Dinosaur remains are found in several localities of the Bostobe Formation (Upper Cretaceous, Santonian–Campanian) in the northeastern Aral Sea region (Kyzylorda Region, Kazakhstan), the richest of which is Shakh-Shakh [1, 2]. Sauropods, theropods Tyrannosauroidea indet., Ornithomimidae indet., Therizi-nosauroidea indet., Caenagnathidae? indet., Dromaeosauridae indet. and Troodontidae indet., ankylosaurs, and hadrosauroids Aralosaurus tuberiferus Rozhdestvensky, 1968 [2–11] are known from there. The provisional identification of Caenagnathidae is based on a dentary from Shakh-Shakh, which was originally classified as a turtle but may belong to the oviraptorosaurs [6, 12, 13]. Dromaeosaurids from Shakh-Shakh were known from ungual phalanges [11] and isolated teeth [6]. Recently, all ungual phalanges described by Yu.V. Suslov from Shakh-Shakh [11] were assigned to the Therizinosauridae [2], however, at least some of them really belong to dromaeosaurids. This paper describes two isolated metatarsal bones of theropods from Shakh-Shakh from the collections of the expedition of the Paleontological Institute of the USSR Academy of Sciences led by A.K. Rozhdestvensky in 1957. One of these bones can be assigned to the oviraptorosaurs of the family Caenagnathidae, the other to the Dromaeosauridae. The first find confirms the presence of the caenagnathids in the vertebrate assemblage of the Bostobe Formation. The material is stored in the collection of the Borissiak Paleontological Institute of the Russian Academy of Sciences (PIN), Moscow.

Specimen PIN, no. 2229/7 is a distal fragment of the right metatarsal III (Fig. 1). The bone shaft widens mediolaterally near the preserved proximal end and narrows in proximal and distal directions. In the widest part, the shaft is only slightly inferior in width to the distal epiphysis. The medial margin of the shaft is more convex than the lateral one. The extensor surface of the shaft is flat near the distal epiphysis and concave more proximal. The free flexor surface of the shaft, not covered by the side metatarsals, is concave. Its width is approximately half of the entire flexor surface of the shaft. The facet for contact with the metatarsal II is slightly larger than the facet for the metatarsal IV. The shaft of the bone is hollow, with thick bone walls (Figs. 1a, 1g). Viewed from the medial or lateral side, the shaft of the bone is curved proximal to the distal epiphysis (Figs. 1b, 1d, 1h, 1j). Viewed from the extensor or flexor side, the distal epiphysis is bent slightly medially, due to which the lateral condyle protrudes more distally than the lateral condyle (Figs. 1c, 1e, 1i, 1k). The distal epiphysis is slightly ginglymoid, with a very shallow groove separating the medial and distal condyles. The distal epiphysis is subrectangular when viewed from the distal end. Its mediolateral width (12.4 mm) is greater than its dorsoventral diameter (11.0 mm). The condyles are rounded when viewed from the medial or lateral side. The articular surface of the medial condyle on the extensor and flexor sides extends more proximally compared to the lateral condyle. The ligament pit of the lateral condyle is large and deep, with well-defined margins. It is located in the center of the lateral condyle. On the medial condyle, the ligament pit is also large, but not so deep and does not have a clearly defined margin.

Fig. 1.
figure 1

Caenagnathidae indet., specimen PIN, no. 2229/7, right metatarsal III, photographs (a–f) and explanatory drawings (g–l), in proximal (a, g), lateral (b, h), extensor (c, i), medial (d, j), flexor (e,k), and distal (f, l) views. Shakh-Shakh, Kyzylorda Region, Kazakhstan; Bostobe Formation, Upper Cretaceous (Santonian–Campanian). Abbreviations: lp, ligament pit; mtII, facet for the metatarsal II; mtIV, facet for the metatarsal IV.

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The metatarsal III, specimen PIN, no. 2229/7 refers to the subarctometatarsalian foot type, in which the side metatarsals (II and IV) do not completely cover the middle metatarsal (III) from the ventral (flexor) side. The cross section of the shaft of metatarsal III with this foot structure has the form of a trapezoid (Figs. 1a, 1g). In the arctometatarsalian foot construction, which is characteristic of Tyrannosauridae, Ornithomimidae, Avimimidae, Alvarezsauridae, and Troodontidae, the side metatarsals completely overlap the middle metatarsal ventrally, and its shaft is triangular in cross section [14]. The subarctometatarsalian foot structure is found in oviraptorosaurs of the family Caenagnathidae and some basal dromaeosaurids or avialans (Microraptorinae, Unenlagiinae) [15]. In particular, specimen PIN, no. 2229/7 is quite similar in structure to the metatarsal III of a unenlagiine Neuquenraptor argentinus Novas et Pol, 2005 from the Coniacian of the Argentina ([16]: text-fig. 7). Specimen PIN, no. 2229/7 is distinguished by a greater width of the free flexor surface of the shaft and a shallower groove separating the distal condyles. All unenlagiines are known only from South America; therefore, the assignment of specimen PIN, no. 2229/7 to this group seems unlikely. An even greater similarity specimen PIN, no. 2229/7 reveals with the metatarsal III of caenagnathids, known from the Upper Cretaceous of Asia and North America. Specimen PIN, no. 2229/7 differs from the metatarsal III of Elmisaurus rarus Osmólska, 1981 from the Maastrichtian of Mongolia [17] by a slightly narrower free ventral side of the shaft and features of the ligament pits, which in E. rarus are deep on both condyles. From the metatarsal III of Citipes elegans (Parks, 1933) from the Campanian of Canada ([18]: text-fig. 2) specimen PIN, no. 2229/7 differs only in the more convex medial margin of the shaft and in the articular surface of the distal condyle, which extends to a lesser extent on the ventral side. Such a significant similarity allows us to attribute confidently specimen PIN, no. 2229/7 to a representative of the family Caenagnathidae.

Specimen PIN, no. 2229/8 is a proximal fragment of the left metatarsal IV coossified with the distal tarsal III (Fig. 2). The sutures between the bones are visible on the medial and proximal sides (Fig. 2). The distal tarsal does not completely cover the proximal side of the metatarsal, not reaching its dorsal (extensor) margin. When viewed from the medial or lateral side, the shaft of the metatarsal IV is significantly expanded at the proximal end and slightly expanded distally. When viewed from the dorsal or ventral side, the shaft gradually tapers distally. The proximal surface is flat and D-shaped when viewed from the proximal end, with a straight medial margin (Figs. 2a, 2g). On the medial side, the proximal end is occupied by a large triangular facet for the proximal end of the metatarsal III. On the lateral side, there is a much smaller triangular facet for the metatarsal V. The extensor surface of the shaft is concave in the transverse plane. Along its medial margin, there is a flat longitudinal facet for the shaft of the metatarsal III, which is separated by a small gap from the proximal facet. The longitudinal facet expands distally, so that near the preserved distal end, its lateral edge is located in the middle of the extensor surface of the shaft. In this place, the shaft of the bone has a triangular cross-sectional shape (Figs. 2f, 2l). The ventral (flexor) edge of the facet for the metatarsal III forms a sharp ridge. The flexor surface of the bone shaft is flat. The extensor and flexor surfaces of the shaft converge to its lateral edge, forming a sharp lateral crest along the entire preserved shaft. The mediolateral width of the proximal epiphysis is 11.4 mm; its dorsoventral diameter is 14.3 mm.

Fig. 2.
figure 2

Dromaeosauridae indet., specimen PIN, no. 2229/8, left distal tarsal III and metatarsal IV, photographs (a–f) and explanatory drawings (g–l), in proximal (a, g), medial (b, h), extensor (c, i), lateral (d, j), flexor (e, k), distal (f, l) views. Kyzylorda Region, Kazakhstan; Bostobe Formation, Upper Cretaceous (Santonian–Campanian). Abbreviations: dtIII, distal tarsal III; lc, lateral crest; mtIII, facet for the metatarsal III; mtV, facet for the metatarsal V; s, suture between the distal tarsal III and metatarsal IV.

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Specimen PIN, no. 2229/8 is almost identical in structure to the metatarsal IV of the dromaeosaurid Velociraptor mongoliensis Osborn, 1924 from the Campanian of Mongolia [19]. In Velociraptor, the distal tarsal III also does not completely cover the proximal surface of the metatarsal IV and there is a small metatarsal V in contact with the proximal end of the metatarsal IV ([19]: text-figs. 15, 16). Besides, in Velociraptor, as in specimen PIN, no. 2229/8, there is a strong bone crest along the lateral margin of the metatarsal IV ([19]: text-fig. 14). In most theropods, including other dromaeosaurids, the lateral surface of the metatarsal IV is flat. In Balaur bondoc Csiki et al., 2010 from the Maastrichtian of Romania, the metatarsals are thick along one margin and thin towards the opposite margin, forming a bone crest along it [20]. However, on the metatarsal IV of Balaur, the crest runs along the medial rather than the lateral margin, contrary to specimen PIN, no. 2229/8. Specimen PIN, no. 2229/8 can be confidently attributed to Dromaeosauridae indet. on the basis of the noted similarity with the metatarsal IV of Velociraptor.