Anchiornis, HUXLEYI

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 : 54-57

AFFILIATION OF ANCHIORNIS HUXLEYI

Affiliation to Maniraptora and Paraves

The morphology of Anchiornis huxleyi can be confidently assigned to the Maniraptora. Several synapomorphies of Maniraptora are present in Anchiornis huxleyi : broad triangular process along the lateral edge of the quadrate that contacts the squamosal and the quadratojugal; absence of the prefrontal, fused parietals, teeth constricted between the root and crown, reduced prezygapophyses on the distal caudal vertebrae, presence of a semilunate carpal; manual phalanx III-3 significantly elongate, and fibular shaπ narrowing abruptly below the iliofibularis tubercle.

In many previous studies a monophyletic Paraves is recovered as a clade that includes Dromaeosauridae , Troodontidae , and Avialae ( Gauthier, 1986; Sereno, 1997, Makovicky and Sues, 1998; Xu et al., 1999; Hwang et al., 2002; Makovicky et al., 2003; Xu and Norell, 2004; Makovicky et al., 2005; Novas and Pol, 2005; Norell et al., 2006; Turner et al., 2007, 2012; Hu et al., 2009; Senter et al., 2012; Brusatte et al., 2014; Foth et al., 2014; Lü and Brusatte, 2015). Anchiornis huxleyi can be referred to Paraves based on the following derived features: shallow maxilla and premaxilla below the external naris, T-shaped lacrimal; dentary symphyseal region in line with the main part of the buccal margin, chevrons anteriorly bifurcate on the distal part of the tail, nearly symmetric furcula, acromion margin of the scapula with a laterally everted anterior edge, coracoid inflected medially from the scapula, forming an L-shaped scapulocoracoid, humerus longer than the scapula, ilium with the pubic peduncle much larger than the ischiadic peduncle, femur with a posterior trochanter, and the distal end of metatarsal III ginglymoid.

Within Paraves, the affiliation of Anchiornis is controversial ( Xu et al., 2009, 2011; Hu et al., 2009; Turner et al., 2012; Senter et al., 2012; Foth et al., 2014). This confusion is caused mainly by the conservative morphology of basal paravians. However, careful examination of new specimens gives us better clue of the affiliation of Anchiornis .

Comparison with Archaeopteryx

Anchiornis huxleyi shares many features in common with Archaeopteryx that are not present in dromaeosaurids and troodontids. Anchiornis resembles Archaeopteryx in having: straight nasal process of the premaxilla, external naris posterior to all premaxillary teeth, fewer than 20 dentary teeth, absence of the coranoid eminence, absence of lateral exposure of the splenial, and absence of an external mandibular fenestra in lateral view. In the postcranial skeleton, Anchiornis shares the following features with Archaeopteryx : slightly bowed metacarpal III, pubic peduncle more ventrally expressed than the ischiadic peduncle, constricted base of the distally located obturator process, absence of a slit on the pubic apron, and proximal end of metatarsal III unconstricted. All of these features are found to be primitive for avialans, either as synapomorphies that are unique for avialans or synapomorphies of a more inclusive coelurosaurian clade that are secondarily lost in deinonychosaurians.

Anchiornis also shares the following features with Archaeopteryx : enlarged orbit, presence of a shallow premaxilla, presence of a promaxillary fenestra, enlarged maxillary fenestra, lacrimal with slender and subequal anterior and posterior processes, dorsal vertebrae with lateral concavities but not pneumatic foramina, scapula significantly shorter and slimmer than humerus, manual phalanx III-3 significantly longer than III-1 and III-2 combined, supraacetabular crest, and ischium with a distally located obturator process. However, these features are also present in many basal troodontids and dromaeosaurids and thus are primitive for Paraves.

Comparison with Troodontidae

Previous studies have assigned Anchiornis as a basal troodontid ( Hu et al., 2009; Turner et al., 2012; Pei et al., 2014). Hu et al. (2009) suggested Anchiornis and others troodontids have the following features in common: large maxillary fenestra, labial surface of dentary bearing a distinct groove, closely packed premaxillary and dentary teeth in the symphyseal region, and dorsal vertebrae and anteriormost caudal vertebrae bearing relatively long and slender transverse processes. Hu et al. (2009) also suggested Anchiornis shares with Mei a large external naris extending posteriorly beyond and dorsal to the anterior border of antorbital fossa, a longitudinal groove along dorsomedial margin of the jugal, unserrated teeth, and a maxillary toothrow approaching the preorbital bar posteriorly. However, a large maxillary fenestra, a large external naris extending posteriorly beyond the anterior border of antorbital fossa, a longitudinal groove along dorsomedial margin of the jugal, unserrated teeth, and long transverse processes on dorsal vertebrae and anteriormost caudal vertebrae, are also present in Archaeopteryx and basal dromaeosaurids ( Wellnhofer, 1974; Mayr et al., 2005; Makovicky et al., 2005; Foth et al., 2014; Pei et al., 2014). The primitive pennaraptoran Epidexipteryx also has closely packed premaxillary and dentary teeth in the symphyseal region ( Zhang et al., 2008). The lateral groove on the dentary and maxillary toothrow approaching preorbital bar are observed in Archaeopteryx and other nontroodontid basal deinonychosaurians such as Buitreraptor ( Wellnhofer, 1974; Makovicky et al., 2005). Thus, these features are all primitive to a more inclusive coelurosaurian clade.

In the phylogenetic analysis of Turner et al. (2012), three characters were recovered to support Anchiornis at the basalmost branch of Troodontidae , which include a dorsoventrally flattened internarial bar, the anteroventrally inclined quadrate and a subarctometatarsalian pes. In scoring based on a new Archaeopteryx specimen ( Foth et al., 2014), the internarial bar of Archaeopteryx is scored as dorsoventrally flat as in troodontids like Sinovenator , Byronosaurus , and Zanabazar , and a dorsoventrally flat internarial bar is also observed in the dromaeosaurid Microraptor ( Pei et al., 2014) , so this feature is no longer restricted to Troodontidae . Both dorsoventrally flat (e.g., Eichstätt and Thermopolis specimens) and rodlike (e.g., Solnhofen specimen) internarial bars are actually observed in Archaeopteryx . The anteroventrally inclined quadrate is also observed in Archaeopteryx and more basal taxa such as Sinosauropteryx and Ornithomimus ( Ji and Ji, 1996; Wellnhofer, 2009; Rauhut, 2014), and thus it is a synapomorphy of a larger clade. The arctometatarsalian pes of Anchiornis is dubious, and new specimens revealed the pes of Anchiornis resembles more the normal pedal configuration of Archaeopteyrx than it does the arctometatarsalian pes of Microraptor and Sinovenator . As discussed above, these three characters no longer adequately support Anchiornis as a member of Troodontidae .

As a Jurassic basal paravian, Anchiornis huxleyi differs from troodontids in several features. These features include: posteriorly tapering ventral ramus of the maxilla, elongation of the forelimb, middle and posterior maxillary teeth sparsely spaced, fewer than 20 dentary teeth, presence of a basisphenoid recess, wide and dorsoventrally low foramen magnum, transition of elongate caudal vertebrae occurring before the 10th caudal vertebra and symmetric metatarsus. These features are primitive for paravians and are found in basal dromaeosaurids and/or basal avialans, but absent in troodontids.

Comparison with Deinonychosauria

Because Anchiornis displays many primitive paravians features that are present in both troodontids and the basal avialan Archaeopteryx , it is necessary to look at derived features of Deinonychosauria in order to distinguish Anchiornis from Troodontidae . Anchiornis does not have a lateral exposure of the splenial and also lacks a specialized raptorial digit II, which differentiates it from deinonychosaurians.

Some features that are regarded as derived characters in deinonychosaurians, such as a subarctometatarsalian pes and a raptorial pedal digit II, were considered to be present in Anchiornis and used as evidence for a troodontid affiliation of this taxon ( Hu et al., 2009). However, careful examination of Anchiornis specimens indicates Anchiornis resembles Archaeopteryx in these pedal characters.

The pes of Anchiornis is reported as subarctometatarsalian ( Xu et al., 2009; Hu et al., 2009). But close examination and comparison of new Anchiornis specimens indicate that the proximal end of metatarsal III is not significantly constricted. Instead, the proximal end of metatarsal III has a subequal width as metatarsals II and IV in anterior view, which resembles Archaeopteryx and most nontroodontid theropods, but different from the subarctometatarsus of basal troodontids (e.g. Sinovenator and Mei) and the type specimen of Sinornithosaurus and Microraptor , where the proximal end of metatarsal III is significantly reduced in width in anterior view compared to metatarsal II and metatarsal IV ( Xu et al., 1999, 2000, 2002; Xu, 2002; Xu and Norell, 2004). Anchiornis was also reported to have a deinonychosaurian-like pedal digit II ( Xu et al., 2009; Hu et al., 2009). Careful examination of more specimens of Anchiornis indicates that pedal digit II of these specimens lacks the typical specialized raptorial condition of deinonychosaurians. In Anchiornis , pedal phalanx II-2 is not reduced in length and lacks a well-developed ventral heel or a constriction at midshaπ, which is different from deinonychosaurians ( Gauthier, 1986; Turner et al., 2012). Ungual phalange II-3 of Anchiornis is only slightly larger than other pedal ungual phalanges, not as distinctly enlarged as in deinonychosaurians. Although the pedal digit II of Anchiornis is more derived than the configuration of basal maniraptoriforms (such as ornithomimosaurians, alva- rezsaurids, and oviraptorosaurians), it still exhibits the primitive features (as mentioned above) of paravians that is also observed in Archaeopteryx (see Turner et al., 2012). As a contrast in deinonychosaurians, even the basal members of each lineage, such as Mahakala and Sinovenator have highly modified and hyperextensible pedal digit II that is different from Anchiornis and Archaeopteryx ( Xu et al., 2002; Turner et al., 2007, 2011, 2012).

We consider Anchiornis huxleyi as a basal avialan based on derived characters it shares with other avialans ( Xu et al., 2009; Godefroit et al., 2013a; Foth et al., 2014), such as a straight nasal process of the premaxilla and the absence of an external mandibular fenestra in lateral view. Anchiornis also lacks typical deinonychosaurian synapomorphies such as lateral exposure of the splenial, and a specialized raptorial pedal digit II, which differentiates Anchiornis from deinonychosaurians.