feel justifies maintaining the two as distinct taxa. Preserved characters have been identified, and are supported here, that can be used to formulate a rigorous differential taxonomic diagnosis for the first time.

Sinornis santensis (BNHM BPV 538) differs from C. yandica (IVPP V9769) 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 ).

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.

Cathayornis aberransis , C. caudatus and C. chabuensis . Although we have presented an argument in favor of the validity of C. yandica , this alone does not completely clarify the taxonomic status of the genus. Three other species have to date been assigned to Cathayornis ; the validity of two species is variably accepted, while the third has only recently been described ( C. chabuensis — Li et al., 2008). Although C. caudatus and C. aberransis have been considered invalid by some ( Sereno et al., 2002; Zhou & Zhang, 2006), the only published record of this was not corroborated by morphological evidence ( Zhou et al., 2008b). These two taxa, and three others ( Longchengornis sanyanensis , Cuspirostrisornis houi , and Largirostrornis sexdentornis ; Hou, 1997) have been synonymized under C. yandica based on the fact the holotype specimens of these taxa were all collected from the same locality, despite the presence of morphologies distinct from C. yandica in some specimens (e.g., Longchengornis, Zhou et al., 2008b ). This suggests a careful taxonomic review of these poorly preserved specimens was not conducted. Accordingly, the taxonomy of C. aberransis and C. caudatus is revisited here. The taxonomic status of these taxa is discussed individually and with caution; the specimens were not accessed first hand so their validity and anatomy is evaluated from published data. However, we argue that this should be adequate: if published data are not sufficient to differentiate a new species from known taxa, then it does not add to our knowledge of the clade, or facilitate future research; a specimen affixed to a name in this way, although unique, still represents a nomen dubium.

Cathayornis aberransis . Known from a single specimen, IVPP V12353 ( Fig. 7A ), C. aberransis comes from the Jiufotang Formation of northeastern China ( Hou et al., 2002); the specimen was studied from published photos of the slab and counterslab ( Hou et al., 2002; Hou, 2003). This taxon was diagnosed by the presence of a crest between the two frontals with processes on the frontal sides (tubercles), numerous teeth in the maxilla, a distally developed sternal carina, sternal outer trabeculae distally ending proximal to the distal end of the xiphoid process (“sternum lateral process no longer than posterior process”), a humerus that is shorter than the ulna, and a distally fused pubis ( Hou et al., 2002). Most of these diagnostic characters are weak because they are widely distributed amongst Mesozoic birds, while some (humerus shorter than ulna, presence of teeth in the maxilla, a distally contacting pubis) are plesiomorphic to Ornithothoraces ( O’Connor et al., 2009). The morphology of the sternum in IVPP V12353 cannot be confirmed and the distal ends of the pubes are clearly non-contacting and thus, while likely joined in a symphysis in life (evident from their expanded distal ends), the pubes were not fused (contra Hou et al., 2002). The frontals are fairly well preserved in dorsal view, whereas typically enantiornithine frontals are preserved in lateral view. A longitudinal ridge appears absent; the frontals are unfused and the slight medial separation of the two bones may have been interpreted as a ridge. The caudolateral corner of the frontals project ventrolaterally, presumably for articulation with the postorbital and squamosal bones ( Fig. 7A ). Given the dorsal view, this feature may be exaggerated in C. aberransis , however because it cannot be readily compared in terms of exact size and shape to other enantiornithines (visible in lateral view in Pengornis , Rapaxavis, LP 4450, GMV 2159, DNHM D2567), this process and morphology are accepted as possibly diagnostic characteristics for this taxon.

While there exists published data supporting the validity of this taxon, albeit weakly, much of the published information has also been shown to be inaccurate and thus studies that incorporate this species must be cautious. Further preparation followed by detailed study is required to verify the validity of this specimen.

Cathayornis caudatus . The holotype and only known specimen of C. caudatus, IVPP V 10917 ( Fig. 7B ), also comes from the Jiufotang Formation, Liaoning, China ( Hou, 1997, 2003; Hou et al., 2002). The taxon is diagnosed as a small Cathayornis species with a transverse groove between the frontal and parietal, more than three teeth in the dentary, a well-developed sternal carina, an elongate tarsometatarsus more than half the length of the tibiotarsus, and a short bony tail lacking a pygostyle ( Hou, 1997). A toothed dentary and sternum with carina are plesiomorphic to Enantiornithes (Chiappe & Walker, 2002) ; based on the relative lengths of the femur and tibiotarsus, IVPP V10917 is only 2–5% smaller than C. yandica (IVPP V9769), so the new specimen is essentially the same size as other Cathayornis . Given the poor preservation of the hindlimb in the holotype of C. yandica , the length of the tarsometatarsus relative to the tibiotarsus cannot be accurately compared between the two taxa. No transverse groove or comparable structure on the frontal, which is preserved in lateral view, can be identified from photographs. A fully fused pygostyle of typical enantiornithine morphology (appears forked dorsally and to have possessed a laterally projected flange) is clearly visible in one of the slabs ( Hou, 1997; Hou et al., 2002; counterslab in Hou, 2003) overlapping the sternum and pelvic elements ( Fig. 7B ). The free caudal vertebrae illustrated by Hou (1997) are reinterpreted as the proximal portion of the left pubic shaft.Although currently available published data fail to support the validity of this taxon, further preparation and study of IVPP V10917 may identify unique morphologies that support C. caudatus as a valid taxon. Until this time, this taxon is regarded as a nomen dubium.

Cathayornis chabuensis . The holotype of C. chabuensis, BMNHC Ph 000110ab, greatly extends the known geographical range of cathayornithiforms (Zhou & Zhang, 2006); the specimen comes from exposures of the Jingchuan Formation at the Chabu Sumu locality near Otog Banner, in the northwest of the Otog basin, Inner Mongolia, China ( Li et al., 2008). The specimen is assigned to Cathayornis on the basis of a longitudinal groove on the dorsal surface of the radius, a narrow intermetacarpal space, and a minor digit formed by single phalanx that closely abuts the first phalanx of the major digit ( Li et al., 2008). This suite of morphologies does not diagnose Cathayornis or even distinguish it from other enantiornithines and thus this taxonomic assignment is unsubstantiated. The presence of a longitudinal groove on the radius is a synapomorphy of a subclade of enantiornithines (Chiappe & Walker, 2002). Most enantiornithines have a minor digit formed by two phalanges, both of which are reduced, the second often to a very small fragment less than 10% the size of the first phalanx (e.g., Eoalulavis lacustris , Longipteryx , Rapaxavis ; Sanz et al., 1996); a second phalanx was likely present in BMNHC Ph000110ab, but may not have been preserved given the disarticulated and incomplete nature of the specimen and the very small size of this phalanx.

The specimen is distinguished from C. yandica by the slightly greater lateral projection of the outer (lateral) sternal trabeculae, as well as the relatively more caudal extension of the keel ( Li et al., 2008). Although the quality of published photographs and illustrations makes it difficult to verify these anatomical details given that the keel is not visible and the very minimal degree of lateral splay of the outer sternal trabeculae that is being compared, the specimen was available for study and these differences have been verified. The more caudal extension of the keel, however, is here interpreted to refer to the caudal extension of the xiphial region (xiphoid process) relative to the outer sternal trabeculae; the outer trabeculae end proximal to the caudal xiphial margin (as in Elsornis keni , Shanweiniao cooperorum ; Chiappe et al., 2006; O’Connor et al., 2009), as opposed to some enantiornithines in which the two processes end at approximately the same level (e.g., Longipteryx , Protopteryx fengningensis ; Zhang & Zhou, 2000) or the outer trabeculae extend beyond the xiphoid process (e.g., Concornis , Rapaxavis ). Another apparent distinction between the two species, a well-developed distal expansion appears absent from the outer sternal trabeculae, whereas C. yandica has large asymmetrical fan shaped expansions (also present in C. caudatus ). The new specimen is further distinguished from previously identified specimens of Cathayornis by its locality, which is over 1000 km away from Chaoyang, Liaoning, where other Cathayornis specimens have been collected. Although this specimen is considered distinct from C. yandica , the characters used to assign this specimen to the cathayornithiforms are unsupported. With limited preserved material for comparison in the holotype of C. chabuensis , the taxonomic assignment of this species may have to await additional discoveries or a better understanding of Cathayornithiformes .

Figures

Figure 7. Holotype of (A), Cathayornis aberransis, IVPP V12353; and (B), Cathayornis caudatus, IVPP V10917. Scale bar is true for both specimens and totals 30 mm.