Compsognathus Temporal range: Late Jurassic, 150.8–145 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Reconstructed skeleton at the Museum of Ancient Life
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Family:	†Compsognathidae
Subfamily:	†Compsognathinae Cope, 1875
Genus:	†Compsognathus Wagner, 1859
Type species
†Compsognathus longipes Wagner, 1859
Synonyms
Compsognathus corallestris Bidar et al., 1972

Compsognathus (/kɒmpˈsɒɡnəθəs/;^[1] Greek kompsos/κομψός; "elegant", "refined" or "dainty", and gnathos/γνάθος; "jaw")^[2] is a genus of small, bipedal, carnivorous theropod dinosaur. Members of its single species Compsognathus longipes could grow to around the size of a chicken. They lived about 150 million years ago, during the Tithonian age of the late Jurassic period, in what is now Europe. Paleontologists have found two well-preserved fossils, one in Germany in the 1850s and the second in France more than a century later. 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 and named C. corallestris.

Many presentations still describe Compsognathus as "chicken-sized" dinosaurs because of the size of the German specimen, which is now believed to be a juvenile. Compsognathus longipes is one of the few dinosaur species whose diet is known with certainty: the remains of small, agile lizards are preserved in the bellies of both specimens. Teeth discovered in Portugal may be further fossil remains of the genus.

Although not recognized as such at the time of its discovery, Compsognathus is the first theropod dinosaur known from a reasonably complete fossil skeleton. Until the 1990s, it was the smallest-known non-avialan dinosaur, with the preceding centuries incorrectly labelling them as the closest relative of Archaeopteryx.

Discovery and species

Compsognathus is known from two almost complete skeletons.^[3] The German specimen (specimen number BSP AS I 563) stems from limestone deposits in Bavaria and was part of the collection of the physician and fossil collector Joseph Oberndorfer. Oberndorfer lent the specimen to paleontologist Johann A. Wagner, who published a brief discussion in 1859, where he coined the name Compsognathus longipes.^[4] Wagner did not recognise Compsognathus as a dinosaur, but instead described it as one of the "most curious forms among the lizards".^[4][5] He published a more detailed description in 1861.^[6] In 1866, Oberndorfer's collection, including the Compsognathus specimen, was acquired by the paleontological state collection in Munich.^[7]

Both the year of discovery and the exact locality of the German specimen are unknown, possibly because Oberndorfer did not reveal details of the discovery to prevent other collectors from exploiting the locality;^[7] later authors have suggested that the German specimen was probably discovered during the 1850s. Weathering of the slab on which the fossil is preserved indicates that it was collected from a pile of waste rock left behind by quarrying.^[8] The specimen either stems from Jachenhausen or the region Riedenburg–Kehlheim.^[8][9] All possible localities are part of lagoonal deposits of the Painten Formation, and date to the latest part of the late Kimmeridgian or the earlier part of the early Tithonian.^[8] In the Jurassic, the region was part of the Solnhofen archipelago. The limestone of the area, the Solnhofen limestone, had been quarried for centuries, and yielded such well-preserved fossils as Archaeopteryx with feather impressions and pterosaurs with imprints of their wing membranes.^[10]

In two publications in 1868 and 1870, Thomas Huxley, a major proponent of Charles Darwin's theory of evolution, compared Compsognathus with Archaeopteryx, which was considered the earliest known bird. Following earlier suggestions by Carl Gegenbaur^[11] and Edward Drinker Cope,^[12] Huxley found that Archaeopteryx was closely similar to Compsognathus, and referred to the latter as a "bird-like reptile". He concluded that birds must have evolved from dinosaurs, an assessment that established Compsognathus as one of the most widely known dinosaurs.^[13][14][5] The specimen has since been studied by many prominent paleontologists, including Othniel Charles Marsh, who visited Munich in 1881. The German paleontologist J.G. Baur, who worked as an assistant of Marsh, removed the right ankle from the slab for illustration and study; this removed part got lost since.^[5][15] Although Baur published a detailed study of the ankle in 1882, which is now the only available source of information of this part of the skeleton, his reconstruction was later found to be inconsistent with corresponding impressions on the slab.^[5] John Ostrom thoroughly described the German specimen as well as the newly discovered French specimen in 1978, making Compsognathus one of the best-known small theropods at that time.^[5]

The larger French specimen (Y85R M4M) was discovered in around 1971 in the Portlandian lithographic limestone of Canjuers near Nice.^[16] It dates to the lower Tithonian, as indicated by ammonite index fossils. As Solnhofen, Canjures was famous for its limestone plates, which were quarried and sold under the name "dalles de Provence". The specimen was originally part of a large private fossil collection of Louis Ghirardi, the owner of the Canjures quarries. The collection, including the Compsognathus specimen, was sold to the National Museum of Natural History in Paris in 1983. Alain Bidar and Gérard Thomel, in a brief 1972 description, announced the new find under a separate species, Compsognathus corallestris.^[17] A more comprehensive description followed in the same year.^[18] According to these authors, the new species differed from the German species in its larger size and modified, flipper-like hand. Ostrom, Jean-Guy Michard and others have since relabeled it as another example of Compsognathus longipes.^[16][19] In 1984, George Callison and Helen Quimby identified the smaller German specimen as a juvenile of the same species.^[20][19]

Collector Heinrich Fischer had originally labeled a partial foot consisting of three metatarsals and a phalanx, from the Solnhofen area, as belonging to Compsognathus longipes. This identification was rejected by Wilhelm Dames, when he described the specimen for the first time in 1884.^[21] Friedrich von Huene, in 1925 and 1932, also found that the foot did probably not belong to Compsognathus itself but to a closely related genus.^{[22][23]: 336} Ostrom, in his 1978 monography, questioned the attribution of this fossil to Compsognathus once more.^[5] Jens Zinke, in 1998, assigned forty-nine isolated teeth from the Guimarota coal mine of Portugal to the genus. Zinke found that these teeth are not identical to those of Compsognathus longipes, having serrations on the front edge, and thus labeled the teeth as Compsognathus sp. (of unknown species).^[24]

Description

For decades, Compsognathus was known as the smallest known non-avian dinosaur,^[5] although some dinosaurs discovered later, such as Mahakala and Microraptor, were even smaller.^[25][26] The German specimen was estimated to be 70–75 cm (28–30 in)^[5] and 89 cm (35 in)^[3] in length by separate authors, while the larger French specimen was estimated at 1.25 m (4 ft 1 in)^[3] and 1.4 m (4 ft 7 in)^[16] in length. The height at the hip has been estimated at 20 cm (7.9 in) for the German specimen and at 29 cm (11 in) for the French specimen.^[3] The German specimen was estimated to have weighed 0.32 kg (0.71 lb)^[27] and 0.58 kg (1.3 lb),^[3] and the French specimen 2.5 kg (5.5 lb)^[3] and 3.5 kg (7.7 lb).^[28] Compared to other compsognathids, the larger French specimen would have been similar in size to larger Sinosauropteryx specimens, but smaller than Huaxiagnathus and Mirischia.^[16]

Compsognathus were small, bipedal animals with long hind legs and longer tails, which they used for balance during locomotion. The forelimbs were smaller than the hindlimbs. The hand bore two large, clawed digits and a third, smaller digit that may have been non-functional.^[16][29] Their delicate skulls were narrow and long, with tapered snouts. The skull had five pairs of fenestrae (skull openings), the largest of which was for the orbit (eye socket),^[30] with the eyes being larger in proportion to the rest of the skull. The lower jaw was slender and had no mandibular fenestra, a hole in the side of the lower jawbone commonly seen in archosaurs.

The teeth were small and pointed, suited for its diet of small vertebrates and possibly other small animals, such as insects. The German specimen had three teeth in each premaxilla (front bone of the lower jaw), 15 or 16 teeth in each maxilla, and 18 teeth in the lower jaw.^[31][5] The French specimen had more teeth, including four in each premaxilla, 17 or 18 in the maxilla, and at least 21 teeth in the dentary.^[16] Compsognathids were unique among theropods in having tooth crowns that curved backwards at two thirds of their height, while their mid-parts were straight; also, the crowns had expanded bases.^[16] In Compsognathus, the frontmost teeth of the upper and lower jaws were unserrated, while those further back had fine serrations on their rear edges. In the German specimen, the crowns were around two times higher than wide in the front of the jaws but diminished in height further back, with the last tooth about as high as wide.^[31] The German specimen also shows a diastema (tooth gap) behind the first three teeth of the premaxilla.^[5] As such a gap was not present in the French specimen, Peyer suggested that additional teeth were possibly present in this region the German specimen.^[16]

The number of digits on the hand of Compsognathus has been a source of debate.^[29] For much of its history, Compsognathus was typically depicted with three digits, as is typical for theropods.^[29][5] However, the type specimen only preserved phalanges from the first two digits, leading to the suggestion that Compsognathus bore only two functional digits, with the third metacarpal being extremely slender and reduced.^[5] Study of the French specimen indicated that the third digit bore at least one or two small phalanges.^[16][29] However, there remains no evidence for an ungual phalanx on the third digit, so the digit may have been reduced and non-functional.^[29]

Integument

Some relatives of Compsognathus, namely Sinosauropteryx and Sinocalliopteryx, have been preserved with the remains of simple feathers covering the body like fur,^[32] prompting some scientists to suggest that Compsognathus might have been feathered in a similar way.^[33] Consequently, many depictions of Compsognathus show them with coverings of downy proto-feathers. However, no feathers or feather-like covering have been preserved with Compsognathus fossils, in contrast to Archaeopteryx, which are found in the same sediments. Karin Peyer, in 2006, reported skin impressions preserved on the side of the tail starting at the 13th tail vertebra. The impressions showed small bumpy tubercles, similar to the scales found on the tail and hind legs of Juravenator.^[34] Additional scales had in 1901 been reported by Von Huene, in the abdominal region of the German Compsognathus, but Ostrom subsequently disproved this interpretation;^[5][35] in 2012 they were by Achim Reisdorf seen as plaques of adipocere, corpse wax.^[8]

Like Compsognathus, and unlike Sinosauropteryx, a patch of fossilized skin from the tail and hindlimb of the possible relative Juravenator starki shows mainly scales, though there is some indication that simple feathers were also present in the preserved areas.^[36] This may mean that a feather covering was not ubiquitous in this group of dinosaurs, or maybe that some species had fewer feathers than others.^[37]

Classification

Originally classified as a lizard, the dinosaurian affinities of Compsognathus were first noted by Gegenbaur, Cope, and Huxley between 1863 and 1868.^[11][12][13] Cope, in 1870, classified Compsognathus within a new clade of dinosaurs, the Symphypoda, which also contained Ornithotarsus (today classified as Hadrosaurus).^[38][39] Later, both genera were found to belong to other groups of Cope's classification of dinosaurs: Compsognathus to the Gonipoda (equivalent to Theropoda, in which it is now classified), and Ornithotarsus to the Orthopoda (equivalent to Ornithischia).^[39] Huxley, in 1870, rejected Cope's dinosaur classification scheme, and instead proposed the new clade Ornithoscelida, in which he included the Dinosauria (comprising several forms now considered as ornithischians) and another new clade, the Compsognatha, which contained Compsognathus as the only member.^[40][41] Later, these groups fell into disuse, although a resurrection of the Ornithoscelida was proposed in 2017.^[42] The group Compsognatha was used for the last time by Marsh in a 1896 publication, where it was treated as a suborder of Theropoda.^[43][16] In the same publication, Marsh erected the new family Compsognathidae.^[43][16] Friedrich von Huene, in 1914, erected the new infraorder Coelurosauria, which includes the Compsognathidae amongst other families of small theropods; this classification remained in use since.^[44][16]

The Compsognathidae are a group of mostly small dinosaurs from the late Jurassic and early Cretaceous periods of China, Europe and South America.^[16] For many years, Compsognathus was the only member known, but in recent decades paleontologists have discovered several related genera. The clade includes Aristosuchus,^[45] Huaxiagnathus,^[46] Mirischia,^[47] Sinosauropteryx,^[32][48] and perhaps Juravenator^[49] and Scipionyx.^[50] At one time, Mononykus was proposed as a member of the family, but this was rejected by Chen and coauthors in a 1998 paper; they considered the similarities between Mononykus and the compsognathids to be an example of convergent evolution.^[51] The position of Compsognathus and its relatives within the coelurosaur group is uncertain. Some, such as theropod expert Thomas Holtz Jr. and co-authors Ralph Molnar and Phil Currie in the landmark 2004 text Dinosauria, hold the family as the most basal of the coelurosaurs,^[52] while others as part of the Maniraptora.^[53][54]

For almost a century, Compsognathus longipes was the only well-known small theropod species. This led to comparisons with Archaeopteryx and to suggestions of an especially close relationship with birds. In fact, Compsognathus, rather than Archaeopteryx, piqued Huxley's interest in the origin of birds.^[55] The two animals share similarities in shape and proportions, so many in fact that two specimens of Archaeopteryx, the "Eichstätt" and the "Solnhofen", were for a time misidentified as those of Compsognathus.^[30] Many other types of theropod dinosaurs, such as maniraptorans, are now known to have been more closely related to birds.^[56]

Below is a simplified cladogram placing Compsognathus in Compsognathidae by Senter et al. in 2012.^[57]

Here is an alternative phylogeny, published by Cau in 2024, with both specimens in bold.