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Paleozoic Partner Highlight


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Many of the first specimens collected in Glacier Bay were from the northern end of Willoughby Island, seen here looking north.

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Glacier Bay National Park & Preserve (Alaska)

Significant, Spectacular, Silurian Fossils

The Silurian limestone (about 425 million years old) of Glacier Bay contain a significant invertebrate megafauna. Fossils are extremely large, with some bivalves approaching 8 inches (20 cm) in length! Their huge size suggests they lived in very favorable conditions during a period of extremely high "bio-productivity" in a warm, shallow-water, shelf environment adjacent to a deep marine basin.

The Upper Silurian rocks of southeastern Alaska are part of the Alexander terrane. Terranes are massive groups of rocks (sometimes called "microcontinents") that form elsewhere, are transported by plate tectonic processes, and later accreted onto another continent. Fossils in terranes are of special interest because they provide clues as to where the rocks originally formed, allowing geologists and paleontologists to reconstruct local paleobiogeography and plate tectonic processes. Many of the Silurian fossils are not North American. They are more similar to fossils found in Siberia and the Ural Mountains (western Russia), suggesting that the Alexander terrane was transported from what is now Eurasia and accreted onto North America! Eurasia was near the equator during the Silurian.


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Leperditiid ostracod from the Willoughby limestone. The large bean-like ostracodes were among the first Silurian fossils identified at Glacier Bay. Scale is in mm.

Early Discovery and Description

Conincidentally three Henry's were involved in the early paleontology studies of Glacier Bay. The first written mention of Silurian megafossils in Glacier Bay was by Henry Platt Cushing (1892) who was on the Henry Fielding Reid expedition to Muir Glacier. Cushing (1895) designated all of the dolomitic limestone around Glacier Bay as the "Glacier Bay limestone". Some of those rocks are known to be younger (Devonian) and are mapped as Black Cap Limestone (Rossman, 1963). On a trip across the bay to Drake Island, Cushing collected Leperditia (an ostracode) and observed cross sections of large gastropods on the glacially polished limestone. Cushing sent his collections to Henry Shaler Williams who determined a probable Paleozoic age for the ostracodes (Cushing, 1892). At the request of Alfred Hulse (surprisingly not "Henry"!) Brooks in 1901, Williams sent the specimens to the US Geological Survey (USGS) where they were examined by Charles Schuchert. Schuchert returned his report to the USGS Director on January 3, 1902. Using Leperditia as an index fossil, Schuchert determined a Late Silurian age for the fossils.

Many of the early publications describing the Silurian fossils of southeastern Alaska and Glacier Bay were written by Edwin Kirk (1884-1955) of the USGS. These specimens were collected on Willoughby Island by Frederick Eugene Wright in 1906 and by Kirk in 1917. Estimates of the rate of glacial retreat in Glacier Bay (Carlson and others, 2002) suggest that the ice retreated from Willoughby and Drake Islands about 1850 providing the early geologists better exposures than those present today.



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Abundant brachiopods and gastropods occur in the Willoughby Limestone on Drake Island.

The largest known Paleozoic bivalve Pycinodesma, was collected and named by Edwin Kirk in 1927 for specimens from Chichagof Island (Freshwater Bay) and Willoughby and Drake Islands (Glacier Bay). Kirk observed specimens at Glacier Bay as large as 12 inches (30 cm) and with shells as thick as 2 inches (5 cm)! He also noted (1927, p. 2) "The pelecypod is an excellent horizon marker, as owing to its tremendously thick shell it can often be recognized in section on weathered rock surfaces where metamorphism and shearing have destroyed other organic remains beyond the possibility of accurate determination." The frilled gastropod, Bathmopterus was collected from northern Willoughby Island by Kirk. Kirk's collections from Willoughby Island are part of a large-shelled molluscan assemblage as well as large thumb-nail-sized ostracodes.



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One of the largest known Paleozoic bivalves, Pycinodesma. A: The original graphic illustration by Kirk (1927). B: the actual specimen in the collections of the Smithsonian Institution National Museum of Natural History (USNM specimen 71277). C: Specimen in outcrop on Willoughby Island in Glacier Bay.


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Kirkospira. This specimen was collected and prepared by Edwin Kirk in 1917. Rohr and Blodgett named the new genus in his honor in 2004.

Discoveries Continue

We examined Kirk's unpublished material and established the large gastropod genus Kirkospira, in honor of Kirk, as well as a new species of gastropod Coelocaulus karlae. Soja and others (2000) described a stromatolite reef on Drake Island.

In 2011, we visited Drake Island and collected from a gastropod and brachiopod-rich outcrop. The gastropods included high-spired murchisonids and Coelocaulus, but the matrix has been recrystallized and the shells could not be broken free. The brachiopods showed preservation of internal calcified structures, and good specimens were recovered. These proved to be a previously unknown brachiopod, Sapelnikoviella santuccii.

Not surprising for rocks that have travelled long distances, the Silurian Willoughby limestone shows low to very high levels of recrystallization (meaning it was "metamorphosed" by high temperature and/or pressure) throughout Glacier Bay. This recrystallization probably obscures some of the fossils present. It is likely that many of the fossils described from similar Alexander Terrane limestones to the south (Heceta limestone), may be present in Glacier Bay waiting to be found.

Article and photographs provided by David M. Rohr (Sul Ross State University; Alpine, Texas) and Robert B. Blodgett (consulting geologist; Anchorage, Alasa).

About Glacier Bay National Park and Preserve

Covering 3.3 million acres of rugged mountains, dynamic glaciers, temperate rainforest, wild coastlines, and deep sheltered fjords, Glacier Bay National Park and Preserve is a highlight of Alaska's Inside Passage and part of a 25-million acre World Heritage Site-one of the world's largest international protected areas. From summit to sea, Glacier Bay offers limitless opportunities for adventure and inspiration. The park lies west of Juneau, Alaska, and can only be reached by plane or boat. The only road merely connects the small town of Gustavus and its airfield to park headquarters at Bartlett Cove. Daily jet service from Juneau to Gustavus is available in the summer and year-round scheduled air service to Gustavus is also provided by a variety of small air taxis and charters. Many visitors arrive in Glacier Bay as passengers on board cruise ships, tour boats, charter boats, and their own private vessels. Others bring their kayak to Glacier Bay on the ferry or rent one in Gustavus or Glacier Bay.

References

Blodgett, R.B., A.J. Boucot, D.M. Rohr, and A.E.H. Pedder. 2010. The Alexander terrane—A displaced fragment of northeast Russia? Evidence from Silurian-Middle Devonian megafossils and stratigraphy. Memoirs of the Association of Australasian Palaeontologists 39:325-341.

Blodgett, R.B., D.M. Rohr, and A.J. Boucot. 2002. Paleozoic links among some Alaskan accreted terranes and Siberia based on megafossils. InTectonic Evolution of the Bering Shelf-Chukchi Sea-Arctic Margin and Adjacent Landmasses, ed. E.L. Miller, A. Grantz, and S.L.Klemperer, 273– 291. Geological Society of America Special Paper 360. Boulder, CO.

Blodgett, R.B., A. J. Boucot, V.V. Baranov, and D.M. Rohr. 2013. Sapelnikoviella santuccii, a new gypidulinid brachiopod genus and species from the upper Silurian of Glacier Bay National Park & Preserve, Southeast Alaska. Memoirs of the Association of Australasian Palaeontologists 44, 65-72.

Brooks, A. H. 1902. Preliminary report on the Ketchikan mining district, Alaska, with an introductory sketch of the geology of southeastern Alaska. U. S. Geological Survey Professional Paper 1, 120 p.

Carlson, P.R. P. Hooge, G. Cochrane, A. Stevenson, P. Dartnell and K. Lee. 2002. Multibeam Bathymetry and Selected Perspective Views of Main Part of Glacier Bay, Alaska .U.S. Geological Survey Open-File Report 02-391.

Cushing, H.P. 1892. Notes on the geology of the vicinity of Muir Glacier. National Geographic Magazine, v. 4:140-146.

Kirk, E. 1927. Pycnodesma, a new molluscan genus from the Silurian of Alaska. Proceedings of the United States National Museum 71, article 20.

Kirk, E. 1928. Bathmopterus, a new fossil gastropod genus from the Silurian of Alaska. Proceedings of the United States National Museum 74, article 18, 1–4.

Rohr, D.M., and R.B. Blodgett. 2003. Kirkospira, a new Silurian gastropod from Glacier Bay, southeast Alaska. In Studies in Alaska by the U.S. Geological Survey, 2001, ed. J.P. Galloway. U.S. Geological Survey Professional Paper 1678 (direct link to PDF): 117–125.

Rohr, D.M., R.B. Blodgett, and J. Frýda. 2003. New Silurian murchisoniid gastropods from Alaska and a review of the genus Coelocaulus. In Short Notes on Alaska Geology 2003, ed. K.H. Clautice and P.K. Davis, 87–93. Alaska Division of Geological and Geophysical Surveys Professional Report 120.

Rossman, D.L. 1963. Geology of the eastern part of the Mount Fairweather quadrangle, Glacier Bay, Alaska, in Contributions to general geology, 1960: U.S. Geological Survey Bulletin 1121-K, p. K1-K57.

Soja, C.M., B. White, A. Antoshkina, S. Joyce, L. Mayhew, B. Flynn, and A. Gleason. 2000. Development and decline of a Silurian stromatolite reef complex, Glacier Bay National Park, Alaska. Palaios, v. 15, no. 4, p. 273-292.

2013 Paleozoic Partner feature articles:

| January: Fossils of the 2013 National Fossil Day Artwork | February: Paleontological Research Institution, Museum of the Earth | March: Falls of the Ohio State Park | April: Field Museum of Natural History, Mazon Creek Collection | May: Prehistoric Trackways National Monument | June: Cincinnati Museum Center | July: Glacier Bay National Park & Preserve | August: University of Michican Museum of Paleontology, Silica Formation Fossils | September: Yale Peabody Museum of Natural History, Beecher's Trilobite Bed | October: Guadalupe Mountains National Park | November: Utah Geological Survey, Millard County Cambrian Fossils | December: Denver Museum of Nature and Science, High-Altitude Mass Extinction |

Last updated: July 1, 2013