Scansoriopterygidae

Scansoriopterygids are among the smallest non-avian dinosaurs known. The juvenile specimens of Scansoriopteryx are the size of house sparrows, about long, while the subadult type specimen of Epidexipteryx is about the size of a pigeon, about long (not including the tail feathers). Scansoriopterygids differentiate from other theropod dinosaurs in part by their extremely long third fingers, which were longer than the first and second digits of the hand. In all other known theropods, the second finger is the longest. At least two species, Yi and Ambopteryx, also had a long "styliform" bone growing from the wrist, which, along with the third finger, helped support a bat-like wing membrane used for gliding. This use of a long finger to support a wing membrane is only superficially similar to the wing arrangement in pterosaurs, even though it is physically more bat-like. == Classification ==

Scansoriopterygidae was created as a family-level taxon by Stephen Czerkas and Yuan Chongxi in 2002. Some scientists, such as Paul Sereno, initially considered the concept redundant because the group was originally monotypic, containing only the single genus and species Scansoriopteryx heilmanni. Additionally, the group lacked a phylogenetic definition. However, in 2008 Zhang et al. reported another scansoriopterygid, Epidexipteryx, and defined Scansoriopterygidae as a clade comprising most recent common ancestor of Epidexipteryx and Epidendrosaurus (=Scansoriopteryx) plus all its descendants. The exact taxonomic placement of this group was initially uncertain and controversial. When describing the first validly published specimen in 2002 (Scansoriopteryx heilmanni), Czerkas and Yuan proposed that various primitive features of the skeleton (including a primitive, "saurischian-style" pubis and primitive hip joint) showed that scansoriopterygids, along with other maniraptorans and birds, split from other theropods very early in dinosaur evolution. This view was supported by a second phylogenetic analysis performed by Zhang et al. in 2008. A subsequent phylogenetic analysis conducted by Agnolín and Novas (2011) recovered scansoriopterygids not as avialans, but as basal members of the clade Paraves remaining in unresolved polytomy with alvarezsaurids and the clade Eumaniraptora (containing avialans and deinonychosaurs). Turner, Makovicky and Norell (2012) included only Epidexipteryx hui in their primary phylogenetic analysis, as a full-grown specimen of this species is known; regarding Scansoriopteryx/Epidendrosaurus, the authors were worried that including it in the primary analysis would be problematic, because it is only known from juvenile specimens, which "do not necessarily preserve all the adult morphology needed to accurately place a taxon phylogenetically" (Turner, Makovicky and Norell 2012, p. 89). Epidexipteryx was recovered as basal paravian that didn't belong to Eumaniraptora. The authors did note that its phylogenetic position is unstable; constraining Epidexipteryx hui as a basal avialan required two additional steps compared to the most parsimonious solution, while constraining it as a basal member of Oviraptorosauria required only one additional step. A separate exploratory analysis included Scansoriopteryx/Epidendrosaurus, which was recovered as a basal member of Avialae; the authors noted that it did not clade with Epidexipteryx, which stayed outside Eumaniraptora. Constraining the monophyly of Scansoriopterygidae required four additional steps and moved Epidexipteryx into Avialae. A monophyletic Scansoriopterygidae was recovered by Godefroit et al. (2013); the authors found scansoriopterygids to be basalmost members of Paraves and the sister group to the clade containing Avialae and Deinonychosauria. Agnolín and Novas (2013) recovered scansoriopterygids as non-paravian maniraptorans and the sister group to Oviraptorosauria. Brusatte et al. (2014) also found Epidexipteryx to be a basal oviraptorosaur along with Pedopenna. The Bayesian analysis of Cau (2018) placed scansoriopterygids in Oviraptorosauria again, while the parsimony analysis placed them in the base of Avialae, and included Xiaotingia in Scansoriopterygidae as sister to the rest of the group. Pittman et al. (2020) again found scansoriopterygids to be basal oviraptorosaurs. Cau (2024) placed Scansoriopterygidae within Anchiornithidae. The cladogram below follows the results of a phylogenetic study by Lefèvre et al., 2014: }} }} }} }} }} Sorkin (2021) argues scansoriopterygids provide evidence that all pennaraptorans evolved from scansorial gliding ancestors. == Provenance and paleoecology ==

The fossil remains of Epidexipteryx, Scansoriopteryx and Yi were all recovered from the Tiaojishan Formation of northeastern China, and the former two were specifically found in the Daohugou Beds. A study published in 2008 refined the possible age range of this formation, finding that the lower boundary of the Tiaojishan was formed 165 Ma ago, and the upper boundary somewhere between 156 and 153 Ma ago. The known scansoriopterygids of the Daohugou biota inhabited a humid, temperate forest made up of a variety of prehistoric trees including species of ginkgo and conifer. The understory would have been dominated by plants such as club mosses, horsetails, cycads, and ferns. The scansoriopterygids would have lived alongside synapsids such as the aquatic Castorocauda, arboreal gliding mammal Volaticotherium and various types of gliding haramiyidans, the rhamphorhynchoid pterosaurs Jeholopterus and Pterorhynchus, as well as a diverse range of insect life (including mayflies and beetles) and several species of salamander. == Paleobiology ==

'' in its environment Climbing In the initial descriptions of the first two scansoriopterygid specimens, scientists studying these animals used several lines of evidence to argue that they were arboreal (tree-climbing), and the first known non-avian dinosaurs with clear climbing adaptations. Zhang and colleagues considered Scansoriopteryx to be arboreal based on the elongated nature of the hand and specializations of the foot. These authors stated that the long hand and strongly curved claws were adaptations for climbing and moving around among tree branches. They viewed this as an early stage in the evolution of the bird wing, stating that the forelimbs became well-developed for climbing, and that this development later led to the evolution of a wing capable of flight. They argued that long, grasping hands are more suited to climbing than to flight, since most flying birds have relatively short hands. Zhang et al. also noted that the foot of Scansoriopteryx is unique among non-avian theropods; while Scansoriopteryx does not preserve a reversed hallux (the backward-facing toe seen in modern perching birds), its foot was very similar in construction to primitive perching birds like Cathayornis and Longipteryx. These adaptations for grasping ability in all four limbs, the authors argued, makes it likely that Scansoriopteryx spent a significant amount of time living in trees. Prior to the discovery of Yi, Italian palaeontologist Andrea Cau had informally suggested that membranes may have been present in Scansoriopteryx, supported by its elongated third finger, due to their similarity to the wing fingers of pterosaurs, a hypothesis he later also applied to Epidexipteryx. ==References==