User:Abyssal/Hoganas Formation


 * 1) Edit fauna
 * 2) Biogeography of Dilophosaurus
 * 3) Source many
 * 4) Links
 * 5) O accent
 * 6) review
 * 7) devise hook

The Hoganas Formation is a stratigraphic formation in the Scania region of southern Sweden. Its rocks are made of sandstones, mudstones rich in kaolinite, and coal. Geologists have divided the formation into three subunits, the Valkara, Bjuv, and Helsingborg Members. These sediments were deposited from the Late Triassic to the Early Jurassic. When the Hoganas first started forming, southern Sweden was a relatively dry environment but by the time deposition concluded the region was much wetter. The Hoganas sediments record the presence of a swampy coastal floodplain in the area.

This floodplain was once home to dinosaurs who left behind an abundance of fossil footprints. The vast majority of these footprints belonged to carnivorous theropods. One tracksite in the Hoganas preserves a large number of such tracks from individuals of various sizes but apparently the same species and may represent evidence for social behavior. One unusual track may be the oldest known fossil of an armored dinosaur.

Lithology and paleoenvironments
The Hoganas Formation is about 250 meters in thickness and divided into three constituent members. From lowest to uppermost these are the Valkara, Bjuv, and Helsingborg Members. The Valkara Member is about 30 m thick and made mostly of mudstone and siltstone interspersed by coal seams and series of ancient soil horizons known as paleosols. It was deposited by calm waters on a floodplain.

The Bjuv Member is another floodplain deposit left by relatively calm waters, but contains a greater portion of coal seams and paleosols. Two of these coal seams are very distinctive and useful for correlating local rock strata. They are simply known as the B and A coals, with B being the lower of the two. The Triassic-Jurassic boundary is contain within the Bjuv Member and probably corresponds to the top surface of the A coal.

Most of the geologic thickness of Hoganas Formation is occupied by the Helsingborg Member. According to Vajda and others, the Helsingborg Member is made up of "heterolites, arenites, and mudstones" interspersed by thin layers of coal. The Helsingborg Member was deposited in shallow coastal waters when sea levels rose and spread into the area once occupied by dry land.

History
Dinosaur tracks were first discovered in the Hoganas Formation during the 1950s in coal mines near the towns of Hoganas and Bjuv. In 1952, E. Bölau published preliminary report on these tracks. He recognized them as theropod traces but did not elaborate on their classification. In the mid 1970s additional Hoganas dinosaur footprints were discovered in a quarry near Vallkara. C. Pleijel reported them to the scientific literature in 1975. The Vallkara tracks are all referrable to the same ichnospecies, which is now known as Kayentapus soltykovensis, and were made by carnivorous dinosaurs.

In the late 1980s, a railroad tunnel was being excavated near Helsingborg. The digging exposed two dinosaur footprints on the ceiling of the tunnel. These may be the first dinosaur footprints ever found on the ceiling of a railway tunnel. Extracting the track was deemed too risky on the grounds that excavation may jeopardize the stability of the tunnel, so scientists cast a replica of the footprint. The local train station now displays the bronze replica to visitors. These tracks were described for the scientific literature by Anders Ahlberg and Mikael Siverson in 1991. Three years later, Polish ichnologist Gerard Gierlinski collaborated with Ahlberg to publish a study of the region's original 1950s-era track discoveries.

In 2004 Jesper Milàn and Gerard Gierlinski reported that one of the supposed 1950s theropod footprint discoveries was probably made by an armored dinosaur. These tracks were so ancient that if their identification is correct, the specimen would be the earliest known armored dinosaur fossils in the world. Records at the Geological Museum of Copenhagen indicate that this track was originally accompanied by three more like it, possibly constituting a single trackway. However, Milàn and Gierlinski were unable to relocate these companion tracks.

Paleofauna
The Hoganas Formation has been the source of many fossil dinosaur footprint discoveries.

In the mid 1970s additional dinosaur footprints were discovered in the Hoganas in a quarry near Vallkara. C. Pleijel reported them to the scientific literature in 1975.

The Vallkara tracks are all referrable to the same ichnospecies, which is now known as Kayentapus soltykovensis, and were made by carnivorous dinosaurs. Even though the tracks all seem to have been left by the same species, they vary in length from 15 to 35 cm. This considerable size variation suggests that the tracks were left by a social group including members of various ages.

Dinosaurs left behind footprints in Hoganas sediments both before and after the Triassic-Jurassic boundry.

Thus it preserves evidence for dinosaurs before and after one of Earth's Big Five mass extinctions.

The possible thyreophoran track has three blunt toes that are roughly equal in length. The angle between the outer toes is about 76 degrees. One of its toe pads and possibly a blunt nail are also subtly perceptible. The track is 26 cm long 3-3.5 cm deep. Its breadth and blunt toes are very different than the narrow, sharp-clawed theropod tracks. The most similar fossils of similar age are putative "proto-stegosaur" tracks from the Hettangian of Poland that Gierlinski referred to Moyenisauripus karaszevskii. This suggests that MGUH 27219 was also left by an extremely early member of the armored dinosaur lineage.

Dilophosaurus

83

The Hoganas Formation has been the source of many fossil dinosaur footprint discoveries. Dinosaur tracks are found in Hoganas Formation sediments dating to both before and after the Triassic-Jurassic boundry. Thus it preserves evidence for dinosaurs before and after one of Earth's Big Five mass extinctions.

Carnivorous dinosaur footprints have been found near Vallkara. These tracks are all referrable to the ichnospecies Kayentapus soltykovensis. The Kayentapus trackmaker is widely regarded to have been similar to Dilophosaurus wetherilli of the American southwest. Even though the Vallkara Kayentapus tracks all seem to have been left by the same species, they vary in length from 15 to 35 cm. This considerable size variation suggests that the tracks were left by a social group including members of various ages.

The possible thyreophoran track has three blunt toes that are roughly equal in length. The angle between the outer toes is about 76 degrees. One of its toe pads and possibly a blunt nail are also subtly perceptible. The track is 26 cm long 3-3.5 cm deep. Its breadth and blunt toes are very different than the narrow, sharp-clawed theropod tracks. The most similar fossils of similar age are putative "proto-stegosaur" tracks from the Hettangian of Poland that Gierlinski referred to Moyenisauripus karaszevskii. This suggests that MGUH 27219 was also left by an extremely early member of the armored dinosaur lineage.

The Hoganas Formation has been the source of many fossil dinosaur footprint discoveries. Dinosaur tracks are found in Hoganas Formation sediments dating to both before and after the Triassic-Jurassic boundry. Thus it preserves evidence for the kinds of dinosaurs that lived both before and after one of Earth's Big Five mass extinctions.

The dinosaur tracks preserved near Vallkara are all referrable to the ichnospecies Kayentapus soltykovensis. The Kayentapus trackmaker is widely regarded to have been similar to Dilophosaurus wetherilli of the American southwest. Even though the Vallkara Kayentapus tracks all seem to have been left by the same species, they vary in length from 15 to 35 cm. This considerable size variation suggests that the tracks were left by a social group including members of various ages.

The possible thyreophoran track has three blunt toes that are roughly equal in length. The angle between the outer toes is about 76 degrees. One of its toe pads and possibly a blunt nail are also subtly perceptible. The track is 26 cm long 3-3.5 cm deep. Its breadth and blunt toes are very different than the narrow, sharp-clawed theropod tracks. The most similar fossils of similar age are putative "proto-stegosaur" tracks from the Hettangian of Poland that Gierlinski referred to Moyenisauripus karaszevskii. This suggests that MGUH 27219 was also left by an extremely early member of the armored dinosaur lineage.

Paleoflora
The ancient Hoganas was a swampy floodplain. Microfossil evidence suggests the presence of a diverse flora ranging from freshwater algae, Botryococcus braunii, and tiny hard-to-identify elements like Michrystridium species to more familiar and conspicuous plant groups like mosses, club mosses, ferns, and gymnosperms. Mosses left behind a variety of microfossils referrable to morphotaxa like Annulispora folliculosa, Cingutriletes clavus, Densoisporites psilatus, and Stereisporites antiquasporis. An even more diverse club moss flora is also recorded in the formation by the morphotaxa Camarozonosporites rudis, Klukisporites neovariegatus, Limbosporites lundbladi, Lycopodiumsporites, Lycopodiumsporites clavatoides, Retitriletes austroclavatidites, Retitriletes semimuris, and Zebrasporites intercriptus. Ferns left behind a variety of microfossils including Acanthotriletes varius, Baculatisporites comaumensis, Cibotiumspora juriensis, Cyathidites australis, Cyathidites minor, Deltoidospora, Gleicheniidites senonicus, Peromonolites densus, Striatella seebergensis, Todisporites major, and Todisporites minor. Gymnosperm morphotaxa included Alisporites parvus, Araucariacites australis, several Classopollis species, Cycadopites, Perinopollenites elatoides, Pinuspollenites, Podocarpidites, Quadraeculina annelaeformis, several Ricciisporites, and Vitreisporites bjuvensis.