Juravenator

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Juravenator
Temporal range: Late Jurassic, 151.5Ma
Juravenator starkae.JPG
Holotype specimen
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Theropoda
Clade: Tyrannoraptora
Genus: Juravenator
Göhlich & Chiappe, 2006
Species

J. starki Göhlich & Chiappe, 2006

Juravenator is a genus of small (75 cm long) coelurosaurian theropod dinosaur, which lived in the area which would someday become the Jura mountains of Germany, about 151 or 152 million years ago. It is known from a single, juvenile specimen.

Discovery and naming[edit]

Close-up of the skull

In the summer of 1998, the Jura-Museum Eichstätt at Eichstätt organised a paleontological expedition to the nearby chalk quarry of Schamhaupten. Near the end of the planned excavations, two volunteers, Klaus-Dieter Weiß and his brother Hans-Joachim Weiß, found a chalk plate in which clear vertebrate remains were visible. A first preparation uncovered the head of a small theropod. However, due to the vulnerability of the bones, removing the hard calcium silicate matrix was slow and expensive. To see whether it was worthwhile to proceed, a CT-scan of the fossil was made. This seemed to show that only the neck and a small part of the rump were still present and accordingly the preparation was discontinued. In 1999 the find was reported in the scientific literature by Günther Viohl.[1] By 2001 the fossil had generated some publicity and was nicknamed Borsti in the German press, a name commonly given to bristle-haired dogs, on the assumption the creature was endowed with bristly protofeathers. In 2003, the new director of the museum, Martina Kölbl-Ebert, decided to finish the preparation. Preparator Pino Völkl then discovered, during seven hundred hours uncovering the remaining bones, that almost the entire skeleton was present.

In 2006 the type species Juravenator starki was named and described by Ursula Göhlich and Luis Chiappe. The generic name is derived from the Jurassic Bavarian Jura Mountains, and a Latin venator, "hunter". The specific name honours the Stark family, owners of the quarry.[2]

The holotype, JME Sch 200, was found in the Malm Epsilon 2, a marl layer of the Painten Formation dating to the late Kimmeridgian, about 151 to 152 million years old. As the bones were accessed from below — the specimen having landed on its back on the seafloor[3] — and the plate was not split further, a counterslab is lacking. The fossil consists of an almost complete articulated skeleton with skull of a juvenile individual. Only the tail end is missing. In small areas impressions or remains of the soft parts are present. The fossil was considered the most complete specimen of a non-avian theropod ever found in Europe.[2]

Description[edit]

Size of the juvenile type specimen, with a human for scale

Juravenator was a small bipedal predator. The holotype of Juravenator represents a juvenile individual, about seventy-five centimetres in length. In 2006 and 2010 Göhlich established some diagnostic traits. The four teeth of the premaxilla in the front of the snout had serrations on the upper third of the back edge of the tooth crown. Between the tooth row of the premaxilla and that of the maxilla there was no hiatus. The maxillary teeth were few in number, eight with the holotype. The depression or fossa for the large skull opening, the fenestra antorbitalis, was long and extended far to the front. The humerus was relatively short. The claws of the hand were high at their bases and suddenly narrowed transversely in the middle. The zygapophyses in the middle of the tail were bow-shaped.[2][4]

Feathers and scales[edit]

Restoration

Juravenator was originally classified as a member of the Compsognathidae, making it a close relative of Compsognathus, which preserved evidence of scales on the tail of one specimen, but also of Sinosauropteryx and Sinocalliopteryx, for which there is fossil evidence of a downy, feather-like covering.[2] However, a patch of fossilized Juravenator skin (from the tail, between the eighth and twenty-second vertebra, and lower hind leg) shows primarily normal dinosaur scales, as well as traces of what may be simple feathers.[5] Paleontologist Xu Xing, in his comments on the find in the journal Nature, initially suggested that the presence of scales on the tail of Juravenator could mean that the feather coat of early feathered dinosaurs was more variable than seen in modern birds. Xu also questioned the interpretation of Juravenator as a compsognathid, suggesting the extensive scaly hide could be a primitive trait. Xu considered it most likely that Juravenator and other primitive feathered dinosaurs simply possessed more extensive scales on their bodies than modern birds, which retain scales only on the feet and lower legs.[6]

Xu's interpretation was supported by further study of the Juravenator fossil. The first follow-up study to the initial description reported that faint impressions of filamentous structures, possibly primitive feathers, were present along the top of the tail and hips.[5] A more in-depth study, published in 2010, included an examination of the specimen under ultra-violet light by Helmut Tischlinger. The examination under UV revealed a more extensive covering of filament-like structures, similar in anatomy to the primitive feathers of other compsognathids, including Sinosauropteryx. The investigation also discovered additional patches with scales, on the snout and the lower leg, and vertical collagen fibres between the chevrons of the tail vertebrae.[4] Achim Reisdorf and Michael Wuttke in 2012 described the taphonomical circumstances of the fossilisation of the holotype of Juravenator starki.[3]

Classification[edit]

While first classified as a member of the Compsognathidae, subsequent studies have found problems with the initial study that produced those findings. Rather than grouping it with Sinosauropteryx and other compsognathids, Butler et al. found that it was not a compsognathid, but rather a basal member of the group Maniraptora.[7] Studies conflict on whether or not compsognathids belong to this later group or are more primitive. Additional work published by Luis Chiappe and Ursula Göhlich in 2010 found that Juravenator was most similar in anatomy to Compsognathus, and that it probably did belong to the Compsognathidae if that is actually a natural group. They also suggested that "compsognathids", including Juravenator, may form a grade of primitive coelurosaurs rather than a monophyletic clade.[4] In 2011 Cristiano dal Sasso and Simone Maganuco published an analysis which recovered the Compsognathidae as a natural group and Juravenator belonging to it as a sister species of Sinosauropteryx.[8] However, a large analysis of coelurosaurs published in 2013 again found Juravenator to be a coelurosaur closely related to, but not a member of, the Compsognathidae. Instead, it was recovered as a close relative of Ornitholestes outside the clade Maniraptoriformes.[9]

Paleobiology[edit]

Comparisons between the scleral rings of Juravenator and modern birds and reptiles indicate that it may have been nocturnal.[10]

References[edit]

  1. ^ Viohl, G. 1999. "Discovery of a new small theropod". Archaeopteryx 17: 15-19
  2. ^ a b c d Göhlich, U.B. and Chiappe, L.M. (2006). "A new carnivorous dinosaur from the Late Jurassic Solnhofen archipelago." Nature, 440: 329-332.
  3. ^ a b Reisdorf, A.G., and Wuttke, M. (2012). "Re-evaluating Moodie's Opisthotonic-Posture Hypothesis in fossil vertebrates. Part I: Reptiles - The taphonomy of the bipedal dinosaurs Compsognathus longipes and Juravenator starki from the Solnhofen Archipelago (Jurassic, Germany)." Palaeobiodiversity and Palaeoenvironments, 92(1): 119-168 doi:10.1007/s12549-011-0068-y
  4. ^ a b c Chiappe, L.M. and Göhlich, U.B. (2010). "Anatomy of Juravenator starki (Theropoda: Coelurosauria) from the Late Jurassic of Germany. Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen, 258(3): 257-296. doi:10.1127/0077-7749/2010/0125
  5. ^ a b Goehlich, U.B., Tischlinger, H., and Chiappe, L.M. (2006). "Juravenator starki (Reptilia, Theropoda) ein neuer Raubdinosaurier aus dem Oberjura der Suedlichen Frankenalb (Sueddeutschland): Skelettanatomie und Weichteilbefunde." Archaeopteryx, 24: 1-26.
  6. ^ Xu, X. (2006). "Scales, feathers, and dinosaurs". "Nature", 440: 287-288.
  7. ^ Butler, R.J., and Upchurch, P. (2007). "Highly incomplete taxa and the phylogenetic relationships of the theropod dinosaur Juravenator starki." Journal of Vertebrate Paleontology, 27(1): 253-256.
  8. ^ Cristiano dal Sasso & Simone Maganuco, 2011, Scipionyx samniticus (Theropoda: Compsognathidae) from the Lower Cretaceous of Italy — Osteology, ontogenetic assessment, phylogeny, soft tissue anatomy, taphonomy and palaeobiology, Memorie della Società Italiana de Scienze Naturali e del Museo Civico di Storia Naturale di Milano XXXVII(I): 1-281
  9. ^ Godefroit, Pascal; Cau, Andrea; Hu, Dong-Yu; Escuillié, François; Wu, Wenhao; Dyke, Gareth (2013). "A Jurassic avialan dinosaur from China resolves the early phylogenetic history of birds". Nature. in press. doi:10.1038/nature12168. PMID 23719374.  edit
  10. ^ Schmitz, L.; Motani, R. (2011). "Nocturnality in Dinosaurs Inferred from Scleral Ring and Orbit Morphology". Science 332. doi:10.1126/science.1200043. PMID 21493820. 

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