SYSTEMATIC PALAEONTOLOGY

Dinosauria Owen, 1842

Saurischia Seeley, 1887

Sauropodomorpha Huene, 1932

Massopoda Yates, 2007

Sauropodiformes Sereno, 2007 (sensu14)

Xingxiulong chengi gen. et sp. nov.

Holotype

LFGT (Bureau of Land and Resources of Lufeng County, Lufeng, Yunnan, China) -D0002, partial skull and mandible; postcranial skeleton including atlas-axis complex, three cervical vertebrae (possibly C7–C9), seven dorsal vertebrae (possibly D8–D14), complete sacral vertebral series, 35 caudal vertebrae, fragments of ribs and chevrons, left ilium, left pubic apron and distal end of right pubis, proximal end of left ischium and distal portions of articulated ischia, both femora, both broken tibiae and proximal ends of fibulae, left astragalus and calcaneum, putative distal tarsals III and IV, and complete left pes and almost complete right pes.

Paratypes

LFGT-D0001, articulated postcranial skeleton including axis, complete cervical (C3–C10), dorsal (D1–D14) and sacral vertebral series, 19 anterior caudal vertebrae (possibly Ca3–Ca21), fragments of cervical and dorsal ribs, nine chevrons, right scapula, right ilium, proximal ends of right pubis and ischium, distal portions of femora, distal end of left tibia, and left astragalus. LFGT-D0003, partial skull with mandible and partial postcranial skeleton including eight cervical vertebrae (possibly C3–C10), 14 dorsal vertebrae (possibly D1–D14), nearly complete sacral vertebral series, fragments of ribs and chevrons, both scapulae, both broken humeri, ulnae and radii, partial carpi and manus, right ilium and fragment of left ilium, left pubis and right pubic apron, broken left femur, right tibia and fibula, left proximal ends of tibia and fibula, broken right astragalus and broken distal tarsals, and partial pes.

Comment

Xingxiulong chengi is estimated to have had a total length of 4–5 m and a hip height of 1–1.5 m based on measurements of preserved skeletons (Fig. 2). Among them, LFGT-D0001 is the smallest, and the holotype (LFGT-D0002) is about the same size as LFGT-D0003 (12–14% larger than LFGT-D0001; see Supplementary Information). The fully fused skull elements and neurocentral sutures as well as the relatively large sizes indicate that the holotype and LFGT-D0003 are probably adults, whereas the visible neurocentral sutures and the relatively small size of LFGT-D0001 indicate that it might be a subadult.

Figure 2: Representative elements of Xingxiulong chengi gen. et sp. nov. and reconstruction of the skeleton. (a) Cervical vertebrae of LFGT-D0001 (3–10); (b) articulated posterior dorsal vertebrae (10–14) and dorsosacral of LFGT-D0001 in lateral and dorsal views; (c) scapula with articulated dorsal vertebrae of LFGT-D0003 in left lateral view; (d) left humerus of LFGT-D0003 in posterior and anterior views; (e) left forelimb of LFGT-D0003 in medial view; (f) right articulated humerus, ulna and radius in medial view, and detail of the proximal end of ulna and radius; (g) left ilium of LFGT-D0002 (photograph and line drawing) in lateral view; (h) right ilium of LFGT-D0003 in lateral view; (i) right ischium of LFGT-D0002 in lateral view; (j) left femur of LFGT-D0002 in anterior, lateral, posterior and medial views; (k) distal end of left tibia of LFGT-D0003 in anterior and distal views; (l) left astragalus of LFGT-D0002 in posterior view; (m) left pes of LFGT-D0002 in lateral and ventral views; (n) right pes of LFGT-D0002 in dorsal and ventral views, with detailed metatarsal I in dorsal view; (o) reconstruction of the skeleton of Xingxiulong chengi gen. et sp. nov. (scaled to the size of the holotype). Abbreviations: 4t, fourth trochanter; alp, anterolateral process; ds, dorsosacral; epls, expanded plate-like summit; it, internal tuberosity; ls, longitudinal sulcus; lt, lesser trochanter; mt I, metatarsal I; mt V, metatarsal V; plp, posterolateral process; pmb, posterior median bulge; pop, postacetabular process; prp, preacetabular process. Dashed lines represent highlighting (c,b, and f) or reconstruction (g and h). Scale bars equal 10 cm in (a–n) and 1 m in (o). Full size image

Type locality and horizon

The specimens were excavated near Sankeshu (Three Trees) Village, Jinshan Town, Lufeng County, Chuxiong Yi Autonomous Prefecture, Yunnan Province, southwestern China (Fig. 1). The specimens were from the base of the Shawan Member of the Lower Jurassic Lufeng Formation17, composed of dark purple silty mudstones.

Etymology

The generic name “Xingxiu”(), meaning constellation in Chinese, is derived from the name of the ancient “Xingxiu Bridge” in Lufeng County, which was built during the Ming Dynasty (1368–1644). The specific name is dedicated to Prof. Zheng-Wu Cheng (1931–2015), for his lifetime contribution to Chinese terrestrial biostratigraphy, including the Lufeng Basin.

Diagnosis

A medium-sized basal sauropodiform with the following unique combination of character states (autapomorphies are marked by *): both surangular and angular extended more anteriorly with respect to the external mandibular fenestra; transversely expanded plate-like summit on top of posterior dorsal vertebrae* (convergent in basal saurischians); four sacral vertebrae, with two primordial sacrals bounded by a dorsosacral and a caudosacral* (convergent in derived sauropodiforms); robust scapula with both ends extremely expanded; ilium with ventral margin of postacetabular process strongly concave*; pubis with elongated proximal pubic plate relative to the pubic apron, with pubic plate approximately 40% of the total length of the pubis*(convergent in basal sauropods); posterolateral process of distal tibia much narrower anteroposteriorly and extended more laterally and distally than anterolateral process*; a median bulge present on the dorsoposterior margin of the astragalus; metatarsal V with strongly expanded proximal end with a proportion of proximal width/total length 0.85*.

Description

The holotype preserves the posterior part of the skull and mandible as well as the articulated proatlas, atlas, and the anterior end of the axis (Fig. 3a–f). The skull is distorted severely and most of the preserved bones are misaligned. Another skull and mandible is adhered to the anterior caudal vertebrae (C9–C13) of the holotype (Fig. 3g,h). Based on its similar size with that of the holotype and the presence of articulated axis (also present in the smaller specimen LFGT-D0001), this skull and mandible are considered to belong to LFGT-D0003.

Figure 3 Skull, mandible and atlas-axis complex of Xingxiulong chengi gen. et sp. nov. (a,b) photograph and interpretative drawing of LFGT-D0002 with skull and atlas-axis complex in left lateral and mandible in ventral views; (c,d) photograph and interpretative drawing of LFGT-D0002 in ventral view; (e,f) photograph and interpretative drawing of LFGT-D0002 in dorsal view; (g,h) photograph and interpretative drawing of LFGT-D0003 in left lateral view. Abbreviations: ana, atlantal neural arch; ang, angular; ar, articular; ax, axis; bo, basioccipital; bpt, basipterygoid process; bt, basal tuber; cau, caudal vertebra; cb, ceratobranchial; d, dentary; ecp, ectopterygoid; f, frontal; inc, intercentrum; j, jugal; l, lachrymal; lc, lateral condyle; lsp, laterosphenoid; ma, maxilla; mc, medial condyle; mpp, medial pyramidal process of the articular; n, nasal; ns, neural spine; od, odontoid; p, parietal; par, paroccipital process; pat, proatlas; pre, prearticular; prf, prefrontal; po, postorbital; popf, postparietal fenestra; ps, parasphenoid; pt, pterygoid; q, quadrate; qj, quadratojugal; r, rib; sa, surangular; sk, fragments of skull roof; so, supraoccipital; sq, squamosal; stf, supratemporal fenestra; tmp, tab-like medial process of the retroarticular process. Black fills represent voids within the skull, dark grey fills represent matrix, and light grey fills represent damage. Scale bars equal 10 cm. Full size image

In LFGT-D0003, only the posterior three quarters portion of the maxilla, which is the posteroventral part of the ascending ramus and the horizontally directed posterior ramus of the left maxilla, has been remained. The vascular foramina have not been preserved. No ridge is present on the lateral surface of the posterior maxilla, differing from Lufengosaurus in which the presence of a maxillary ridge is one of its autapomorphies18. Eleven short alveoli are preserved in the maxilla, but their length would be longer due to the covering from the tooth in the mandible (Fig. 3g,h). The lachrymal has a prominent flange ventral to its dorsal end along the anterior margin of the shaft (Fig. 3g,h); a similar condition is also present in Lufengosaurus, Adeopapposaurus Massospondylus, and Riojasaurus13,18,19,20, but absent in Yunnanosaurus, Jingshanosaurus, and more derived sauropodiforms. The postorbital has a relatively long contact with the anterior surface of the dorsal ramus of the jugal, similar to that in Lufengosaurus. In contrast, the postorbital has a simple tongue-and-groove contact with jugal in Yunnanosaurus21. The jugal is a triradiate bone and consists of an anterior ramus, a dorsal ramus, and a posterior ramus. The dorsal ramus projects posterodorsally and diverges from the posterior ramus at an angle of approximately 80°, resembling although slightly smaller than that of Plateosaurus and Thecodontosaurus, whereas it is larger than that of Lufengosaurus, Jingshanosaurus, Mussaurus (50°), Yunnanosaurus and Xixiposaurus (60°), and Chuxiongosaurus (70°)3,9,21,22,23. The quadratojugal is a slender element that gives rise to an anterior ramus and a dorsal ramus (Fig. 3g,h). In lateral view, the two rami diverge from each other at an angle slightly less than 90°, contrary to the angle of 45° in Lufengosaurus, 60° in Yunnanosaurus, and 110° in Jingshanosaurus. The paired quadrates are well preserved in the holotype. Its distal articular surface is composed of a lateral condyle and a medial condyle, which are separated by a sharp ridge (Fig. 3c,d). The lateral condyle is subtriangular in shape, whereas the medial condyle is smaller, semicircular, and situated more ventrally than the lateral one, similar to that of Lufengosaurus and Yunnanosaurus but differing from that of Plateosaurus in which the lateral condyle is more ventrally offset than the medial one. A postparietal fenestra is developed along the suture between the supraoccipital and the parietal (Fig. 3e,f). The supraoccipital slopes anteriorly. The basipterygoid processes are long, slender, and ventrolaterally projected, resembling that of Plateosaurus. They diverge from each other at an angle of approximately 80°. In contrast, the basipterygoid processes of Lufengosaurus and Jingshanosaurus are short, robust, and nearly parallel with each other.

Both the angular and the surangular extend anteriorly beyond the external mandibular fenestra (Fig. 3g,h), resembles the condition in some basal sauropodomorphs such as Adeopapposaurus and Plateosaurus24, but unlike the short extension of these elements in Lufengosaurus, Yunnanosaurus, and Jingshanosaurus. The articular possesses a large pyramidal process as the medial extension of the glenoid region. Posteriorly, a dorsomedially projected and tab-like process is developed on the medial surface of the retroarticular process (Fig. 3a–d). This feature is also reported to be present in Coloradisaurus and is considered as an uncertain autapomorphy13. A similar medial process is present in some other basal sauropodmorphs from Lufeng, such as Jingshanosaurus and an unnamed new specimen (pers. observ.).

Based on overlapping preservation, Xingxiulong can be safely said to possess 10 cervical, 14 dorsal, four sacral, and more than 35 caudal vertebrae. Both the proatlases are preserved almost in life position in the holotype (Fig. 3a–f). Anteriorly, they are situated at the dorsolateral corner of the foramen magnum. The atlas consists of the intercentrum, odontoid, and paired neural arches; however, these elements have been distorted and lack critical anatomical details. The centrum of the axis is relatively short with respect to its dorsoventral height, and has moderately compressed lateral and ventral surfaces (Fig. 2a). All cervical centra are proportionally shorter; the length/height ratio of the anterior cervical centra ranges from 2.5 to 3, and this ratio decreases posteriorly, which resembles that of Lufengosaurus. This contrasts with Jingshanosaurus, Massospondylus, Adeopapposaurus, and Leyesaurus with a range of 3–425,26,27. The ventral keel is present in cervicals 4 to 9 and becomes more prominent along the cervical series, as in Jingshanosaurus Massospondylus, Adeopapposaurus, Coloradisaurus, and Lufengosaurus. Cervical lamination is poorly developed in Xingxiulong; although the prezygodiapophyseal lamina is present in C7 to C9, it is not as developed as in derived sauropodiforms. Neural spine tables are well-developed on the posterior cervicals.

The dorsal vertebrae are slightly amphicoelous, as in other non-eusauropod sauropodomorphs. The blade-like ventral keels are well developed in D1 to D3 and are absent in the middle and posterior series, as seen in most basal sauropodomorphs. The neural spines of the anterior and the last three dorsals are dorsoventrally low, anteroposteriorly short, and expanded to plate-like summits (Fig. 2b). The anterior dorsal neural spine expansion is commonly developed in sauropodomorph dinosaurs; however, it rarely occurs at the posterior dorsal vertebrae, and only developed in some basal saurischians (e.g. Herrerasaurus and Eoraptor). Hence, we suggest this feature as a possible autapomorphy of Xingxiulong. A projecting posterodorsal corner of the neural spine is present in the middle-posterior dorsals, forming a concave posterior margin shared with that of some other basal sauropomorphs but contrasting with the straight posterior margin of Lufengosaurus, Riojasaurus, Jingshanosaurus, and Yunnanosaurus. Lamination of the dorsal vertebrae is well developed. Notably, a prezygodiapophyseal lamina is developed in D3 to D7 but absent in the posterior dorsals, and the condition in the anterior dorsals is unclear because of the poor preservation. The prezygodiapophyseal lamina is absent in the mid-dorsals in most basal sauropodmorphs, but is present in Plateosaurus, Sarahsaurus and Mussaurus, and in eusauropods22.

The sacrum consists of four sacral vertebrae (Fig. 4a–d), all of which are preserved in articulation with the ilium. This four-sacral condition is rare in basal sauropodomorphs but resembles that of Melanorosaurus28, Leonerasaurus29, and the basal sauropods such as Barapasaurus30 and Shunosaurus31. Based on their morphology and relative position, the anterior-most element is interpreted as a dorsosacral, the middle two elements as primordial sacrals, and the posterior one as a caudosacral. The first sacral vertebra (dorsosacral) is placed between the anterior end of the pubic peduncle and the acetabulum of the ilium. It is not fully fused to the subsequent sacral vertebra (first primordial), with the demarcation between them being clear. The transverse process is not entirely fused to the sacral rib; nonetheless, they are sutured tightly with each other to form a single complex that extends anterolaterally and contacts the ilium. The second sacral vertebra (first primordial sacral) is located slightly anterior to the level of the ischial peduncle of the ilium. The third sacral vertebra (second primordial sacral) is placed between the ischial peduncle and the postacetabular process of the ilium. It should be noted, however, that the length of second primordial sacral centrum is approximately equal to the first primordial sacral and the two elements are fused with each other. The transverse process projects posterolaterally, resembles the morphology of most basal sauropodomorphs. The fourth sacral vertebra (caudosacral) is located at the level of the postacetabular process of the ilium. The caudosacral rib is fully fused to the transverse process and extended posterolaterally as the second primordial sacral, forming a lateral expansion to contact the ilium, as observed in Plateosaurus. In contrast, the caudosacral rib of Leonerasaurus is directed anterolaterally29. The neural spines of the sacral series exhibit the same dorsal transversely expansion as in the posterior dorsals.

Figure 4: Sacral vertebrae and pubis of Xingxiulong chengi gen. et sp. nov. The complete sacral series (LFGT-D0002) in dorsal (a) and ventral (b) views; the complete sacral series (LFGT-D0003) in dorsal (c) and right lateral (d) views; left pubis (LFGT-D0003) in anterior (e), posterior (f), lateral (g), and medial (h) views. Abbreviations: cs, caudosacral; ds, dorsosacral; il, ilium; is, ischium; isp, ischial peduncle of ilium; of, obturator foramen; pap, pubic apron; pop, postacetabular process; ppl, pubic plate; pup, pubic peduncle of ilium; s1, the first primordial sacral; s2, the second primordial sacral; sr, sacral rib. Scale bar equals 10 cm. Full size image

Based on the preserved elements, Xingxiulong possesses more than 35 caudal vertebrae. The caudal vertebrae are tall and robust, and the lateral surface of their centra is concave, as the typical morphology seen in other basal sauropodomorphs (see Supplementary Information). It seems that all the caudal centra have amphicoelous articular facets. The anterior transverse processes are dorsolaterally directed, anteroposteriorly elongated, and dorsoventrally flat, whereas the posterior elements are more slender and nearly horizontal oriented. Lamination of the caudal vertebrae is poorly developed, as in other basal sauropodomorphs; only the prezygodiapophyseal laminae are present in caudal 3 to caudal 8 in LFGT-D0001. The neural spines are tall, posteriodorsally directed, and laterally compressed.

The scapula is characterized by both remarkably expanded proximal and distal ends, with the proximal broader than the distal, as well as a robust scapular shaft (Fig. 2c). The width of the proximal expansion is approximately 56% the total length of the scapula and the distal end is less expanded with a ratio of 49%. A similar robustness is also observed in Antetonitrus and Lessemsaurus; however, contrary to Xingxiulong, the distal end of Antetonitrus and Lessemsaurus is more expanded than the proximal. A number of non-eusauropod sauropodomorphs (e.g. Lufengosaurus, Jingshanosaurus, Plateosaurus, and Anchisaurus) has the similar condition to Xingxiulong that the proximal expansion is more developed than the distal, although the scapula of them is more gracile14,32. In contrast, Yunnanosaurus displays a scapula with more pronounced expansion of the distal end compared with the proximal end. The scapular shaft is broad, with its minimum width being 19–20% the total length; this proportion is identical to that of Jingshanosaurus, whereas greater than that of most sauropodomorphs including some basal sauropods such as Isanosaurus and Shunosaurus, which have narrower scapular shafts with ratios varying between 15–17%14,32. Antetonitrus and Lessemsaurus, however, exhibit a more broadened scapular shaft.

The humerus has a poorly developed internal tuberosity on the medial surface of the proximal end (Fig. 2d,e), differing from the well-developed internal tuberosity in majority of basal sauropodomorphs (e. g. Adeopapposaurus, Coloradisaurus, Lufengosaurus, and Yunnanosaurus). The length of the ulna is about 61% the length of the humerus (Fig. 2e,f), similar to that of Yunnanosaurus and Jingshanosaurus but contrasting with Lufengosaurus in which this ratio is approximately 68%. Proximally, the ulna is expanded both lateromedially and craniocaudally, with the development of the anteromedial and anterolateral processes. The two processes delimit a shallow radial fossa, resembling that of most basal sauropodomorphs but contrasting with the deep radial fossa in derived sauropodiforms and sauropods. Compared with the ulna, the radius is a slender element (Fig. 2e,f). It is approximately 54% the length of the humerus.

The ilium is similar in overall morphology to that of other basal sauropodomorphs (Fig. 2g,h). The preacetabular process does not project beyond the anterior end of the pubic peduncle. The postacetabular process extends posterolaterally with a subsquare-shaped distal end, contrasting with the pointed postacetabular process of a number of sauropodomorphs. In lateral view, the postacetabular process exhibits a strongly concave ventral margin (between the postacetabular process and the posterior margin of the ischial peduncle), distinguishing it from that of other basal sauropodomorphs. Some taxa such as Plateosaurus, Massospondylus, Lufengosaurus, Antetonitrus and Lessemsaurus possess an almost straight to slightly convex ventral margin, whereas in other taxa such as Adeopapposaurus, Jingshanosaurus, and Yunnanosaurus this margin is more convex. Therefore, this feature is a possible autapomorphy of Xingxiulong. A posterior projecting heel is present on the distal end of the ischial peduncle of the ilium. The pubis is characterized by a relatively long pubic plate and a short pubic apron, with the former being approximately 40% of the total pubic length (Fig. 4e–h), which has not been reported in other basal sauropodomorph in which generally the pubic plate is short, but resembles the condition in some basal sauropods. In anterior view, the lateral margin of the pubic apron is concave. The distal end of the pubis is expanded both lateromedially and anteroposteriorly, with its anteroposterior depth approximately 16% of the total length of the pubis. The obturator plate of the ischium possesses a longitudinal sulcus on its lateral surface (Fig. 2i).

The femur has a lesser trochanter located distal to the distal margin of the femoral head (Fig. 2j), differing from Yunnanosaurus and Jingshanosaurus in which the proximal tip is level with the femoral head. In anterior view, the lesser trochanter is placed more close to the mid-line of the mediolateral axis of the femoral shaft with respect to the lateral margin, as seen in most basal sauropodomorphs but unlike the condition in more derived sauropodiforms including Antetonitrus and Melanorosaurus. The fourth trochanter is located close to the centre of the posterior surface of the femur, contrasting with the more medially placed fourth trochanter in Lufengosaurus, Adeopapposaurus, Coloradisaurus, Riojasaurus and more derived taxa (e.g. Anchisaurus, Aardonyx, Lessemsaurus and Antetonitrus)27. The distal end of the tibia possesses a posterolateral process that is much narrower anteroposteriorly and extends more laterally and distally than the anterolateral process (Fig. 2k); in contrast, the posterolateral process of other basal sauropodomorphs is similar in width with or slightly narrower than the anterolateral element and they almost share the same lateral extension (e.g. Plateosaurus, Massospondylus, Coloradisaurus, Lufengosaurus, and Jingshanosaurus) or the former projects less laterally than the latter (e.g. Sarahsaurus, Yunnanosaurus, Mussaurus, and sauropods). The posterior surface of the astragalus bears a dorsally convex bulge close to the mid-line that is lower than the ascending process (Fig. 2l). This feature is also reported in Mussaurus, which has been suggested to be one of the autapomorphic features of this taxon22; the bulge of Xingxiulong seems to be less developed and located more laterally than in Mussaurus, although it is still more pronounced than that of Plateosaurus and Blikanasaurus. All the pedal elements are well preserved in the holotype (Fig. 2m,n). Metatarsal I is a robust element; its mid-shaft is mediolaterally wider than that of other metatarsi (see Supplementary Information). Metatarsal V has a markedly expanded proximal end, with its width being approximately 85% of the total length and forming a triangle profile. This differs from other sauropodomorphs including derived taxa in which this ratio ranges from 50% to 77%15. Although it appears that metatarsal V of Antetonitrus has the same proximal expansion; unfortunately, the lack of its distal end precludes the comparison between the two taxa. Hence, we assume this feature as a potential autapomorphy of Xingxiulong. The pedal phalangeal formula is 2-3-4-5-1, as in other basal sauropodomorphs.