Halszkaraptorinae

Lore & Background

The first known remains of halszkaraptorines were found in sandstone sediments in 1970 during a Polish-Mongolian expedition at the Barun Goyot Formation, Gobi Desert. In 1982, Polish palaeontologist Halszka Osmólska described them as the holotype for Hulsanpes perlei, honoring Mongolian paleontologist Altangerel Perle. These fossils were tentatively interpreted as belonging to Deinonychosauria. In 1992, dromaeosaurid-like fossils from the Tugriken Shire locality of the Djadokhta Formation were described in 2007 by Alan H Turner and team as Mahakala omnogovae, regarded as a primitive dromaeosaurid based on body proportions.

In 2015, the Royal Belgian Institute of Natural Sciences received a small theropod fossil specimen from private collections in England and Japan. Through negotiations involving Eldonia and Mongolian authorities, it was returned to the Institute of Paleontology and Geology, Mongolian Academy of Sciences. The specimen had been illegally poached from the Djadokhta Formation (Ukhaa Tolgod or Bayn Dzak locality). In 2017, Andrea Cau and colleagues formally described it as Halszkaraptor escuilliei, honoring Halszka Osmólska and François Escuillié. With this naming, the subfamily Halszkaraptorinae was coined to include Halszkaraptor, Hulsanpes, and Mahakala.

Reader's Guide

Halszkaraptorinae represents a small-bodied, long-necked group of theropod dinosaurs from the Late Cretaceous of Mongolia. Their significance lies in the ongoing debate about whether they were semiaquatic, a rare lifestyle among non-avian dinosaurs. In 2017, a comparison of Halszkaraptor fossils with bones of extant crocodilians and aquatic birds suggested semiaquatic adaptations, including forelimb use for swimming and long necks for foraging. However, subsequent studies have challenged this: a 2024 analysis of skull morphology and bite forces indicated Halszkaraptor was likely an insectivore with exceptional low-light vision, not a piscivore. Other researchers, such as Brownstein in 2019, argued that features do not directly support swimming ability, while Fabbri and colleagues in 2022 noted low bone density inconsistent with semiaquatic animals. Cau rebutted these criticisms, pointing out that swans also have low bone density yet are semiaquatic. The group's classification also remains fluid, with some analyses placing them within Avialae. Their legacy is as a key taxon for understanding the ecological diversity and evolutionary transitions among dromaeosaurids.

Did You Know?

• Halszkaraptorinae is known only from Late Cretaceous strata in Mongolia.
• The holotype of Halszkaraptor escuilliei was illegally poached and later returned to Mongolia through negotiations.
• A 2024 study suggested Halszkaraptor was an insectivore with exceptional low-light vision, not a fish-eater.

A Contested Path to Recognition

The halszkaraptorine fossil record was built piecemeal over nearly five decades, and one chapter of that story carries a stain of illegality. The first remains were recovered in 1970 by a joint Polish-Mongolian expedition working in the sandstone beds of the Barun Goyot Formation, Gobi Desert. They went undescribed for twelve years, until Halszka Osmólska erected Hulsanpes perlei in 1982, naming it for Mongolian paleontologist Altangerel Perle and tentatively assigning it to Deinonychosauria. In 1992, dromaeosaurid-like material was collected at the Tugriken Shire locality in the Djadokhta Formation; Alan Turner and colleagues finally published it in 2007 as Mahakala omnogovae, interpreting its primitive status mainly from body proportions. The most turbulent episode began in 2015, when a small theropod specimen surfaced at the Royal Belgian Institute of Natural Sciences after passing through English and Japanese private collections. Associated documents revealed it had been illegally poached from the Djadokhta Formation. Following negotiations among the institute, Eldonia, and Mongolian authorities, the fossil was returned to the Institute of Paleontology and Geology. Andrea Cau and co-workers described it in 2017 as Halszkaraptor escuilliei, honoring Osmólska and François Escuillié for their mediation, and coined the subfamily Halszkaraptorinae to group it with Hulsanpes and Mahakala.

A Duck-Sized Body with Distinctive Skeletal Signatures

Halszkaraptorines were modest in size, reaching lengths comparable to a modern duck, yet their skeletons carry a suite of diagnostic traits that set them apart from other small theropods. The neck was notably elongated, and the proximal caudal vertebrae display articular processes oriented in a distinctive configuration, along with prominent zygapophyseal laminae that form the posterior walls of each vertebral segment. The forelimb bones show a flattened ulna with a sharply defined posterior margin, while the pelvis bears a shelf-like supratrochanteric process on the ilium. In the hand, the shaft of metacarpal III is transversely as thick as that of metacarpal II, an unusual proportion. The femur's posterodistal surface carries an elongate fossa bounded by a lateral crest, and the proximal half of metatarsal III is unconstricted and markedly convex on its anterior face. Together these characters form the formal diagnosis of the subfamily, distinguishing halszkaraptorines from other dromaeosaurids and unenlagiids.

The Semiaquatic Question

Whether halszkaraptorines spent significant time in water remains one of the most hotly debated questions in small theropod paleobiology. In 2017, Andrea Cau and colleagues compared Halszkaraptor's bones with those of living crocodilians and aquatic birds, concluding the dinosaur was an obligate biped on land but also a capable swimmer that used its forelimbs to paddle and its long neck to probe for prey in water. However, a 2024 study by Tse, Miller, and Pittman examined skull morphology and bite forces across dromaeosaurids and found that Halszkaraptor's rapid bite was ill-suited to a fish-eating diet. Instead, they proposed it was an insectivore hunting small invertebrates, possibly in low-light conditions, given its relatively large orbits among dromaeosaurids. Skeptics have mounted further objections: Brownstein (2019) argued the cited features do not directly demonstrate swimming ability and suggested the animal might simply be a basal dromaeosaur with transitional traits, a claim Cau rebutted the following year. Hone and Holtz (2021) noted that flattened unguals do not inherently indicate aquatic competence, while Fabbri and colleagues (2022) pointed to the dinosaur's low bone density as inconsistent with a semiaquatic lifestyle. Cau countered that swans, too, have low bone density yet feed semi-aquatically.

Shifting Positions on the Tree of Life

Halszkaraptorinae occupies a fluid position in theropod phylogeny, and its exact placement has shifted as new taxa and analyses have emerged. The subfamily is formally defined as the most inclusive clade containing Halszkaraptor escuilliei but excluding Dromaeosaurus albertensis, Unenlagia comahuensis, Saurornithoides mongoliensis, and the vulture Vultur gryphus. Traditionally it has been treated as a basal lineage within Dromaeosauridae, though some researchers have questioned whether it might instead belong to Unenlagiidae. All known material comes exclusively from Late Cretaceous deposits in Mongolia, making the group geographically restricted. A 2022 phylogenetic analysis by Lee and colleagues, conducted alongside the description of Natovenator, placed the group in a particular configuration relative to other small theropods. More dramatically, Motta and colleagues in 2025 elevated Halszkaraptorinae to family rank and recovered it, together with Unenlagiidae, Anchiornithidae, and Archaeopteryx, within Avialae itself. This would push the group considerably closer to the origin of modern birds than earlier interpretations suggested, underscoring how rapidly our understanding of basal theropod relationships continues to evolve.

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