Theropod dinosaurs are the only group of carnivorous predatory dinosaurs, and include the largest predators to have walked on dry land. Many of the characteristics that were believed to be exclusive to birds already appeared more or less developed in many of these theropods. For example, the pneumatization of their bones: theropods (and also sauropods, but we’ll talk about that another day) had some hollow bones, filled with air sacs, so that they improved their breathing and lightened their weight in a very remarkable way. . They were lighter than they look!
Although the most famous of theropod dinosaurs is the Cretaceous Tyrannosaurus rex , in the Jurassic the Allosaurus stands out. This carnivorous dinosaur was described in 1877 by the American paleontologist Othniel Charles Marsh , who gave it the name Allosaurus , which means “strange reptile” . It lived at the end of the Jurassic period, approximately 156 to 144 million years ago, in what is now North America and Eurasia. Several species have been defined, although the best known is Allosaurus fragilis , from the United States and Portugal. It is a very well known theropod thanks to the fossils of up to 44 individuals found so far, many of them in Utah. In Europe, however, it seems that allosaurids were also abundant. It is for all these reasons that it is popularly known as “the Jurassic lion”.
It was a robust theropod with large serrated teeth and claws, which as an adult could reach 9 meters in length and up to 3 meters in height, as well as weighing more than a ton. It had hands with three fingers and bulky hind legs. Its large tail would serve as a counterweight. On occasions, so many specimens have been found together that it has been suggested that they may have had a gregarious life.
The skull of Allosaurus could measure up to almost a meter in length , although due to being so light it may seem small compared to that of other large theropods such as Tyrannosaurus or carcharodontosaurs. As a distinguishing feature, allosaurids had a pair of ridges above the eyes , as well as two parallel lines of ridges on the nasal bone running along the top of the skull. These ornamentations, especially the small crests, being covered by scales, would have seemed larger. Among the proposed functions, its possible use as a sunshade stands out, as a kind of visor to protect the eyes from sunlight, or they may even have had some social function. They had a long snout, and the entire skull was lightened by wide holes and fenestrae, which also served as anchors for their muscles and sensory organs. Their jaws were full of up to 60 teeth, sharp and with a serrated edge, thanks to which they were capable of tearing the meat of their prey.
The most famous Allosaurus specimen was discovered in 1991 in Wyoming and is known by the name “Big Al” . It is a practically complete and partially articulated specimen, around 10 meters long. Much information about his paleobiology was obtained from his study, such as the fact that he was a young adult, or that he had suffered many injuries and infections during his life. In paleontology, pathologies that affect bones can be recognized and studied in a similar way, comparing them with those observed in current animals, allowing part of the life of these specimens to be reconstructed. The branch of paleontology that is dedicated to this is Paleopathology.
The allosaurs must have preyed on a wide variety of dinosaurs and other Jurassic animals of all sizes, since they coexisted with huge herbivorous dinosaurs such as Stegosaurus, Brachiosaurus or Diplodocus . However, in the case of very large or dangerous species, such as large sauropods, they would possibly only attack old, young or sick individuals. Another option would be the use of a cooperative hunting strategy, which could only occur if they lead a gregarious life, in herds or groups.
Thanks to the study of their paleohistology (the study of their bones under a microscope ), it has been determined that they had a high metabolism , grew rapidly and could reach ages between 22 and 28 years, very similar to what has been obtained for other large theropods such as Tyrannosaurus.
In the Iberian Peninsula , abundant remains of allosaurid bones and teeth have been found, especially in Portugal , where the deposits with remains of Allosaurus are very important, even defining a species, Allosaurus europaeus . Teeth similar to his have also been found in Jurassic sites in provinces such as Teruel or Valencia. One more example that the mythical dinosaurs were closer than we usually think.
References:
Marsh, OC 1877. Notice of new dinosaurian reptiles from the Jurassic formation. American Journal of Science and Arts, 14: 514-516.
Malafaia, E. et al. 2007. New remains of Allosaurus fragilis (Theropoda: Carnosauria) from the Andrés site (Late Jurassic; Central-West Portugal). In: Paleontological Quarry: 255-271. Provincial Council of Cuenca, Cuenca: 398 pp.
Madsen, JH, Jr. 1976. Allosaurus fragilis: A Revised Osteology. Utah Geological Survey Bulletin 109. Salt Lake City: Utah Geological Survey.
