Sinocalliopteryx (CAGS-IG-T1) Description and Comparison

The skull of CAGS-IG-T1 includes both maxillae, right nasal, right lacrimal, right prefrontal, right jugal, left palatine and vomer, and fragmentary right dentary (Figure 2A, B). The left maxilla, shown in medial view, has at least ten alveoli, six of which hold teeth. Given that the anterior ramus is incomplete, the maxillary tooth count probably exceeded ten by one or two. The most posterior maxillary alveolus is ventral to or slightly anterior to the anterior end of the maxillary-lacrimal contact. The anterior ramus of the maxilla is demarcated by the transition to the posterodorsally-oriented ascending ramus. The posterodorsal process of the maxilla is dorsoventrally deeper than the horizontal ramus. Near the posterior end, the posterodorsal process splits into the larger and longer lateral prong and the smaller and shorter medial prong, between which the lacrimal was clasped. The medial surface of the maxilla above the palatal shelf is smooth and not excavated into the conspicuous maxillary antrum and promaxillary recess as in other theropods [7]. The antorbital fossa has a distinct margin. The maxillary fenestra is absent.

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larger image TIFF original image Download: Figure 2. Sinocalliopteryx gigas (CAGS-IG-T1), partial skeleton. A, B; skull; C, dorsal vertebrae and ribs. Arrow points to partly covered Confuciusornis humerus; D, associated feet; E, associated pedal phalanges and unguals; F, articulated tail with filamentous integument. Abbreviations: C, centrum; Ch, chevron; Dr, dorsal rib; In, integument; Ju; jugal; La, lacrimal; Mt, metatarsal; Mx, maxilla; Na, nasal; Pal, palatine; Pfr, prefrontal; Ph, phalanx; Pm, premaxilla; Po, postorbital; Tc, tooth crown; Un, ungual; V, vomer. Scale bars in C–F equal 10 cm in 1 cm increments. https://doi.org/10.1371/journal.pone.0044012.g002

A lacrimal duct is present dorsal to the anterior margin of the preorbital bar. The dorsal edge of the lacrimal is inflated into a longitudinal, dorsally low cornual process. The prefrontal is as long as the anterior ramus of the lacrimal. The postorbital process of the right jugal was anteriorly displaced and now sits on the right and left maxillae. The process retains a groove along its anterior margin that would have received the postorbital. The vomer is dorsoventrally deeper than the horizontal ramus of the maxilla and has a dorsally convex margin. The palatine contacts the maxilla posterior to the maxillary tooth row and posterior to the anterior margin of the antorbital fenestra. The dentary is preserved for the anteroposterior length of three tooth positions.

Six dorsal vertebrae are preserved in one of the slabs (Figure 2C). The neurocentral sutures are visible in all of these vertebrae, but the sutures are not completely open because the neural arches and the centra are tightly knit. All of the dorsal vertebrae lack pleurocoels as in other compsognathids [8]–[14]. Six left dorsal ribs are preserved with the vertebral series. In a separate slab, two dorsal ribs, nine medial gastralia, and at least ten lateral gastralia surround the abdominal contents of this specimen. The abdominal contents are between the dorsal ribs and the gastralia and partially overlapped by these elements (Figure 3A–F). The right and left ischia as well as the abdomen of Sinocalliopteryx CAGS-IG-T1 (along with the gastralia and the abdominal contents) have all shifted posteriorly relative to their position in life. Two elements of the abdominal contents (scapulocoracoid and sternum) lie on a horizontal plane between the left and right ischia (Figure 3C). Two slabs contain caudal vertebrae (Figure 2F). One of the two slabs contains the 11th to 15th caudal vertebrae with L-shaped haemal arches. The other slab contains an articulated series of 13 mid- to distal-caudal vertebrae, of which 11 are entirely preserved. In that slab, only the first two vertebrae have dorsoventrally low neural spines. In comparison with the holotype of Sinocalliopteryx [4], the most anterior vertebra in the series represents the 16th caudal vertebra. All but the last two of the vertebrae are associated with L-shaped haemal arches. In the same slab, filamentous integument is preserved along both the dorsal and ventral margins of the tail (Figure 2F). The qualities of preservation and preparation on the specimen do not permit microscopic comparison of the integument. The neurocentral sutures are closed in all mid- to distal-caudal vertebrae.

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larger image TIFF original image Download: Figure 3. Abdominal contents of Sinocalliopteryx gigas (CAGS-IG-T1). A, B; block containing Confuciusornis (blue) and unidentified ornithischian (red) remains. C, Close up of Confuciusornis sternal and pectoral elements (small box in B); D, E; associated skeleton of Confuciusornis (large box in B); F, proximal Confuciusornis humerus (arrow in Figure 2). Abbreviations: C, carpal; Dr, dorsal rib; Fu, furcula; Gs, gastralia; H, humerus; Il, ilium; Is, ischium; Mu, manual ungula; Ms, miscellaneous ornithischian bone; Ph, phalanx; Pu, pubis; Ra, radius; Sc, scapulocoracoid; St, sternum. Scale bars in A, B equal 10 cm in 1 cm increments; C, F in 1 mm increments. https://doi.org/10.1371/journal.pone.0044012.g003

The forelimb elements are scattered across two slabs. The main forelimb slab has the partially articulated right forearm and hand. The radius, metacarpal II, metacarpal III, and manual phalanx I-1 are complete, whereas other manual elements are overlain on one another such that identification is difficult. Metacarpal III is less than half as wide transversely as metacarpal II. This is the case in Compsognathus and Sinocalliopteryx [4], [14], [16], but differs from Huaxiagnathus, Nqwebasaurus, and Sinosauropteryx, in which metacarpal II is half as wide transversely as metacarpal III [10], [11], [13]. Although the full length of metacarpal II cannot be measured, it is as long as or slightly longer than manual phalanx I-1, as in Compsognathus, Huaxiagnathus, Juravenator, Scipionyx, and the holotype of Sinocalliopteryx [4], [9], [13], [14], [16], [17] but not as in Sinosauropteryx in which manual phalanx I-1 is substantially longer [10]. Manual phalanges II-1 and II-2 and the ungual for the digit are preserved near the metatarsals in a separate slab.

Both right and left metatarsals are preserved in a single slab. All of the metatarsals are present for the left foot, whereas the right foot is represented by only metatarsals II–IV (Figure 2D). In the left foot, metatarsal I is 24% of the length of metatarsal III. Metatarsal V is reduced to a curved splint less than half the length of metatarsal IV. Metatarsals II, III, and IV are cylindrical and straight. Distal to the metatarsals is a complete digit III, and two phalanges of digit I. Additional pedal phalanges are in the distal foot slab. The right pedal phalanges II-1, II-2, III-1, III-2, III-3, IV-3, IV-4, and pedal unguals II–IV are present (Figure 2E).

CAGS-IG-T1 clearly represents a compsognathid, distinguished by the nasal excluded from the antorbital fenestra by the maxilla and lacrimal, the absence of pleurocoelus in the dorsal vertebrae, and the manual phalanx I-1 nearly as long as metacarpal II [7], [10], [12], [14], [18]. CAGS-IG-T1 is anatomically almost identical to JMP-V-05-8-01 (the holotype of Sinocalliopteryx gigas; [4]) and therefore referable to Sinocalliopteryx gigas. CAGS-IG-T1 is larger based on the postcranial measurements (Table 2). The size difference between the two specimens is relatively greater in the length of the metatarsals than in the radius or height of the maxilla, presumably due to allometric growth. Although the original diagnosis of Sinocallipteryx does not include any characters preserved in CAGS-IG-T1, this specimen and the holotype of Sinocalliopteryx gigas can be distinguished from the similarly-sized, contemporaneous compsognathid Huaxiagnathus [13] based on several features of the maxilla: 1) The maxilla not as tall dorsoventrally in both specimens of Sinocalliopteryx as it is in Huaxiagnathus, in which the maxilla is two thirds taller at maximum than the anterior ramus; 2) The dorsal margin of the posterodorsal process of the maxilla forms an acute angle with the alveolar margin in Sinocalliopteryx whereas in Huaxiagnathus, the dorsal margin of the process is subparallel to the alveolar margin; 3) The maxillary fenestra is absent in Sinocalliopteryx, whereas the fenestra appears to be present in Huaxiagnathus [13]; 4) The anterior margin of the antorbital fenestra is dorsal to the seventh or eighth maxillary tooth position in Huaxiagnathus [13], whereas the anterior margin of the fenestra is dorsal to at least the ninth or possibly the tenth tooth position in Sinocalliopteryx.