Halszkaraptor

Halszkaraptor is a genus of dromaeosaurid dinosaur from the Late Cretaceous of Mongolia, resembling a waterfowl in appearance. Its only known species is *Halszkaraptor escuilliei*. The holotype skeleton shows features seen in modern crocodilians and aquatic birds, suggesting a semiaquatic lifestyle, although some scientists dispute this interpretation. Phylogenetic analysis places it in the basal subfamily Halszkaraptorinae, alongside *Mahakala* and *Hulsanpes*.

The holotype likely came from the Djadochta Formation at Ukhaa Tolgod in southern Mongolia. It was illegally removed by fossil poachers in or before 2011, then passed through collectors in Japan and Great Britain before being acquired by the Eldonia company, run by fossil dealer François Escuillié. Escuillié recognized it as a new species and brought it to the Royal Belgian Institute of Natural Sciences in Brussels in 2015, where paleontologists Pascal Godefroit and Andrea Cau verified its authenticity. They used synchrotron radiation at the European Synchrotron Radiation Facility to scan the fossil. Cau and other researchers then described the genus in a study published in *Nature*. The fossil was later returned to Mongolian authorities.

The specimen, MPC D-102/109, came from orange sandstone of the Bayn Dzak Member of the Djadochta Formation, dating to the late Campanian, around seventy-five million years old. It is a relatively complete skeleton with skull, found in 2017 and not further prepared at that time. Fossil dealers had exposed the left side of the skeleton. The synchrotron scan showed that bones continued into the rock, indicating the specimen was likely not a chimaera (an artificial composite), though the top of the snout had been restored with plaster and some elements reattached with glue. The skeleton is largely articulated and uncompressed, representing a subadult about one year old.

The type species was named in 2017 by Andrea Cau, Vincent Beyrand, Dennis F. A. E. Voeten, Vincent Fernandez, Paul Tafforeau, Koen Stein, Rinchen Barsbold, Khishigjav Tsogtbaatar, Philip John Currie, and Pascal Godefroit. The genus name combines a reference to the late Polish paleontologist Halszka Osmólska, who worked on many Mongolian expeditions and named the related *Hulsanpes*, with the Latin *raptor* meaning "robber." The species name honors Escuillié for making the specimen available to science.

Halszkaraptor was about the size of a mallard duck. Its head measured about 7 cm, neck 20 cm, back 13 cm, and sacrum 5 cm. The describing authors identified several distinguishing traits, including autapomorphies (unique derived characters). The premaxilla forms a flattened snout that makes up 32% of the snout length and holds eleven teeth. The jugal bone is rod-shaped, with its ascending branch only a tenth of the bar behind the eye socket, not reaching the orbit. The neck is extremely elongated, half the length from snout to sacrum. The postzygapophyses (rear joint processes) of the neck vertebrae lack epipophyses. The neural spines on the neck vertebrae are extremely reduced: on the second to fifth vertebrae they are low ridges, and later neck vertebrae lack them entirely. In the second to fifth neck vertebrae, the normally paired postzygapophyses are fused into a single lobe-shaped process. The neural spines of the tail vertebrae are extremely shortened: the first three tail vertebrae have low bumps, and later tail vertebrae lack them completely. The chevrons at the tail base are large with a pentagonal profile. The first phalanx of the third finger is 47% the length of the third metacarpal.

A unique combination of non-unique traits is also present. The external bony nostril sits behind the main body of the premaxilla, at its connection to the front branch of the maxilla. The descending branch of the postorbital bone is rod-shaped. The total number of neck and back vertebrae is twenty-two. Only the seventh, eighth, and ninth neck vertebrae have pleurocoels (pneumatic depressions). The transition between the tail base and middle tail occurs at the seventh to eighth vertebra. The third finger is longer than the second.

The snout is elongated but transversely expanded at the front, giving a spoon-shaped profile from above. It is also flat, with a width 180% of its height. The top profile in side view is hollow. The expanded area is formed by a relatively long premaxilla, which is internally excavated by air chambers. From a larger rear chamber, neurovascular channels run through the entire bone, including the top, not just the sides as in *Neovenator*. These channels likely housed electro-sensory organs. Each premaxilla holds eleven teeth, a record among dinosaurs. Theropods usually have four premaxillary teeth; the previous record was seven, seen in spinosaurids. In Halszkaraptor, the premaxillary teeth are very closely packed, touching each other, and are very elongated and gradually recurving. The maxillary teeth, estimated at twenty to twenty-five, are more robust, curve only at the tips, and are spaced farther apart. They are more transversely flattened with an oval cross-section. The dentary of the lower jaw also holds an estimated twenty to twenty-five teeth.