Alvarezsaur Evolution and Systematics

Alvarezsauria is a group of bizarre maniraptoran theropods whose latest-branching members possess remarkably avian homoplasies (e.g., a lightly built, kinetic skull, a keeled sternum, a fused carpometacarpus, and a retroverted pubis and ischium). This group underwent clade-specific body size miniaturization during their evolutionary history in their body size, visual and auditory specialization, and limb specialization for fossorial and cursorial lifestyles. Alvarezsaurs with the above unusual features and evolutionary trends have received much attention in terms of functional morphology and evolution.  

I focus on the evolutionary trend in their anatomy, locomotor ability, and behavior.

Photograph: a nearly complete alvarezsaurid skeleton with an avian-like sleeping position, Jaculinykus yaruui, we described recently [Kubo et al., 2023, PLOS ONE]

Evolutionary pattern of homoplasy in Theropod Dinosaurs

Coelurosauria is a group of theropods, including Tyrannosaurus rex and living birds, which  established a high taxonomic diversity and significant morphological modifications. The evolutionary history of coelurosaurs further documents the convergent evolution of ecological traits such as body size, herbivorous diets, flight. Among them,  a unique homoplasy, arctometatarsalian condition, evolved independently across five or six coelurosaur lineages. This ecomorphology exhibits unique arrangements of metatarsals II–IV at the ankle joint, whose metatarsal III is displaced plantarly relative to metatarsals II and IV. 

I focus on the evolutionary trend and its temporal pattern in Coelurosauria and outside Coelurosauria.

Our paper recently reported convergent evolution of the arctometatarsalian arrangement occurred during mid-Cretaceous interval (ca. 119–93 Ma) [Kubo and Kobayashi, 2025, Royal Society Open Science]. This evolution is also probably restricted to Coeluroasauria, suggesting a phylogenetic constraint that likely prevented modifications to metatarsal arrangement outside Coelurosauria.

Convergent Evolution and Developmental Constraint of Foot Musculoskeletal Morphology in the Lineage leading to Birds 

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