What about European alvarezsauroids?

Xu et al. (1) reported the alvarezsauroid Linhenykus monodactylus from Inner Mongolia, a confirmed monodactyl Mesozoic dinosaur. Xu et al.’s (1) phylogenetic hypothesis for Linhenykus suggests that it is sister to a clade that includes all other Cretaceous Laurasian alvarezsauroids. On this basis, Xu et al. (1) discussed the paleobiogeographic history of Alvarezsauroidea, concluding that the lineage originated in Asia and then spread via successive dispersal events to South and North America [the earliest known record of this lineage is from the Jurassic of China (2)].

Although there is little doubt that Linhenykus is important and interesting from both phylogenetic and functional standpoints, here, we draw attention to three serious shortcomings of the study by Xu et al. (1). First, diagnostic alvarezsauroid remains, although fragmentary, have been described from the European Cretaceous and are well-documented and corroborated (3, 4). Irrespective of their eventual phylogenetic position (awaiting the discovery of better preserved fossils), these specimens will seriously affect Xu et al.’s (1) paleobiogeographic hypothesis. Xu et al. (1) did not discuss the known European records of these dinosaurs, except for a passing mention in SI Appendix. Second, Xu et al.’s (1) use of clade names and referral of Linhenykus to Parvicursorinae are inconsistent with their own previous work and will cause confusion. Choiniere et al. (2) originally defined this clade using a fixed, node-based definition that specifically excludes Linhenykus (2). In their present study, Xu et al. (1) contradicted this by using Parvicursorinae for a more inclusive, branch-based clade that includes Parvicursor (5) but not Patagonykus. This needs to be noted and corrected in subsequent works. Third, Xu et al.’s (1) diagnosis for Linhenykus is currently insufficient to distinguish this taxon from the similarly aged Mongolian Parvicursor (5), named more than 15 y ago by Karhu and Rautian [Borrisiak Paleontological Institute of the Russian Academy of Sciences (PIN) no. 4487/25]. The only difference clear to us between the overlapping preserved elements of these two taxa is a slight discrepancy in size.