Mononykus olecranus (Perle et al, 1993) Late Cretaceous ~70 mya, 1 m in length was a flightless alvarezsaurid theropod.

Only digit I remained on the hand, which was otherwise stunted, except for the proximal ulna (the elbow) which was enlarged. The hindlimbs were elongated. The tibia had two cnemial crests and the femur/tibia joint was very flexible. Metatarsal 3 did not reach the tarsus.

Shuvuuia deserti (Chiappe, Norell and Clark 1998, Late Cretaceous; IGM 100/1304) is a smaller alvarezsaurid that retains vestigial digits 2 and 3 and a longer tail. This alvarezsaurid pattern is characterized by the strong but stunted forelimbs likely used to cling as insect-eating 'tick-birds' to larger dinosaurs. (IGM 100-0977 is a larger specimen)

Halszkaraptor escuilliei (Cau et al. 2017; Late Cretaceous) was originally considered an aquatic dromaeosaur related to Mahakala, known from scraps. Here Halszkaraptor nests with Shuvuuia between Haplocheirus and Sinornithoides. Manual digit 3 was the longest, but the thumb had the largest claw. The naris was displaced posteriorly and additional teeth lined the premaxilla.

Natovenator polydontus (Lee et al 2022; Late Cretaceous) was originally considered a duck-like or penguin-like underwater swimmer due to the orieintation of its dorsal ribs and wider than deep torso. It was buried in eolian sands. Here it nests with another small swift runner with weak pectoral limbs, Halszkaraptor.

Chiappe LM, Norell MA and Clark JM 1998. The skull of a relative of the stem-group bird Mononykus. Nature, 392(6673): 275-278.

Choiniere JN, Xu X, Clark JM, Forster CA, Guo Y, Han F 2010. A basal alvarezsauroid theropod from the Early Late Jurassic of Xinjiang, China. Science 327 (5965): 571–574.

Lee S et al (7 co-authors) 2022. A non-avian dinosaur with a streamlined body exhibits potential adaptations for swimming. Nature communications biology .https://doi.org/10.1038/s42003-022-04119-9

Perle A, Norell MA, Chiappe LM and Clark JM 1993. Flightless bird from the Cretaceous of Mongolia. Nature 362:623-626.

Perle A, Chiappe LM, Rinchen B, Clark JM and Norell 1994. Skeletal Morphology of Mononykus olecranus (Theropoda: Avialae) from the Late Cretaceous of Mongolia. American Museum Novitates 3105:1-29.

wiki/Mononykus
wiki/Halszkaraptor
wiki/Shuvuuia
wiki/Natovenator