Xixianykus zhangi, Xu & Wang & Sullivan & Hone & Han & Yan & Du, 2010

Xu, Xing, Wang, De-You, Sullivan, Corwin, Hone, David W. E., Han, Feng-Lu, Yan, Rong-Hao & Du, Fu-Ming, 2010, A basal parvicursorine (Theropoda: Alvarezsauridae) from the Upper Cretaceous of China, Zootaxa 2413, pp. 1-19 : 2-13

Xixianykus zhangi gen. et sp. nov.

Etymology. Generic name from Xixia (area of origin), and onyx (Greek), claw; specific name is in honor of Prof. Zhang Wentang, who has contributed greatly to the study of paleontology in Henan Province. The taxon name is pronounced as ‘SHEE-shya-nye-kus jong-eye’.

Holotype. XMDFEC V0011 (Xixia Museum of Dinosaur Fossil Eggs of China), a partial, articulated postcranial skeleton including five posterior dorsal vertebrae, a complete synsacrum, two anterior caudal vertebrae, a few dorsal ribs and gastralia, both ilia, the right pubis and ischium, and most of the right hindlimb ( Figs 2, 3). A cast of the holotype is housed at the Institute of Vertebrate Paleontology & Paleoanthropology, Beijing (cast catalog number: IVPP FV1788 )

Type locality and horizon. Zhoujiagou, Yangcheng, Xixia County, Henan Province. Majiacun Formation. The fossil-bearing beds are considered to be late Coniacian to Santonian in age based on invertebrate and plant fossils, including bivalves, gastropods, conchostracans, and ostracods ( Wang et al. 2008).

Diagnosis. Xixianykus zhangi differs from other known alvarezsaurids in possessing the following autapomorphies: sacral rib-transverse process complexes and zygapophyses fused to form separate anterior and posterior laminae; distinct fossa dorsal to antitrochanter on lateral surface of ilium; short ridge along posterior surface of pubic shaft near proximal end; distinct depression on lateral surface of ischium near proximal end; sharp groove along posterior surface of ischium; distal end of femur with transversely narrow ectocondylar tuber that extends considerable distance proximally as sharp ridge; transversely narrow tibial cnemial crest with sharp, ridge-like distal half; lateral margin of tibiotarsus forms step near distal end; fibula with substantial extension of proximal articular surface onto posterior face of posteriorly curving shaft; distal tarsals and metatarsals co-ossified to form tarsometatarsus; and sharp flange along anteromedial margin of metatarsal IV near proximal end.

Description and comparisons. Some fusion features of this specimen suggest a relatively advanced ontogenetic stage. The sacral vertebrae are fused to form a synsacrum, the astragalus and calcaneum are coossified both to each other and to the tibia, and the distal tarsals are coossified with the metatarsals to form a tarsometatarsus. However, neurocentral sutures are visible on most of the adequately preserved vertebrae, indicating that the individual had not reached full skeletal maturity ( Irmis 2007). With a femoral length of 70 mm ( Table 1), XMDFEC V0011 is smaller than the vast majority of adult non-avian theropod specimens ( Turner et al. 2007), but some further growth would have occurred if the animal had survived to reach adulthood.

The axial skeleton is represented by the five posteriormost dorsal vertebrae (identified as D9 through D13), a nearly complete synsacrum, and the two anteriormost caudal vertebrae. All but the most posterior of the preserved dorsal vertebrae are opisthocoelous, each bearing a convex, knob-like anterior projection that occupies only the central part of the articular surface ( Fig. 4A–C). The last dorsal vertebra has a biconvex centrum as in Mononykus and Shuvuuia ( Chiappe et al. 2002) . The centra of all these dorsal vertebrae are considerably longer than either wide or tall (D9 has a centrum whose length is more than double either the height or width of the posterior end). All five dorsal centra are laterally compressed, but the degree of compression gradually decreases toward the posterior end of the dorsal series. The last dorsal centrum displays only marginal compression. The ninth dorsal centrum has a sharp ventral midline ridge, whereas the others each have a narrowly rounded ventral surface ( Fig. 4D). Pleurocoels are not present in any of the preserved dorsal centra. The neural arches of D9 and D10 are centered on the centra, but those of the other dorsal vertebrae are shifted posteriorly. Xixianykus resembles other parvicursorines in that the parapophyses each sit at the end of a long stalk, and are level with the diapophyses ( Chiappe et al. 2002). The parapophyseal and diapophyseal processes are merged into a horizontal lamina on the last dorsal vertebra, though the former process is distinctly shorter than the latter and the articular facets of the two processes remain separate. This suggests that the last dorsal rib was probably double-headed. Two additional features of the dorsal vertebrae are also typical of parvicursorines ( Chiappe et al. 2002), but otherwise rare among theropods: the neural spines are low and relatively robust, and the zygapophyseal facets are strongly inclined, facing somewhat medially in the case of the prezygapophyses and somewhat laterally in the case of the postzygapophyses ( Fig. 4A–C).

The synsacrum is formed by seven sacral vertebrae (S1–S7). The single most anterior and three most posterior ones are laterally compressed, but the second through fourth are considerably wider than tall. In contrast, all sacral vertebrae are laterally compressed in other alvarezsaurids ( Chiappe et al. 2002). In S6 and S7 of Xixianykus the degree of lateral compression is extreme, pinching the ventral surfaces of the two successive centra into a prominent, laminar ventral keel that also extends onto the posterior portion of the fifth sacral centrum ( Fig. 5). This ventral keel is probably a diagnostic feature for the Alvarezsauridae ( Chiappe et al. 2002) , but its ventral prominence and longitudinal extent vary among taxa. In Patagonykus , the ventral keel occupies only the two most posterior sacral vertebrae ( Novas 1997), whereas in previously described Asian taxa the keel covers the three most posterior sacrals. In Xixianykus the ventral surface of the part of the synsacrum anterior to the keel is concave in lateral view ( Fig. 5B–D), a feature also seen in other parvicursorines ( Longrich & Currie 2009). The ventral surfaces of the first through fifth sacral centra also bear a shallow, wide longitudinal groove ( Fig. 5B). A groove is also present in an unnamed alvarezsaurid from the Upper Cretaceous of Tugriken Shireh, Mongolia (referred to as the Tugriken Shireh alvarezsaur in Longrich & Currie 2009) and in Alvarezsaurus ( Bonaparte 1991) , but is shallower and shorter (covering fewer sacral vertebrae) in these taxa. Xixianykus also differs from the Tugriken Shireh alvarezsaur ( Longrich & Currie 2009) in lacking a ventral keel on the first sacral centrum. The sacral rib-transverse process complexes and the zygapophyses of the anterior three sacral vertebrae, and of the fourth through sixth sacral vertebrae, are coossified with each other to form separate anterior and posterior horizontal laminae ( Fig. 5A, B). The anterior lamina is transversely narrow but the posterior one is much wider, conforming to the posteriorly divergent ilia. Two small sacral fenestrae are present within the anterior lamina, and three more within the posterior. The third fenestra is located within an anterior extension of the large posterior lamina that is transversely very narrow and thus is barely visible in ventral view. Each fenestra is located near the transition between two sacral vertebrae. Lateral to the third fenestra is a much larger opening between the anterior and posterior laminae, which is roofed dorsally by the medially inclined iliac blade. The five fenestrae have smaller openings on the dorsal surfaces of the laminae than on the ventral surfaces. Although the dorsal surface of the synsacrum is obscured by the ilia anteriorly, it is clear that the neural spines of S4 through S6 are fused to each other, so that at least the posterior part of the synsacrum bears a median dorsal ridge.

Only two partial anterior caudal vertebrae are preserved and they represent the first and second caudal vertebrae (Ca1 and Ca2). They are procoelous, each bearing a prominent ball-like condyle on the posterior articular surface of the centrum. The centra are laterally compressed, and Ca1 bears a ventral midline ridge extending over all but the posterior end of the centrum ( Fig. 5B). The neural arch is preserved only on Ca1 and is located on the anterior half of the centrum ( Fig. 5C, D), as in other parvicursorines ( Chiappe et al. 2002). The slightly anterolaterally oriented transverse processes of Ca1 are long (length of each process nearly three times width of centrum) and strap-like as in Parvicursor ( Karhu & Rautian 1996) . They are anteriorly displaced on the centrum as in other alvarezsaurids, and their broad bases even extend slightly forward beyond the contact between the first caudal centrum and the synsacrum ( Fig. 5A, B), a feature possibly also present in P.remotus ( Karhu & Rautian 1996) .

Both ilia are preserved, but neither is complete. As in other parvicursorines, the ilium is strongly inclined medially to contact the neural spines of the sacral vertebrae anteriorly, although the posterior end of the ilium diverges laterally away from the midline ( Fig. 5A). The anterior margin of the iliac blade is slightly convex. The preacetabular process is relatively shallow compared to the more posterior portion of the iliac blade. The anteroventral corner of the iliac blade extends a significant distance ventrally, even protruding beyond the pubic peduncle. The posterior half of the ventral margin of the preacetabular process curves medially to slightly underlap the lateral margin of the anterior lamina of the synsacrum. The postacetabular process is largely broken away on each ilium. Immediately posterior and medial to the ischial peduncle of the ilium is a robust crest defining the brevis fossa, which tapers strongly toward the posterior end ( Fig. 5B). The medial margin of the crest bears an articular facet for the large posterior lamina of the synsacrum. In lateral view, the ventral margin of the postacetabular process continues at nearly the same level onto the antitrochanter, not displaying the step-like configuration seen in most other non-avian theropods including basal alvarezsaurids. However, this feature is also seen in other parvicursorines in which the morphology of this region is known. The pubic peduncle extends anteroventrally from the acetabular region and has a sub-triangular lateral profile, as in other parvicursorines but in contrast to the sub-rectangular outline of the pubic peduncle in basal alvarezsaurids and most other non-avian coelurosaurs ( Fig. 5C, D). The pubic peduncle is also unusual in being deflected medially relative to the iliac blade rather than lying in the same plane. It is difficult to assess whether this condition occurs in other alvarezsaurids, although it may be characteristic at least of parvicursorines. The articular surface of the pubic peduncle is composed of two facets: a small one oriented anterolaterally and a large, posteroventrally directed one that is transversely convex but much longer anteroposteriorly than wide transversely. The supracetabular crest originates midway along the length of the pubic peduncle and terminates over the anterior margin of the ischial peduncle ( Fig. 5C, D). The anterior portion of the supracetabular crest extends farther laterally than the posterior portion, as in Mononykus ( Chiappe et al. 2002) , but in Xixianykus the crest terminates more posteriorly than in Mononykus and its lateral edge is convex in dorsal view. In more basal alvarezsaurids such as Patagonykus ( Novas 1997) , the supracetabular crest originates posterior to the pubic peduncle and its lateral edge is nearly straight in dorsal view ( Fig. 5A). The small ischial peduncle is nearly transversely oriented and projects only slightly below the ventral edge of the postacetabular process. Lateral to the ischial articular facet is a large and robust antitrochanter, which is strongly expanded laterally as in Mononykus ( Perle et al. 1994) . However, the antitrochanter of Xixianykus seems to differ from that of Mononykus in that the contact surface for the femur faces almost straight anteriorly, rather than somewhat laterally. Accordingly, the antitrochanter of Xixianykus is nearly perpendicular to the edge of the supracetabular crest in dorsal view. There is a distinct fossa on the lateral surface of the iliac blade dorsal to the antitrochanter, a feature not previously reported in any alvarezsaurid. The acetabulum is partially closed by an extensive medial wall.

The right pubis is preserved, with only the distal end missing. The pubis is similar to that of other parvicursorines ( Chiappe et al. 2002) in being closely appressed to the ischium throughout its entire length ( Fig. 6). The proximal end is strongly laterally compressed, as in other alvarezsaurids ( Hutchinson & Chiappe 1998), and forms a distinctive preacetabular tubercle like that seen in other parvicursorines ( Hutchinson & Chiappe 1998). In Xixianykus , however, the tubercle is laterally compressed and anteroposteriorly short. The iliac peduncle is laterally concave and medially convex in proximal view as in other parvicursorines ( Hutchinson & Chiappe 1998). The iliac articular facet is much longer anteroposteriorly than transversely, and is inclined slightly medially. The acetabular fossa of the pubis is sub-circular in proximal view, is inclined lateroventrally, and is bordered laterally by a distinct crest. The ischial peduncle is short and robust. Its subcircular facet is similar in size to the acetabular fossa, as in basal alvarezsaurids ( Novas 1997). In other parvicursorines, this facet is much smaller than the acetabular fossa ( Hutchinson & Chiappe 1998). The orientation of the ischial facet relative to the long axis of the pubic shaft is intermediate between the conditions seen in basal alvarezsaurids and in other parvicursorines. In the former group, the facet is nearly parallel to the long axis of the pubic shaft, but in the latter group it forms a sharp angle with the pubic shaft. In Xixianykus the lateral surface of the proximal part of the pubis has two concavities, a large one occupying most of the available area and a small one on the lateral surface of the ischial peduncle. The medial surface of the proximal part of the pubis is slightly convex, but a proximodistally oriented shallow groove separates the ischial peduncle of the pubis from the rest of the surface. Immediately distal to the ischial peduncle is a distinct obturator notch ( Fig. 6A, B). Distal to the notch is a sharp ridge along the posterior surface of the pubic shaft, close to the medial side, that is 4 mm in length. The shaft is mostly straight in lateral view except that the distal portion curves slightly anteriorly, a derived feature apparently also present in some other parvicursorines including Shuvuuia and the North American specimen described by Hutchinson & Chiappe (1998). In anterior or posterior view, the pubic shaft of Xixianykus is sigmoidal ( Fig. 6C, D) as in Patagonykus ( Novas 1997) . The most proximal part of the pubic shaft is considerably laterally compressed, but most of the pubic shaft is sub-triangular in cross section, with flat posterior and medial surfaces and a convex lateral surface. However, the most distal portion is sub-circular in cross section as in Shuvuuia and Patagonykus ( Chiappe et al. 2002) . The pubis lacks a pubic apron, as in other parvicursorines.

The right ischium is mostly preserved, lacking only the distal end. The ischium has a transversely oriented iliac articular surface, which extends along the dorsal edge of an antitrochanteric flange that juts laterally from the posterior end of the proximal expansion of the ischium. Accordingly, the proximal end of the ischium is L-shaped in proximal view ( Fig. 6E), a condition probably shared with other parvicursorines but poorly preserved in most known specimens. The iliac peduncle has a slightly concave anterior margin and a convex posterior margin in proximal view. The acetabular portion of the ischium is very narrow transversely, but is much longer anteroposteriorly than in other parvicursorines ( Chiappe et al. 2002). As in other alvarezsaurids, the proximal portion of the ischial shaft has a considerable posterior curvature as it passes distally. The ischial shaft is laterally compressed and is considerably wider than the pubis in lateral view, a feature reminiscent of other maniraptorans. In more basal theropods and other parvicursorines the ischial shaft is sub-equal to or even narrower than the pubic shaft in lateral view, though the latter differ from the former in having an ischial shaft that is laterally compressed rather than rod-like. Immediately below the acetabular portion of the ischium is a fossa on the lateral surface. An eminence arising from the pubic peduncle separates this fossa from a second, more elongate fossa located further distally on the lateral surface. The Tugriken Shireh alvarezsaur also has a fossa bordered by a posterolateral ridge along the ischial shaft (N. R. Longrich pers. comm.). The posterior surface of most of the ischial shaft is somewhat flat and thus the ischial shaft is sub-rectangular in cross section, but the more distal portion is sub-triangular in cross section due to the ridged posterior surface. There is a distinct groove on the posterior surface of the flat middle portion, bounded by two ridges respectively located along the posteromedial and posterolateral edges of the ischial shaft ( Fig. 6D). The medial ridge is sharp. The distal part of the ischial shaft also curves anteriorly, giving the shaft a sigmoidal appearance in lateral view; in anterior or posterior view, the shaft is also sigmoidal, with the proximal end curving laterally and the distal end medially.

The right femur is completely preserved ( Fig. 7A–F). It is estimated to be about 55% of the trunk length (i.e., the estimated distance from the cervico-dorsal transition to the middle of the acetabulum). As in other alvarezsaurids, the proximal end of the femur is approximately L-shaped in proximal view, with the femoral head offset medially and slightly anteriorly ( Fig. 7E). A well-developed trochanteric crest is formed by the completely fused greater and lesser trochanters. The anterior portion of the crest is much thinner than the posterior portion. The anterior margin of the lesser trochanter curves medially to border a large shallow fossa anterior to the femoral head on the medial surface of the femoral shaft. The femoral head is separated from the trochanteric crest by a distinct groove, but lacks the longitudinal posterior groove seen in many other theropods ( Xu 2002). A ridge emanating from the proximal end of the femur runs along the posterolateral margin of the femoral shaft for nearly its entire length. The ridge is more prominent near the mid-length of the shaft. A second, much sharper ridge, emanating about 10 mm distal to the proximal end of the femur and running distally for a distance of 14 mm along the posteromedial margin, is probably a reduced fourth trochanter as in Mononykus , Patagonykus , and Alvarezsaurus ( Chiappe et al. 2002) . There is an elongate fossa lateral to the distal half of the fourth trochanter. The distal portion of the shaft is anteroposteriorly narrower than the proximal portion but transversely much wider. The medial distal condyle of the femur is transversely much wider than the lateral distal condyle. Unlike other parvicursorines, a distally closed popliteal fossa is absent; instead, the medial condyle extends laterally in the form of a small lateral tuber, partially closing the canal between the medial and lateral distal condyles ( Fig. 7F). This represents the incipiently developed stage of a fully closed popliteal fossa. Distally the medial and lateral condyles are separated by an extremely shallow groove, which does not extend onto the anterior surface of the femur. The transversely very narrow lateral condyle projects considerably beyond the medial condyle as a distal peak, as in all alvarezsaurids ( Chiappe et al. 2002). In Xixianykus this peak is strongly laterally compressed, though its lateral surface is damaged. The ectocondylar tuber of the lateral condyle is very narrow transversely and extends a considerable distance posteriorly, so that the tuber is about twice as high as transversely wide. This contrasts with the condition in most other theropods, including other alvarezsaurids, in which the tuber is about as high as it is wide. The ectocondylar tuber is sub-hemispherical, with the medial surface convex and the lateral surface flat. The lateral surface of the ectocondylar tuber also appears to be relatively flat in some other parvicursorines such as Mononykus , in contrast to the convex lateral surface seen in most other non-avian theropods. The tuber sends a high and sharp ridge proximally along the lateral margin of the popliteal fossa for about 10 mm.

The astragalus and calcaneum are coossified with the distal end of the tibia to form a tibiotarsus ( Fig. 7G– L). The preserved right tibiotarsus is complete, and measures about 130% of the femoral length. The medial condyle projects proximally to the same level as the cnemial crest. In other parvicursorines, it projects even further proximally. The fibular condyle of Xixianykus is lower than the medial condyle and cnemial crest. In proximal view, the medial condyle is well defined by a sharp groove anteriorly ( Fig. 7K), which extends onto the medial surface of the shaft for a short distance. The fibular condyle is cube-like, and is separated from the cnemial crest by a broad, smooth lateral indentation as in Mononykus ( Perle et al. 1994) . In Patagonykus and many other theropods ( Novas 1997), there is a small accessory condyle anterior to the fibular condyle within the lateral indentation. Unlike in other parvicursorines, the cnemial crest of Xixianykus is transversely very narrow, and its distal half is sharply ridged ( Fig. 7G, I). A shallow groove separates the cnemial crest from a small condyle at the anteromedial corner of the proximal end of the tibiotarsus. Distal to this small condyle is a distinct fossa. A 13-mm-long fibular crest fades off at both ends, and posterior to the crest is a weak parallel ridge. The crest and ridge together define a longitudinal groove, the distal end of which is pitted. The distal end of the tibiotarsus is transversely widened to a significant degree. As in Mononykus ( Perle et al. 1994) , the ascending process of the astragalus occupies only the lateral half of the anterior surface of the distal end of the tibiotarsus, and its base is distinctly depressed relative to the main body of the astragalocalcaneum. The medial half of the main body is distally excavated to form a distinct notch. Unusually, the lateral margin of the distal portion of the tibiotarsus is interrupted by an abrupt step due to a sudden lateral protrusion at the distal end ( Fig. 7C, D).

The right fibula is nearly complete. The fibula curves strongly in the posterior direction as it extends distally ( Fig. 7M–P). The posterior edge of the proximal end is wider than the anterior edge. The proximal articular surface has a large extension onto the posterior margin of the shaft, where it forms a shallow fossa ( Fig. 7P). The medial and posterior surfaces of the shaft are flat, and the lateral and anterior ones convex, near the proximal end. A 5-mm-long tubercle for M. iliofibularis is present along the lateral surface of the fibular shaft. The tubercle is somewhat step-wise at both ends, and its outer edge curves posteriorly to help define a shallow, wide groove on the posterior fibular surface. The fibula thins significantly in the distal direction and seems to taper off immediately distal to the tubercle, suggesting that it was much shorter than the tibiotarsus even when intact. A significantly shortened fibula appears to characterize the Parvicursorinae ( Chiappe et al. 2002).

The distal tarsals and the proximal ends of the right metatarsals II and IV are co-ossified to form a tarsometatarsus, and the co-ossification extends distally for a distance of at least 5 mm ( Fig. 7Q–U). In Patagonykus , distal tarsal 3 and the proximal ends of metatarsals II and III have been suggested to be partially fused, but sutures are clearly present between the elements ( Novas 1997). The tarsometatarsus of Xixianykus is clearly longer than the femur although the distal end is broken away. The preserved length of the tarsometatarsus is 68.4 mm, slightly shorter than the femur, but comparisons to other parvicursorines suggest that its length when complete was probably about 74 mm. The tarsometatarsus is a slender element, its minimum transverse width being less than 7% of its estimated length. The proximal articular surface is subrectangular in outline, and is slightly less than twice as transversely wide as anteroposteriorly deep ( Fig. 7Q). The proximal end of the tarsometatarsus is only slightly wider transversely than the more distal portion, but is about twice as deep anteroposteriorly. In proximal view, the proximal articular surface has a sub-linear anterior margin and a markedly convex posterior margin. The proximal articular surface of the tarsometatarsus bears adjacent medial and lateral fossae, the medial one being larger and deeper than the lateral one ( Fig. 7Q). They are separated by a low, wide eminence. This morphology somewhat resembles the condition in relatively derived birds (e.g., specimen of Crossoptilon auritum, IVPP collection) but differs from that in some non-avian theropods, in which the proximally fused tarsometatarsus has a flat proximal articular surface (e.g., IVPP V14009 ). There is a distinct sub-triangular articular facet on the posterolateral margin of the tarsometatarsus, situated near the proximal end and probably representing the articular facet for metatarsal V. The tarsometatarsus displays a modified arctometatarsal condition (metatarsal III fails to reach the proximal end of the metatarsus), as in other parvicursorines ( Longrich & Currie 2009). A groove between the shafts of the second and fourth metatarsals extends over most of the length of the anterior surface of the tarsometatarsus, but terminates distally at the sharply pointed proximal end of the third metatarsal. By contrast, the posterior surface is mostly flat. The second and fourth metatarsals define only a short posterior midline groove extending over less than the middle third of the length of the metatarsus, in contrast to the many other theropods in which the metatarsus has a long, conspicuous groove along the posterior surface.

The transverse width of the proximal half of metatarsal II is considerably greater than its anteroposterior depth. The distal part of this metatarsal bears a weak flange on the anterior surface, and a ridge near the medial margin of the posterior surface. The distal portion of the shaft is deeper anteroposteriorly than wide transversely, and its posterior surface is flat. Metatarsal III is estimated to be less than half as long as metatarsals II and IV, and its tapering proximal end intrudes between the other two metatarsals. Most of metatarsal III has no exposure on the posterior surface of the tarsometatarsus. The preserved proximal portion is triangular in cross section, with a slightly transversely concave anterior surface as in other Asian parvicursorines and a sharply ridged posterior surface as in the North American alvarezsaurids ( Longrich & Currie, 2009). As with metatarsal II, the proximal half of metatarsal IV is anteroposteriorly compressed. About 10 mm distal to the proximal end is a 6-mm-long sharp ridge along the anterolateral margin ( Fig. 7R, T, U). The distal half of the shaft of metatarsal IV is anteroposteriorly deeper than transversely wide. A distinct ridge arises near the mid-length of the shaft and runs distally along the anterior surface. Along the posterior surface an oblique ridge arises more proximally, close to the medial margin of the shaft, and reaches the lateral margin as it passes distally. The distal portion of metatarsal IV diverges laterally, and bears a shallow notch on the medial margin in posterior view.