TWO NEW EUDROMAEOSAURS FROM KHULSAN (CENTRAL MONGOLIA) REVEAL MODERN-LIKE FAUNAL PREDATORY STRUCTURE AMOUNG NON-AVIAN DINOSAURS

In modern ecosystems, closely related taxa of similar body size commonly coexist throughout part or all of their ranges. While anthropogenic factors have caused the ranges of many species to contract, historical records show that even apex predators are capable of coexistence without competitive exclusion. Such patterns, however, are rare among non-avian dinosaur faunas – typically, localities only attest to the presence of a single predatory species, and when multiple are present, they differ in body size. The faunal structure of dromaeosaurid dinosaurs from Mongolia has been suggested to show a similar pattern, in which a single small halszkaraptorine and a single mid-sized velociraptorine are present at localities from which both are known. Here, we describe two new eudromaeosaur specimens, one from the Khulsan locality (Barun Goyot Formation) and one from Zos Wash locality (–near Ukhaa Tolgod, Djadokhta Formation), that contradict this interpretation. The first, IGM 100/981, represents a new taxon that is distinguished from all other eudromaeosaurs by possession of dual posterior surangular foramina, a suranuglar shelf that is totally dorsal in orientation, and anteriorly displaced pleurocoels on the dorsal centra, as well as a unique combination of mandibular, dental, and postcranial characters. The second,IGM 100/3503, appears referrable to Velociraptor, although it differs from other Velociraptor mongoliensis specimens in some respects such as the absence of a dorsal ectopterygoid recess. It is distinct from the coeval Tsaagan mangas, the only eudromaeosaur hitherto known from Ukhaa Tolgod. IGM 100/3503 preserves ulnar papillae for feather attachment, and therefore preserves the only direct evidence of feathering in a eudromaeosaur. Phylogenetic analysis places IGM 100/981 as a eudromaeosaur that is not sister to Shri devi, confirming that two species are present at Khulsan. IGM 100/3503 is too incomplete to place confidently, but its clear distinction from Tsaagan mangas similarly attests to the presence of two eudromaeosaur species at Ukhaa Tolgod. Given that these specimens are almost identical in body size to the dromaeosaurs they coexisted with, they demonstrate that multiple closely related and morphologically similar dinosaurian predators existed in some Mesozoic faunas, and calls into question specimen referrals based on size and provenance rather than shared derived characters.