Gansus zheni, Liu, Di, Chiappe, Luis M., Zhang, Yuguang, Bell, Alyssa, Meng, Qingjin, Ji, Qiang & Wang, Xuri, 2014

Liu, Di, Chiappe, Luis M., Zhang, Yuguang, Bell, Alyssa, Meng, Qingjin, Ji, Qiang & Wang, Xuri, 2014, An advanced, new long-legged bird from the Early Cretaceous of the Jehol Group (northeastern China): insights into the temporal divergence of modern birds, Zootaxa 3884 (3), pp. 253-266 : 254-259

Gansus Hou and Liu, 1984

Emended diagnosis. Non-ornithurine ornithuromorphs having a hooked omal projection of the sternolateral process of the coracoid; sternum with a caudomedially curved outermost (lateral) trabecula; intermembral index (length of humerus + ulna)/(length of femur + tibiotarsus) between 0.9 and 1.1; intermetacarpal space terminating distal to the distal end of metacarpal I; metatarsal II shorter than metatarsal IV (metatarsal II extends distally only as far as the base of the trochlea of metatarsal IV); proximal pedal phalanges longer than distal pedal phalanges; pedal digit IV longer than digit III.

Included species. Gansus yumenensis Hou and Liu, 1984 ; Gansus zheni sp. nov.

Holotype. BMNHC-Ph1342, complete and articulated specimen preserved in ventral view ( Fig. 1 ).

Referred specimen. BMNHC-Ph1318 and BMNHC-Ph1394, complete and articulated specimens preserved in ventral view ( Fig. 2 ).

Etymology. The specific name zheni honors Dr. Zhen Shuonan, for his paleontological legacy to the Beijing Museum of Natural History.

Stratigraphic and geographic occurrence. Sihedang Town, Lingyuan City, Liaoning Province, China; Jiufotang Formation, Early Cretaceous, Aptian (approximately 120 Ma) ( He et al. 2004).

Diagnosis. A medium-sized gansuid bird distinguishable from Gansus yumenensis by the following combination of characters: broader U-shaped furcula with an interclavicular angle of approximately 60 degrees (as opposed to ~ 40 degrees for Gansus yumenensis ); cnemial crests of tibiotarsus extending distally less than in Gansus yumenensis ; manual digit II (major digit) proportionally shorter than in Gansus yumenensis ; pedal digit IV slightly longer than digit III (pedal digit IV is markedly longer than digit III in Gansus yumenensis ); ratio of pedal digit III/tarsometatarsus about 1.1 (it is approximately 1.0 in Gansus yumenensis ); claws of pedal digits III and IV lacking a prominent pendant flexor tubercle (present in Gansus yumenensis ).

Description. Based on the complete fusion of compound bones, we regard BMNHC-Ph1342, BMNHC- Ph 1318, and BMNHC-Ph1394 as adult individuals, with a body mass of approximately 238 g. based on the midshaft diameter of the tibiotarsus ( Liu et al. 2012). These specimens have a mesorostral, subtriangular-shaped skull in which the delicate rostrum is slightly longer than half of the total length of the skull. The internal naris is small, rostrally tapered, and tear-shaped. Its caudal margin overlaps the rostral level of the much larger and triangular antorbital fenestra. Very small teeth are present in the dentary ( Fig. 3 ), but it is unclear whether they were also present in the premaxilla and/or maxilla. Caudal to the round orbit, a displaced quadrate shows that the orbital process of this bone was narrow, pointing rostrodorsally. The dentary is generally straight, but not as long as that of Yanornis martini, in which the dentary extends for approximately two-thirds of the skull’s length ( Zhou & Zhang 2001). The angular is slightly curved and robust. Unlike other Jehol ornithuromorphs, such as Hongshanornis longicresta Zhou & Zhang, 2005 and Jianchangornis microdonta Zhou et al., 2009, no pre-mandibular ossification is visible in any of the two studied specimens.

The articulated vertebral column is composed of 11-12 cervical vertebrae. The number of thoracic and synsacral vertebrae cannot be determined given that they are covered by the sternum and a mass of gastroliths. However, a few posterior thoracic vertebrae preserved either caudal to the sternum or with relief visible through this thin bone, show centra that are laterally excavated by broad fossae ( Fig. 4 ) typical of many basal birds. The visible portion of the synsacrum of BMNHC-Ph1318 also shows a broad ventral groove running along the entire exposed surface. A small number of free caudals are preserved in BMNHC-Ph1342, although their specific morphology cannot be determined. The caudal series ends in a small pygostyle formed by the fusion of three to four vertebrae.

The coracoid is strut-like, and approximately 50% and 40% the length of the scapula and the humerus, respectively ( Table 1 ). The coracoid possesses a distinct procoracoid process and a hooked laterodistal process ( Fig. 4 ) that is more pronounced than that noted for Yixianornis grabaui Zhou & Zhang, 2001 by Clarke et al. (2006). The slender scapular blade is gently curved and the acromion is short. The delicate U-shaped furcula has an intraclavicular angle of approximately 60?, a subcircular cross-section, and lacks a hypocleideum ( Fig. 4 ). In these respects, the furcula of Gansus zheni resembles that of Gansus yumenensis and Yixianornis grabaui (see You et al. 2006 & Clarke et al. 2006).

*Measurements of Gansus yumenenis were taken from different specimens because no single specimen is completely preserved.

Like in most basal ornithuromorphs, the sternum is large, longer than wide, and keeled throughout. The length: width ratio of the sternum is approximately 2.0, thus comparable to that of other ornithuromorphs. The cranial margin of this bone forms a broad, inverted V outline, with a prominent and triangular-shaped laterocranial process (BMNHC-Ph1394); a similar process was reported in Gansus yumenensis by Li et al. (2011). Central on its lateral margin, the sternum develops a subrectangular, squared-off lateral process. Gastroliths and other bones largely cover the caudal margin of this bone, thus making it impossible to fully determine the precise morphology of this important region of the skeleton. Nonetheless, it can be seen that the distal end of the sternum bears two relatively short trabeculae. The outermost (lateral) trabecula is gently curved caudomedially, defining a short caudal notch ( Fig. 4 ), this condition is consistent with that of Gansus yumenensis . While it is uncertain whether the medial trabeculae of the sternum of Gansus zheni defined caudal fenestrae, the exposed portion of the caudal sternum indicate that if they indeed formed caudal fenestrae, these openings would have been located more caudally, and not at the same level as the fenestrae cranial margin, than in Gansus yumenensis . The thoracic ribs are slender and slightly curved, lacking uncinate process. These processes are known for a variety of basal birds, including many ornithuromorphs, such as Hongshanornis longicresta, Archaeorhynchus spathula Zhou & Zhang, 2006, and Schizooura lii Zhou, Zhou & O’Connor, 2012 (see Zhou & Zhang 2005; 2006; Zhou et al. 2012), however they have not been reported in the many known specimens of Gansus yumenensis .

The humerus has a cranially convex and moderately developed deltopectoral crest, which extends more than one-third the total length of the bone. The distal margin of the deltopectoral crest gradually merges with that of the shaft ( Fig. 4 ). A laterally concave intermuscular line marks the dorsal half of this crest. The cranioventral corner of the proximal end bears a broad transverse ligamental groove that is distally defined by a prominent, cranially raised protuberance. A distinct ligamental pit excavates the distal margin of this protuberance. The distal condyles of the humerus are not well preserved in any of the two specimens but a scar for the brachial impression is clear. The ulna is straight and subequal in length to the humerus ( Table 1 ). Proximally, this bone exhibits a prominent bicipital tubercle and a deep but narrow brachial impression. The carpometacarpus is fully fused both proximally and distally, a condition comparable to that recorded for basal ornithuromorphs such as Schizooura lii (see Zhou et al. 2012) and in the more advanced Gansus yumenensis (e.g., You et al. 2006; Li et al. 2011).

The alular metacarpal lacks an extensor process. The major metacarpal is much thicker than the minor metacarpal and both define a narrow intermetacarpal space, thinner than the width of the latter. The alular digit, including a small terminal claw, does not reach distally to the end of the carpometacarpus. The major digit is longer, but still shorter than the carpometacarpus. The proximal phalanx of this digit has a dorsally thickened cranial margin and a thin, craniocaudally expanded corpus ( Fig. 4 ). This phalanx is followed by a slender and somewhat shorter phalanx, which in turn is followed by a small claw. The minor digit is greatly reduced and limited to a small, wedge-shaped phalanx similar to that observed in Hongshanornis longicresta by Chiappe et al. (2014).

The ilia are largely covered by other elements in both specimens. The acetabulum is small, about 1/10 the length of the bone and its dorsocaudal portion bears an antitrochanter. The slender pubis is compressed craniocaudally, ending in a rounded, distal expansion ( Fig. 5 ); the distal third of the bone is strongly curved. Both pubes form a short distal symphysis. The ischium is slender, distally tapered, and less than half the length of the pubis; its distal 1/3 curves ventrally. The lateral surface of the ischium bears a central ridge and its cranial margin expands into a triangular process. Fragments of gastralia are visible between and around the ischia and pubes of the holotype specimen.

The hindlimb of Gansus zheni is longer than its forelimb, with a forelimb:hindlimb ratio of 0.97. Such a ratio is similar to that in Yixianornis grabaui (0.98), smaller than in Archaeorhynchus spathula (1.35), and greater than in Hongshanornis longicresta (0.79). The femur is subequal in length to the tarsometatarsus and slightly more than half the length of the tibiotarsus. The femur: tarsometatarsus ratio of Gansus zheni is 0.96, which is similar to Hongshanornis longicresta (1.0) and Gansus yumenensis (1.04), and lower than other those of ornithuromorphs such as Archaeorhynchus spathula (1.85), Yixianornis grabaui (1.57), and Yanornis martini Zhou & Zhang, 2001 (1.37). The tibiotarsus of Gansus zheni bears two distinct cnemial crests that extend along the proximal one-fourth of the bone ( Fig. 5 ), somewhat less than the cnemial crests of Gansus yumenensis . The lateral cnemial crest curves laterally. The distal end of the tibia is fully fused to the proximal tarsals. A deep extensor groove excavates the craniomedial surface of the distal end ( Fig. 5 ); a raised medial ridge strongly defines this sulcus. The fibula is needle-shaped and long. The proximal half is preserved and the distal portion is only recognized by a groove on the tibia. The slender bone extends to nearly the midshaft of the tibia, while in Yixianornis grabaui and Yanornis martini it is shorter and reduced ( Zhou & Zhang, 2001; Clarke et al., 2006).

The tarsometatarsus is completely fused. On the right element of the holotype, there is a tarsometatarsal proximal foramen piercing the bone between metatarsals III and IV. Distally, the trochlea of metatarsal II ends much more proximally than the trochleae of either metatarsal III or IV. The trochlea of metatarsal IV ends more proximally than that of metatarsal III. In dorsal view, the trochleae of metatarsal II and IV are somewhat behind that of metatarsal III. The pedal digits are long and slender. The ratio of pedal digit III/tarsometatarsus is about 1.1, which is similar to that of Gansus yumenensis (1.0). The feet are completely preserved and in articulation in both specimens. Like in Gansus yumenensis , the proximal phalanges of all pedal digits are longer than any of their respective distal phalanges, and the ungual phalanges are small and relatively straight ( You et al. 2006). Nonetheless, the ungual phalanges of Gansus zheni differ from those of Gansus yumenensis in that they lack the prominent flexor tubercles present in the latter species ( Li et al. 2011). Unlike most other basal ornithuromorphs, digit IV is slightly longer than digit III. This rather unusual condition is shared with Gansus yumenenis , although in the latter the ratio of digit IV/digit III is greater and equal to 1.2. Pedal digit III is followed in length by digit II and digit I ( Fig. 5 ), which are approximately 80% and 30% the length of digit III, respectively.

There are 14–15 and about 30 poorly sorted gastroliths preserved in BMNHC-Ph1342 and BMNHC-Ph1318, respectively ( Fig. 6 ). These stomach stones vary greatly in size and are minimally rounded. Their size is much greater than the gastroliths preserved in other birds from the Jehol Biota (e.g., Yanornis martini, Archaeorhynchus spathula, Hongshanornis longicresta ).

Figures

FIGURE 1. Photograph and interpretive drawing of the holotype of Gansus zheni (BMNHC—Ph 1342). Abbreviations: co—coracoid, cv—cervical vertebrae, fem—femur, fi—fibula, fu—furcula, g—gastroliths, h—humerus, pu—pubis, r—radius, sc—scapula, st—sternum, ti—tibiotarsus, tmt—tarsometatarsus, u—ulna; roman numerals refer to the manual and pedal digits.

FIGURE 2. Photograph and line drawing of a referred specimen (BMNHC—Ph 1318) of Gansus zheni. Abbreviations as in Fig. 1.

FIGURE 3. Details of the skull of Gansus zheni. A. Holotype BMNHC—Ph 1342 showing teeth in dentary. B. BMNHC-Ph 1318. Abbreviations: af—antorbital fenestra, d—dentary, pm—premaxilla, q—quadrate.

FIGURE 4. Thoracic girdle and limb of Gansus zheni (BMNHC—Ph 1342). Abbreviations as in Fig. 1.

FIGURE 5. Pelvic girdle and limbs of Gansus zheni (BMNHC—Ph 1342). Abbreviations as in Fig. 1.

FIGURE 6. Details of thoracic region of Gansus zheni showing gastroliths in BMNHC—Ph 1318 (A) and BMNHC—Ph 1342 (B). Abbreviations as in Fig. 1.

Tables

TABLE 1. Selected measurements of Gansus zheni (BMNHC—Ph 1342, BMNHC—Ph 1318) and Gansus yumenenis (You et al. 2006) (in mm).

Taxon	BMNHC-Ph1342	BMNHC-Ph1318 Gansus yumenenis *
Skull	53.28	45.42
Humerus	54.56(l) 53.37(r)	53.56(l) 52.28(r) 48.0(l)
Ulna	56.10(l) 55.08(r)	? 54.08(r) 48.8(l)
Radius	54.91(l) 54.33(r)	? 50.24(r) 46.8(l)
Carpometacarpus	27.12(l) 25.58(r)	? 21.62(r) 23.4(l)
Coracoid	20.74(l) 20.73(r)	22.91(l) 21.20(r) 19.2(l)
Scapula	40.38(l)?	?? 41.7(l)
Femur	36.41(l)?	34.50(l) 34.56(r) 30.0(r)
Tibiotarsus	64.41(l) 66.11(r)	63.78(l) 65.44(r) 65.8(r)
Tarsometatarsus	37.85(l) 38.11(r)	36.92(l) 36.45(r) 36.3(l)
Pedal digit I-1	8.37(l) 8.17(r)	7.67(l) 7.51(r) 8.2(r)
Pedal digit I-2	3.92(l) 4.21(r)	4.14(l) 4.58(r) 4.2(r)
Pedal digit II-1	15.04(l) 14.87(r)	13.69(l) 12.87(r) 13.9(r)
Pedal digit II-2	13.46(l) 14.39(r)	11.98(l) 12.66(r) 11.9(r)
Pedal digit II-3	4.59(l) 5.09(r)	4.57(l) 4.54(r) 5.2(r)
Pedal digit III-1	15.51(l) 14.33(r)	13.47(l) 13.76(r) 14.2(r)
Pedal digit III-2	10.97(l) 11.04(r)	10.36(l) 10.44(r) 9.4(r)
Pedal digit III-3	10.16(l) 9.85(r)	8.82(l) 8.70(r) 8.7(r)
Pedal digit III-4	4.11(l) 4.83(r)	4.73(l) 4.15(r) 4.8(r)
Pedal digit IV-1	10.59(l) 10.95(r)	10.42(l) 9.82(r) 12.0(r)
Pedal digit IV-2	9.09(l) 8.66(r)	8.49(l) 8.25(r) 9.7(r)
Pedal digit IV-3	8.51(l) 8.46(r)	8.01(l) 8.25(r) 9.4(r)
Pedal digit IV-4	8.16(l) 8.34(r)	7.92(l) 7.05(r) 9.4(r)
Pedal digit IV-5	3.45(l) 3.46(r)	3.73(l) 3.90(r) 4.9(r)