Region 7: Hawai'i

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 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 charcoalized imprints of trees in or between lava flows.

Many of the organic remains described from Hawai’i can be called “subfossils,” meaning that they are less than 10,000 years old (the standard—though arbitrary—age definition for fossils). In practice, such materials are treated like “true” fossils and provide the same kind 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. 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 seed arrivals came with migratory birds, either in their stomachs, or stuck to their feathers or skin. A smaller number of organisms drifted on air or on floating plant matter in ocean currents. Those that survived the voyage and were able to reproduce in their new environment were also able to move into new ecological niches because competitors for those resources were few. This gave rise to an adaptive radiation of species: the creation of multiple new species from a colonizing ancestor.

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, and only one terrestrial mammal, the Hawaiian hoary bat. In contrast, there are many native species of terrestrial birds, 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 near-shore reef species. These factors—long distance travel and subsequent species radiation—give Hawai’i a very unusual and highly endemic group of organisms.

Figure 3.42: An extinct Hawaiian duck known as the turtle-jawed moa-nalo (<em class='sp'>Chelychelychen quassus</em>). This flightless bird became extinct prior to European contact with Hawai’i. DNA analysis places its arrival on the islands at 3.6 million years ago.

Figure 3.42: An extinct Hawaiian duck known as the turtle-jawed moa-nalo (Chelychelychen quassus). This flightless bird became extinct prior to European contact with Hawai’i. DNA analysis places its arrival on the islands at 3.6 million years ago.

Figure 3.43: The Hawaiian Islands exhibit numerous examples of coevolution between bird and plant species. A) Extinct black mamo (<em class='sp'>Drepanis funerea</em>), and B) modern <em class='sp'>Trematolobelia</em> plant on O’ahu.

Figure 3.43: The Hawaiian Islands exhibit numerous examples of coevolution between bird and plant species. A) Extinct black mamo (Drepanis funerea), and B) modern Trematolobelia plant on O’ahu.

Before the introduction of continental species by humans, Hawaiian terrestrial ecosystems lacked grazing mammals and were characterized by large—often flightless—grazing ducks and geese (Figure 3.42). The top predators were raptors, and plants lost the chemical defenses needed to guard against large herds of grazing mammals. Carnivorous caterpillars and plants can still be found in Hawaiian forests. Without diverse insect pollinators, nectar-sipping birds co-evolved curved bills to match long curved flowers like that of the Trematolobelia plant (Figure 3.43).

The oldest terrestrial fossils in Hawai’i are found in lake sediments at the bottom of Ulupau crater on O’ahu. This fossil occurrence is an unusual one for Hawai’i, and does not fall into one of the three more common modes of preservation described above. The island does not have many lakes, as its base is made up of porous lava. Eleven species of extinct birds have been identified there, dating to 400,000 years ago.

The richest fossil site in the islands is Makauwahi Cave on Kaua’i. Makauwahi is a karstic cave system in limestone containing a sinkhole lake, in which sediments accumulated over the last 10,000 years, providing excellent preservation. The hundreds of fossil organisms identified at Makauwahi include more than 40 species of birds (half of which are now extinct), 15 or more species of native land snails (Figure 3.44), all now extinct, and scores of endemic plants. In addition to extinct species, many species of living plants and animals found in the fossil record no longer grow at low elevation, rather, they are found only in high-elevation refuges in remote parts of the islands.

Figure 3.44: Extinct Hawaiian land snails. A) <em class='sp'>Leptachatina</em> sp., roughly 12 millimeters (0.5 inches) long. B) <em class='sp'>Orobophana juddii</em>, roughly 6 millimeters across (0.2 inches).

Figure 3.44: Extinct Hawaiian land snails. A) Leptachatina sp., roughly 12 millimeters (0.5 inches) long. B) Orobophana juddii, roughly 6 millimeters across (0.2 inches).

Radiocarbon dating of specimens at Makauwahi Cave documents the arrival of humans—and the continental species they introduced—and the relationship between these new arrivals and the native organisms. Prehistoric native species began to disappear as the first human colonists arrived; lowland birds, along with large flightless geese and ducks, were among the earliest extinctions. These species disappear from sedimentary layers shortly after the appearance of human-introduced rat fossils and other evidence of human habitation. More than half of Kaua’i’s 140 historically described native bird species are now extinct. One of the most curious is the Kaua’i mole duck, a bird with unusually small eyes but very large nerve passages to its bill, leading paleontologists to believe that it was nearly blind and may have inhabited caves, or was perhaps a nocturnal feeder.

What makes Hawai’i’s extinct fossil species so surprising and important is the recency of many of the extinctions. Hawaiian fossil assemblages describe the unusual suite of organisms that once inhabited Hawai’i, and help us better understand precisely when and how these species became extinct. Hawai’i’s highly diverse prehuman landscape has been completely transformed, with the decline or extirpation of most native species and their replacement with a small number of introduced species. These human-caused extinctions began about 1000 years ago, accelerated 200 years ago, and the extinction rate continues to increase even today.

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” (Figure 3.45).

Figure 3.45: Lava molds of tree trunks at Lava Tree State Monument, Hawai’i.

Figure 3.45: Lava molds of tree trunks at Lava Tree State Monument, Hawai’i.

Marine fossils in Hawai’i are not widespread, but can be very abundant where they do occur, along the coasts of Kaua’i, Oahu, 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.