Mammal Fossils

Mammal Taxa Recovered from
Caves of Southeast Alaska

Class Mammalia                     Mammal common names

Order Insectivora
  Family Soricidae
    Sorex cf. monticolus           Dusky Shrew

Order Chiroptera
  Family Vespertilionidae
    Myotis spp.                    Myotis (small bat)

Order Primates
  Family Hominidae
    Homo sapiens                   Human

Order Rodentia
  Family Sciuridae
    Marmota caligata               Hoary Marmot
    Glaucomys sabrinus             Northern Flying Squirrel

  Family Castoridae
    Castor canadensis              Beaver

  Family Muridae
    Peromyscus cf. keeni           Keen's Mouse
    Clethrionomys cf. rutilus      Northern Red-backed Vole
    Phenacomys cf. intermedius     Heather Vole
    Microtus cf. pennsylvanicus    Meadow Vole
    Microtus cf. oeconomus         Tundra Vole
    Microtus longicaudus           Long-tailed Vole
    Lemmus sibiricus               Brown Lemming
    Synaptomys borealis            Northern Bog Lemming

  Family Erethizontidae
    Erethizon dorsatum             Porcupine

Order Cetacea                      Great Whale

Order Carnivora
  Family Canidae
    Canis familiaris               Domestic Dog
    Alopex lagopus                 Arctic Fox
    Vulpes vulpes                  Red Fox

  Family Ursidae
    Ursus americanus               Black Bear
    Ursus arctos                   Brown Bear

  Family Mustelidae
    Martes americana               Marten
    Mustela erminea                Ermine
    Mustela vison                  Mink
    cf. Gulo gulo                  Wolverine
    Lontra canadensis              River Otter

  Family Otariidae
    Eumetopias jubatus             Steller's Sea Lion

  Family Phocidae
    Phoca vitulina                 Harbor Seal
    Phoca hispida                  Ringed Seal

Order Artiodactyla
  Family Cervidae
    Odocoileus hemionus            Mule Deer
    Rangifer tarandus              Caribou

  Family Bovidae
    cf. Saiga tatarica             Saiga

Mammals were the first fossil animals to be found in the caves of Southeast Alaska, and they have provided the bulk of the information about ancient environments. This is in part because of their limited mobility among the islands compared to birds and fishes. Large mammals appear to be able to colonize islands fairly quickly once suitable habitats become available, whereas small mammals have great difficulty.

The table at the left lists all mammal species that have been found thus far (of all ages) from the caves of Southeast Alaska. Some species have only been recovered from island caves, whereas others have only been found in caves on the mainland.

Prior to our work, if was generally believed by geologists and biologists that Southeast Alaska was rendered uninhabitable during the Ice Age, and that all animals in the region are postglacial immigrants. This was well-stated in the last biological study of the region prior to the 1990s (by Dr. David R. Klein in 1965):

During the Wisconsin glaciation the present land areas of the coastal regions of Alaska bordering the Gulf of Alaska were virtually completely overridden by ice. The now existing flora and fauna of the region have presumably become established in the 10,000 years since the recession of the ice. . . . The present distribution of mammals in this region, although complicated by the phenomenon of insularity, reflects the sequence of their arrival and their relationship to specific refugia.

Dr. Klein hoped to explain the curious distribution of bears in the Alexander Archipelago. Only brown bears live on islands north of Frederick Sound (i.e. Admiralty, Baranof, and Chichagof Islands), whereas only black bears inhabit islands south of Frederick Sound (Prince of Wales and surrounding islands). This made sense in terms of postglacial colonization since brown bears came from Asia and black bears came from North America. Dr. Klein continued:

The presence of the brown bear on all large islands northwest of Frederick Sound including Kodiak Island and the islands of Prince William Sound suggests that this bear had access to these areas in a very early postglacial period, possibly before the sea level had risen substantially and prior to the arrival of the black bear and most ungulates. The present lack of movement of brown bears across the water channels separating these islands also supports the thesis that these bears reached the islands when access was easier than at present. Possibly access to the northern islands of the Alexander Archipelago was via the coastal route from the Prince William Sound region. . . . The failure of brown bears to occupy the islands south of Frederick Sound when access became available may be a result of prior occupancy by the black bear. A previously established species obviously has an advantage over a similar form attempting to occupy the same ecological niche.

Without a historical record, the present lack of brown bears on Prince of Wales Island was logically attributed to competition and an inability of this species to colonize the island at the end of the Ice Age. But the premise that brown bears never inhabited Prince of Wales and other southern islands was proven incorrect by the discovery of a fossil record. The bear skeletons found in El Capitan Cave in 1990 included both black and brown bears, and more brown bear remains were soon found in Blowing in the Wind and Bumper caves. Since that time, brown bear remains have also been found in caves on Dall and Coronation Islands. In On Your Knees Cave, brown bear remains date to both before and after the Last Glacial Maximum. This means that brown bears have a long history in the southern Alexander Archipelago, and they may have have even survived there during the Last Glacial Maximum rather than being postglacial immigrants.


The caribou antler at left was found in On Your Knees Cave. The caribou radius above was found in a high dome in El Capitan Cave by caver Pete Smith.

Several other species were found in early postglacial cave deposits of Prince of Wales Island that do not live on the island today. A red fox lower jaw and upper canine (photo below) and a fragment of a wolverine molar were found in El Capitan Cave. A caribou fragment was found in Bumper Cave. An arctic fox skeleton (photo below) and more bones of red fox and caribou were found at On Your Knees Cave. These discoveries, like that of brown bear, suggested that the Prince of Wales Island mammal fauna was not simply the result of postglacial colonization (else why did so many immigrants quickly die out). This was further confirmed by the older record at On Your Knees Cave. Arctic fox and red fox were recovered that dated to the glacial maximum (and to the preceding time interval for arctic fox), so they were survivors from the Ice Age. Brown bear, black bear, and caribou were found that dated to before, as well as after, the glacial maximum, but it is still unclear whether they survived the glacial maximum on coastal refugia or merely recolonized the islands afterward. In the case of brown bear, a genetic study of modern populations suggests that a population was isolated in Southeast Alaska during the Last Glacial Maximum.


Arctic fox (upper jaw and skeleton at right) was an important bone accumulator at On Your Knees Cave. Red fox (lower jaw, El Capitan Cave) was also present.

One of the most surprising discoveries at On Your Knees Cave was bones and teeth of ringed seal--a small, polar seal that is specialized for living and denning on sea ice (see photos below). So many seal remains were recovered that we named one of the two crawlways in that cave the Seal Passage. In the modern circum-arctic region ringed seals feed on fish that life under the ice, and they are preyed upon by polar bears and arctic foxes. Polar bears quietly sneak up on their snow-covered dens then try to crush them before the seals can escape into the water through their access hole. The foxes scavenge bear kills and catch seal pups. These predators will walk hundreds of miles across the sea ice in search of the seals.

All the the ringed seal fossils date to the Last Glacial Maximum (as discussed under Radiocarbon Dating). A few remains of harbor seal and Steller's sea lion have also been recovered. How did these marine mammal bones get into On Your Knees Cave, which is over a kilometer from the ocean? Either foxes or bears must have dragged them to their den. Some of the adult remains appear to have been chewed by bears (based on large puncture marks). Many juvenile seal teeth (with the enamel barely developed) have also been found. Since foxes are much more likely to cache food in their dens than bears, they are probably responsible. Whether bears chewed on the remains before or after they were brought to the cave is unknown. No bones of polar bear have been identified from any of the caves, but the presence of ringed seal and arctic fox suggests that it too was in the area during the Last Glacial Maximum. (Polar bears usually den in snow caves rather than rock caves.) It is also possible that brown bears participated in hunting or scavenging the seals. We hope in the future to discover bear remains that date to the glacial maximum so that the identity of the bears involved can be better understood.


Ringed seal bones and teeth (above and small molar at right), all dating to the Last Glacial Maximum, were brought to On Your Knees Cave by foxes or bears. Note the bite mark that matches a bear canine on the left side of the upper humerus. Steller's sea lion (canine tip at right) and harbor seal are also present, and they date to before, during, and after the glacial maximum (and are still present around Prince of Wales Island today).

In addition to ringed seals, the best index fossils in the caves are the rodents. (Index fossils reveal the age of the sediment because they were only present during a limited time interval.) Hoary marmot bones are abundant at On Your Knees Cave (see photo below), and two teeth were also found at Devil's Canopy Cave. All the marmot remains date to before the Last Glacial Maximum. Marmots do not live on Prince of Wales Island today, and they have never been found in postglacial deposits in the archipelago. One marmot molar was found in Hole 52 on the mainland, where marmots still live at higher elevations today. But on Prince of Wales Island marmot is an index fossil for the middle Wisconsin interstadial (the period prior to the Last Glacial Maximum). Two other rodents that are also limited to the middle Wisconsin interstadial are heather vole (see photo below) and brown lemming. They have not been radiocarbon dated, but they are very abundant in On Your Knees Cave and have never been found in postglacial deposits. The only microtine rodent living on Prince of Wales Island today is long-tailed vole, and it is found in sediments from both before and after the Last Glacial Maximum. But since this species is the least cold adapted of any of these rodents, it must have died out with the others and recolonized the island after the ice melted.


Hoary marmot (left) and heather vole (above) are the most abundant species from On Your Knees Cave. They all date to before the Last Glacial Maximum. The most common elements are isolated teeth. Long-tailed voles and brown lemmings are other common rodents.

Long-tailed vole is a forest dweller. Hoary marmots like rock piles to live in. Heather voles live in alpine areas. Brown lemming is a tundra animal. The fact that these four rodents coexisted on Prince of Wales Island suggests that the plant community was mixed or transitional prior to the Last Glacial Maximum. Below are links to two plots that show the distribution of these four rodents through a one-meter-thick cross-section in the First Room of the Bear Passage of On Your Knees Cave. Each grid square is 1/3 meter across and 5 cm thick. Colors represent the relative abundances of two rodent species, and the number of fossils of each species is also indicated for each grid square. The deeper sediments are older, and the upper sediments are younger. In the first plot, it can be seen that heather voles increase in number relative to marmots over time. In the second plot, long-tailed voles are seen to dominate over brown lemmings during most of the sequence, but lemmings become dominant at the final approach of the Last Glacial Maximum. So the relative abundances of these rodents suggest that the climate was getting much colder as the glacial maximum approached (as would be expected). Some of the uppermost long-tailed vole fossils may be postglacial in age (after this species recolonized the island), but this sedimentary sequence dates mainly to the middle Wisconsin interstadial.

    Hoary marmot / Heather vole     Long-tailed vole / Brown Lemming

 

Fossil Carnivores of
Southeast Alaska

Prince of Wales Island Wrangell Mainland
Postglacial (13,000 yr BP to present)

Ermine
Mink
Marten
River otter
Wolverine
Arctic fox
Red fox
Brown bear
Black bear

Ermine
River otter
Black bear
Last Glacial Maximum
(24,000 to 13,000 yr BP)
Arctic fox
Red fox
Ringed Seal
-- No record --
Middle Wisconsin
(40,000+ to 24,000 yr BP)
River otter
Arctic fox
Red fox
Brown bear
Black bear
-- No record --

Fossil Rodents of
Southeast Alaska

Prince of Wales Island Wrangell Mainland
Postglacial (13,000 yr BP to present)
Northern flying squirrel
Beaver
Keen's mouse
Meadow vole
Long-tailed vole
Hoary marmot
Northern flying squirrel
Keen's mouse
Red-backed vole
Meadow vole
Long-tailed vole
Northern bog lemming
Porcupine
Last Glacial Maximum
(24,000 to 13,000 yr BP)
-- None present --
-- No record --
(certainly none present)
Middle Wisconsin
(40,000+ to 24,000 yr BP)
Hoary marmot
Heather vole
Long-tailed vole
Brown lemming
-- No record --

It is interesting to compare the carnivores and rodents of Prince of Wales Island and the Alaskan mainland. The Prince of Wales Island fossil record is well known from many cave sites, whereas only two brief postglacial excavations have been made on the mainland near Wrangell. This collecting bias is certainly why more carnivores are known from the island caves (see table at left). Carnivores are present in much smaller numbers than rodents in animal communities, so so a large fossil collection is needed to identify them all. Additional carnivores are known to live on the Alaskan mainland.

In spite of this collecting bias (favoring island species), far more rodents are known from postglacial sites on the mainland than from Prince of Wales Island (see table at right). The record at On Your Knees Cave suggests that all rodents in the region died out during the Last Glacial Maximum. Recolonization of the islands by rodents proceeded from the mainland (probably in the simple manner described by Dr. Klein for bears). But rodents have great difficulty crossing ocean barriers, and only a small subset of mainland species have succeeded in colonizing the offshore islands. The cave faunas represent all the living rodents that are native to Prince of Wales Island (plus meadow vole), whereas the mainland list is far from complete (red squirrels, beavers, bushy-tailed wood rats, muskrats, and meadow jumping mice also live on the mainland near Wrangell but have not yet been found in cave sediments).

Fossil mammals are discussed further in the sections on Radiocarbon Dating and Stable Isotopes, and additional photos of mammal fossils are available on the individual cave site descriptions.

   
© 2002 by Timothy H. Heaton