Feathers and Leathers: A Newly Discovered Jurassic Experiment in Theropod Flight

A new peculiar theropod dinosaur, Ambopteryx longibranchium, has been named and taxonomically classified after a fossil unearthed in the Liaoning Province of northeast China reveals a divergence from the expected wing morphology of its contemporaries. The early Upper Jurassic fossil’s upper extremities formed an elongated styloid-like extension, or ‘styliform element,’ in addition to elongated manual digits, which are together expected to have provided support for bat-like wings. In an exciting discovery reported in Nature Magazine in May of 2019, remnants of a membranous tissue were preserved around the upper extremities, which the scientists interpret to have formed the gliding surface of wings. Membranous wings were previously unidentified in theropods, the dinosaur clade that gave rise to modern birds. This fossil shows that other evolutionary experiments with flight existed in this group during the Jurrassic. The confirmation of the presence of the styliform element supports the conclusion that the elongated hand structure of this dinosaur and others closely related, like Yi qi, were more likely used as a flight apparatus rather than as a specialized feeding mechanism or for display or signaling purposes.

The evolutionary development observed in this study separates Ambopteryx from the early avian ancestral lineage, but shows are still phylogenetically related. Ambopteryx joins Yi in a specialized and unique group of theropods called Scansoriopterygids. It is expected that this group of Paravian dinosaurs branched off from the avian groups before Archeopteryx and other feathered-flight experimentalists emerged.

Phylogenetic diagram from Nature Magazine (May 9, 2019) relating Ambopteryx to theropods, raptors, and the avian lineage.

Min Wang, Jingmai K. O’Connor, Xing Xu & Zhonghe Zhou; Nature volume 569, pages256–259 (2019)

The three winged taxonomical orders, Pterosauria, Chiroptera and Aves, exhibit specific and distinct variations on the forelimb morphology that supports wings utilized for powered flight, as opposed to volant flight or gliding. Each group has a unique skeletal scaffolding that supports the soft-tissue that comprises the wing. Pterosaurs developed an elongated fourth digit that served as the primary of a membranous wing, with three small digits remaining. Bats exhibit an extended hand-like support of membranous wings, where four bony digits are elongated. Feathered bird wings are supported by a robust leading edge formed by what looks like fused fingers. As is firmly established through the fossil record, modern birds arose from the theropod lineage, which suggests that theropods would have developed feathered flight and was evidenced by fossils like Archeopteryx. The new species is a divergence from this logical evolutionary trajectory, suggesting that membranous wings were perhaps developed and then lost. The researchers hypothesize that these bat-like wings allowed for an early evolutionary experiment in volant flight, or limited airborne gliding. The skeletal styliform element that would provide skeletal support for the gliding surface or wings has not been found in any other groups of dinosaurs except Yi and Ambopteryx, but is found in living vertebrates. In extant species like the mammalian sugarglider or modern bats, as well as flying lizards such as the Draco genus, the styliform element supports a patagium, which could be described as a skin flap that is associated with gliding-type of flight, often extending from the forelimbs to the hindlimb and forms the aerodynamic surface for gliding or flight. It is not generally associated with powered flight, however. 

Ambopteryx anatomy

Ambopteryx is a strange-looking dinosaur, even by Mesozoic standards. There are many skeletal elements that the researchers concluded were unique to this organism. Overall, Ambopteryx exhibits an avian-like body plan. The fossil preserved soft tissue like dense long feathers around the neck and shoulder girdle, the wing-associated membrane, and an unidentified tissue matrix around much of the skeleton. This slight organism is calculated to weigh under a pound and stand only 12 inches in body length, as can be correlated by femur circumference.

Stomach contents provided another new discovery:  these dinosaurs were likely omnivores.  Scansopterygid feeding strategy was previously unknown. Twenty gastroliths, small stones swallowed and retained temporarily in the stomach (or gizzard in some modern birds), were found juxtaposed with small skeletal fragments, together suggesting carnivory as part of the feeding mode of this organism. Gastroliths are common in the extinct sauropods, as well as extant birds, amphibians and reptiles, and can range in size and number depending on the organism. Fossilized gastroliths can be identified by their distinction in rock type and provenance from the formation in which a fossil is buried. They are generally rounded or sub-rounded and the extruding faces or surfaces of the stones are polished down. They must also be found in close association with a fossil to be named ‘gastrolith.’ In this case, the stones were 2-4mm in diameter, rounded and found in a collection ventral to the vertebrae. 

The identification of fossils in general is based on very specific and detailed description of the morphology of the organism. Ambopteryx was distinguished based on a few key characteristics. The ulna of Ambopteryx is two times wider than the radius. It exhibits a pygostyle, which is a fused bone that constitutes a short tail that is common in non-avian theropods. Hip structure and morphology of dinosaurs often plays a part in discerning phylogeny. Here, the ischium bone of the pelvic girdle exhibits a projection on the posterior side of what is called the acetabulum, looking like a rod-like extension off the hip running parallel to the tail. These features place Ambopteryx in the Scansopterygid group, but separate it from early avian-bound theropod.

This discovery opens a new dialogue on the evolution of flight in one of the most intriguing groups of dinosaurs, the theropods. No membranous wings have been observed in Cretaceous-aged theropods until Ambopteryx, indicating that either the fossil record is biased against the preservation of these soft-tissue, non-feathered wings, or that membranous wings were a short-lived mutation that did not ultimately survive the evolutionary forces of selection acting during the Jurassic. The intrigue of the Mesozoic Era has not yet exhausted itself as scientists continue to uncover bizarre dinosaurs with unique behaviors and morphologies, and recover new phylogenetic relationships between modern and extinct organisms on Earth throughout time.

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