Megaraptora have puzzled paleontologists since the naming of Megaraptor namunhuaiquii from Patagonia in 1998. The group belongs to the coelurosaurian theropods, the same broad radiation that eventually gave rise to birds, yet its members grew to enormous sizes while retaining strikingly long, powerful arms tipped with enormous claws. Their skulls were long, low, and lightly built, and their skeletons were conspicuously slender compared with those of their massive relatives. The new work examines a key feature that may explain this paradox: postcranial skeletal pneumaticity, the invasion of air sacs — diverticula of the respiratory system, homologous to those found in living birds — into the vertebral column, ribs, and other bones. In modern birds this system lightens the skeleton and supports the efficient flow-through lungs that power flight and sustained activity. In megaraptorids, the study shows, it became an evolutionary engine for gigantism without the usual mass penalty.

The research took a multiproxy approach, combining two independent lines of evidence. The first relied on recently published body-mass estimations derived from limb bone circumferences, a standard method in paleontology because the strength of the femur and tibia scales tightly with the load they must carry. The second line of evidence came from segmented computed tomography scans of fossil bones, which allowed the internal cavities left by air sacs to be measured directly in three dimensions. CT data of the sacral and caudal vertebrae and the chevrons — the haemal arches beneath the tail — of the enigmatic Patagonian megaraptoran Aoniraptor libertatem, together with cervical and dorsal vertebrae and ribs of the recently described giant Maip, revealed that pneumatic cavities occupy a huge proportion of total bone volume. In practical terms, these animals carried skeletal frameworks riddled with air, probably reducing their skeletal weight considerably compared with solid-boned contemporaries.

The contrast with other theropods is stark. Comparable measurements taken from non-megaraptoran species, including the dorsal and sacral vertebrae of the small coelurosaur Bicentenaria and the cervical and dorsal vertebrae and ribs of the carcharodontosaurid Taurovenator, show notably lower volumes of internal air spaces. In other words, even the giant carcharodontosaurids — the “shark-toothed” predators that dominated Gondwanan ecosystems before the Cenomanian-Turonian boundary roughly 95 million years ago — built their bulk in a fundamentally heavier way. Megaraptorids were not simply growing large; they were growing large on a different structural plan, one in which the skeleton itself contributed far less to overall body mass than in the competing heavy clades such as megalosauroids, carcharodontosaurids, and tyrannosaurids.

Body-mass estimates reinforce this picture. Previous authors had computed masses from femoral circumference, and the new study adds estimates based on tibial circumference, applying the widely used scaling relationships that link proximal limb bone dimensions to body mass in terrestrial tetrapods. The results indicate that megaraptorids were between 40 and 70 percent lighter than non-avian theropods of similar body length. Combined with previously established body lengths, this implies that megaraptorans deviate systematically from the mass expected for their size class within Theropoda as a whole. A predator stretching to lengths comparable with a tyrannosaur, but weighing a fraction as much, would have moved through its world in a very different manner — with different energetic demands, different locomotor capabilities, and possibly different hunting strategies built around the group’s trademark long, clawed forelimbs rather than sheer crushing power.

Crucially, the study also traces how this lightness evolved. Megaraptorans did not begin as hollow-boned giants. The analysis confirms that pneumaticity increased progressively through the phylogeny of the group: younger, late-branching megaraptorans became both bigger and more pneumatized over time. This trend has previously been linked to one of the most significant faunal turnovers of the middle Cretaceous — the extinction of carcharodontosaurids at the Cenomanian-Turonian boundary, which opened the top-predator niche in South American ecosystems. As the giant carnosaur dynasty collapsed, megaraptorans appear to have shifted into the vacancy, escalating in size. What the new study adds is the insight that the rise in pneumaticity and the rise in body mass may be intimately coupled rather than coincidental. Each increment of evolutionary size was paid for, at least in part, in bone volume replaced by air.

The physiological machinery behind this trend is well understood from living birds. In birds, cervical, thoracic, and abdominal air sacs ventilate rigid, flow-through lungs, and their diverticula invade the vertebrae, ribs, sternum, and even the limb bones, creating the characteristic pneumatic foramina visible on fossil bones. Among non-avian dinosaurs, pneumaticity appears in a patchwork of lineages — saurischian sauropodomorphs and theropods independently acquired postcranial skeletal pneumaticity, and recent work has even documented it in alvarezsaurians and therizinosaurs. But megaraptorids pushed the system to an extreme for large predators. Their vertebrae and ribs, emptied of marrow and converted into a network of chambers, represent one of the most extreme expressions of skeletal pneumaticity known among giant predatory dinosaurs, second only in spirit to the baroque pneumatization that allowed sauropods to hoist their enormous necks.

The evolutionary payoff of a lightened skeleton would have been considerable. A reduced skeletal mass lowers the overall cost of supporting and moving the body, potentially allowing a megaraptorid to achieve the length of a multi-ton carnosaur with the metabolic and biomechanical burden of a substantially smaller animal. Lighter skeletons may also have permitted the elongated, muscular forelimbs that define the group, since every kilogram saved in the torso is a kilogram available for limb machinery. The study’s author notes that megaraptorans deviate from the expected theropod body-mass trajectory precisely as their pneumaticity deepens, suggesting an evolutionary trend toward a “gracile” model of large predation — a way of being huge that traded dense bulk for structural efficiency. In ecosystems where their competitors were massive, solidly built ambush predators, megaraptorids may have occupied a parallel role as faster, lighter, more active hunters, dispatching prey with claw-bearing hands and lightly constructed skulls rather than the bone-pulverizing bites of tyrannosaurs.

The implications extend beyond megaraptorids themselves. By quantifying how much air-filled bone can reduce body mass in a giant predator, the study provides a caution and a tool for anyone estimating the masses of extinct animals from skeletal dimensions. Mass estimates based on bone circumference are generally robust, but reconstructions that assume a typical relationship between length and mass may systematically overestimate pneumatized species. Conversely, CT-based volumetric analyses of fossil bones, like those deployed here on Aoniraptor, Maip, Bicentenaria, and Taurovenator, offer a way to detect such departures and to map the distribution of pneumaticity across the dinosaur tree with unprecedented precision. As scanning technology spreads through paleontological collections worldwide, comparisons of internal bone volume are likely to become a routine complement to traditional anatomical description.

The megaraptorid story is also a reminder that evolutionary success on the predator stage has never had a single template. For much of the Cretaceous, the largest predatory niches were contested between lineages that solved the problem of gigantism in strikingly different ways. Carcharodontosaurids grew vast through sheer skeletal mass; abelisaurs remained compact and heavily muscled; tyrannosaurids in the northern hemisphere culminated in the dense, robust architecture of Tyrannosaurus rex. Megaraptorids, on this new evidence, chose air. When the carcharodontosaurids vanished, the lightest of the giants was ready to step into their place, having spent millions of years quietly hollowing out its bones as it grew. The result, preserved in Patagonian sandstones as slender vertebrae pocked with pneumatic foramina, is one of paleontology’s most elegant experiments in predator design — a demonstration that even among dinosaurs, sometimes the lightest build could carry the heaviest crown.