Positional Variation in Pedal Ungulas of North American Ornithomimids (Dinosauria, Theropoda): A Response to Brownstein (2017)

Bradley Mcfeeters,Thomas M Cullen,Michael J Ryan

doi:10.18435/vamp29283

Bradley Mcfeeters, Thomas M Cullen + Show 1 more

Open Access

https://doi.org/10.18435/vamp29283

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Abstract

Positional variation is documented in ornithomimid pedal unguals from the Dinosaur Park and Horseshoe Canyon Formations of Alberta, Canada, and characters for identifying the position of isolated ornithomimid pedal unguals are discussed. Ungual morphology has been used recently to argue for the coexistence of two distinct ornithomimosaurs, a basal taxon and distinctly more derived taxon, in the Early Cretaceous Arundel Clay of Maryland, USA. However, these conclusions are based on misconceptions of the morphology and positional variability of ornithomimosaur unguals. Some characters previously cited as diagnostic of ornithomimosaur unguals are not actually observed in this clade, or are more homoplastically distributed among theropods. Other characters proposed to distinguish between the two pedal ungual morphs in the Arundel Clay material are shown in the Albertan ornithomimid material to consistently distinguish the different ungual positions within the pes of one individual. Claims of multiple distinct ornithomimosaur taxa in the Arundel Clay are premature, as the two pedal ungual morphotypes more likely represent positional variation in a single taxon.

Highlights

The ornithomimosaur pes is regarded as an important source of taxonomically informative anatomical information, with several recently named species diagnosed almost exclusively by pedal characters (Serrano-Brañas et al 2016; Tsogtbaatar et al 2017; Hunt and Quinn 2018)
A review of pedal anatomy of multiple ornithomimosaur taxa from the late Campanian-early Maastrichtian of Alberta reveals a consistent pattern of pedal ungual positional variation
Among Early Cretaceous basal ornithomimosaurs, the pedal unguals are most completely represented in Nedcolbertia justinhofmanni (Kirkland et al 1998), Nqwebasaurus thwazi (Choiniere et al 2012, Sereno 2017), and Hexing qingyi (Jin et al 2012)

Summary

INTRODUCTION

The ornithomimosaur pes is regarded as an important source of taxonomically informative anatomical information, with several recently named species diagnosed almost exclusively by pedal characters (Serrano-Brañas et al 2016; Tsogtbaatar et al 2017; Hunt and Quinn 2018). 3e) as the partial right pedal ungual III of CMN 12069 is more likely a partial left pedal ungual IV (Fig. 3I), possibly belonging to CMN 12068 (no catalogue number is directly associated with it), on the basis of its asymmetrical shape, abbreviated proportions, flexor fossa and ventral spurs located close to the projected distal tip of the ungual, and the relatively large angle between ventrolateral and ventromedial edges. Brownstein (2017) recently described isolated ornithomimosaur postcranial elements collected from the Early Cretaceous Arundel Clay of Maryland He identified NHRD-AP 2014.s.195, NHRD-AP 2014.s.197, NHRDAP 2014.s.198, NHRD-AP 2016.v.1104, and USNM PAL 529423 as ornithomimosaur pedal unguals based on the presence of a flexor fossa and relatively straight ventromedial edges forming keels, in some of these specimens he could not confirm both characters (Brownstein 2017: 7). It is unlikely that NHRD-AP 2014.s.196, which lacks this structure, represents an ornithomimosaur manual ungual. Brownstein (2017:11) explicitly rejected positional variation to explain the differences between the two ungual morphotypes that were recognized from the Arundel Clay, claiming that in “ornithomimosaurs where the proximal faces of more than one pedal ungual is exposed and documented

Approximately equal on lateral and medial sides

DISCUSSION AND CONCLUSIONS

LITERATURE CITED