Close up of Princeton Chert outcrop showing volcanic ash (white layer at base), peaty coal (dark layer), and Chert layers (grey). Layer 36 is labelled.

The Princeton Chert is a fossil locality in British Columbia, Canada, which comprises an anatomically preserved flora of Eocene Epoch age, with rich species abundance and diversity. It is located in exposures of the Allenby Formation on the east bank of the Similkameen River, 8.5 km (5.3 mi) south of the town of Princeton, British Columbia.[1]

History

The Princeton Chert (Ashnola shale in older sources) and its fossils have been known since the 1950s,[2][3] but have attracted increased attention in the late 1970 and on.[4] This may be due to the rare type of silica permineralized fossil Lagerstätten found, which has preserved plants and animals in minute 3D detail, with exceptional internal cellular detail.[5] This has meant anatomical descriptions and reconstruction of whole plants from isolated parts has been possible in many species.[6][7] Few plant fossils elsewhere in the world exhibit such excellence in both preservation and diversity. Similar aged fossil beds in Eocene lake sediments are found elsewhere in British Columbia, including in Driftwood Canyon Provincial Park near Smithers in northern British Columbia, the McAbee Fossil Beds west of Kamloops, about 160 km (160,000 m) NNW of the Princeton Chert beds, and the Klondike Mountain Formation around Republic, Washington, south of Princeton.

Location and geologic setting

The Princeton Chert is an interbedded sequence consisting of coal, shale, volcanic ash, and chert in the Allenby Formation.[8][9][10][11] 49 chert layers, ranging in thickness from 1 to 55 cm (0.39 to 21.65 in) have been recognized and described, though each is not unique in organisms preserved. Despite this, trends are evident throughout the outcrop, with certain taxa appearing and disappearing with time.[12][13]

The Princeton Chert was originally considered to be Middle Eocene based on data from mammals, freshwater fish, and potassium-argon dates.[14][15][16] Recently, more accurate radiometric techniques provided a date of 48.7 mya,[12] placing the Princeton Chert in the Ypresian stage (47.8–56.0 mya), consistent with the whole Allenby Formation being now dated radiometrically as being Early Eocene.[10][17][18]

The climate at this time was warm; it had reached a maximum during a series of warming events during the Early Eocene with the Princeton Chert likely deposited after the Eocene Thermal Maximum 2 and during the Early Eocene Climatic Optimum.[10][19][20][21] During this time the sea warmed approximately 4 °C and terrestrial temperatures were several degrees warmer than today, meaning little or no ice was present at the poles. The temperature difference between poles and equator was small.[22][23] This long term warmth is thought to be due to increased greenhouse gases, particularly CO2 trapping more heat.[21][24] The reason for this sudden increase in CO2 is unknown, but it is hypothesised that it was due to an increase in ocean floor being recycled via volcanic arcs and metamorphic decarbonation reactions.[23] This happened because the ocean between India and Asia was disappearing and being replaced by the Himalayas and the Tibetan plateau due to the collision of tectonic plates. Also at the time, Australia, which was joined to Antarctica, was beginning to move northwards.[25]

The Princeton Chert fossils indicate that the area was an aquatic ecosystem, growing in tropical to subtropical conditions.[12] More recent analysis of the fossil flora, however reconstructs for the Princeton Chert flora a moist warm temperate climate with mean annual temperature 13.1 ± 3.1 °C, with mild winters (cold month mean temperature 5.3 ± 2.8 °C), and mean annual precipitation 114 ± 42 cm per year.[20] Several of the smaller chert layers are separated by volcanic ash layers, indicating nearby volcanic activity. It is thought that fossils were pervaded with silicic acid due to this volcanic activity.[26] Subsequently, water charged with minerals flowed from springs or geysers into the low lying basin where the Princeton chert was located. Here, the water surrounded organisms as they grew, along with plant debris which had been accumulated.[12] Many organisms were preserved in situ, in the lake or small pond environment in which they lived.[27] The preservation must have been rapid, due to the minute cellular detail which has been conserved. This sequence of events is thought to have been replicated up to 50 times, as the basin allowed peat to re-accumulate each time,[12] producing the multiple layers.

Fossil biota

Sampling into the Princeton Chert has been carried out, but presently the data has not been analysed in detail.[12] Across the outcrop, trends in taxa can be seen; in the topmost layers fossil organs of Metasequoia milleri[9] cease to be represented, yet Pinus (pine) and monocotyledons increase in number. There is a huge increase in ferns, such as Dennstaedtiopsis, after a huge ash fall, though few angiosperms occur in these layers. A large number of angiosperms have been found along with several types of conifers, ferns, and several unidentified fossils from various families.[28]

In situ lacustrine fossils

The array of floral and faunal fossils found in the Princeton Chert has offered unequivocal evidence that it was a lacustrine or lake environment. The plant fossils found show many structural and anatomical adaptations to an aquatic environment, including a reduced vascular system, aerenchyma in tissues (air spaces to provide buoyancy), and protoxylem lacunae surrounded by a ring of cells with thickened inner walls.[27][12] Further evidence is provided by the fossils’ clear affinities with modern aquatic angiosperms. Many extant plants show these adaptations and are similar to the organisms found in the chert. For example, water lilies (Allenbya, Nymphaeaceae), water plantains (Alismataceae), arums (Keratosperma, Araceae) and rushes and sedges (Ethela, Juncaceae/Cyperaceae) are just some of the angiosperms found both today and in the Princeton Chert.[29][30] Seeds have also been found which share adaptations with living aquatics.[27][12][6] On the other hand, terrestrial fossils have rarely been found. The few that are, are represented mainly by seeds, some of which may have been transported by birds.[9][28][31]

Additional support for the aquatic nature of the Princeton Chert deposits comes from animal fossils. Several fossils of a freshwater fish, Amia (bowfin), have been found in the shale overlying the plant deposits, along with remains of the freshwater fishes Amyzon and Libotoniusm, and a soft-shelled turtle.[27][32] Once the lacustrine nature of the fossils has been established, it seems fairly likely they were preserved in situ, especially considering the method of preservation. The growth position, large number of plant organs of the same type, preservation of delicate plant material, and presence of rooted axes all provide further evidence for the preservation of plants where they grew.

Fungi

Pathogenic fungi have been recorded on the leaves and other organs of some vascular plants. Fossil Uhlia palms have tar spot fungi on their leaves described as Paleoserenomyces. These fungi are in turn parasitized by a mycoparasite, Cryptodidymosphaerites princetonensis.[33] Symbiotic mycorrhizal relationships have also been discovered in roots of Pinus and Metasequoia. In Metasequoia these associations have been compared to extant mycorrhizae, and found to be very similar.[12]

References

  1. ↑ Miller, C.N. (1975). "Silicified cones and vegetative remains of Pinus from the Eocene of British Columbia". Contributions from the Museum of Paleontology, University of Michigan. 24 (10): 101–118.
  2. ↑ Arnold, C. A. (1955). "A Tertiary Azolla from British Columbia". Contributions from the Museum of Paleontology, University of Michigan. 12 (4): 37–45.
  3. ↑ Boneham, R.F. (1968). "Palynology of three Tertiary coal basins in south-central British Columbia". Ph.D. Dissertation, University of Michigan, Ann Arbor, Michigan.
  4. ↑ Greenwood, D.R.; Pigg, K.B.; Basinger, J.F.; DeVore, M.L. (2016). "A review of paleobotanical studies of the Early Eocene Okanagan (Okanogan) Highlands floras of British Columbia, Canada and Washington, USA". Canadian Journal of Earth Sciences. 53 (6): 548–564. Bibcode:2016CaJES..53..548G. doi:10.1139/cjes-2015-0177. hdl:1807/71961.
  5. ↑ Pigg, KB; Stockey, RA (1996). "The significance of the Princeton chert permineralized flora to the middle Eocene upland biota of the Okanogan Highlands". Washington Geology. 24: 32–36.
  6. 1 2 Cevallos-Ferriz, S; Stockey, RA (1988). "Permineralized fruits and seeds from the Princeton Chert (Middle Eocene) of British Columbia: Araceae". American Journal of Botany. 75 (8): 1099–1113. doi:10.2307/2444092. JSTOR 2444092.
  7. ↑ Lepage, BA; Currah, RS; Stockey, RA (1994). "The fossil fungi of the Princeton chert". International Journal of Plant Sciences. 155 (6): 828–836. doi:10.1086/297221. S2CID 85107282.
  8. ↑ Basinger, JF (1976). "Paleorosa similkameenensis, gen. et sp. nov., permineralized flowers (Rosaceae) from the Eocene of British Columbia". Canadian Journal of Botany. 54 (20): 2293–2305. doi:10.1139/b76-246.
  9. 1 2 3 Basinger, JF (1984). "Seed cones of Metasequoia milleri from the Middle Eocene of southern British Columbia". Canadian Journal of Botany. 62 (2): 281–289. doi:10.1139/b84-045.
  10. 1 2 3 Moss, PT; Greenwood, DR; Archibald, SB (2005). "Regional and local vegetation community dynamics of the Eocene Okanagan Highlands (British Columbia - Washington State) from palynology". Canadian Journal of Earth Sciences. 42 (2): 187–204. Bibcode:2005CaJES..42..187M. doi:10.1139/E04-095.
  11. ↑ Archibald, S.B.; Greenwood, D.R.; Smith, R.Y.; Mathewes, R.W.; Basinger, J.F. (2012). "Great Canadian Lagerstätten 1. Early Eocene Lagerstätten of the Okanagan Highlands (British Columbia and Washington State)". Geoscience Canada. 38 (4): 155–164.
  12. 1 2 3 4 5 6 7 8 9 Stockey, RA (2001). Briggs, DEG; Crowther, PR (eds.). The Princeton Chert In: Palaeobiology II. Oxford, Blackwell Science. pp. 359–362.
  13. ↑ Stockey, RA (1987). "A permineralized flower from the middle Eocene of British Columbia". American Journal of Botany. 74 (12): 1878–1887. doi:10.2307/2443970. JSTOR 2443970.
  14. ↑ Russell, LS (1935). "A middle Eocene mammal from British Columbia". American Journal of Science. 29 (169): 54–55. Bibcode:1935AmJS...29...54R. doi:10.2475/ajs.s5-29.169.54.
  15. ↑ Wilson, MVH (1980). "Eocene lake environments: Depth and distance-from-shore variation in fish, insect, and plant assemblages". Palaeogeography, Palaeoclimatology, Palaeoecology. 32: 21–44. Bibcode:1980PPP....32...21W. doi:10.1016/0031-0182(80)90029-2.
  16. ↑ Hills, LV; Baadsgaard, H (1967). "Potassium-argon dating of some Lower Tertiary strata in British Columbia". Canadian Petroleum Geologists Bulletin. 15: 138–149.
  17. ↑ Ickert, RB; Thorkelson, DJ; Marshall, DD; Ullrich, TD (2009). "Eocene adakitic volcanism in southern British Columbia: Remelting of arc basalt above a slab window". Tectonophysics. 464 (1–4): 164–185. Bibcode:2009Tectp.464..164I. doi:10.1016/j.tecto.2007.10.007.
  18. ↑ Dillhoff, RM; Dillhoff, TA; Greenwood, DR; DeVore, ML; Pigg, KB (2013). "The Eocene Thomas Ranch flora, Allenby Formation, Princeton, British Columbia, Canada". Botany. 91 (8): 514–529. doi:10.1139/cjb-2012-0313.
  19. ↑ Bohaty, SM; Zachos, JC (2003). "Significant Southern Ocean warming event in the late middle Eocene". Geology. 31 (11): 1017–1020. Bibcode:2003Geo....31.1017B. doi:10.1130/g19800.1.
  20. 1 2 Greenwood, DR; Archibald, SB; Mathewes, RW; Moss, PT (2005). "Fossil biotas from the Okanagan Highlands, southern British Columbia and northeastern Washington State: climates and ecosystems across an Eocene landscape". Canadian Journal of Earth Sciences. 42 (2): 167–185. Bibcode:2005CaJES..42..167G. doi:10.1139/E04-100.
  21. 1 2 Zachos, JC; Dickens, GR; Zeebe, ZE (2008). "An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics". Nature. 451 (7176): 279–283. Bibcode:2008Natur.451..279Z. doi:10.1038/nature06588. PMID 18202643.
  22. ↑ Greenwood, D.R.; Wing, S.L. (1995). "Eocene continental climates and latitudinal temperature gradients". Geology. 23 (11): 1044–1048. Bibcode:1995Geo....23.1044G. doi:10.1130/0091-7613(1995)023<1044:ECCALT>2.3.CO;2.
  23. 1 2 Pearson, PN (2010). "Increased atmospheric CO2 during the middle Eocene". Science. 330 (6005): 763–764. Bibcode:2010Sci...330..763P. doi:10.1126/science.1197894. PMID 21051620. S2CID 20253252.
  24. ↑ Bijl, P. K.; Houben, A. J. P.; Schouten, S.; Bohaty, S. M.; Sluijs, A.; Reichart, G.; Damsté, S.; Brinkhuis, H. (2010). "Transient Middle Eocene Atmospheric CO2 and Temperature Variations". Science. 330 (6005): 819–821. Bibcode:2010Sci...330..819B. doi:10.1126/science.1193654. hdl:1874/385803. PMID 21051636. S2CID 206528256.
  25. ↑ Scotese, C. R. (2003) Paleomap Project. [Online]. Available from: http://www.scotese.com/ [Accessed 18/03/2012].
  26. ↑ Mustoe, G. E. (2010). "Cyclic sedimentation in the Eocene Allenby Formation of south-central British Columbia and the origin of the Princeton Chert fossil beds". Canadian Journal of Earth Sciences. 48 (1): 25–43. doi:10.1139/e10-085.
  27. 1 2 3 4 Cevallos-Ferriz, SRS; Stockey, RA; Pigg, KB (1991). "Princeton chert: evidence for in situ aquatic plants". Review of Palaeobotany and Palynology. 70 (1–2): 173–185. doi:10.1016/0034-6667(91)90085-H.
  28. 1 2 Stockey, RA; Wehr, WC (1996). "Flowering plants in and around Eocene lakes of the interior". In Ludvigsen, R. (ed.). Life in Stone: A natural history of British Columbia's Fossils. Vancouver, UBC Press. pp. 234–247.
  29. ↑ Erwin, DM; Stockey, RA (1989). "Permineralized monocotyledons from the Middle Eocene Princeton chert (Allenby Formation) of British Columbia: Alismataceae". Canadian Journal of Botany. 67 (9): 2636–2645. doi:10.1139/b89-340.
  30. ↑ Smith, SY; Stockey, RA (2003). "Aroid seeds from the Middle Eocene Princeton chert (Keratosperma allenbyense, Araceae): comparisons with extant Lasioideae". International Journal of Plant Sciences. 164 (2): 239–250. doi:10.1086/346164. S2CID 84905098.
  31. ↑ Phipps, CJ; Osborn, JM; Stockey, RA (1995). "Pinus Pollen Cones from the Middle Eocene Princeton Chert (Allenby Formation) of British Columbia, Canada". International Journal of Plant Sciences. 156 (1): 117–124. doi:10.1086/297232. S2CID 84598167.
  32. ↑ Wilson, MVH (1982). "A new species of the fish Amia from the Middle Eocene of British Columbia". Palaeontology. 25 (2): 413–424.
  33. ↑ Currah, RA; Stockey, RA; LePage, BA (1998). "An Eocene tar spot on a fossil palm and its fungal hyperparasite". Mycologia. 90 (4): 667–673. doi:10.2307/3761225. JSTOR 3761225.
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