Fossils of Hawaiʻi

Types of Hawaiian fossils

Most fossils occur in sedimentary rocks, but almost all of Hawai'i consists of igneous rock. Nevertheless, Hawai'i does have a fossil record, and most of these fossils are often found in three unusual geological settings:

• Inside lava tubes or caves.
• In limestones formed by the coral reefs surrounding the islands, which, when exposed to the air (when sea level falls or the island is uplifted) can become karst.
• As charcoalified imprints of trees in or between lava flows.

Subfossils

Many of the organic remains described from Hawai'i can be called subfossils. Subfossils are remains or traces of past life that are less than 10,000 years old, which is the standard (though arbitrary) age definition for fossils. In practice, such materials are treated like “true” fossils and provide the same kinds of information.

The fossil record of the Hawaiian Islands preserves a 400,000-year history of island biodiversity. Fossils of plants, birds, fish, terrestrial and marine invertebrates, and a lone native terrestrial mammal paint a picture of surprisingly diverse Hawaiian ecosystems prior to the arrival of the first humans.

Trace fossils

Trace fossils in Hawai'i are most often represented by the trunks and branches of trees that have been consumed by molten lava. When lava flows through a forested area, the molten liquid chills and solidifies almost instantly when it comes in contact with large vegetation. The lava is still quite hot—enough to ignite and burn the trees, leaving behind a mold of the former tree trunk. Lava flows commonly deflate and subside after solidification, and tree molds can protrude above the frozen surface of the flow, leaving behind “lava trees.”

Marine fossils

Marine fossils in Hawai‘i are not widespread, but can be very abundant where they do occur, along the coasts of Kaua'i, O'ahu, Molokai, Lanai, and Maui. All are found in limestones formed by uplifted Pleistocene or Holocene coral reefs and can be studied either from coastal outcrops or from cores taken by ship. More than 150 species of mollusks (bivalves and gastropods) have been reported in these reef limestones, together with numerous fossil corals.

How did organisms colonize Hawai'i?

As the most isolated archipelago in the world, the Hawaiian Islands were colonized by a relatively small number of species that could successfully disperse across the ocean by flying, floating, or being blown by wind. Most plants arrived as seeds carried by migratory birds, either carried in the birds' stomachs or stuck to their feathers or skin. A smaller number of organisms drifted on air or floated on plant matter in ocean currents.

Organisms that survived the voyage to Hawai'i and reproduced in their new environment were able to move into new ecological niches because there were few competitors for resources. The lack of competition gave rise to an adaptive radiation of species. An adaptive radiation is the creation of multiple new species from a colonizing ancestor. Long distance travel coupled with subsequent species radiation have given Hawai'i a very unusual and highly endemic group of organisms.

The long distance and duration of the trip to Hawai'i favored certain types of organisms and selected strongly against others. Thus, there are no native Hawaiian terrestrial reptiles and amphibians. There is only one terrestrial mammal, the 'ōpe'ape'a or Hawaiian hoary bat. In contrast, there are many native species of birds, terrestrial invertebrates, and plants. Even in the marine realm, the abundance of species is skewed toward those that could travel across the open ocean, and a similar adaptive radiation occurred following the arrival of early nearshore reef species.