The pedal digital formula of P. djadokhtaensis is 0-3-4-4-0. The first phalanx of each digit is the largest, followed by relatively thin, wedge-shaped phalanges that terminate in a hoof-like ungual.
PHYLOGENETIC ANALYSIS
Methods
The phylogeny of hadrosauroids has undergone major revisions in the last decade, as numerous new taxa have been discovered and described. A number of large-scale, taxonomically inclusive phylogenetic analyses have also been conducted (e.g., You et al., 2003; Horner et al., 2004; Norman, 2004; Evans and Reisz, 2007; Sues and Averianov, 2009; Prieto-Márquez, 2010b; Godefroit, Bolotsky, and Lauters, 2012; Godefroit, Escuillié, et al., 2012; McDonald, 2012; Wu and Godefroit, 2012). In order to provide a preliminary hypothesis with regard to the phylogenetic position of Plesiohadros, the type and referred material were scored into a modified version of the data matrix published by Sues and Averianov (2009). The analysis of Prieto-Márquez (2010b) and modifications thereof were not considered because most iterations have focused on more derived hadrosauroids. The Sues and Averianov (2009) matrix was expanded to include the recently described taxa Jinzhousaurus yangi and Tethyshadros insularis, which were scored for all characters in the matrix using descriptions in the literature (Barrett et al., 2009; Dalla Vecchia, 2009; Wang et al., 2011). The majority of character scorings were left unchanged (although some scorings for specific taxa were modified, see data matrix in Appendix 7.1). All characters were considered unordered. The character matrix for the analysis includes 42 taxa and 138 characters. The data matrix was analyzed using TNT (Goloboff et al., 2008) under the default setting TBR branch-swapping algorithm with 10,000 random addition sequences. Bremer Decay and bootstrap analyses were conducted to assess the robustness of the results.
Results
The analysis resulted in 10 most-parsimonious trees (MPTs), each with a tree length of 309 steps, a Consistency Index (CI) of 0.505, and a Retention Index (RI) of 0.869. The strict consensus cladogram is shown in Figure 7.24. Plesiohadros djadokhtaensis is recovered as the sister taxon to the least inclusive clade containing Lophorhothon atopus and Hadrosauridae (sensu Sereno, 2005 and Horner et al., 2004). This position is supported by eight unambiguous synapomorphies, including: a broadly arcuate premaxilla that constricts abruptly behind oral margin (character 3: state 1); the presence of a free ventral flange on the jugal that is dorsoventrally constricted beneath infratemporal fenestra (character 60: state 1); free ventral flange is rounded or lobate (character 61: state 1); free ventral flange is small, with a ratio of depth of jugal at constriction below infratemporal fenestra to length of free ventral flange between 0.7 to 0.9 (character 62: state 1); angle between postorbital bar and jugular bar is acute (character 63: state 1); mandibular condyle of quadrate is subtriangular in distal view with a lateral condyle expanded anteroposteriorly relative to the medial condyle (character 67: state 1); a long diastema length of the dentary, greater than one-fifth the length of the tooth row (character 80: state 1); and the coronoid process is formed almost entirely by the dentary with the surangular reduced to thin sliver along posterior margin and does not reach distal end of coronoid process (character 85: state 1). An interesting result of the analysis is that P. djadokhtaensis does not belong to Hadrosauridae (sensu Horner et al., 2004), despite its probable Late Campanian age, but is a derived non-hadrosaurid hadrosauroid.
7.21. Left manus of Plesiohadros djadokhtaensis (MPC-D100/745) in (A) caudal; and (B) cranial views. Scale bars equal 10 cm. Abbreviations: mtc-II, metacarpal II; mtc-III, metacarpal III; mtc-IV, metacarpal IV; mtc-V, metacarpal V; ung-IV, ungual of digit IV.
The analysis also supports the hypothesis of Weishampel and Jainu (2011) that Bactrosaurus and Levnesovia are part of the least inclusive clade containing Telmatosaurus and hadrosaurids (Weishampel et al., 1993). This clade (=Hadrosauridae sensu Weishampel et al., 1993; and Godefroit, Escuillié, et al., 2012) on the cladogram is supported by three unambiguous synapomorphies: the apex of the maxilla (in lateral view) is at or rostral to center (character 52: state 1); there is no contact between the ectopterygoid and the jugal (character 56: state 1); and the absence of a surangular foramen (character 89: state 1). The newly recovered clade formed by Telmatosaurus, Bactrosaurus, Levnesovia, and Tanius is also supported by three unambiguous synapomorphies: the presence of a premaxillary foramen (character 4: state 1); a lunate shape to the posterior margin of the external nares (character 15: state 0); and a tooth crown that is dominated by the presence of one primary ridge and faint secondary ridges (character 99: state 1). The recovery of this “bactrosaur” clade supports the hypothesis of Sues and Averianov (2009) of a Late Cretaceous radiation of derived non-hadrosaurid (sensu Horner et al., 2004) hadrosauroids in Asia.
7.22. Left distal tibia and fibula of Plesiohadros djadokhtaensis (MPC-D100/751). Left distal tibia in (A) cranial; (B) caudal; (C) medial; (D) ventral; and (E) lateral views. Scale bars equal 10 cm. Left distal fibula in (F) lateral; (G) medial; and (H) cranial views. Scale bars equal 5 cm. Abbreviations: ast, astragalus; ca.p, cranial ascending process of the astragalus; cda.f, facet for caudal ascending process of astragalus; cda.p, caudal ascending process of the astragalus; fb.f, fibula facet; fb.m, fibula maleolus; l.m, lateral maleolus of tibia; m.m, medial maleolus of tibia; tb.c, tibia contact.
7.23. Left pes of Plesiohadros djadokhtaensis (MPC-D100/751) in (A) dorsal; and (B) ventral views. Scale bars equal 5 cm. Abbreviations: mt II–IV, metatarsals of the second through fourth digits; p.ph, proximal pedal phalanges; pnu.ph, penultimate pedal phalanges; ung, unguals.
DISCUSSION
Plesiohadros djadokhtaensis is characterized by a unique combination of plesiomorphic and derived characters relative to more basal iguanodontians and hadrosaurids, as well as an autapomorphic prefrontal that flares dorsolaterally to form a rugose, everted, wing-like rim around the rostrodorsal orbital margin, which may have even supported a small horn-like keratinous projection in life. Similar circumorbital rugosity has been described on the postorbital of the hadrosauroid Jeyawati from the Turonian of western North America (McDonald et al., 2010).
Plesiohadros djadokhtaensis is the first hadrosauroid identified from the fossiliferous Djadokhta Formation in Mongolia, and the first new member of this clade named from the Late Cretaceous of Mongolia in three decades. Remarkably, this is only the second report of remains referable to Hadrosauroidea from these deposits, despite extensive sampling and research (Dashzeveg et al., 2005; Dingus et al., 2008). The only previous report of hadrosauroid material from this formation was the brief note on numerous articulated infant hadrosauroid skeletons from the Tögrögiin Shiree locality by Barsbold and Perle (1983). Together, these represent the only Campanian occurrences hadrosauroids