Pedopenna, Xu & Zhang, 2005

Pei, Rui, Li, Quanguo, Meng, Qingjin, Norell, Mark A. & Gao, Ke-Qin, 2017, New Specimens Of Anchiornis Huxleyi (Theropoda: Paraves) From The Late Jurassic Of Northeastern China, Bulletin of the American Museum of Natural History 2017 (411), pp. 1-66 : 58-59

COMPARISONS WITH PEDOPENNA , XIAOTINGIA, EOSINOPTERYX, AND AURORNIS

Pedopenna daohugouensis ( Xu and Zhang, 2005) is a Jurassic paravian reported in 2005 based on hindlimbs from the Daohugou area of the Yanliao Biota. Pedopenna differs from Anchiornis from the proportions of pedal phalanges ( Xu and Zhang, 2005). The morphology of the hindlimb of Pedopenna is conservative, and is very similar to the Jianchang paravians. However, the type specimen (which is also the only specimen) of Pedopenna is very poorly preserved and prevents further comparison.

Three Jurassic paravians, Xiaotingia , Eosinopteryx , and Aurornis , have been reported from the Jianchang area following the discovery of Anchiornis huxleyi ( Xu et al., 2011; Godefroit et al., 2013a, 2013b). All of those other taxa are based on single specimens. Xiaotingia was reported in 2011, based on a specimen with a complete skull and a partial postcranial skeleton, missing the tail, partial pectoral girdle, and distal hindlimb. Eosinopteryx and Aurornis were reported in 2013, both with complete cranial and postcranial skeletons. However, several bones of these three specimens are shattered and split like many other western Liaoning dinosaurs, so they provide only limited anatomical information.

These Jianchang paravians ( Xiaotingia , Eosinopteryx , and Aurornis ) closely resemble Anchiornis huxleyi in a number of characters: straight nasal process of the premaxilla, posteriorly placed external naris, relatively short rostral ramus of the maxilla, ventrally displaced promaxillary fenestra, dentary with a distinct lateral groove, sheetlike posteroventral process of the dentary, mandible that is significantly deeper posteriorly than anteriorly, short deltopectoral crest, straight ulna and radius, and extremely expanded proximal end of the fibula. An extremely short ischium is also shared by Anchiornis , Eosinopteryx , and Aurornis . All recently reported paravians from Jianchang County likely form a monophyletic group based on their anatomical similarities (also see Foth et al., 2014).

In the original diagnoses for Xiaotingia , it differs from Anchiornis in having the following unique features: maxillary posterior ramus has a depth at midlength exceeding that of the dentary; surangular has a small lateral exposure and forms a wide, flat dorsal surface over the posterior part of the mandible; large surangular foramen extends over more than 6% of the total mandibular length; posterior end of the mandible is blunt and dorsoventrally expanded; anteriormost caudal centra are less than half as long as the posterior dorsal centra; metacarpal III is more robust than metacarpals I and II; and manual phalanx II-2 is longer than metacarpal II ( Xu et al., 2011). In Anchiornis , the subantorbital-fenestra portion of the maxilla (to be clear, the posterior ramus [ventral ramus] of the maxilla is actually thin, but a distance exists between the ventral ramus and the antorbital fenestra in all specimens of Anchiornis ) is dorsoventrally deep and the posterior end of the mandible is also dorsoventrally expanded, as in Xiaotingia . The proportional differences of the caudal vertebrae, an elongate surangular foramen, and manual phalanx II-2 longer than metacarpal II are likely a result of allometric variation. However, other diagnostic features of Xiaotingia , such as the surangular with little lateral exposure, forming a wide dorsal surface over the posterior part of the mandible, and a much more robust metacarpal III, are not observed in any reported Anchiornis specimens. Xiaotingia also differs from other Jianchang paravians in having a proportionally longer ischium.

Eosinopteryx is diagnosed by: short snout; lacrimal with a longer posterior process; short tail of 20 caudal vertebrae; chevrons reduced to small rodlike elements below the proximal 8th or 9th caudals; ilium with a proportionally long, low, and distally tapering postacetabular process; pedal unguals shorter than corresponding penultimate phalanges; and absence of rectrices and feathers on the metatarsus ( Godefroit et al., 2013a). In this study we find not all these features are diagnostic. The snout of YFGP-T5197 (type specimen of Eosinopteryx brevipenna ) is broken and incomplete, and thus cannot be used for comparison. Moreover, the anteroposterior length of the preorbital portion of the skull is about 1.5–2 times that of the orbit, which is the same as in Anchiornis . The lacrimal of BMNHC PH 804 has both long anterior and posterior processes, but both processes are extremely slender and easy to break, so the longer posterior process of the lacrimal is not a reliable diagnostic feature of Eosinopteryx . Reduced chevrons below the proximal 8th or 9th caudal vertebrae are observed in other Anchiornis specimens, such as BMNHC PH 804 and BMNHC PH 822. Anchiornis specimens have variation in proportions of pedal phalanges, even within the same specimen, such as in BMNHC PH 804 (see above). A long and low posterior (= postacetabular) process of the ilium is also present in Anchiornis (e.g., IVPP V14378 , BMNHC PH 804, BMNHC PH 822, and BMNHC PH 823). The absence of rectrices and feathers on the metatarsus is possibly a preservational artifact, as many Anchiornis specimens only have feathers associated with few bones instead of the entire body. The characters mentioned above cannot be regarded as solid diagnosis to distinguish Eosinopteryx from Anchiornis . However, a short tail does differentiate Eosinopteryx from Anchiornis . Normally more than 30 caudal vertebrae are preserved in Anchiornis . Even if only 22 caudal vertebrae are observed in PKUP V1068, it clearly does not represent the life condition of that specimen. Moreover, the 12th–14th caudal vertebrae reach the maximum length in Eosinopteryx , as in most Anchiornis specimens, while the 6th exposed caudal vertebra reaches the maximum length in PKUP V1068. The morphology of Eosinopteryx is mostly identical to that of Anchiornis , except for the short length of the tail in the former. We regard Eosinopteryx as a valid taxon, but firsthand examination of the specimen is necessary to further confirm this conclusion.

As mentioned by Godefroit et al. (2013b), Aurornis xui (YFGP-T5198) differs from Anchiornis huxleyi in having an elongate subnarial process of the premaxilla, an elongate posterior process of the lacrimal, a relatively shorter and more gracile humerus compared to the femur, a postacetabular process that is quadrangular in lateral view, the presence of a hooked ischium, an elongate metatarsal I that is not reduced to a splint, and the shorter penultimate phalanges of the pedal digits ( Godefroit et al., 2013b: supplementary information). However, these features are inadequate to differentiate Aurornis from Anchiornis as a distinct new taxon. Many cranial bones of YFGP-T5198 are shattered, and the presumed subnarial process is not confluent with the remainder of the premaxilla ( Godefroit et al., 2013b: fig. 2a ), and thus likely a misidentification. The elongate piece of bone exposed between the shattered nasal and maxilla is possibly the vomer or the ventral ramus of the adjacent maxilla, but this assertion requires further examination of the specimen to validate it. BMNHC PH 804 also has an elongate posterior process of the lacrimal as in YFGP-T5198. Although the long posterior process of the lacrimal is not observed in all Anchiornis specimens, it is likely a preservational artifact, since this process is extremely slender and difficult to preserve. The humerus of YFGP-T5198 is relatively shorter and more slender than the femur, with a femur/ humerus ratio of 1.13, but it is within the range that is observed in other Anchiornis specimens (discussed above). The posterior (= postacetabular) process of the right ilium of YFGP-T5198 is more squared than in other Anchiornis , but the left posterior (= postacetabular) process of YFGP-T5198 is dorsoventrally shallow and has a posteroventrally sloping dorsal edge as in other Anchiornis specimens (e.g., IVPP V14378 , LPM- B00169, BMNHC PH 804, BMNHC PH 822, and BMNHC PH 823). The elongation of metatarsal I in Aurornis is also likely a preservational artifact, as no complete metatarsal I is preserved/exposed in any reported Anchiornis specimens. Anchiornis also shows variation in proportions of pedal phalanges, even within the same specimen (BMNHC PH 804), and thus the shorter penultimate phalanges of the pedal digits also cannot differentiate Aurornis from Anchiornis .

Figures

FIGURE 2. Mounted slab of BMNHC PH804.