Horseshoe Canyon Formation

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The Horseshoe Canyon Formation at its type locality in Horseshoe Canyon, near Drumheller, Alberta. The dark bands are layers of coal.

The Horseshoe Canyon Formation is a sequence of sedimentary rocks of late Cretaceous period , in the Canadian province of Alberta minded is. This formation is known for its rich fauna of fossil vertebrates ; Its dinosaur fossils are particularly famous . Stratigraphically , it belongs to the Edmonton Group , along with the Whitemud Formation, the Battle Formation and the Scollard Formation. The Horseshoe Canyon Formation is up to 230 meters thick and consists of claystones , sandstones and coal-bearing shale . It was deposited during the late Campanian (83.5 to 70.6 mya ) and the early Maastrichtian (70.6 to 65.5 mya); Outcrops are particularly found in the area around Drumheller and further north along the Red Deer River near Trochu, but also in the city of Edmonton . The formation is named after the Horseshoe Canyon ( type locality ), a canyon about 3 kilometers long that is 17 kilometers southwest of Drumheller on Highway 9. The name ("Horseshoe Gorge") describes the horseshoe-shaped course of the gorge.

Geological history

The Horseshoe Canyon Formation forms the lowest lithostratigraphic unit of the Edmonton Group. The Whitemund, Battle and Scollard formation follow above it. Beneath the Horseshoe Canyon Formation lie the marine deposits of the Bearpaw Formation, showing that this area was covered by a sea, the Western Interior Seaway, a few million years ago . During the deposition time of the Horseshoe Canyon Formation, the sea withdrew more and more . While the upper third of the formation shows strong marine influences, the lower two thirds consist of sediments deposited by rivers in large flood plains. The vast majority of vertebrate fossils come from the deposits of these flood plains. However, the retreat of the sea was by no means complete - the constantly changing sea levels created a variety of different environments at different times and places: In addition to the flood plains, there were marine habitats near the coast, deltas, lagoons, estuaries and mudflats. The coal layers indicate peaty swamps.

Fossil fauna

The formation's vertebrate fauna is well documented. Finds of fish include, among others, sharks , rays , sturgeons , bonefish and bald pike . The plesiosaur Leurospondylus lived in the sea , while turtles, Champsosaurus and crocodiles such as Leidyosuchus and Stangerochampsa lived in fresh water . The land fauna was formed, among other things, by mammals from the group of the multituberculate and the marsupial Didelphodon ; however, the dominant land vertebrates were the dinosaurs. Among the herbivores, the hadrosaurids in particular dominated , from which half of all dinosaur fossils found originated: the species Edmontosaurus regalis , Saurolophus osborni and Hypacrosaurus altispinus are known . Also common were Ceratopsier with the types Pachyrhinosaurus canadensis , Anchiceratops ornatus , arrhinoceratops brachyops and eotriceratops xerinsularis . Rare were Ankylosaurier as Euoplocephalus tutus and Edmontonia longiceps or Pachycephalosaurier as Stegoceras edmontonense . Another ornithic is the ornithopod Parksosaurus warrenae . Among the theropods , ornithomimids such as Dromiceiomimus brevitertius , Ornithomimus edmontonicus, and Struthiomimus altus were particularly common. The top predator was the tyrannosaurid Albertosaurus sarcophagus , which could only be detected in this formation. Smaller predators include Ricardoestesia isosceles , the Caenagnathids Chirostenotes pergracilis , the Troodontids Troodon formosus, and the Dromaeosaurids Atrociraptor marshalli , Dromaeosaurus sp. and Saurornitholestes sp. The remains of the chicken-sized Alvarez saurid Albertonykus borealis , the smallest known dinosaur in North America , were also found in the Horseshoe Canyon formation . The fossil evidence supports the hypothesis of the origin of this unusual dinosaur clade in South America and its spread over North America to Asia.

In addition, the arthropod fauna of the Upper Cretaceous habitat is well documented by amber finds in the coal layers of the Horseshoe Canyon Formation near Edmonton and Drumheller . The rich fauna identified so far from amber (botanical fossils are extremely rare) confirms the habitats already deduced from the vertebrate finds.

Individual evidence

  1. ↑ Gibson, DW, 1977: Upper Cretaceous and Tertiary coal-bearing strata in the Drumheller-Ardley region, Red Deer River Valley, Alberta . In: Geological Survey of Canada, pp. 76-35.
  2. ^ David A. Eberth: Edmonton Group . In: Currie, Philip J. and Padian, Kevin (herg.). (Ed.): Encyclopedia of Dinosaurs . Academic Press, San Diego 1997, ISBN 0-12-226810-5 , pp. 199-204.
  3. ↑ Nicholas R. Longrich, and Philip J. Currie (2008): " Albertonykus borealis , a new alvarezsaur (Dinosauria: Theropoda) from the Early Maastrichtian of Alberta, Canada: implications for the systematics and ecology of the Alvarezsauridae". In: Cretaceous Research . doi : 10.1016 / j.cretres.2008.07.005
  4. ^ McKellar & Wolfe: Canadian Amber. In: Biodiversity of fossils in amber from the major world deposits. Ed .: D. Penney, Manchester (UK), ISBN 978-0-9558636-4-6 , pp. 149-166.