Spinosaurus aegyptiacus was a gigantic predator that roamed the river systems of North Africa during the Late Cretaceous period, approximately 100 to 94 million years ago. This dinosaur is easily recognized by its distinctive features, including an enormous dorsal sail and an elongated, narrow skull. With estimated lengths of around 14 to 15 meters, it ranks among the largest known carnivorous dinosaurs. Its unique semi-aquatic lifestyle has complicated the question of its closest living relative, requiring a trace of its lineage deep into the evolutionary past.
The Immediate Answer: Birds
The most direct and scientifically accurate answer is that the closest living relatives of Spinosaurus are modern birds, belonging to the class Aves. This may seem counter-intuitive given the dinosaur’s massive size and crocodile-like snout, but it reflects a deep truth in evolutionary history. Spinosaurus was a member of the Theropoda, the group of two-legged, mostly carnivorous dinosaurs. Birds are not merely related to dinosaurs; they are the sole surviving lineage of the dinosaurs themselves.
All species of modern birds are equally related to Spinosaurus. They share a common ancestor that lived much more recently than the common ancestor Spinosaurus shares with any other modern animal. This classification places the massive, sail-backed river predator firmly on the same evolutionary branch as a modern sparrow or eagle. The distinction between extinct non-avian dinosaurs and living avian dinosaurs is a matter of nomenclature, not a fundamental break in the family tree.
Tracing the Lineage: The Archosaur Family Tree
To understand why birds are the closest relatives, it is necessary to examine the Archosauria clade, which includes dinosaurs, pterosaurs, and crocodilians. This ancient group split into two primary branches long before the appearance of Spinosaurus. One branch, the Pseudosuchians, led to modern crocodiles, alligators, caimans, and gharials.
The other major branch is the Avemetatarsalians, which includes all dinosaurs and their descendants, the birds. Spinosaurus, as a Theropod, sits firmly within the Avemetatarsalian lineage, sharing defining skeletal features with other dinosaurs. The common ancestor that Spinosaurus shares with a bird is therefore a dinosaur that lived much later in time than the distant common ancestor it shares with a crocodile.
Paleontologists use specific skeletal characteristics to map these relationships. Features in the hip structure and the shape of the ankle bones distinguish the dinosaur-bird line from the crocodile line. The structure of these joints shows that the evolutionary path leading to crocodiles diverged many millions of years before Spinosaurus even existed.
Aquatic Traits and Evolutionary Convergence
The confusion regarding Spinosaurus’s closest relative stems from its striking resemblance to modern crocodilians, a textbook example of evolutionary convergence. Convergence occurs when unrelated species independently evolve similar traits to adapt to shared environmental pressures or niches. In this case, the shared pressure was a semi-aquatic, fish-eating lifestyle.
The skull of Spinosaurus was long, low, and narrow, with high-set nostrils and specialized teeth, features that mirror the snouts of fish-eating crocodilians like the modern gharial. These cranial adaptations provide a mechanical advantage for gripping slippery prey, but they evolved separately in the two groups. Although the jaws were functionally similar to those of crocodilians, the underlying bone structure was fundamentally different, reflecting its theropod heritage.
Spinosaurus possessed other unique adaptations for its life in the water that distinguish it from crocodiles. Recent fossil discoveries revealed that its tail was deep and paddle-like, with long vertebral spines creating a large, flexible fin for aquatic propulsion. Furthermore, the bones were unusually dense, a condition known as osteosclerosis, which is seen in many diving animals to help control buoyancy.
These specialized features allowed Spinosaurus to exploit a unique predatory niche in its riverine habitat. While its appearance and lifestyle converged with that of crocodilians, its deep evolutionary roots remain firmly planted in the dinosaur lineage. The massive sail-backed predator shared its most recent common living ancestor with a bird, despite looking like a terrestrial crocodile.