Ornithomimidae, Marsh, 1890

Nottrodt, Rachel & Farke, Andrew, 2021, New data on the distal tarsals in Ornithomimidae, Acta Palaeontologica Polonica 66 (4), pp. 789-796 : 790-791

publication ID	https://doi.org/ 10.4202/app.00884.2021
persistent identifier	https://treatment.plazi.org/id/03EE87A9-FFB0-3050-492A-94655377FA83
treatment provided by	Felipe
scientific name	Ornithomimidae
status

Treatment

Ornithomimidae indet.

Material.— RAM 6794, an associated and partially articulated partial postcranial skeleton, including pelvis, sacrum, some anterior caudal vertebrae, and both hind limbs. RAM 6794 was collected at locality RAM V1998008, within the middle unit of the upper Campanian, Upper Cretaceous Kaiparowits Formation, Grand Staircase-Escalante National Monument, Garfield County, Utah, USA. Detailed locality data are on file at the Raymond M. Alf Museum.

Description.— RAM 6794 represents a relatively small individual, as compared to other ornithomimids from the upper Campanian of North America. The femur measures 327 mm in maximum length, approximately 20% smaller than the 411 mm long femur in an Ornithomimus edmontonicus ( Sternberg, 1933) with a body length of 3.6 m ( Zelenitsky et al. 2012; TMP 1995.110.1). The most complete metatarsal, right MT IV, is 236 mm long. This is approximately 35% smaller than the Kaiparowits Formation MNA Pl. 1762A, in which the equivalent element is 365 mm long ( Decourten and Russell 1985).

The remainder of the description here focuses nearly exclusively on the tarsal complex in RAM 6794, which is most relevant to the questions under consideration. The astragalus and calcaneum are fused with each other, with a faint recessed area indicating the point of contact ( Fig. 2A 2 View Fig , A 3 View Fig , A 6). This complex is firmly attached to the tibia, but the contact with the tibia remains open. The fibula is not fused with the astragalus-calcaneum complex, either.

During collection and preparation, the left distal tarsals were kept in place against their corresponding astragalus and calcaneum, leaving the distal surface visible ( Fig. 1A View Fig ), whereas the right distal tarsals were retained atop the corresponding metatarsals, leaving the proximal surface exposed ( Fig. 1B View Fig ). Thus, the following description is a composite of observations from both the left and right distal tarsals, which provide distal and proximal views of the distal tarsals, respectively.

Distal tarsal 3 underlies the medial third of the astragalus and overlies the posterior (plantar) surfaces of the proximal ends of metatarsals II and III, with a small portion of the lateral edge also overlying metatarsal IV ( Figs. 1B 1 View Fig , 2A View Fig 1 View Fig ). The tarsal bone has a somewhat pear-shaped outline in proximal and distal views, with the narrower end directed medially (lies along the medial border), and the broader, lateral end only narrowly separated from distal tarsal 4 ( Fig. 1A View Fig 2 View Fig , B 1 View Fig ). The distal articular surface of distal tarsal 3 is relatively flattened, divided into medial and lateral halves by a transverse ridge that trends antero-posteriorly ( Fig. 1A View Fig 2 View Fig ). This ridge accommodates the proximal surface of metatarsals II and III. The proximal articular surface is strongly convex, imparting a lenticular cross-sectional profile to the bone ( Figs. 1B View Fig 3 View Fig , 2A View Fig 4). Visually, distal tarsal 3 appears taller than distal tarsal 4; however, the bones are roughly similar in thickness when measured. In the part that remains in position relative to the astragalus, a modest gap separates the two bones, indicating the distal tarsals are not fused to each other ( Fig. 1A View Fig 2 View Fig ).

Distal tarsal 4 underlies the middle portion of the astragalus, does not articulate with the calcaneum, and overlies the proximal posterior (plantar) surface of metatarsal 4 alone ( Figs. 1B 1 View Fig , B 5, 2A 1, A 2, A 5). The position of distal tarsal 4 leaves visible the anterior and very small portions of the posterior surface of the proximal end of metatarsal IV ( Figs. 1B 1 View Fig , 2A View Fig 1 View Fig ). This tarsal is divided into two parts, with a medial portion that forms most of the element and a narrower, laterally directed flange. The distal surface of the overall tarsal is relatively flat, with a slight bit of concavity towards the ventral edge of this surface and a slight convexity towards the dorsal edge (this morphology is somewhat obscured by matrix in Fig. 1A View Fig 2 View Fig ). Like distal tarsal 3, distal tarsal 4 has a prominent lenticular cross-section, with the proximal surface strongly rounded ( Figs. 1B View Fig 4, 2A 6). In combined articulation, distal tarsals 3 and 4 together produce a continuous, rounded proximal surface, with the peak of the rounding positioned slightly ventral, relative to the main bodies of the tarsals. The combined effect is a subtly saddle-shaped articular surface between proximal and distal tarsals, with the long axis of the “saddle” directed mediolaterally.

Remarks.— RAM 6794 can be assigned to Ornithomimidae on the basis of the arctometatarsalian pes ( Figs. 1 View Fig and 2 View Fig ), in which the proximal end of metatarsal III is constricted and is not visible anteriorly where metatarsals II and IV articulate ( Holtz 1995), because this is the sole unambiguous synapomorphy of the family ( Kobayashi and Lü 2003). Additionally, the medial expansion of distal tarsal 3 and straight pedal unguals in RAM 6794 also support an assignment to Ornithomimidae . The distal tarsals will be referred to as distal tarsal 3 (dt3) and distal tarsal 4 (dt4) following the naming convention of Ossa-Fuentes et al. (2020). The ornithomimids from the Kaiparowits Formation have not yet been fully described, and their relationship with other taxa in North America at the time is still unexplored ( Zanno et al. 2013). Claessens and Loewen (2016) suggested they may be referable to Ornithomimus sp. , but this has not yet been tested in detail.