Ornithomimosaurs from the Nemegt Formation of Mongolia: manus morphological variation and diversity

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Highlights

  • Morphologically different ornithomimosaur mani are discovered from Mongolia.

  • Nemegt ornithomimosaurs are highly diversified by their manus variation.

  • Various paleoecological niches are presumably occupied in the Nemegt Formation.

  • The different manual unguals may be used for different functions related to feeding.

Abstract

The Upper Cretaceous Nemegt Formation of Mongolia is rich in well-preserved dinosaurs and Ornithomimosauria is one of the most common taxa in the formation. Three ornithomimosaur taxa, Anserimimus planinychus, Deinocheirus mirificus, and Gallimimus bullatus, have been discovered from the formation so far. However, the recently discovered specimens suggest there is even greater morphological variation of ornithomimosaurs in the Nemegt Formation than are presently recognized. This study focuses on the structures of manual elements among Nemegt ornithomimosaurs and reveals their remarkable diversity. The manual structures of seven individuals, including aforementioned three known taxa and four new individuals, are morphologically distinct from each other. Numerical analyses on metacarpals, phalanges, and unguals also support high morphological diversity of the Nemegt ornithomimosaurs. The large diversity of manual morphology may be related to large variety of palaeoecological niches were prevailed in the Nemegt ecosystem.

Introduction

The Upper Cretaceous Nemegt Formation (upper Campanian – lower Maastrichtian) of Mongolia is one of the most fossiliferous formations in the world (Gradziński, 1970, Osmόlska, 1980, Currie, 2016). Ornithomimosaurs are one of the most diverse theropod dinosaur groups and their remains are abundant within the formation (Hurum and Sabath, 2003, Makovicky et al., 2004). They are generally characterized by their medium to large body size, proportionately small skulls with large orbits, elongate forelimbs with weakly developed mani, and cursorially adapted powerful hindlimbs (Osborn, 1917, Norell et al., 2001, Makovicky et al., 2004, Lee et al., 2014). To date, four definitive ornithomimosaurs have been described from the Nemegt Formation of Mongolia; namely, Anserimimus planinychus from Bügiin Tsav, Deinocheirus mirificus from Altan Uul III and IV, and from Bügiin Tsav, Gallimimus bullatus from most of localities of the Nemegt Formation, and a new ornithomimid (aff. Anserimimus planinychus Barsbold, 1988) from Tsagaan Khushuu (Kobayashi and Barsbold, 2006, Bronowicz, 2011) (Fig. 1). Deinocheirus mirificus belongs to the clade Deinocheiridae (Lee et al., 2014), and the other three are positioned within the cursorial clade Ornithomimidae (Makovicky et al., 2004).

Although body designs of the cursorial ornithomimids are extremely similar, there are distinct differences in the hand structure (Kobayashi and Barsbold, 2006). In fact, the forelimb structures of ornithomimosaurs, specifically for derived ornithomimosaurs, are important for phylogenetic analysis (Kobayashi and Barsbold, 2006). Members of this group show high morphological variation in their manual structures, the phalangeal proportions and the shapes of their unguals, including curvature and robustness. These variations may be related to functional diversity (Osmόlska et al., 1972, Nicholls and Russell, 1981, Nicholls and Russell, 1985, Makovicky et al., 2004, Kobayashi and Barsbold, 2005a, Kobayashi and Barsbold, 2005b, Choiniere et al., 2012, Lee et al., 2014, Claessens and Loewen, 2016).

In the last few decades, intensive fieldwork at the Nemegt Formation localities have collected hundreds of isolated bones and several semi-articulated or articulated specimens of ornithomimosaurs. Additional species may be present in the Nemegt Formation based on the hand structure (e.g., the ornithomimid from Ulaan Khushuu locality, Kobayashi et al., 2010). Although previous studies have compared the structures and relative length proportions of ornithomimosaur hand elements, statistical analysis has not been conducted yet (Kobayashi and Barsbold, 2006). Here, morpho-functional differences among Nemegt ornithomimosaurs are demonstrated by qualitative morphological comparisons and by quantitative statistical analyses.

The purposes of this study are: 1) to describe manual elements of new specimens from the Nemegt Formation and compare them statistically; 2) to demonstrate the morphological disparity of Nemegt ornithomimosaurs within Ornithomimosauria.

Section snippets

Specimen sampling

Measurements and the photographic dataset includes 26 specimens, containing 13 species, including 1 unnamed, but described specimen (Bissekty ornithomimid) and 7 undescribed specimens of ornithomimosaurs (Table S1). Multiple specimens are used for Gallimimus bullatus (2), Ornithomimus edmontonicus (2), Sinornithomimus dongi (2), and Struthiomimus altus (3). The three named taxa from the Nemegt Formation are Anserimimus planinychus (MPC-D 100/300), Deinocheirus mirificus (MPC-D 100/18), and

Geological settings and occurrences

The Upper Cretaceous Nemegt Formation (upper Campanian – lower Maastrichtian) is distributed at Altan Uul, Bügiin Tsav, Guriliin Tsav, Hermiin Tsav, Khuree Tsav, Nemegt, Tsagaan Khushuu, and Ulaan Khushuu localities (Efremov, 1954, Gradziński et al., 1969, Gradziński et al., 1977, Martinson, 1982, Jerzykiewicz and Russell, 1991, Eberth, 2017, this volume; Fanti et al., 2017, this volume) (Fig. 1). All of the ornithomimosaur specimens described herein were collected from three different

Metacarpals

Metacarpals (mcI, mcII, and mcIII) of ornithomimosaurs generally have the following features: metacarpal II longer than metacarpals I and III; robust metacarpal I; slender metacarpal III; proximally weakly arched metacarpals; a slightly concave or flat proximal articular surfaces of metacarpals; and proximally closely adherent to each other (Fig. 3). However, many other features such as the length ratios of metacarpals, the contact surfaces between metacarpals, the deviation and the rotation of

Manual morphological variations and functional implications

The comparisons of Nemegt ornithomimosaur hands revealed that manual structures of three known taxa and four new individuals are distinct from each other. Morphological diversity of ornithomimosaur hands is discussed here, along with their possible functional implications.

The cluster of derived ornithomimosaurs in the scatter-plot diagram reflects the character used in the phylogenetic analysis (showing “sub-equal metacarpals” [Makovicky et al., 2004]) (Fig. 4A). At the same time, this diagram

Conclusions

The Upper Cretaceous Nemegt Formation of Mongolia has a higher diversity of ornithomimosaur dinosaurs than any other places where ornithomimosaur specimens have been discovered. Seven morphologically different mani of ornithomimosaurs are identified from the formation in this study. The quantitative and statistical analyses demonstrate large manus variations within the Nemegt ornithomimosaurs. Moreover, Nemegt ornithomimosaurs contribute to about half of the manual morpho-functional disparity

Acknowledgments

We grateful to Mr. Ken Hayashibara and the Mongolian Academy of Sciences for their support of the Japan-Mongolian Joint Paleontological Expedition (HMNS-MPC) and especially the field crews of 1995, 1997 and 2006 years, as well as members of the Polish-Mongolian and the Soviet-Mongolian Paleontological Expeditions. In addition, we also express our thanks to the following people; technicians, Mr. Ch. Bayardorj for his skillful preparation, S. Ulziitseren and A. Khaliun for their help to collect

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