Margaret formation

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The Ellesmere Island in Canada's far north, indicating important sites

The Margaret Formation is a lithostratigraphic formation that is common in the Arctic of North America. It consists largely of sand and clay / silt stones , which represent deposits of a former river delta and date to the early to mid- Eocene around 50 million years ago. Two fossil-bearing horizons are embedded in these deposits, which contain numerous remains of the former fauna and flora. These allow a species-rich primeval forest to be reconstructed, which was inhabited by an early mammal fauna and developed in a warm, temperate climate under humid conditions. Due to its high northern distribution at the time, this community cannot be compared with any biotope today.

Geological framework

The Margaret Formation is part of the Eureka Sound Group , which extends in the arctic region of North America between 77 and 82 ° north latitude and is open in several areas of Ellesmere Island and adjacent islands of the Canadian Arctic archipelago . However, it is sometimes called differently, so the names Iceberg Bay Formation (on Axel-Heiberg Island ), Cyclic Member (on Banksinsel ) or the informal name Member IV have been scientifically introduced. The age of the Eureka Sound Group dates back to the late Cretaceous ( Campanium or Maastrichtian ). It is stored in the Sverdrup Basin , a long-lived basin structure of the North American Arctic that was formed in the Paleozoic over 300 million years ago. Traditionally, the group is divided into four formations, the top of which is the Margaret Formation, whose ages range from the Lower to the Middle Eocene . The Buchannan Lake Formation , which belongs to the younger Middle Eocene and from which a petrified forest has been preserved at Geodetic Hill on Axel-Heiberg Island , is only locally formed on the Margaret Formation . Main formation time of the Eureka Sound Group was during the Eurekasischen orogeny , in the Paleocene began and up to the Upper Eocene continued and shaped the entire layer sequence. This mountain formation influenced large parts of the Arctic and was caused by the pressing of what is now the northern part of Ellesmere Island against the Greenland plate by the northward drifting North American plate .

Structure and lithology

The Margaret Formation consists of cross-layered, gray to white-gray colored sand , silt and clay stones , in the upper half of which individual layers of coal up to 15 m thick are embedded. In this sequence of sandstones and claystones, mixed with coal seams, isolated volcanic ash layers were detected, which are usually only a few centimeters thick. The entire formation is between 600 and 2000 m thick. Within the Margaret Formation, two fossil-bearing horizons could be detected, both of which are located in the upper, coal-bearing section: on the one hand, the older and more fossil-rich fauna from the Lower Eocene, which can be compared with the Wasatchium faun complex of central North America, on the other hand, a less extensive fauna from the early Middle Eocene , which has its counterpart in the Bridgerium Fauna Complex of North America.

Fossil finds

The fossils from the Margaret Formation are very extensive, but often fragmented and small pieces. This applies above all to the remains of the vertebrates , some of which are relatively difficult to identify, only a fraction, around 230 pieces, have been identified so far. The richest discovery site, which is located in the central area of ​​Ellesmere Island, has around 80 identifiable pieces. Due to their character, the fossil layers are classified as concentrate deposits. Despite the fragmentary tradition, the find horizons of the Margaret Formation are among the special ones of that time, as they make it possible to decipher a fossil biotope in the far north .

flora

The remains of the vegetation have been handed down in the form of leaf prints, seeds and pollen, but often have a stratigraphically greater range than the remains of the animal world. Partly there are also petrified tree trunks, which have been preserved mainly in the coal-bearing layers at several outcrops of the Margaret Formation. They have been preserved particularly extensively in the area of ​​the Strathcona Fjord in the southwest of Ellesmere Island. Here tree stumps with a diameter of 1.5 m and a height of up to 1.8 m could be observed, with about 83 trees on 2,261 m², which mainly belong to the primeval sequoia. The macroflora from the fossil-bearing areas of the Margaret Formation is particularly meaningful. This often and analogously to the stumps includes leaves of primeval sequoia, but also relatives of the trees of life , horsetail and birch plants . Elm , linden and plane trees have also been identified, as well as ginkgo from the upper horizon . An analysis of the macro flora of the entire region (Ellesmere and Axel-Heiberg Island) from 2019 differentiates a total of 65 species of flowering seeders for the period of the end of the Paleocene and the beginning of the Eocene , including 62 species of dicotyledonous species , the remainder being distributed over the years Monocot plants , as well as around a dozen species of naked samers and 5 fern-like plants . The presence of spruce , oak , ash and viburnum is also known from the microflora .

fauna

Fossils of birds from the Margaret Formation: A – D: proximal phalanx of Gastornis ; E – F: distal upper arm fragment of presbyornis (F is an example from the Wasatch Formation in Wyoming); G – H: phalanx of an indeterminate bird

Among the vertebrates which come fish on multiple orders, including members of the pike and billfish . While only freshwater fish are known in the eastern Arctic of Canada, in the western, so on Banks Island, with sand sharks ( Striatolamia and Carachias ), requiem sharks ( Physogaleus ) and eagle rays ( Myliobatis ) there are also saltwater inhabitants . Amphibians are rare and represented by the more than 100 cm long salamander Piceoerpeton . The remains of the reptiles are more extensive, nine families of turtles could be identified here alone . These range from gigantic shapes like Hadrianus with a back armor up to 64 cm long to those with a leathery shell like Apalone to the small, mostly less than 20 cm large pond turtles ( Emydidae ). In addition, scale reptiles have survived, but some of them have not yet been scientifically determined. What is significant, however, is the occurrence of a monitor lizard , the northernmost distribution of which is now at 46 ° north latitude. The same applies to the alligator Allognathosuchus , a smaller representative of the armored lizards with a skull only 15 cm long, which is the first representative of the crocodiles in the Arctic and at the same time the northernmost find of such an animal. On the other hand, birds appear relatively seldom , of which only two genera are clearly identified. Thus, the large, flightless is Gastornis about covered with a 6.2 cm long phalanx, along with the related with today's geese and shore areas colonized occurs presbyornis among other remnants of the humerus in appearance.

Around two dozen genera of mammals have been documented so far. The evidence of a representative of the extinct Multituberculata , a very primitive group of mammals that already appeared in the Mesozoic , is unique . Ectypodus represents the northernmost known member. The only find includes a tooth only 3.5 mm long, which is divided into eleven segments. All other remains belong to the higher mammals . Arcticanodon , one of the Palaeanodonta to be counted animal is an ancestor of pangolins . As such it was probably equipped with suitable for digging forelegs. The lower jaw finds that have been handed down show the most original features of this group to date. The small mammals mainly include rodents , which include Paramys , Microparamys and Strathcona , the latter occurring with two different species. The fossil remains include an edentulous lower jaw and several isolated molars. Of the giant gliders , only the Plagiomenidae family is represented with possibly several genera, including Ellesmene , which was also found on the east coast of the Axel-Heiberg-Insel. The extensive find material consists mainly of remains of the upper and lower jaw, the closest comparative finds are around 4000 km south in central North America. A very early primacy, Ignacius , could also be proven. The hippopotamus coryphodon from the group of large, herbivorous or omnivorous and semi-aquatic pantodonts , to which a lower jaw and around 80 other finds can be assigned, is one of the largest and most commonly known representatives . The early ungulate Pachyaena from the group of the Mesonychia represents a carnivore, while real predators with the Palaeonictis and Prolimnocyon belonging to the Creodonta are passed down. However, early felines such as Viverravus and distant ancestors of today's dogs with miacis were also present. The insectivore-like Palaeosinopa, represented by some teeth, lived semiaquatically like a modern otter . With Prodiacodon , a representative of the possibly two-legged and now extinct Leptictida also lived in the far north, but the only identified find so far is only one tooth. The unpaired ungulates are quite rich in shape . The early and still relatively small Brontotheriidae occur here ; Eotitanops and Palaeosyops are proven with only about half a dozen finds , but the latter is only known from the upper fauna horizon. The Taporoidea are more frequently represented with around 40 finds, including the medium-sized, but already modern-looking tapiroid Thuliadanta , of which mainly a partial skull and several teeth are present, but also heptodons with individual jaw remains . The Homogalax belonging to the Isectolophidae has also been proven to be a primeval odd-toed ungulate ; here, too, mainly parts of the lower jaw and teeth have survived.

Age position

The formation of the coal layers and the associated lower discovery horizon of the Margaret Formation is most likely to be correlated with the Early Heocene temperature maximum , which occurred 50 to 53 million years ago. The vertebrate fauna can generally be assigned to the Wasatchium fauna complex from North America in the middle latitudes, which is particularly evidenced by the occurrence of Coryphodon , Palaeonictis , Miacis and Pachyaena . However, other genera are also recorded here for the first time, such as Eotitanops or the very early tapiroids. The lower fossil horizon has an age of 52.6 million years, which was obtained radiometrically on zirconium minerals from a volcanic ash and which corresponds relatively well to the biostratigraphic data. Magnetostratigraphic investigations also showed a position of the lower horizon within the paleomagnetic event C24 , while the transition from Paleocene to Eocene more than 55 million years ago are assigned to events C25 and C26 .

The upper horizon is more recent and dated to the beginning of the Middle Eocene and can be assigned to the Bridgerium. The evidence of Palaeosyops , Ignaticius and some turtle remains speak for this . Absolute dates are not yet available.

Landscape reconstruction

According to reconstructions, the sites of the Margaret Formation, especially the very rich ones on Ellesmere Island, were also located north of the Arctic Circle at the time of formation in the early Eocene with a presumed position of 74 to 80 ° north latitude. Today's region of Ellesmere Island and neighboring areas is exposed to the arctic climate with an annual average temperature of −19 ° C and high winter snowfalls and glaciers. Numerous plants that have been found, such as the primeval sequoia or the ginkgo, are found in subtropical areas today , just like today's crocodiles live in tropical and subtropical climates. The same applies to turtles, which tend to love warmth and need a high average summer temperature so that the egg clutches are hatched. Furthermore, tapirs tend to be mammals that inhabit tropical landscapes, and because of their conservative tribal history , this is also assumed by their relatives. Due to this and based on pre exempt isotope studies on three different vertebrate species, the services a strong matching result, researchers reconstruct an annual average temperature of 4 to 8 ° C, which rose in the summer to 19 to partly above 20 ° C, but at the winter freezing remained and did not fall below −3.5 ° C.

The cross-layered sand and clay / silt stones that make up the Margaret Formation are to be interpreted as the remains of a large river delta with numerous arms. On the basis of the remains of flora, a dense forest interspersed with primeval sequoias can be reconstructed, the treetops of which were 25 to 40 m high; Today comparable coniferous forests can be found in western North and South America but also in eastern Asia . In addition, isotope measurements on wood residues also showed that the climate was clearly humid , with an annual precipitation of approximately 1500 mm, more than two thirds of which fell during the summer months. From this, in connection with the reconstruction of the river delta, it could be deduced that this jungle was periodically flooded, which is also indicated by the deposits themselves, which do not reveal any increased iron deposits , which would indicate longer dry phases. The entire landscape was close to the coast, with the river flowing into an almost closed sea, comparable to today's Baltic Sea and which covered most of the northern polar region.

The moderate temperatures and the species-rich jungle are atypical for regions close to the poles, a comparable biotope does not exist in today's landscape. In addition, due to its high northern location, this community was exposed to the conditions of the polar day with months of exposure to light and the polar night with equally long darkness; around 100 days are assumed for both. It is therefore not completely clear whether the animals of this jungle were present all year round or spent the winter time with the polar night south of the Arctic Circle, similar to what is still the case with recent reindeer today. Isotope measurements were carried out on several teeth of Coryphodon and other unpaired ungulates, which showed only slight fluctuations in the ratio of the heavy oxygen isotope 18 O to the lighter 16 O (δ 18 O). This suggests that the animals obtained their water from resources that existed locally under similar conditions and does not lead to the assumption that they undertook larger annual migrations in which a more unstable δ 18 O ratio resulted due to the different available water sources would have to. On the other hand, stronger variances in the δ 13 C ratio ( carbon isotope 12 C versus 13 C) could be determined, which can be attributed to greater fluctuations in the annual biomass production. This may have resulted in shortages of food crops during the dark winter months. In addition, finds of young animals are known from both Coryphodon and Thuliadanta , which the researchers also advocate with a year-round inhabited site.

Research history

George Nares

The first fossil finds in the Margaret Formation, especially on Ellesmere Island, were made in 1875 by an expedition of the Royal Navy under the leadership of Captain George Nares . This led to an extensive collection of fossil leaves. The first remains of petrified forests in the northeast of Ellesmere Island were discovered by the American soldier David Legge Brainard during the expedition of Adolphus Greely, famous for numerous deaths due to illness, between 1881 and 1883; another such forest was found in the Stenkul Fjord in the south of the same island known by the geologist Per Schei in the early 1900s . Other smaller discoveries followed, which in 1973 resulted in the discovery of the important fossil forest on Strathcona Fjord by Mary R. Dawson . This was also the beginning of scheduled geological field research, especially in the eastern Arctic of Canada, by the Geological Survey of Canada , which Dawson undertook with numerous colleagues and which lasted a good four decades. Other highlights were the exploration of the extremely fossil-rich site on the Bay Fjord in the central area of ​​Ellesmere Island since the 1970s, but also the discovery of the petrified forest on Geodetic Hill on Axel-Heiberg Island in 1983 by geologist Brian Ricketts, which, at 42 million years old, is somewhat younger than the forests of the Margaret Formation. These discoveries led to the description of numerous new taxa, the evaluation of the fossil material continues to this day.

literature

  • Jaelyn J. Eberle and David R. Greenwood: Life at the top of the greenhouse Eocene world - A review of the Eocene flora and vertebrate fauna from Canada's High Arctic. Geological Society of America Bulletin; January / February 124 (1/2), 2012, pp. 3–23

Individual evidence

  1. ^ A b Brian D. Ricketts: New formations in the Eureka Sound Group, Canadian Arctic Islands. Current Research, Part B: Geological Survey of Canada Paper 86-1B, 1986, pp. 363-374
  2. ↑ a b c d e f g h i j k l m Jaelyn J. Eberle and David R. Greenwood: Life at the top of the greenhouse Eocene world - A review of the Eocene flora and vertebrate fauna from Canada's High Arctic. Geological Society of America Bulletin; January / February 124 (1/2), 2012, pp. 3–23
  3. ^ Brian D. Ricketts and DJ McIntyre: The Eureka Sound Group of eastern Axel Heiberg Island: new data on the Eurekan Orogeny. Current Research, Part B: Geological Survey of Canada Paper 86-1B, 1986, pp. 405-410
  4. ↑ a b Christopher J. Williams, Ben A. LePage, Arthur H. Johnson and David R. Vann: Structure, Biomass, and Productivity of a Late Paleocene Arctic Forest- Proceedings of the Academy of Natural Sciences of Philadelphia, 158 (1) , 2009, pp. 107-127
  5. ↑ a b Lisa Tauxe and David R. Clark: New palaeomagnetic results from the Eureka Sound Group: Implications for the age of early Tertiary Arctic biota. Geological Society of America Bulletin 99, 1987, pp. 739-747
  6. ↑ Jane E. Francis: A 50-Million-Year-Old Fossil Forest from Strathcona Fiord, Ellesmere Island, Arctic Canada: Evidence for a Warm Polar Climate. Arctic 41 (4), 1988, pp. 314-318
  7. Jump up ↑ Christopher K. West, David R. Greenwood and James F. Basinger: The late Paleocene to early Eocene Arctic megaflora of Ellesmere and Axel Heiberg islands, Nunavut, Canada. Palaeontographica, Section B: Palaeobotany - Palaeophytology 300 (1–6), 2019, pp. 47–163
  8. ↑ Aspen Padilla, Jaelyn J. Eberle, Michael D. Gottfried, Arthur R. Sweet, and J. Howard Hutchison: A Sand Tiger Shark-Dominated Fauna from the Eocene Arctic Greenhouse. Journal of Vertebrate Paleontology 34 (6), 2014, pp. 1307-1316
  9. ↑ Thomas A. Stidham and Jaelyn J. Eberle: The palaeobiology of high latitude birds from the early Eocene greenhouse of Ellesmere Iceland, Arctic Canada. Scientific Reports 6, 2016, 20912 doi: 10.1038 / srep20912
  10. ↑ K. Christopher Beard and Mary R. Dawson: Northernmost Global Record for Multituberculata from the Eocene of Ellesmere Island, Arctic Canada. Journal of Vertebrate Paleontology 34 (6), 2014, pp. 1476-1480
  11. ↑ Kenneth D. Rose, Jaelyn J. Eberle and Malcolm C. McKenna: Arcticanodon dawsonae, a primitive new palaeanodont from the lower Eocene of Ellesmere Island, Canadian High Arctic. Canadian Journal of Earth Sciences 41, 2004, pp. 757-763
  12. ^ Mary R. Dawson: Early Eocene rodents (Mammalia) from the Eureka Sound Group of Ellesmere Island, Canada. Canadian Journal of Earth Sciences 38, 2001, pp. 1107-1116
  13. ^ A b Mary R. Dawson, Malcolm C. McKenna, K. Christopher Beard and J. Howard Hutchinson: An Early Eocene Plagiomenid Mammal from Ellesmere and Axel Heiberg Islands, Arctic Canada. Kaupia 3, 1993, pp. 179-192
  14. ^ Mary R. Dawson: Coryphodon, the northernmost Holarctic Paleogene pantodont (Mammalia), and its global wanderings. Swiss Journal of Paleontology 131, 2012, pp. 11-22
  15. ↑ Jaelyn J Eberle and Malcolm C McKenna: Early Eocene Leptictida, Pantolesta, Creodonta, Carnivora, and Mesonychidae (Mammalia) from the Eureka Sound Group, Ellesmere Island, Nunavut. Canadian Journal of Earth Sciences 39 (6), 2002, pp. 899-910
  16. ↑ Jaelyn J. Eberle: Early Eocene Brontotheriidae (Perissodactyla) from the Eureka Sound Group, Ellesmere Island, Canadian High Arctic - Implications for Brontothere origins and high latitude dispersal. Journal of Vertebrate Paleontology, 26 (2), 2006, pp. 381-386
  17. ↑ Jaelyn J. Eberle: A new 'tapir' from Ellesmere Island, Arctic Canada - Implications for northern high latitude palaeobiogeography and tapir palaeobiology. Palaeogeography, Palaeoclimatology, Palaeoecology 227, 2005, pp. 311-322
  18. ↑ Jaelyn J. Eberle and David A. Eberth: Additions to the Eocene Perissodactyla of the Margaret Formation, Eureka Sound Group, Ellesmere Island, Arctic Canada. Canadian Journal of Earth Sciences 52, 2015, pp. 123-133
  19. ↑ Lutz Reinhardt, Harald Andruleit, Solveig Estrada, Friedhelm Henjes Art, Karsten beep John, Werner von Gosen, Donald W. Davis and Bill Davis: Altered Volcanic Ashes in Paleocene / Eocene Eureka Sound Group sediment (Ellesmere Iceland, Arctic Canada) -New Stratigraphic Tie-Points? GeoCanada 2010, Calgary
  20. ↑ Jaelyn J. Eberle, Henry C. Fricke, John D. Humphrey, Logan Hackett, Michael G. Newbrey and J. Howard Hutchison: Seasonal variability in Arctic temperatures during early Eocene time. Earth and Planetary Science Letters 2010, pp. 1–6 doi : 10.1016 / j.epsl.2010.06.005 .
  21. ^ Brian A. Schubert1 *, A. Hope Jahre1, Jaelyn J. Eberle2, Leonel SL Sternberg3 and David A. Eberth: A summertime rainy season in the Arctic forests of the Eocene. Geology 40 (6), 2012; Pp. 523-526
  22. ^ J. Eberle, H. Fricke and J. Humphrey: Lower-latitude mammals as year-round residents in Eocene Arctic forests. Geology 37 (6), 2009, pp. 499-502