Phuwiangvenator yaemniyomi, Samathi & Chanthasit & Sander, 2019

Samathi, Adun, Chanthasit, Phornphen & Sander, P. Martin, 2019, Two new basal coelurosaurian theropod dinosaurs from the Lower Cretaceous Sao Khua Formation of Thailand, Acta Palaeontologica Polonica 64 (2), pp. 239-260 : 242-248

Phuwiangvenator yaemniyomi sp. nov.

Figs. 3–14 .

Zoobank LCID: urn:lsid:zoobank.org:act:AA829C5F-84C6-4287-BED

-50DDC551A815

Etymology: In honor of Sudham Yaemniyom, former geologist of the Department of Mineral Resources, Bangkok, who found the first dinosaur bone of Thailand in 1976 at Phu Wiang Mountain.

Holotype: SM-PW9B, a partial skeleton consisting of a dorsal vertebra, three fused sacral vertebrae, right metacarpal II, right manual phalanges and unguals, right and left tibiae, left astragalocalcaneum, left metatarsal I, right metatarsals II–IV, right pedal phalanges and unguals. The bones were found in an area of about 5 square meters.

Type locality: Phu Wiang Site 9B, Phu Wiang Mountain, Khon Kaen Province, Thailand ( Fig. 2 ).

Type horizon: Sao Khua Formation, probably upper Barremian, Lower Cretaceous.

Referred material.—SM-PW9A, an atlantal intercentrum and right astragalocalcaneum which were found together and approximately 300 m away from the holotype (Phu Wiang Site 9A).

Diagnosis.— Phuwiangvenator is a megaraptoran diagnosed by the following autapomorphies: (i) short sulci on the sacral vertebrae ventrally along the anterior and posterior part of the centrum; (ii) the anterior rim of metatarsal IV slopes from proximolaterally to distomedially. This way, the distomedial corner of the proximal articular surface of metatarsal IV in anterior view is much lower than the articular surface of metatarsal III. In addition, the distomedial corner is lower than in any other known theropod.

Description.— General morphology: Phuwiangvenator was a mid to large-sized theropod with an estimated body length of 6 m. The skeletal proportion are shared by other theropods of corresponding sized such as Australovenator .

Atlantal intercentrum: The atlantal intercentrum is well preserved, only the posterior portion is slightly eroded. The atlantal intercentrum is concave anteriorly, and high dorsoventrally. The odontoid concavity in the dorsal surface is smooth. The distinct ventrolateral process on the atlantal intercentrum is not present ( Fig. 3A ).

Dorsal vertebra: The dorsal vertebra (SM-PW9B-12) was recovered ( Fig. 3B ). The neural arch is missing and possibly was not fused to the centrum. The centrum looks similar to that of Fukuiraptor (FPDM-V-8381; AS personal observations) and Gualicho ( Apesteguía et al. 2016) . The centrum has no pleurocoel and no ventral and lateral rugosities in the anterior and posterior end of the centrum, the same as in Gualicho ( Apesteguía et al. 2016) . The centrum is longer than high with a lateral depression on the lateral sides. The anterior face of the centrum is concave, whereas the posterior is flat (platycoelous). The centrum shape is circular anteriorly and posteriorly. The vertebra of Phuwiangvenator are highly constricted, hour-glass shaped in ventral view and very concave in the ventral part in lateral view which is same as in dorsal vertebrae of Fukuiraptor , Aerosteon , Allosaurus , Tyrannosaurus , Ornitholestes , Coelurus , and Zuolong ( Madsen 1976; Brochu 2003; Carpenter et al. 2005b; Choiniere et al. 2010). This differs from caudal vertebrae which normally are less concave in the ventral part in lateral view and less constricted in ventral view compared with dorsal vertebrae. The lack of a parapophysis on the centrum suggest that it belongs to the middle to posterior portion of the dorsal series. The highly constricted posterior dorsal vertebral centrum is present in Phuwiangvenator as well as in allosauroids, megaraptorids, and derived tyrannosauroids ( Novas et al. 2013). The dorsal vertebra of Phuwiangvenator also differs from the sacral vertebrae in the constriction of the centrum in ventral and lateral views. There is no keel or groove on the centrum ventrally. There is no facet for the chevron, so it is clearly not a caudal vertebra. As seen on broken surfaces, the dorsal vertebra appears to have a camellate internal structure.

Sacral vertebrae: Three incomplete fused sacral vertebrae are preserved (SM-PW9B-39). The anterior face of the centrum is flat. The centra are possibly sacral 1, 2, and 3 based on the probable sacral ribs which are situated on the anterodorsal side of the centrum. The neural arches of sacral 1 and 2 are badly preserved and still covered by the matrix. The quality of preservation of the centra is generally good, especially on the right side. The left side is compressed. The centra are flat ventrally, there is no keel and no constriction on the sacrals ventrally. There are small, short sulci on the anterior and posterior part of the sacral 2 and one sulcus on the anterior part of the sacral 3 (the posterior part has no sulci) ventrally ( Fig. 4 ). There are no pleurocoels or any

Fig. 5. Coelurosaurian theropod Phuwiangvenator yaemniyomi gen. et sp.

nov. from the Early Cretaceous Sao Khua Formation of Phu Wiang, Khon Kaen Province, Thailand. SM-PW9B-6, right metacarpal II in dorsal (A 1),

ventral (A2), distal (A3), and medial (A4) views.

foramina. The centra are longer than high. The sacral rib articulation is located low for sacral 3 anteriorly, at middle for sacral 1, and anteriorly in scaral 2.

Manual phalanges and unguals: The manual phalanges and unguals consist of manual phalanx I-1 (SM-PW9B-3); manual ungual I-2 (SM-PW9B-19); metacarpal II (SM-PW9B-6, Fig. 5); manual ungual II-3 (SM-PW9B-23); manual phalanx III-1 (SM-PW9B-4); manual phalanx III-2 (SM-PW9B-7); manual phalanx III-3 (SM-PW9B-5); and manual ungual III-4 (SM-PW9B-21) ( Fig. 6, 7 ).

In general, manual phalanges are referred to the Theropoda based on the presence of deep, well-defined collateral ligament pits and the presence of an extensor fossa on the dorsal surface proximal to the distal articulation ( Rauhut 2003).

Only the distal part of the right metacarpal II (SM-PW9B-6) is preserved. It is long and gracile. The metacarpal is dorsoventrally flattened and resembles the metacarpal II of the basal tyrannosauroid Kileskus from Siberia ( Averianov et al. 2010) and identified here to be a distal part of the right metacarpal II. The distal end of metacarpal II is asymmetrical; it is slightly expanded and divided

A1 B 1

I into two distinct condyles. The distal surface of the medial condyle is larger than the lateral condyle, as in Kileskus and Australovenator ( White et al. 2012) . The medial condyle protrudes slightly more distally than the lateral condyle. The pits for the collateral ligaments are not well developed on both condyles. There is no distinct extensor pit on the anterior surface of the shaft proximal to the distal condyle, only a small concavity is present.

The manual phalanx I-1 (SM-PW9B-3) has deep, well-defined collateral ligament pits which are larger on the medial side than the lateral one. There is an extensor fossa on the dorsal surface proximal to the distal articulation. The shaft is proximodistally elongate, shows a high ratio of proximodistal length to transverse width and has a ginglymoid articulation which is asymmetrical and elongated mediodistally. The lateral condyle is about the same height dorsoventrally but shorter proximo-distally than the medial condyle, as in Australovenator , in which the lateral condyle is also shorter proximo-distally than the medial condyle ( White et al. 2012). The lateral condyle is expanded proximally. The proximal end is asymmetrical, with the medial articular surface larger than the lateral surface. The proximal articular surface is transversely wider on its ventral margin than on its dorsal margin. A ridge on the proximal articular surface slopes slightly laterally rather than being in the middle of the articular surface. This feature is also present in Gualicho ( Apesteguia et al 2016). The manual phalange I-1 of Phuwiangvenator has a longitudinal ventral furrow and longitudinal ridges, and it is convex ventrally in proximal view ( Fig. 7A ).

C II 1 D1 E1 III F1 G1 50 mm G2 E2 F2 D2 III C 2 II A 2 I B 2 A 3 B 3 I C3 II D3 E 3 F3 G3 II

The manual phalanx III-1 (SM-PW9B-4) has well-defined collateral ligament pits. There is a shallow, poorly defined extensor fossa on the dorsal surface proximal to the distal articulation. There is no shallow furrow on the ventral surface of the phalanx. The shaft is proximodistally short and relatively robust. The ratio of proximodistal length to transverse width is approximately 2.8. The bone also has a ginglymoid articulation and is asymmetrical and elongated mediodistally. Two prominent ligament scars on the ventral surface proximal to the proximal articulation and one facet for articulation on the proximal end are present as in Allosaurus .

The manual phalanx III-2 (SM-PW9B-7) is not well preserved especially on the distal articulation. However, this bone shows deep, well-defined collateral ligament pits on the condyles and no extensor fossa on the dorsal surface proximal to the distal articulation. It is relatively shorter than the manual phalanx III-2 of other theropods.

The manual phalanx III-3 (SM-PW9B-5) has deep, well-defined collateral ligament pits on both condyles. There is no extensor fossa on the dorsal surface proximal to the distal articulation. There is no shallow furrow on the ventral surface of the phalanx. The bone is proximodistally elongate and slender, the ratio of proximodistal length to transverse width is 4. The bone has a ginglymoid articulation which is symmetrical. Two ligamental scars on the ventral surface proximal to the proximal articulation are present but not prominent or well defined. The shaft is slender, and its proximal part is extended ventrally. Two facets for articulation on the proximal end are present. Digit III is much more slen- der compared with Australovenator ( White et al. 2012) and Allosaurus ( Madsen 1976) . Phalanx III-3 is as long and slen- der as phalanx III-3 of Australovenator ( White et al. 2012) .

The manual unguals consist of three unguals in this study. The manual ungual I-2 (SM-PW9B-19) is large, missing the distal portion and the surface of the lower half of the left side. There is a small depression on the right side. In proximal view, the articular surface is almost symmetrical. The medial condyle has almost the same height dorsoventrally as the lateral condyle, but it is slightly broader than the lateral condyle. Based on the distal articular surface of phalange I-1, we suggest that this ungual is an ungual I-2 and belongs to the first digit. The proximal end is slightly broader mediolaterally than the distal-most preserved portion. The prominent, sub-oval, mound-like flexor tubercle is present proximally on the ventral surface ( Fig. 7 ). The vascular grooves are present on the lateral and medial surfaces, and are symmetrical. The proximal height/width ratio is 2.4. The manual ungual II-3 (SM-PW9B-23) is medium to large sized and missing the distal portion. The vascular groove is preserved on the medial side. It is symmetrical, the proximal end and the distal-most preserved portion are slightly equal in width mediolaterally. The sub-oval, mound-like flexor tubercle is present proximally on the ventral surface. The proximal height/width ratio is 2.67. The manual ungual III-4 (SM-PW9B-21) is small to medium sized and missing the distal portion. The vascular groove cannot be observed due to the preservation. The articular surface is symmetrical. The proximal end is slightly broader mediolaterally than the distal-most preserved portion. A prominent, sub-oval, mound-like flexor tubercle is present proximally on the ventral surface. The proximal height/width ratio is 2.27.

Tibiae: The right tibia is complete but fractured in the distal portion. The left tibia is not complete, lacking the tibial shaft, only the proximal and distal ends are preserved. The proximal part of the right tibia is better preserved than that of the left one. The tibial shaft is long and almost straight. The cnemial crest is expanded. The anterolateral process of the lateral condyle of Phuwiangvenator curves ventrally as a point process as in Neovenator (Brusatte et al. 2008) , Australovenator ( Hocknull et al. 2009) , Allosaurus (UMNH VP 7148, 7922, 7932, 7938, 7939, 7940; AS personal observations), Lythronax (UMNH VP 20200; AS personal observations), Teratophoneus (UMNH VP 16690; AS personal observations), and Tyrannosaurus ( Brochu 2003) , but not in Fukuiraptor and Vayuraptor . The presence of this process may be more widespread than previously thought as suggested by Novas et al. (2013). The tibia shows derived states, the elongated and expanded medial malleolus and a distally expanded lateral malleolus ( Brusatte and Sereno 2008; Figs. 8 , 9 ).

The morphology of the distal cnemial process of the tibia is rounded. The lateral condyle is small relative to the tibial shaft. It offset from the lateral side of the proximal end of the tibia by a notch (posterior cleft) posteriorly. The fibular crest (the fibular flange) of the tibia is present on the proximal half as a pronounced longitudinal ridge. The fibular crest is clearly offset from the proximal articular facet of the tibia. It does not connect with the proximal end of the tibia (the ridge continuing from the fibular flange to the proximal articular surface of the tibia is absent). The fibular crest morphology is teardrop-shaped in lateral view. The primary nutrient foramen is present and situated posterior to the distal portion of the fibular crest. In the proximal end, the posterior rim of the lateral condyle is short and does not reach the same level as the medial condyle. The lateral malleolus of the distal

B

tibia is overlapped by the calcaneum. The distal end of the tibia is triangular in outline, flattened anteroposteriorly, and strongly expanded laterally and medially. The lateral malleolus of the distal expansion extends laterally and extends further distally than the medial malleolus. The anteromedial buttress for the astragalus is a bluntly rounded vertical ridge on the medial side. The tibia length of Phuwiangvenator is more than 12 times its anteroposterior width at mid-length. This is a synapomorphy of megaraptorans and coelurosaurs Porfiri et al. 2014) but also present in Spinosaurus ( AS personal observations; see Discussion; SOM: table 17).

Astragalocalcaneum.—The left astragalocalcaneum (SM-PW9B-18) is nearly complete, lacking only the body of the ascending process of the astragalus ( Fig. 10B ). The astragalus is 103 mm wide, the calcaneum is 25 mm wide. The astragalar width/calcaneum width ratio is 4.12. The morphology of the ascending process of the astragalus is laminar. It is approximately 90 mm long estimated from the scar for the reception of the ascending process of the astragalus on the distal end of the tibia, approximately 15% of the total length of the tibia. However, this scar does not necessarily correspond to the shape and height of the ascending process of the astragalus ( Rauhut and Xu 2005; Rauhut 2012). It is offset from the anterior border of the astragalar body by a shallow groove. The ascending process height relative to the depth of the astragalar body is approximately 1.7 times. The ascending process is slightly transversely narrow compare to more derived coelurosaurs, but it is wider than in Allosaurus (UMNH VP 11003; AS personal observations). The ascending process of the astragalus arises from more than half of the breath of the astragalar body. A round fossa at the base of the ascending process of the astragalus is present.

The cranioproximal process is present, but small compared to other theropods. A distinct anterior development of the lateral condyle (in ventral view) of the astragalar body is present. A proximolateral extension of the astragalus is also present as in Fukuiraptor ( AS personal observations), but with some damaged on its tips.

The trapezoidal outline of the bone in distal view looks similar to Australovenator and the megaraptoran astragalus NMV P150070 ( Benson et al. 2012b). In distal view, the astragalar body is concave anteriorly and straight posteriorly. Its surface is flat on the lateral half and convex on the medial half.

The astragalus condyle is significantly expanded proximally on the anterior side of the tibia and faces anterodistally. The orientation of the distal condyles of the astragalus is 30–45° anteriorly. A horizontal groove across the astragalar condyle is present anteriorly. The development of the articular surface for the distal end of the fibula in the astragalus is reduced and situated laterally. A posterolateral crest and a posteromedial crest on the astragalus are not present. The articulation between the ascending process and the fibula is not preserved. Both astragalus and calcaneum are unfused. The calcaneum bears a well-developed facet for the tibia. The calcaneum transverse development is moderate, being approximately 24% (left) and 23% (right) of the width of the astragalus.

The right astragalocalcaneum ( SM-PW9 A-B17) is referred to the same taxon and the same individual as the holotype of Phuwiangvenator since it perfectly matches with the right tibia of the holotype and shows the same size and characters as the left astragalocalcaneum ( Fig. 10A ; SOM: tables 6–9) .

Metatarsals: The metatarsals of Phuwiangvenator comprises a left metatarsal I (SM-PW9B-1) and right metatarsals II–IV (SM-PW9B-42, 43, and 44, respectively).

The left metatarsal I (SM-PW9B-1) has well-defined collateral ligament pits which are deeper on the right side than on the left one. An extensor fossa on the dorsal surface proximal to the distal articulation is present. There is one shallow fossa on the ventral surface proximal to the distal articulation. It is proximodistally elongate. The ratio of proximodistal length to transverse width is approximately 5.2. The bone is asymmetrical and lacks a ginglymoid articulation ( Fig. 11 ).

The metatarsal II is subequal in width to the Mt III and Mt IV proximally (see Rauhut 2003). The shape of the proximal end of the metatarsal III has a deep notch or “hourglass” shaped or “notched” outline which is most pronounced in avetheropods ( Carrano et al. 2012). At the broken part, distally below the proximal end, the posterior part of the shaft of the metatarsal III is strongly pinched between the shafts of metatarsals II and IV. In proximal view, metatarsal III is anteroposteriorly oriented, not anterolateral to posteromedial. The anteroposterior orientation of the metatarsal III can be found in Australovenator , Chilantaisaurus , and Neovenator , but not in Tanycolagreus and Ornitholestes . The anterior rim of metatarsal IV slopes from proximolaterally to distomedially. This way, the distomedial corner of the proximal articular surface of metatarsal IV in anterior view is much lower than the articular surface of metatarsal III ( Fig. 12 ).

Pedal phalanges and unguals: The pedal phalanges consist of phalanges II-1, II-2, III-2, III-3, IV-1, and IV4 (SM-PW9B-5–9 and SM-PW9B-45, respectively; Fig. 13 ). The pedal phalanges are as in normal theropods in shape.

The pedal unguals in this study consist of unguals I-2, II-3, III-4, and IV-5 (SM-PW9B-11, 22, 19, and 10, respectively). Specimens SM-PW9B-22 and SM-PW9B-19 are medium to large sized and missing the distal portion. They have a rounded cross-section that is symmetrical. SM-PW9B-10 is complete. It has flat ventral surface as in Australovenator ( Fig. 14 ). Two vascular grooves are present on both sides. The flexor tubercle is poorly developed. It is higher dorsoventrally than broader mediolaterally and weakly curved. In general, the pedal unguals have the same shape as in other theropods. They do not show extreme dorsoventral flattening in lateral view as in Spinosaurus and some ornithomimosaurs.

Stratigraphic and geographic range.— Type locality and horizon only.

Figures

Fig. 3. Coelurosaurian theropod Phuwiangvenator yaemniyomi gen. et sp. nov. from the Early Cretaceous Sao Khua Formation of Phu Wiang, Khon Kaen Province, Thailand. A. SM-PW9A-1, atlantal intercentrum in anterior (A1) and posterior (A2) views. B. SM-PW9B-12, dorsal vertebra in left lateral (B1), anterior (B2), ventral (B3), and posterior (B4) views.

Fig. 4. Coelurosaurian theropod Phuwiangvenator yaemniyomi gen. et sp. nov. from the Early Cretaceous Sao Khua Formation of Phu Wiang, Khon Kaen Province, Thailand. SM-PW9B-39, anterior sacral vertebrae in right lateral (A1) and ventral (A2) views; line drawing in ventral view (A3). Arrows indicate ventral sulci, light grey, broken part; dark grey, matrix.

Fig. 2. Map of Thailand (A) and close-up of northeastern Thailand (B) showing the location of Phu Wiang locality, Khon Kaen Province (square) and Phu Wat locality, Nong Bua Lamphu Province (star). Dark gray, Sao Khua Formation outcrops, light gey, Phu Kradung and Khok Kruat formations.

Fig. 7. Coelurosaurian theropod Phuwiangvenator yaemniyomi gen. et sp. nov. from the Early Cretaceous Sao Khua Formation of Phu Wiang, Khon Kaen Province, Thailand. A. SM-PW9B-3, manual phalanx I-1 in ventral (A1) and proximal (A3) views; line drawing in ventral view (A2). B. SM-PW9B-19, manual ungual I-2 in medial (B1), distal (B2), lateral (B3), proximal (B4), dorsal (B5), and ventral (B6) views.Arrow indicates the longitudinal ventral furrow.

Fig. 8. Coelurosaurian theropod Phuwiangvenator yaemniyomi gen. et sp. nov. from the Early Cretaceous Sao Khua Formation of Phu Wiang, Khon Kaen Province, Thailand. A. SM-PW9B-41, right tibia in anterior (A1), lateral (A2), posterior (A3), medial (A4), proximal (A5), and distal (A6) views. B. SM-PW9B-40, proximal left tibia in lateral view. Arrow indicates vertical ridge on the tibia.

Fig. 9. Coelurosaurian theropod Phuwiangvenator yaemniyomi gen. et sp. nov. from the Early Cretaceous Sao Khua Formation of Phu Wiang, Khon Kaen Province, Thailand. A. SM-PW9B-41, right tibia in proximal view. B. SM-PW9B-40, left tibia in distal view. Light grey indicates broken bone.

Fig. 10. Phuwiangvenator yaemniyomi gen. et sp. nov. from the Early Cretaceous Sao Khua Formation of Phu Wiang, Khon Kaen Province, Thailand. SM- PW9A-B17, right (A) and SM-PW9B-18, left (B); astragalocalcaneum in anterior (A2, B2), medial (A3, B3), distal (A4, B4), and lateral (A5, B5) views; line drawings in anterior view (A1, B1). Dashed lines are the reconstruction of the ascending processes of the astragali.

Fig. 11. Coelurosaurian theropod Phuwiangvenator yaemniyomi gen. et sp. nov. from the Early Cretaceous Sao Khua Formation of Phu Wiang, Khon Kaen Province, Thailand. SM-PW9B-1, left metatarsal I in lateral (A1), posterior (A2), anterior (A3), and medial (A4) views.

Fig. 12. Coelurosaurian theropod Phuwiangvenator yaemniyomi gen. et sp. nov. from the Early Cretaceous Sao Khua Formation of Phu Wiang, Khon Kaen Province, Thailand. SM-PW9B-44 (A), SM-PW9B-43 (B), SM-PW9B-42 (C); metatarsals in anterior (A1–C1) and proximal (A2–C2) views; line drawings in anterior (A3–C3) and proximal (A4–C4) views; metatarsal III in medial (B5) and distal (B6) views.

Fig. 13. Coelurosaurian theropod Phuwiangvenator yaemniyomi gen. et sp. nov. from the Early Cretaceous Sao Khua Formation of Phu Wiang, Khon Kaen Province, Thailand. SM-PW9B-11 (A), SM-PW9B-7 (B), SM-PW9B-8 (C), SM-PW9B-22 (D), SM-PW9B-5 (E), SM-PW9B-9 (F), SM-PW9B-19 (G), SM-PW9B-6 (H), SM-PW9B-45 (I), SM-PW9B-10 (J); right pedal phalanges (B, C, E, F, H, I) and unguals (A, D, G, J), in ventral (A1–J1) and dorsal (A2–J2) views; line drawings in dorsal view (A3–J3). Dashed lines are the reconstruction of pedal phalanges III-1, IV-2, and IV-3 and pedal unguals II-3 and III-4.

Fig. 14. Coelurosaurian theropod Phuwiangvenator yaemniyomi gen. et sp. nov. from the Early Cretaceous Sao Khua Formation of Phu Wiang, Khon Kaen Province, Thailand. SM-PW9B-10, right pedal ungual IV-5 in lateral (A1) and ventral (A2) views.