 |
| Choyrodon barsboldi Gates, Tsogtbaatar, Zanno, Chinzorig & Watabe, 2018 DOI: 10.7717/peerj.5300 |
Abstract
We describe a new iguanodontian ornithopod, Choyrodon barsboldi gen. et sp. nov. from the Albian-aged Khuren Dukh Formation of Mongolia based on several partial skeletons interpreted to represent a subadult growth stage based on osteohistological features. This new taxon is diagnosed by many autapomorphies of the maxilla, nasal, lacrimal, opisthotic, predentary, and surangular. Choyrodon displays an unusual combination of traits, possessing an open antorbital fenestra (a primitive ornithopod trait) together with derived features such as a downturned dentary and enlarged narial fenestra. Histological imaging suggests that the type specimen of Choyrodon would have been a subadult at the time of death. Phylogenetic analysis of two different character matrices do not posit Choyrodon to be the sister taxon or to be more primitive than the iguanodontian Altirhinus kurzanovi, which is found in the same formation. The only resolved relationship of this new taxon is that it was hypothesized to be a sister-taxon with the North American species Eolambia caroljonesa. Though discovered in the same formation and Choyrodon being smaller-bodied than Altirhinus, it does not appear that the former species is an ontogimorph of the latter. Differences in morphology and results of the phylogenetic analyses support their distinction although more specimens of both species will allow better refinement of their uniqueness.
 |
| Figure 4: Type specimen of Altirhinus. (A) In lateral view; (B) close up of lacrimal and maxilla showing the region that would contains the antorbital fenestra in Choyrodon. Scale bar equals 10 cm. Photograph credit: Khishigjav Tsogtbaatar. |
 |
Figure 2: Reconstructed skull of Chyorodon type specimen MPC-D 100/801. (A) Line drawing reconstruction of based on elements present in MPC-D 100/801. The gray filled region near the maxilla, nasal, and lacrimal represents an uncertain relationship of the bones due to incomplete specimens. The frontal–prefrontal contact has been estimated on this figure, as it is not easily seen on the original specimen. (B) Bones of MPC-D reconstructed. Study sites: Ang, angular; Aof, antorbital fenestra; Dn, dentary; Exo, exoccipital; F, frontal; Ju, jugal; La, lacrimal; Mx, maxilla; Na, nasal; Pf, prefrontal; Pmx, premaxilla; Po, postorbital; Qj, quadratojugal; Qu, quadrate; Sq, squamosal; Su, surangular. Scale bar equals 10 cm. Illustration credit and photograph credit: Terry Gates. |
Systematic Paleontology
Dinosauria (Owen, 1842)
Ornithischia (Seeley, 1888)
Ornithopoda (Marsh, 1881)
Iguanodontia (Dollo, 1888) (sensu Sereno, 1986)
Choyrodon gen. nov.
Choyrodon barsboldi sp. nov.
Holotype: MPC-D 100/801: A partial disarticulated skull containing right and left premaxillae, right maxilla, right ectopterygoid, right palatine, left pterygoid, right and left jugals, right and left lacrimals, right nasal, nearly complete skull roof (including frontals, a single prefrontal (right?), postorbitals, squamosals, parietals, laterosphenoids, orbitosphenoids, prootics, exoccipitals, opisthotics, and supraoccipital), right and left quadratojugals, right quadrate, right and left dentaries, partial predentary, right and left surangulars, right and left angulars, metacarpals, and cervical ribs.
Etymology: Choyrodon—Choyr, a city near the Khuren Dukh locality where this taxon was discovered; don—latin, meaning tooth, a common ending for ornithopod dinosaur taxa; barsboldi—named after Dr. Rinchen Barsbold, a leading dinosaur paleontologist of Mongolia and leader of the paleontology expedition that discovered the first remains of this species.
Locality, horizon, and geologic age: The quarry from which the holotype specimen derives is located within a thick brown–black organic rich siltstone at the Khuren Dukh locality of the Lower Member of the Khuren Dukh Formation (sensu Ito et al., 2006). Approximate age is middle to late Albian (Nichols, Matsukawa & Ito, 2006). Exact locality information is on file at the MPC.
Diagnosis:
Iguanodontian ornithopod distinguished by the following autapomorphies (marked with an asterisk) and unique combination of characters: anterodorsal process of maxilla dorsoventrally broad relative to total height of maxilla, with a length to height ratio of 1 and the dorsal margin of this process reaching and extending along a portion of the ventral bony naris; anterodorsal margin of maxilla horizontal*; nasal possessing low rise on dorsal surface positioned at posterior extent of nasal fenestra*; expansion of the distal lateral process of premaxilla; dorsoventrally thickened bone on posterior surface of nasal below dorsal rise; elongate, hypertrophied external narial fenestra with nasals comprising a small portion of posteroventral margin; antorbital fenestra; lacrimal bearing rounded anterior margin and lobate shape in lateral view*; squamosal processes of postorbital deflected posterodorsally; quadrate notch located at midheight of element; posterior process of opisthotic? wraps over to contact broadly the posterior face of supraoccipitals*; predentary with flattened articulation surface across entire ventral margin*; predentary with series of paired foramina below oral margin throughout lateral length of element*; distal end of dentary deflected ventrally; surangular displays two deep osteological folds on the lateral surface ventral to second foramen*; prepubic process of pubis deflects ventrally.
Conclusion:
The new iguanodontian, Choyrodon barsboldi, from the Albian Khuren Dukh locality of the Khuren Dukh Formation of Mongolia is described here based on approximately seven autapomorphic traits found throughout the skull in addition to a unique combination of several skull traits found in other iguanodontians. Of the autapomorphic traits, some of the most notable include an apparent overlap of the opisthotic onto the supraoccipital and two osteological folds on the anteroventral surangular. Choyrodon shares with more basal taxa an open antorbital fenestra, yet also possesses more derived traits such as a downturned dentary and an enlarged narial fenestra. An enlarged narial fenestra is also found in, and a defining feature of, the taxon A. kurzanovi, which was discovered from the same formation as Choyrodon. Osteohistology indicates that one specimen of Choyrodon was a subadult individual still actively growing at the time of death. Given that all specimens are approximately the same size we assume that all specimens share a similar growth stage.
Dismissing the number of unique features characterizing Choyrodon, we attempted to test the hypothesis that Choyrodon represents a subadult growth stage of the contemporary taxon Altirhinus by conducting a phylogenetic analysis in which we inferred that the antorbital fenestra of Choyrodon would close with skeletal maturity. Hypothesized relationships between Choyrodon and Altirhinus did not change in these iterations. Although we cannot rule out the hypothesis that specimens of Choyrodon are subadult specimens of Altirhinus we find no conclusive evidence to support an assignment. Firstly, given that the growth stage of the holotype of Altirhinus is unknown it cannot be documented that these species are represented by different ontogenetic stages, and although specimens of both species are known from the same formation, stratigraphic placement is not a justifiable reason for inferring taxonomic identity. Moreover, we note that many of the morphological differences are not consistent with ontogenetic changes observed in other hadrosauroids. Finally, closure of the antorbital fenestra between subadult and adult growth stages is undocumented in iguanodontians to date, and when taken in sum none of our phylogenetic tests, including those incorporating the potential of antorbital fenestra closure in mature Choyrodon, recovered Choyrodon as having diverged earlier than Altirhinus, a finding that would be consistent with it being a younger growth stage (Campione et al., 2013; Fowler et al., 2011). Thus, we find the weight of evidence currently supports the hypothesis that Choyrodon is a distinct taxon. However, we acknowledge that without definitively overlapping ontogenetic stages for comparison, an ontogenetic hypothesis cannot be disproved. Until such discoveries come to light we find it most conservative to follow the current morphological and phylogenetic evidence in considering these taxa as distinct species.
Taphonomically, the holotype specimen of Choyrodon (MPC-D 100/801) displays incredibly well-preserved, weathering crack marks caused from subaerial exposure—one of the best examples of this taphonomic modification yet reported on a dinosaur fossil. Given the interpretation of the paleoenvironment as an organic-rich, wet riparian system, the bone condition seems to contrast with assumptions of taphonomic potential where such a humid system would rot instead of crack (Scherzer & Varricchio, 2010). Although Behrensmeyer (1978) noted that microenvironments are what drives bone decay since there is not a close association between the overarching environment a bone lies in and the patterns of degradation. More research on modern weathering regimes is required to more fully understand the implications of this phenomenon.
Terry A. Gates, Khishigjav Tsogtbaatar, Lindsay E. Zanno, Tsogtbaatar Chinzorig and Mahito Watabe. 2018. A New iguanodontian (Dinosauria: Ornithopoda) from the Early Cretaceous of Mongolia. PeerJ. 6:e5300. DOI: 10.7717/peerj.5300
