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	Ancient Insect-Plant Relationship Persists through Time (October. 2000)

Ancient Insect-Plant Relationship
Persists through Time

Smithsonian researchers and their collaborators have turned back the geologic clock on the well-known herbivore-host interaction between beetles and the leaves of gingers, heliconias and their relatives in the Zingiberales, a taxonomic order of flowering plants. Their paper "Timing the Radiations of Leaf Beetles: Hispines on Gingers from the Latest Cretaceous to Recent" was published in the July 14 issue of Science magazine. They discovered damage characteristic of particular beetles, known as rolled-leaf hispines, in 11 fossil specimens of gingers dated at the latest Cretaceous (66 million years ago) and early Eocene (52-53 million years ago) of North Dakota and Wyoming.

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Damage from an adult of a rolled-leaf hispine beetle, Aslamidium semicircularum, inflicted on a leaf of Calathea sp. (Marantaceae), a ginger relative. This photograph was taken from 900 m at Cerro Campana, Panamá Province, Republic of Panamá, and is similar to stereotyped damage found in the early Eocene and Late Cretaceous of Wyoming and North Dakota. (Photo: Donald M. Windsor.)

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Eocene Cephaloleichnites
(Photo: Conrad Labandeira.)

Like the tracks left by lawnmowers when cutting grass, modern rolled-leaf hispines are known to produce characteristic damage as they eat the surface of the leaves. Rolled-leaf hispines feed on gingers, heliconias, and their plant relatives in the order Zingiberales. Only 2.5 to 3 millimeters wide, these telltale feeding marks are preserved in fossilized plants, providing evidence for the presence of this particular beetle lineage that goes back 66 million years. Hispines -- the larger group of beetles that includes the rolled-leaf beetles and eleven other lineages -- have been preserved as "body fossils" in deposits that are 20 million years younger than the fossilized plants that preserve the signs of their feeding activity.

Although the rolled-leaf lineage is one of the few hispine subgroups that lacks a "body fossil" record, it is known now from the fossilized remains of their eating activity. Thus, by discovering fossilized feeding activity, the researchers were able to determine that the hispine beetles in general and rolled-leaf beetles in particular were in existence 20 million years earlier than previously thought. These findings demonstrate that these beetles co-existed with dinosaurs and outlived them, while never changing their diet of Zingiberales.

The Researchers

The scientists credit the discovery in part to a rare interdisciplinary collaboration between paleobiologists, botanists, entomologists and tropical ecologists, as well as to the vast research collections of natural history museums. NMNH scientists Dr. Conrad Labandeira (Department of Paleobiology), Dr. W. John Kress (Department of Botany), Dr. Charles L. Staines (Department of Entomology), Ashley L. Allen (Department of Paleobiology) co-authored the paper with Dr. Donald M. Windsor (Smithsonian Tropical Research Institute), Dr. Kirk R. Johnson (Denver Museum of Natural History), and lead author Dr. Peter Wilf (a post-doctoral fellow at NMNH when the discovery was made who is now at the University of Michigan's Museum of Paleontology).

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Heliconia platystachys
(Photo: Carl C. Hansen.)

Drs. Conrad Labandeira (on the left) and Peter Wilf at a fossil leaf quarry in the greater Green River Basin of southwestern Wyoming. The cylindrical structure on the top of the hill is an abandoned eagle's nest. (Photo: Peter Wilf.)

The Circuitous Route of Research

In 1998, Peter Wilf and Conrad Labandeira, both in the NMNH's Department of Paleobiology, were examining plant fossils from the early Eocene epoch that they had collected in the Great Divide Basin of southwestern Wyoming when they came across a fossil ginger with unusual damage. "It looked like small pellets lined up in rows parallel to the veins of the leaf," reported Labandeira. Since gingers are tropical plants, he and Wilf consulted hispine beetle specialist Donald Windsor from the STRI and Zingiberales expert W. John Kress of the NMNH's Department of Botany, as well as other colleagues. None of them had seen anything like it. "That mystery led us to look at other kinds of insect damage," Labandeira said, "and to an interest in knowing and studying the insects which feed on gingers."

A literature search turned up papers written in the 1970s and 1980s by Donald R. Strong, Jr., now at the University of California, Davis, Bodega Marine Laboratory, and colleagues, describing the modern association and distinctive chewing marks of the beetle larvae on the young, rolled leaves of Zingiberales in Central America. Strong's papers prompted Wilf to examine Eocene fossil gingers for chewing marks. The telltale marks were not only present in several fossil specimens, but turned out to be identical to those illustrated by Strong and attributable only to rolled-leaf hispines.

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Bite tracks of rolled-leaf beetle larvae, Chelobasis, on a leaf of Heliconia, in Chiriauí Province, Panama. (Photo: W. John Kress.)

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Damage from rolled-leaf hispine beetles on a leaf of Heliconia darienensis from the US National Herbarium. Scale at left is in millimeters. The specimen was collected in the Republic of Panama. (Photo: Peter Wilf.)
Pictured at right - hispine beetle,
Aslamidium semicircularum.

Hispine specialist Charles Staines, of the NMNH's Department of Entomology, confirmed this diagnosis, and a survey of the NMNH's National Herbarium by Ashley Allen, a volunteer in the Department of Paleobiology, turned up numerous examples of hispine damage on modern Zingiberales that were similar to the fossil specimens. The original "pellet" damage that spurred the whole investigation still remains a mystery.

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Meanwhile, Labandeira and Wilf were pursuing another project with Kirk Johnson of the Denver Museum of Natural History, to study the loss of insect species across the Cretaceous-Tertiary boundary, 65 million years ago. That is the time at which a cataclysmic disturbance, brought about by the impact of a giant asteroid or comet, caused the extinction of dinosaurs, ammonites and many other plant and animal groups including, according to Johnson's work, about 80 percent of the plant species in North America. While examining the latest Cretaceous gingers from North Dakota collected by Johnson, Labandeira found the same chewing pattern Wilf had diagnosed on Eocene gingers. "We've got it now in the Cretaceous," he told Wilf in an excited phone call.

Implications for Understanding
Today's Diversity

Even more startling than the discovery itself, according to Labandeira, is the fact that these associations represent a relatively derived (non-primitive) group of beetles. "This implies that some of the existing relationships between flowering plants and their beetle herbivores were launched at the same time or not long after the great diversification of flowering plants in the Late Cretaceous," he said. Wilf noted also that this adaptive evolution of beetles following the diversification of flowering plants might have been an important factor in causing the astonishing diversity of beetles: today, there are 38,000 described living species of leaf beetles, the larger group that includes hispines.

[photo]
Hispines Mating
(Photo: David Furth.)

Naming:
A Part of the Process

The first order of business for a paleontologist who discovers a unique, identifiable fossil even when the organism itself is not preserved is to give the fossil a name so that it can be referred to accurately and consistently in discussions, publications, and in the context of taxonomy. Fossilized organisms, whether plant or animal, are referred to as “body fossils” and are proof positive of the presence of that organism in the past. The behavioral products or “work” of an animal, whether a footprint, a burrow, or a nest, is also solid evidence of its existence. Frequently, behavioral evidence can also be associated with a group of species, or more rarely to only one species, and may thus merit a name for recording purposes. Examples are the helical burrows of the extinct beaver from Nebraska (Daimonelix), or characteristically shaped termite mounds (Termitichnus), or even brood cells made by bees in the soil (Celliforma). The last two types are good environmental indicators as well as evidence of past insect life. Scientists have applied this reasoning for a long time and have developed rules for naming such ichnofossils (ichnos is Greek, for “trail” or “track”). Wilf, Labandeira, and their colleagues coined a taxonomic name for their discovery, Cephaloleichnites strongi. Why Cephaloleichnites strongi? The new species name is a tribute to Donald R. Strong, Jr., the evolutionary ecologist who first sorted out the plan-insect interactions of hispine beetles and gingers during the late 1970s to early 1980s. The genus name Cephaloleichnites is a reminder that this ancient vestige is not a “body fossil” but is instead an ichnofossil

Labandeira and Wilf note that the newly-discovered antiquity of the hispine beetle-Zingiberales association and its continuation to the present makes this plant-animal interaction similar to organisms commonly referred to as "living fossils" (coelacanths, horseshoe crabs, and ginkgo trees are commonly called "living fossils"). The association is also an extreme example of conservatism over evolutionary time, both of phylogenetic conservatism in plant-insect associations and of ecological niche conservatism within a specialized feeding group. The researchers further believe that the paper demonstrates the importance of the fossil record of insect-plant interactions in shedding light on the timing and ecological context of the diversification of insects, particularly in light of the paucity of fossil insects in certain time periods. "The damage can provide valuable data that otherwise would be unavailable if one depended only on the 'body fossil' record of insects," said Labandeira.

Broader Significance of the Research

Access to well cared for museum collections and a network of natural scientists leads to broad findings. Wilf, Labandeira, and their colleagues were able to use collections and expertise from a number of sources, emphasizing the value of maintaining museum collections and supporting the free and ready exchange of scientific information among scholars.

Wilf further noted that the research underscores the importance of conserving tropical forests for comparison between modern and ancient species and associations. "This and previous research have raised questions about the patterns and timing of the evolution of plant-insect associations that are important for understanding why the world is the way it is today," he said. "If the forests are destroyed, we'll never know the answers."

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Rainforest along the Río Negro near Santa Lucía, Venezuela. (Photo: Anna Weitzman.)

This research was funded in part by the Smithsonian Institution's Office of Fellowships and Grants, the Walcott Fund and Evolution of Terrestrial Ecosystems of the National Museum of Natural History, and the Michigan Society of Fellows.

To learn more visit these sites:

Genera Zingiberarum, from the
NMNH Department of Botany

Department of Paleobiology

NMNH Department of Entomology

The Smithsonian Tropical Research Institute (STRI)

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