When “dragons” really existed: here is the dinosaur that looks like a hybrid between a bird and a bat

Scansoriopterygidae  (meaning “creeping wings”) is an  extinct family of  climbing and gliding maniraptoran dinosaurs   . Scansoriopterygids are known from five  well-preserved fossils, representing four species, found in the fossil beds  of the Tiaojishan Formation  (dating from the Middle to Late  Jurassic Period  ) of  Liaoning  and  Hebei  ,  China :

Scansoriopteryx heilmanni  (and its likely synonym  Epidendrosaurus ninchengensis  ) was the first non-avian dinosaur discovered that had clear adaptations to an arboreal or semi-arboreal lifestyle: it is likely that they spent much of their time in trees. Both specimens displayed characteristics that indicated they were juveniles, making it difficult to determine their exact relationship to other dinosaurs and non-avian  birds . It was not until the description of  Epidexipteryx hui  in 2008 that an adult specimen was known. In 2015 the discovery of another larger adult specimen belonging to the  Yi qi speciesshowed that scansoriopterygids were not only climbers but also had adaptations that could have been used for gliding flight.  The recently discovered (in 2019)  Ambopteryx also demonstrates this .  The previously described  Pedopenna may also be a member of this clade. ^[1]

Scansoriopterygians are among the smallest known non-avian dinosaurs. Juvenile specimens of  Scansoriopteryx  are about the size of  house sparrows  ,  ^[2]  about 16 cm (6.3 in) long, while the adult type specimen of  Epidexipteryx  is about the size of a  pigeon  , about 25 cm (10 in) long. 9.8 inches) (excluding tail feathers). ^[3]

Scansoriopterygids differ from other theropod dinosaurs in part by their extremely long third toe, which was longer than the first and second digits on the hand. In all other known theropods, the second toe is the longest. At least two species,  Yi  and  Ambopteryx  , also had a long “styliform” bone growing from the wrist, which, together 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  , although physically it is more bat-like. ^[4]

Other characteristics shared within the group include short, tall skulls with downturned lower jaws and large front teeth, and long arms. Tail length, however, varied significantly among scansoriopterygids. Epidexipteryx  had a short tail (70% of torso length), anchoring long tail feathers, while  Scansoriopteryx  had a very long tail (more than three times as long as the torso) with a short spray of feathers at the tip. All three described scansoripterygid specimens retain trace fossil feathers covering their bodies. ^{[2]  [5]  [6]}

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  . Furthermore, the group lacked a  phylogenetic definition  . ^[7]  However, in 2008 Zhang  et al. reported another scansoriopterygid,  Epidexipteryx  , and defined Scansoriopterygidae as a  clade  including the most recent common ancestor of  Epidexipteryx  and Epindrosaurus  (=Scansoriopteryx  ) plus all of its descendants. ^[6]

The exact taxonomic placement of this group was initially uncertain and controversial. Describing the first validly published specimen in 2002 (  Scansoriopteryx heilmanni  ), Czerkas and Yuan hypothesized that various primitive skeletal features (including a primitive, “Saurischian-style” pubis and a primitive hip joint) show that scansoriopterygids, along with other maniraptorans and birds, separated from other theropods very early in dinosaur evolution. ^[5]  However, this interpretation has not been followed by most other researchers. In a   2007  cladistic analysis of relationships among coelurosaurs  ,  it was a member of the  clade  Avialae  . ^[8]  This view was supported by a second phylogenetic analysis performed by Zhang  et al. in 2008.

A subsequent phylogenetic analysis 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  ). ^[9]

Turner, Makovicky, and Norell (2012) only included  Epidexipteryx hui  in their primary phylogenetic analysis, as one adult specimen of this species is known; as for  Scansoriopteryx  /  Epidendrosaurus  , the authors were concerned that including it in the primary analysis would be problematic, because it is only known from juvenile specimens, which “do not necessarily retain all of the adult morphology needed to accurately place a taxon phylogenetically” (Turner, Makovicky and Norell 2012, p.89). Epidexipteryx  was recovered as a basal paravia that did not belong to Eumaniraptora. The authors noted that its phylogenetic position is unstable; binding  Epidexipteryx huias a basal avialan it required two additional steps compared to the more 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 has no clade with  Epidexipteryx  , which remained outside Eumaniraptora. Limiting the monophyly of Scansoriopterygidae required four additional steps and displaced  Epidexipteryx  into Avialae. ^[10]

A monophyletic Scansoriopterygidae was recovered by Godefroit  et al. (2013); the authors found that scansoriopterygids were nearly basal members of Paraves and the sister group of the containing clade Avialae and Deinonychosauria. ^[11]  Agnolín and Novas (2013) recovered scansoriopterygids as non-paravian maniraptorans and the sister group of Oviraptorosauria. ^[12]  Brusatte  et al. (2014) also found  Epidexipteryx  to be a basal oviraptorosaur along with  Pedopenna  . ^[13] Bayesian analysis by Cau (2018) re-placed scansoriopterygids in Oviraptorosauria, while parsimony analysis placed them in the basis of Avialae and included  Xiaotingia in Scansoriopterygidae as a sister to the rest of the group. ^[14]  Pitman  et al  . (2020) again found that scansoriopterygids were basal oviraptorosaurs. ^[15]

The following cladogram follows the results of a phylogenetic study by Lefèvre  et al. , 2014:  ^[16]

Sorkin (2021) argues that scansoriopterygids provide evidence that all pennaraptorans evolved from scansorial gliding ancestors. ^[17]

Origin and palaeoecology

The fossil remains of  Epidexipteryx  ,  Scansoriopteryx  , and  Yi  have all been recovered from the  Tiaojishan Formation  of northeastern China, and the first two have been found specifically in the  Daohugou Beds  . A study published in 2008 refined the possible age range of this formation, finding that the lower limit of the Tiaojishan was formed 165 Ma ago and the upper limit somewhere between 156 and 153 Ma ago. ^[18]

The known scansoriopterygids from the Daohugou biota inhabited a moist, temperate forest composed of a variety of prehistoric trees including  ginkgo species  and  conifers  . The understory would have been dominated by plants such as  mosses  ,  horsetails  ,  cycads  and  ferns  . ^[19]

Scansoriopterygids would have lived alongside  synapsids  such as the  aquatic Castorocauda,  ​​Volaticotherium  , a  gliding arboreal mammal, and various types of  gliding Haramiyidans  , the  rhamphorhynchoid  pterosaurs  Jeholopterus  and  Pterorhynchus  , as well as a diverse range of insects (including  mayflies  and  beetles  ) and several species of  salamander  . ^{[20]  [21]}

Paleobiology:

Climbing

In initial descriptions of the first two scansoriopterygid specimens, scientists studying these animals used several lines of evidence to argue that they were arboreal (tree climbers) and the earliest known non-avian dinosaurs with clear climbing adaptations.

Zhang and colleagues considered  Scansoriopteryx  arboreal based on the elongated nature of the hand and foot specializations. These authors argued that the long hand and strongly curved claws were adaptations for climbing and moving through tree branches. They saw it as an early stage in the evolution of the bird’s wing, stating that the forelimbs had become 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 better suited for climbing than flying, as most flying birds have relatively short hands. Zhang  et al. also noted that the foot of  Scansoriopteryx it is unique among non-avian theropods; while  Scansoriopteryx does not retain an inverted big toe (the rear-pointing toe seen in modern perching birds), its foot was very similar in construction to primitive perching birds such as  Cathayornis  and  Longipteryx  . These adaptations for grasping in all four limbs, the authors argued, make it likely that  Scansoriopteryx  spent a significant amount of time living in trees. ^[2]

In describing  Scansoriopteryx  , Czerkas and Yuan also described evidence for an arboreal lifestyle. They noted that, unlike all modern bird hatchlings, the forelimbs of  Scansoriopteryx  are longer than the hind limbs. The authors argued that this anomaly indicates that the forelimbs played an important role in locomotion even at an extremely early stage of development. Scansoriopteryx has a better preserved foot than the Epidendrosaurus  type,   and the authors interpreted the big toe as inverted, the condition of a backward-pointing toe being widespread among modern tree-dwelling birds. Furthermore, the authors pointed to the stiffened tail of Scansoriopteryx as a tree-climbing adaptation. The tail may have been used as a prop, much like the tails of  modern woodpeckers  . Comparison with the hands of modern climber species with an elongated third toe, such as  iguanid lizards  , also supports the tree-climbing hypothesis. In fact, Scansoriopteryx ‘s hands   are much better adapted to climbing than the modern   tree-climbing  hoatzin pup. ^[5]

Feathers 

Both juvenile scansoriopterygid specimens retain impressions of down-like simple feathers, especially around the hand and arm. The longer feathers in this region led Czerkas and Yuan to hypothesize that adult scansoriopterygids may have had reasonably well-developed wing feathers that could have aided in rudimentary jumping or gliding, although they ruled out the possibility that Scansoriopteryx could have achieved powered flight   . Like other  maniraptorans , scansoriopterygids had a carpal semilunate (crescent-shaped wrist bone) that allowed for a bird-like bending motion in the hand. Even if powered flight had not been possible, this movement could have aided maneuverability in jumping from branch to branch. ^[5]

Adult  Epidexipteryx  lacked preserved feathers around the forelimbs, but retained simple feathers on the body and long, ribbon-like feathers on the tail. The tail feathers, probably used in display, consisted of a central shaft (rachis) and an unbranched vane (unlike modern feather vanes, which are broken into smaller filaments or tips). ^[6]

Yi  also keeps feathers. These are remarkably very simple for a member of  Pennaraptora  (a clade of which scansoriopterygids are usually considered members), being “brush-like”, with long, quill-shaped bases topped with sprays of finer filaments. Feathers covered most of the body, starting near the tip of the snout. The feathers on the head and neck were long and formed a thick coat, and the feathers on the body were even longer and denser, making it difficult for scientists to study their detailed structure.

Sliding membranes

At least two species,  Yi qi  and  Ambopteryx longibrachium  , have evolved a  patagium  , supporting it with an elongated third toe and a single styliform wrist bone resembling similar structures in  flying squirrels  ,  bats  ,  pterosaurs  , and  anomalies  . Although  propatagi  are known in  birds  and similar dinosaurs, scansoriopterygids were the only dinosaurs known to develop true membranous wings, particularly due to the presence of already quite derived feathers. ^{[4]  [22]  [23]  [24]}

Prior to  Yi ‘s discovery  , Italian paleontologist Andrea Cau had informally suggested that membranes may be present in  Scansoriopteryx  , supported by its elongated third toe, due to their resemblance to the wing digits of pterosaurs,  ^[25]  a hypothesis that later he also applied to the Epidexipteryx. _  ^[26]

References