The fossil record of Mesozoic and Paleocene pennaraptorans

Abstract

An unabated surge of new and important discoveries continues to transform knowledge of pennaraptoran biology and evolution amassed over the last 150+ years. This chapter summarizes progress made thus far in sampling the pennaraptoran fossil record of the Mesozoic and Paleocene and proposes priority areas of attention moving forward. Oviraptorosaurians are bizarre, nonparavian pennaraptorans first discovered in North America and Mongolia within Late Cretaceous rocks in the early 20th century. We now know that oviraptorosaurians also occupied the Early Cretaceous and their unquestionable fossil record is currently limited to Laurasia. Early Cretaceous material from China preserves feathers and other soft tissues and ingested remains including gastroliths and other stomach contents, while brooding specimens and age-structured, singlespecies accumulations from China and Mongolia provide spectacular behavioral insights. Less specialized early oviraptorosaurians like Incisivosaurus and Microvenator remain rare, and ancestral forms expected in the Late Jurassic are yet to be discovered, although some authors have suggested Epidexipteryx and possibly other scansoriopterygids may represent early-diverging oviraptorosaurians. Long-armed scansoriopterygids from the Middle-Late Jurassic of Laurasia are either early-diverging oviraptorosaurians or paravians, and some have considered them to be early-diverging avialans. Known from five (or possibly six) feathered specimens from China, only two mature individuals exist, representing these taxa. These taxa, Yi and Ambopteryx, preserve stylopod-supported wing membranes that are the only known alternative to the feathered, muscular wings that had been exclusively associated with dinosaurian flight. Thus, scansoriopterygid specimens—particularly those preserving soft tissue—remain a key priority for future specimen collection. Dromaeosaurids and troodontids were first discovered in North America and Mongolia in Late Cretaceous rocks. More recent discoveries show that these animals originated in the Late Jurassic, were strikingly feathered, lived across diverse climes and environments, and at least in the case of dromaeosaurids, attained a global distribution and the potential for aerial locomotion at small size. China and Mongolia have yielded the most dromaeosaurid and troodontid specimens and taxa, but Gondwanan troodontids are almost unknown compared to southern dromaeosaurids, so the fidelity of this biogeographical signal is worth further exploration. Discovery of well-preserved Middle-Late Jurassic material will be crucial for understanding the origin of key dromaeosaurid and troodontid traits, with the controversial anchiornithines potentially already offering this if their troodontid status can be solidified. In line with the preferences of most theropod palaeontologists, birds are defined herein as members of Avialae, including stem and crown taxa, whilst Aves herein refers to crown-group birds (see Pittman et al., chapter 1, for the precise definition of Avialae adopted; elsewhere, typically among ornithologists, Aves refers to stem and crown taxa whilst Neornithes refers to crown-group birds). Despite taphonomic bias against avialans in the fossil record, their Early Cretaceous record is fairly robust largely due to the high taxonomic and ecological diversity preserved within the rich Jehol deposits of northeastern China. Archaeopteryx (and possibly the controversial Middle-Late Jurassic anchiornithines) show what some of the earliest birds were like, but better-preserved soft tissues hold the key to understanding their substantially different anatomy and flight capabilities to crowngroup birds (Aves). The Late Cretaceous–early Paleocene fossil record of crown birds is especially poor, and improved sampling will be necessary to clarify our understanding of avialan survivorship, ecological selectivity, and recovery across the end-Cretaceous mass extinction. Deposits of Eocene age, such as Messel and Green River, have been especially useful for documenting the early evolutionary history of crown birds. However, the discovery of new Cretaceous and/or Palaeogene bird-bearing lagerstätten from Gondwana will be important for accurately determining ancestral biogeographic patterns.