Imperobator ("powerful warrior") is a genus of probable unenlagiid paravian theropod dinosaurs, that lived during the Maastrichtian age of the Late Cretaceous in what is now James Ross Island in Antarctica. Imperobator is one of only two non-avian theropods known from Antarctica, crossing over to the landmass when it was part of Gondwana. The only described specimen was found in 2003 by an expedition launched by the University of California Museum of Paleontology and initially described as a dromaeosaur in 2007. The fossil was formally described as a new genus in 2019, and later searches reported more fossils from the site including teeth and skull bones. It was initially suggested that Imperobator may be one of the largest known paravians, comparable in size to gigantic dromaeosaurids such as Utahraptor and Austroraptor, but subsequent anatomical revisions suggested a body length similar to that of Neuquenraptor and Deinonychus. Because the preserved material appeared to lack the characteristic "sickle claw" of dromaeosaurs on the second digit, Imperobator was initially classified as a basal paravian of uncertain affinities, though later researchers found support for unenlagiine affinities. It was discovered in the Cape Lamb Member strata of the Snow Hill Island Formation, which bears a variety of other fossils, many of them unique as they evolved in the isolation of Antarctica after the breakup of Gondwana. Imperobator coexisted with the ornithopod dinosaur Morrosaurus and bird Antarcticavis in addition to a menagerie of mosasaurs, plesiosaurs, and a pterosaur.
Discovery and naming
Fossils of a large theropod dinosaur were unearthed in December 2003 by a fossil hunting expedition that had been mounted by the University of California Museum of Paleontology to the Naze Peninsula of James Ross Island, Antarctica to find fossils. The peninsula bears several fossiliferous outcrops of marine sediment. These belong to the Cape Lamb Member of the Snow Hill Island Formation, which dates to the early Maastrictian age of the Late Cretaceous (~71 mya). The fossils were unearthed in 2003, consisting mostly of an incomplete isolated left pes including a portion of the tibia, an incomplete astragalus, a partial calcaneus and fibula, ungual, partial phalanges and metacarpals, though teeth are known as well. They were then deposited at the University of California Museum of Paleontology under catalog number UCMP 276000. However, additional fossils were located in the facilities of Eastern Washington University and the South Dakota School of Mines and Technology that pertain to UCMP 276000 including skull fragments which may be from the premaxilla, maxilla, and/or dentary as well as a caudal vertebra, more teeth, and pedal elements. The third Antarctic Peninsula Paleontology Project expedition in 2011 and 2016 to the same locality found even more fossils from the UCMP 276000 individual, such as a tooth, incomplete pedal ungual, cranial fragments, and indeterminate bone shards, now in the collections of the American Museum of Natural History under number AMNH FARB 30894. According to Lamanna et al. (2019), an ongoing description of the novel remains is in the works. The fossils were first reported in published literature in 2005, with the authors theorizing that the specimen was of a "primitive holdover of the original Gondwanan dinosaur assemblage", noting its less derived characteristics compared to other Maastrichtian dromaeosaurs. A more detailed paper on the theropod specimen was published in 2007, which believed that it was of a dromaeosaurid, dubbing it the "Naze dromaeosaur" after the site in which it was found. The idea of the fossils being from a dromaeosaur is contentious, as the pes lacks the distinct sickle claw in addition to other characteristics of dromaeosaurs. The specimen was formally described as the holotype of a new genus and species, Imperobator antarcticus, by American paleontologists Ricardo Ely and Judd Case in 2019. The generic name derives from the Latin for "powerful warrior". The specific name refers to the continent in which the specimen was discovered.
Description The holotype specimen measures approximately 45 centimetres (18 in) in length, and it was initially estimated that Imperobator would have measured 2 metres (6.6 ft) tall, comparable to the size of the largest dromaeosaurs such as Utahraptor and Austroraptor. However, the previous estimate was criticized for its lack of specification about the parameters for calculations, and reexaminations of the phalanx III-1 and distal metatarsal III of Imperobator suggested that they were much smaller than those of Austroraptor and more similar in size to those of Neuquenraptor and Deinonychus, which led to the revision of its body length estimate to around 2–3 metres (6.6–9.8 ft). Despite prior assignment to Dromaeosauridae, Imperobator has since been assigned to the clade Paraves due to certain characteristics that differ from those of dromaeosaurids, including the lack of a sickle claw, the smooth surface of the distal metatarsal II and the lack of an ungual on the second pedal digit. Undescribed cranial material preserves teeth from the maxilla and dentary, which were long, curved, and bladed like in other carnivorous paravians.
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