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6 February 2015 New Elmisaurine Specimens from North America and Their Relationship to the Mongolian Elmisaurus rarus
Gregory F. Funston, Philip J. Currie, Michael E. Burns
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Abstract

New specimens from Canada confirm the presence of elmisaurines in North America and shed light on the relationship of Leptorhynchos elegans to Mongolian forms. These specimens have hindlimb elements previously unknown from elmisaurines in the Dinosaur Park Formation, including tibiae and pedal phalanges. Metatarsal anatomy is sufficiently different to merit a generic distinction from Elmisaurus rarus, and both can be distinguished from Caenagnathus collinsi and Chirostenotes pergracilis. Differences between these taxa include body size, degree of coossification of the tarsometatarsus, and development of cruciate ridges of the metatarsal III. Histological analysis confirms that these differences are not correlated with ontogenetic age of the specimens. The results support the informal separation of caenagnathids based on metatarsal structure, and allow comments on paleobiological differences between caenagnathids and oviraptorids.

Introduction

The first caenagnathid remains discovered in North America were a pair of hands described by Gilmore (1924). Although Gilmore suggested that they were ornithomimid in nature, Osborn (1924) noticed that they were similar to those of Oviraptor philoceratops Osborn, 1924. More unusual “ornithomimid-like” material was identified by Sternberg (1932) and Parks (1933). Sternberg (1940) described an edentulous mandible he attributed to Aves, as a new order “Caenagnathiformes”. Only much later (Osmólska 1976) would it be established that all of these fossils are oviraptorosaurian. Osmólska (1981) then described Elmisaurus rarus Osmólska, 1981, based on three specimens collected by the Polish-Mongolian Paleontological Expedition in 1970. She noted that there were similarities between Chirostenotes pergracilis Gilmore, 1924 and Macrophalangia canadensis Sternberg, 1932 and suggested that they might be synonymous. She also speculated that Elmisaurus rarus was closely related but distinct from Chirostenotes pergracilis, based on the fusion of the metatarsals. Currie and Russell (1988) demonstrated that Chirostenotes and Macrophalangia were, in fact, synonymous, based on a skeleton with a manus and pes. Currie (1989), prompted by the discovery of a fused proximal metatarsus in Dinosaur Provincial Park, reexamined the material described by Parks (1933) and expanded the range of elmisaurines to North America. Further work by Currie et al. (1993) and Sues (1997) established that Chirostenotes pergracilis was closely related to Caenagnathus collinsi (Sternberg, 1940), but distinct from the Mongolian Elmisaurus rarus.

Many specimens have been collected in Dinosaur Provincial Park (Alberta, Canada) that are almost indistinguishable anatomically from the Mongolian Elmisaurus rarus, although the North American tarsometatarsi are more gracile. They were referred to as Elmisaurus elegans (Parks, 1933) by Currie (1989, 1990, 1997). Sues (1997) subsumed “Elmisauruselegans into Chirostenotes, as a separate species, Chirostenotes elegans, and included material from “Caenagnathus sternbergi” in this species. Varricchio (2001) described MOR 752, a partial left foot from the Hell Creek Formation of Montana, and referred it to Elmisaurus elegans, rejecting the synonymy of Sues (1997). Sullivan et al. (2011) reevaluated ROM 43250 and placed it in its own genus, Epichirostenotes Sullivan, Jasinski, and Van Tomme 2011. They did not comment on the status of the “Elmisauruselegans material from Dinosaur Provincial Park, but did note that the synonymy with Chirostenotes by Sues (1997) was tenuous. Questions remain, therefore, regarding whether Elmisaurus is present in North America, and the relationship between Chirostenotes and Elmisaurus.

Recently, Longrich et al. (2013) suggested that “Elmisauruselegans material was sufficiently distinct from Elmisaurus rarus to erect a new genus, Leptorhynchos. A reevaluation of undescribed material from the Dinosaur Park and Frenchman Formations, presented here, confirms this suspicion. Although there are enough differences between Elmisaurus rarus and “Elmisauruselegans to justify the establishment of a new genus for the latter, Longrich et al. (2013) made ROM 781 (a tarsometatarsus that has none of the characters included in the diagnosis) the holotype. It is questionable whether the tarsometatarsus of the holotype can be associated with the dentaries that were used to establish the diagnosis. Furthermore, Longrich et al. (2013) used the smaller sizes of the mandibles to associate them with ROM 781, but some elmisaurine metatarsi are comparable in size to Chirostenotes (Table 1). Although it is conceivable that Leptorhynchos may be a nomen dubium, this name was not replaced in this paper with another generic name that would add to the confusion that has already existed for more than half a century. All specimens that have been previously referred to the species “Ornithomimuselegans (Parks 1933), “Elmisauruselegans (Currie 1989, 1990, 1997; Varricchio 2001), and “Chirostenoteselegans (Sues 1997) will be referred to as Leptorhynchos elegans (Parks, 1933), in this paper. However, it is possible that a new generic name may be necessary should a skeleton be discovered with a dentary and metatarsus showing that the association made by Longrich et al. (2013) is incorrect.

Table 1.

Measurements (in mm) of elmisaurine and caenagnathine metatarsi from Campanian-Maastrichtian of Canada. *, width refers to transverse (mediolateral) breadth, and depth refers to anteroposterior thickness; e, estimated; MT, metatarsal; P, pathological.

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Institutional abbreviations.—MOR, Museum of the Rockies, Bozeman, Montana, USA; MPC-D, Mongolian Paleontological Center, Mongolian Academy of Sciences, Ulaan Baatar, Mongolia; RSM, Royal Saskatchewan Museum, Regina, Saskatchewan, Canada; TMP, Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta, Canada; UALVP, University of Alberta Laboratory of Vertebrate Paleontology, Edmonton, Alberta, Canada; ZPAL, Institute of Paleobiology, Polish Academy of Sciences, Warsaw, Poland.

Geological setting

Most elmisaurine material from North America comes from the (Upper Campanian) Dinosaur Park Formation of Alberta. The Dinosaur Park Formation is characterized by sandstone and mudstone rocks, but the sedimentological provenance of the specimens described here is not known. Other formations that have produced elmisaurine material are the Hell Creek Formation (Upper Maastrichtian) of Montana and the Frenchman Formation (Upper Maastrichtian) of Saskatchewan.

Material and methods

There are more than a dozen North American specimens that are currently referable to Elmisaurinae. All of the material is excellently preserved, but it varies in terms of crushing and completeness. Much of the material represents isolated single or compound elements (i.e., fused tarsometatarsi), but one specimen (TMP 2000.012.0008) includes multiple associated elements. Most of the specimens are partial tarsometatarsi that include more than one