MEANING: Jaculus claw
PERIOD: Late Cretaceous
CONTINENT: Asia
Jaculinykus is a small alvarezsaurid theropod from the Late Cretaceous of what is now Mongolia. The name is a reference to the Jaculus, a small dragon in Greek mythology. 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 Cretaceous. The Cretaceous is a geological period that lasted from about 145 to 66 million years ago. It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of the entire Phanerozoic. The name is derived from the Latin creta, "chalk", which is abundant in the latter half of the period.
The Cretaceous was a period with a relatively warm climate, resulting in high eustatic sea levels that created numerous shallow inland seas. These oceans and seas were populated with now-extinct flora and fauna, while dinosaurs continued to dominate on land. The world was ice free, and forests extended to the poles. During this time, new groups of mammals and birds appeared. During the Early Cretaceous, flowering plants appeared and began to rapidly diversify, becoming the dominant group of plants across the Earth by the end of the Cretaceous, coincident with the decline and extinction of previously widespread gymnosperm groups.
The Cretaceous (along with the Mesozoic) ended with the Cretaceous–Paleogene extinction event, a large mass extinction in which many groups, including non-avian dinosaurs, pterosaurs, and large marine reptiles, died out. The end of the Cretaceous is defined by the abrupt Cretaceous–Paleogene boundary (K–Pg boundary), a geologic signature associated with the mass extinction that lies between the Mesozoic and Cenozoic Eras.
Jaculinykus is an alvarezsaur. Alvarezsauria is a group of small theropod dinosaurs that originated in Asia in the Jurassic, and became more prominent in the Cretaceous, spreading to the Americas and Europe. The size of the derived members range between 0.5 and 2 m, making them some of the smallest known non-avian dinosaurs. Haplocheirus was the largest and most primitive member of the Alvarezsauroidea, suggesting a pattern of miniaturization as time went on.
Alvarezsaurs are known for their long legs, suggesting they were great runners, and their extremely short, yet robust, arms. Derived members are characterized by digit reduction. Over the course of their evolution, one finger on each hand, along with its corresponding claw, grew larger, while the others became reduced. Eventually, the most derived species were left a single large claw tipping each short arm, with the remaining fingers being so small as to seem vestigial.
These changes have been interpreted as adaptations for digging. One possible interpretation suggests that alvarezsauroids fed on insects, using their claws to dig under tree bark. This is consistent with their long snout and small teeth. Another interpretation suggests that they used their claws to break into ant and termite colonies.