Jaculinykus

MEANING: Jaculus claw

PERIOD: Late Cretaceous

CONTINENT: Asia

Like other Alvarezsaurids, Jaculinykus had long legs, and enlarged first digit claws on each hand, which were probably used for digging and tearing. It also had a long tube-shaped snout filled with tiny teeth. These adaptations indicate that it probably had an insectivorous diet, feeding on colonial insects such as termites.

Abstract from paper: Alvarezsauria is a group of early-branching maniraptoran theropods that are distributed globally from the Late Jurassic to the latest Cretaceous. Despite recent increases in the fossil record of this group, the scarcity of complete specimens still restricts interpreting their detailed anatomy, ecology, and evolution. Here, we report a new taxon of derived alvarezsaur, Jaculinykus yaruui gen. et sp. nov., from the Late Cretaceous of Mongolia, which represents a nearly complete and articulated skeleton. Our phylogenetic analysis reveals that Jaculinykus belongs to the sub-clade of Alvarezsauridae, Parvicursorinae, and forms a mononphyletic group with Mononykus and Shuvuuia. Its well-preserved manus has only two fingers, composed of a hypertrophied digit I and greatly reduced digit II, which implies an intermediate condition between the tridactyl manus of Shuvuuia and monodactyl manus of Linhenykus. This highlights a previously unrecognized variation in specialization of alvarezsaurid manus. Notably, the preserved posture of the specimen exhibits a stereotypical avian-like sleeping position seen in the troodontids Mei and Sinornithoides. Evidence of this behavior in the alvarezsaur Jaculinykus suggests that stereotypically avian sleeping postures are a maniraptoran synapomorphy, providing more evidence of bird-like traits being distributed broadly among avian ancestors.

Jaculinykus is from the Late Cretaceous. The Cretaceous is the third and final geological period of the Mesozoic Era, with the Late Cretaceous making up roughly the second half of it, lasting from about 100 to 66 million years ago. It was a time of significant evolutionary change, with dinosaurs reaching their greatest diversity before the mass extinction at the end of the Cretaceous.

The Cretaceous was a period with a relatively warm climate, though the Late Cretaceous experienced a global cooling trend, caused by falling levels of carbon dioxide in the atmosphere. The continents were nearing their present positions, but high sea levels flooded low-lying regions, turning Europe into an archipelago, and forming the Western Interior Seaway in North America. These seas were home to a variety of marine reptiles, including mosasaurs and plesiosaurs, while pterosaurs and birds shared the skies.

On land, dinosaurs continued to thrive and diversify during the Late Cretaceous, producing many of the most well-known goups, including tyrannosaurs, hadrosaurs, and pachycephalosaurs. Established Cretaceous dinosaur clades like the ceratopsians, ankylosaurs, and dromaeosaurs continued to flourish. Sauropod species consisted almost exclusively of titanosaurs, which seemed to be confined to the Southern Hemisphere for much of the Late Cretaceous. Flowering plants and grasses diversified and spread, becoming the dominant flora similar to what we see today.

The Cretaceous (along with the Mesozoic) ended with the Cretaceous-Paleogene (K-Pg) extinction event, a large mass extinction in which many groups, including non-avian dinosaurs, pterosaurs, and large marine reptiles, died out. This event, likely triggered by an asteroid impact, is marked by the abrupt K-Pg boundary, a distinct geologic layer separating the Mesozoic and Cenozoic Eras. In its aftermath, mammals and avian dinosaurs rapidly diversified, becoming the dominant land animals of the Cenozoic Era.

Jaculinykus is an alvarezsaur. Alvarezsaurs are a group of small, bird-like theropod dinosaurs that originated in Asia in the Jurassic, and became more prominent in the Cretaceous, spreading to the Americas and Europe. Their exact placement within the theropod family tree has been debated, but alvarezsaurs are generally considered to be highly specialized maniraptorans.

While basal alvarezsauroids show less extreme adaptations and more typical theropod anatomy, the derived alvarezsaurids proved to be highly specialized. They were small, lightly built, and fast-running animals, typically under 2 meters in length. Their skeletons reveal long hindlimbs adapted for speed, bird-like skulls with slender jaws, and a keeled sternum for strong muscle attachment - traits that suggest a very active lifestyle. One of their most distinctive features is their forelimbs - extremely short but robust, with a single dominant claw and reduced digits. Their exact diet remains uncertain, but many paleontologists believe they were insectivores, using their strong arms to break into tough nests like those of termites.

Despite their unusual build, alvarezsaurs represent a successful and diverse branch of maniraptoran evolution, highlighting the varied ecological roles small theropods played in Mesozoic ecosystems.