Cathayornis yandica

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 : 11-16

Cathayornis yandica vs. Sinornis santensis

Locality. The holotype specimens of S. santensis and C. yandica were described from different localities in the Early Cretaceous Jiufotang Formation (upper Jehol Group: 120 Ma; Zhou, 2006), approximately 10 km apart, with Chaoyang City as the closest major geographic indicator ( Sereno et al., 2002; Zhou & Hou, 2002; Zhou & Zhang, 2006). Although this has been used to distinguish the two taxa, differentiation based on inferred geological age, or geographical separation, makes assumptions about a species range or success. Among extant birds there are many examples of species that have huge geographical ranges, thus such assumptions are for obvious reasons inherently weak. The error associated with dating sediments is typically large and limiting an extinct taxon based on knowledge of an extant analogue bears no validity other than a proposed possibility. Comparisons with geographical ranges of similar (e.g., diet, ecology, size, etc) modern taxa or average species duration through time may be used as valid arguments for erecting a new taxon, however, such arguments should be made only when they can be justified and well supported by cited data. In the case of S. santensis and C. yandica , no detailed justification has been provided.

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 , 3B ), 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. 3A ). 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 D2950/1, 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. 3B ), 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. 3B ). 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. 5B ) 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 V9769, 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 V9769a), a small space is clearly visible in ventral view (IVPP V9769b; Fig. 5A ). As in other enantiornithines, the minor metacarpal is proximally contiguous with the pisiform process (IVPP V9769b), 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. 5B ). 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. 5B ).

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. 5A ) while in S. santensis it is curved with a concave ventral margin ( Fig. 5B ) (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. 6A ). 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. 7.7H) 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 D2950 /1). Visible in the cast of the IVPP V9769b 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).

Cathayornis yandica was described as lacking an antitrochanter on its pelvis while this structure is reportedly present in S. santensis (Zhou & Hou, 2002) . This difference is also very difficult to determine because of the lack of fusion and complete disarticulation of the pelvic girdle in the holotype of C. yandica and the absence of a wellpreserved ischium in this specimen; however, because the antitrochanter is typically located just where the ischium and ilium contact, the presence of an antitrochanter cannot be ruled out. A small triangular antitrochanter is clearly present in the holotype of S. santensis ; the structure is located primarily on the ilium, consistent with other Early Cretaceous enantiornithines (e.g., CAGS-IG-05–CM-06; JOC pers. obs.). This region of the ilium is only visible on the left in the holotype of C. yandica , but damaged so that no antitrochanter appears present.

Zhou & Hou (2002) also diagnosed C. yandica as having a rim over the craniodorsal margin of the acetabulum; this feature is clearly visible on the holotype of S. santensis extending over the entire cranial and dorsal margins of the acetabulum, contiguous with the antitrochanter ( Fig. 6A ). In C. yandica this rim can only be seen on the craniodorsal margin and is cut off where the ilium is damaged so that it cannot be determined if the crest continued and was also contiguous with an antitrochanter, as in S. santensis . A supracetabular process (the ‘dorsal antitrochanter’ of Sereno et al., 2002) is also present in both specimens ( Fig. 6A,B,C ).

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. 6A ). 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. 6C,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. 6A ). Overall, the morphology of C. yandica is more strap-like than that of S. santensis ( Fig. 6D ).

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.

Figures

Figure 1. Photograph of the slab and counterslab of Sinornis santensis: (A), BNHM BPV 538a; and (B), BNHM BPV 538b. Scale bar equals 10 mm. Photographs from Zhou Zhonghe (A) and Luis Chiappe (B).

Figure 3. Camera lucida drawings of the skull of: (A) Cathayornis yandica (from cast of IVPP V9769a); (B) Sinornis santensis (from cast of BNHM BPV 538a).Abbreviations:den, dentary; frn, frontal; jug, jugal; max, maxilla; pmx, premaxilla; spl, splenial; sur, surangular. Question marks denote equivocal determinations.

Figure 4. Pygostyle of (A) Cathayornis yandica (IVPPV9769a); (B) Sinornis santensis(cast of BNHM BPV 538a).Scale trueforA and B.

Figure 5. Carpometacarpus of (A), Cathayornis yandica (IVPP V9769b, left manus); and (B), Sinornis santensis (cast of BNHM BPV 538a, right manus). Scale bars equal 5 mm.

Figure 6. (A), cast of the pelvic girdle in Sinornis santensis BNHM BPV 538a; (B), left ilium of Cathayornis yandica IVPP V9769b; (C), left ilium IVPP V9769a; (D), right ilium IVPP V9769a.