Although not recognized as such at the time of its discovery, Compsognathus is the first dinosaur known from a reasonably complete skeleton. Today, C. longipes is the only recognized species, although the larger specimen discovered in France in the 1970s was once thought to belong to a separate species, C. corallestris. Until the 1980s and 1990s, Compsognathus was the smallest known dinosaur and the closest supposed relative of the early bird Archaeopteryx. Thus, the genus is one of the few dinosaur genera to be well known outside of paleontological circles.
For decades, Compsognathus was famed as the smallest dinosaur known; the specimens collected were around 1 meter (3 ft) in length. However, dinosaurs discovered later, such as Caenagnathasia, Microraptor and Parvicursor, were even smaller. Compsognathus is estimated to have weighed around 3 kg (6.5 lb).
Compsognathus was a small, bipedal animal with long hind legs and a longer tail, which it used for balance during locomotion. The forelimbs were smaller than the hindlimbs and featured three digits equipped with solid claws suited for grasping prey. Its delicate skull was narrow and long, with a tapered snout. The skull had five pairs of fenestrae (skull openings), the largest of which was for the orbit (eye socket). The eyes were large in proportion to the rest of the skull.
The lower jaw was slender and had no mandibular fenestra, a hole in the lower jawbone commonly seen in archosaurs. The teeth were small but sharp, suited for its diet of small vertebrates and possibly other small animals, such as insects. Its frontmost teeth (those on the premaxilla) were unserrated, unlike those further back in the jaw. Scientists have used these dental characteristics to identify Compsognathus and its closest relatives.
Compsognathus is known from two nearly complete skeletons, one from Germany that is 89 cm long (35 in) and another from France that is 125 cm (49 in). The physician and fossil collector Joseph Oberndorfer discovered the German specimen (BSP AS I 563) in the Solnhofen lithographic limestone deposits in the Riedenburg-Kelheim region of Bavaria during the 1850s. The limestone of the Solnhofen area has also yielded such well-preserved fossils as Archaeopteryx with feather impressions and some pterosaurs with imprints of their wing membranes that are dated to the lower Tithonian age. Johann A. Wagner described the specimen briefly in 1859 and in more detail in 1861, when he coined the name Compsognathus longipes. In early 1868, Thomas Huxley hypothesized that the specimen was closely related to the dinosaurs, and in 1896, Othniel Marsh recognized the fossil as a true member of that group. John Ostrom thoroughly redescribed the species in 1978, making it one of the best-known small theropods at that time. The German specimen is on display at the Bayerische Staatsammlung für Paläontologie und historische Geologie (Bavarian State Institute for Paleontology and Historical Geology) in Munich, Germany. The larger French specimen (MNHN CNJ 79) was discovered in 1972 in the Portlandian lithographic limestone of Canjuers near Nice in southeastern France. It dates to the lower Tithonian. Although Bidar originally described the specimen as a separate species called Compsognathus corallestris, Michard and others have since relabeled it as another example of Compsognathus longipes. Quimby identified the smaller German specimen as a juvenile of the same species. In 1983, the Muséum national d'histoire naturelle in Paris acquired the French Compsognathus fossil; Michard thoroughly studied it there. Scientists originally identified a partial foot, also from Solnhofen, as belonging to a Compsognathus, but later research has disproved this. Zinke has assigned teeth from the Kimmeridgian Guimarota formation of Portugal to the genus.
The Compsognathus specimen discovered in Germany in the 19th century featured only two digits on each forelimb, leading scientists to conclude that this was how the creature appeared in life. However, the fossil discovered later in France revealed the manus (hands) to have had three digits, similar to other members of compsognathid genera. The fossilization of the German Compsognathus had simply failed to preserve the specimen's forefeet. Bidar supposed that the French specimen had webbed forefeet, which would look like flippers in life. In the 1975 book The Evolution and Ecology of the Dinosaurs, L. B. Halstead depicts the animal as an amphibious dinosaur capable of feeding on aquatic prey and swimming out of reach of larger predators. Ostrom debunked this hypothesis by showing conclusively that the French specimen was nearly identical to the German specimen in every aspect but its size. Peyer confirmed these conclusions.
The remains of a lizard in the German specimen's thoracic cavity show that Compsognathus preyed on small vertebrates. Marsh, who examined the specimen in 1881, thought that this small skeleton inside Compsognathus belly was an embryo, but in 1903, Franz Nopcsa concluded that it was a lizard. Ostrom identified the remains as belonging to a lizard of the genus Bavarisaurus, which he concluded was a fast and agile runner due to its long tail and limb proportions. This in turn led to the conclusion that its predator, Compsognathus, must have had sharp vision and the ability to rapidly accelerate and outrun the lizard. The Bavarisaurus is in a single piece, indicating that Compsognathus must have swallowed its prey whole. The French specimen's gastric contents consist of unidentified lizards or sphenodontids.
For nearly a century, Compsognathus was the only well-known small theropod. This led to comparisons with Archaeopteryx and to suggestions of a relationship with birds. In fact, Compsognathus, rather than Archaeopteryx, piqued Huxley's interest in the origin of birds. The two animals share many similarities in shape, size and proportions, so many in fact that a featherless skeleton of an Archaeopteryx was for many years misidentified as a Compsognathus. Many other dinosaurs, including Deinonychus, Oviraptor and Segnosaurus, are now known to have been more closely related to birds.
No feathers or feather-like covering have been preserved with Compsognathus fossils, in contrast to Archaeopteryx, which was found in the same sediments; many depictions of Compsognathus thus show it without feathers. However, the only feathers found in association with Archaeopteryx are the large ones on the wings and tail; the short ones that likely covered the body have rarely been preserved. Von Huene reported the presence of a fossilized patch of skin in the abdominal region of the German Compsognathus, but Ostrom later disproved this. Relatives of Compsognathus, namely Sinosauropteryx and Sinocalliopteryx, have been preserved with the remains of simple feathers covering the body like fur, indicating that Compsognathus might have been feathered in a similar way. In contrast, a patch of fossilized skin from the tail and hindlimb of another presumed compsognathid genus, Juravenator, only shows scales, with no indication that feathers were present in the preserved areas. This may mean that feather covering was not ubiquitous in this group of dinosaurs, though a 2007 re-evaluation by Butler and Upchurch cast doubt on the assignment of Juravenator to the same family as Compsognathus.
More recently, the animal has appeared in the movies The Lost World: Jurassic Park and Jurassic Park III. In The Lost World: Jurassic Park, one of the characters incorrectly identifies the species as "Compsognathus triassicus", combining the genus name of Compsognathus with the species name of Procompsognathus, a distantly related small carnivore featured in the Jurassic Park novels. The "compys" are depicted as social animals that hunt in packs which makes them capable of taking down human-sized prey through overwhelming numbers. This behavior was invented by the creators of Jurassic Park as there is indeed no indication from the scientific point of view that Compsognathus (or Procompsognathus) had such social behavior.