Ancient Weevil Pupal Cases: Trace Fossils from Australia’s Pleistocene

Curious pupal cases made by prehistoric weevils, together with worm burrows, are found as trace fossils in rock exposures of the Upper Bridgewater Formation along the western coastline of the Eyre Peninsula in South Australia (Flint, 1992; Flint and Rankin, 1991; Rankin and Flint, 1992).  According to Parker and Flint (2005), the Upper Bridgewater Formation is a middle to late Pleistocene aeolian calcarenite (a wind-blown, consolidated gritty calcareous sandstone).  These trace fossils are found inland from the coast for a distance of about 40 km.  Microscopic analysis of these ancient pupal cases shows they are made of gritty sand and gravel that were cemented by calcite over thousands of years.

Fig. 1. Fossil pupal cases from the Bridgewater Formation resemble small elongated eggs.  These cases have a hole where the fossil organism exited.  The trace fossils are characterized by their strong cementation and a hollow interior.  Scale in mm. Specimen from the S. W. Veatch collection. Photo by S. W. Veatch.

These cases are thought to have contained the pupae Leptopius duponti, a medium-size, soil-inhabiting weevil or snout beetle of the family Curculionidae. The Curculionidae are one of the largest families of organisms, with at least 44,000 described species (Grimaldi and Engel, 2005).  Adults of most species of this family have a characteristic elongate snout or nostrum.  At the end of this well-developed snout is a small pair of mandibles for biting and chewing food.

Taxonomic Classification:
Kingdom        Animalia
Phylum          Arthropoda
Class             Insecta
Order             Coleoptera
Suborder       Polyphaga
Superfamily Curculionoidea
Family           Curculionidae
Subfamily Leptopiinae
Genus           Leptopius
Species         duponti

The adult female Leptopius duponti not only relishes the foliage of acacia trees as food, but also carefully lays her eggs on the leaves.  When the larva hatch, they move underground to feed on roots. When they are ready to pupate, they form a chamber or pupal case out of the soil.  After their metamorphosis, they cut a hole near one end of their pupal case to leave and then burrow to the surface, where they quickly climb the acacia trees to feed.

The pupal cases are usually too delicate to survive for any length of time, but, occasionally, some of the empty cases remain underground where they become petrified by calcite.  (Tilley et al., 1997).  Some of these pupal cases in the Upper Bridgewater Formation are estimated to be 40,000 to 100,000 years old.

Fig. 2. Leptopius duponti is common in Australia, where they are called “wattle pigs.” 
The body length of Leptopius duponti averages 20 mm. 
These slow-moving weevils are plant eaters.
Photo by David Nelson. Used with permission.

References cited:

Flint, R.B., 1992, Elliston, South Australia, Sheet SI3-6, South Australia Geological Survey, 1:250,000 series, explanatory notes.

Flint, R.B. and Rankin, L.R., 1991, Kimba, South Australia, Sheet SI53-7, South Australia Geological Survey, 1:250,000 series, explanatory notes.

Grimaldi, D and Engle, M. S., 2005, The Evolution of Insects: New York, Cambridge University Press, 689 p.

Parker, A.J. and Flint, R.B., 2005, Yardea, South Australia Sheet SI53-3, Geological Survey of South Australia, 1:250,000 series, explanatory notes.

Rankin, L.R., and Flint, R.B., 1992, Streaky Bay, South Australia Sheet SI53-2, South Australia Geological Survey, 1:250,000 series, explanatory notes.

Tilley, D. B., Barrows, T.T., and Zimmerman, E.C., 1997, Bauxitic insect pupal cases from northern Australia.  Alcheringa 21, p. 157-160.