Bauxitic insect pupal cases from northern Australia

Bauxitic insect pupal cases of northern Australia D. B. TILLEY, T. T. BARROWS AND E. C. ZIMMERMAN

TILLEY, D. B., BARROWS, T. T., & ZIMMERMAN, E. C., 1996: cases of northern Australia. Alcheringa

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. Bauxitic insect pupal .

Within the Weipa bauxite deposit of northern Queensland, Australia, are rare ovoid structures averaging 49 x 30 mm in size and of a similar composition to the surrounding bauxite. Comprised of a thin shell with an opening near one end, these structures encase ordinary bauxitic pisoliths and are referred to as ovate compound pisoliths. A small proportion of them are completely hollow and have no apparent opening into their chamber. Similar calcareous structures may be found along the West Coast of the Eyre Peninsula, South Australia. These calcareous structures are believed to have been constructed by the larval stage of the weevil Leptopius duponti, for protection during pupation. The calcareous structures have a similar size and shape to ones composed of bauxite, suggesting that ovate compound pisoliths may have been constructed by a similar organism, possibly belonging to the genus Leptopius.

David B. Tilley, Cooperative Research Centre for Landscape Evolution and Mineral Exploration, c/o Department of Geology, Australian National University, Canberra ACT 0200, Australia; Timothy T. Barrows, Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia; Elwood C. Zimmerman, Division of Entomology, CSIRO, GPO Box 1700, Canberra ACT 2601, Australia; received .

Keywords: Adelognatha, bauxite, Curculionidae, Leptopius duponti, pisolith, puparia, 1

pupation cell.

CONSTITUTING a minor part of the Weipa bauxite in northern Australia are shell and tubelike structures of bauxitic composition, ranging in length from 2 to 5 cm. The internal chamber of such structures commonly contains ordinary bauxitic pisoliths or oöliths, however some are completely empty. Usually, there is at least one opening into the chamber. Openings may be partially or totally occluded with bauxite. These structures are designated bauxitic compound pisoliths and are known locally at Weipa as "mother pisolites" or "rattlers". Four different types of compound pisolith were identified in the Weipa area (Fig. 1). The nodular forms are found at the base of the bauxite and externally resemble the underlying ferruginous kaolin-rich nodules. Common compound pisoliths which exist mainly at the top of the bauxite are smoother and more ellipsoidal in shape than nodular ones. Less common varieties are the root-like tubular compound pisoliths and the large ovate type. Common and nodular compound pisoliths are attributed to the bauxitisation of kaolin-rich nodules (Tilley et al., 1995), while the tubular kind are believed to be rhizoliths; root-like concretions formed when a particular type of rainforest plant selectively extracted silica, leaving behind an encrustation of bauxite. The following paper deals specifically with ovate compound pisoliths and their comparison with similar calcareous structures found along the Eyre Peninsula in South Australia.

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Description Ovate compound pisoliths are found in small groups, sporadically distributed throughout the bauxite deposit. They may be found at any level in the bauxite and soil horizons. They are the largest type of pisolith known in the Weipa area, averaging 49 x 30 mm (Table 1 and 2). Much smaller ovate compound pisoliths can be found, however their similar size and shape to common compound pisoliths, makes them difficult to distinguish. A common feature to all ovate compound pisoliths, regardless of their size, is their thin shell. Often there is an opening or evidence for one, offset from one end of the compound pisolith. When such an opening is present, ordinary bauxitic pisoliths generally occupy the inside chamber. As with common compound pisoliths, when an opening is absent, the internal chamber is invariably devoid of material. Ovate compound pisoliths from the Weipa-Andoom railway cutting, displaying examples of both empty and infilled chambers are shown in Fig. 2 (A-D). Compared with ordinary bauxitic pisoliths, there is a general lack of concentric banding in ovate compound pisoliths. Mineralogical analyses were performed on ovate compound pisolith shells and the surrounding material using X-ray diffraction (XRD) in conjunction with SIROQUANT (Taylor and Clapp, 1992); a quantitative XRD phase analysis program (Table 3). The analyses showed that the shells have a similar composition to the material which immediately surrounds them. Those which occur in the soil horizon are composed of soil material, while those that exist in the bauxite horizon are composed of bauxite.

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Discussion Lea (1925) published a well-illustrated account of similar, but calcareous, structures found along about 500 km of the coast of South Australia and as far inland as about 60km. These pupal cases possess at least one opening into their inner chamber. Lea reported that: "It was assumed that they were pupal cases or cocoons of insects, that had been altered by the action of lime water; but of what insect was unknown till recently, when Mr. J. R. Ryan, of Ceduna, who had brought in many of the specimens, sent others which proved that the original cases, or at least many of them, were constructed by large weevils, Leptopius duponti, some of them being actually in their original cases. Mr. Ryan wrote: 'They are found almost everywhere on the coast, inland as well as near the seaside, and can be dug up just under the surface of the ground. Those I gathered were mostly lying on the ground.....' " Fig. 2 (E-H) exhibit the pupal cases of Leptopius duponti from the Elliston area, South Australia. Leptopius is an endemic genus of perhaps more than a 100 mostly large Australian weevils. Species are found in all states, and more than 30 species occur in Queensland. The adult weevils feed upon the foliage of broad-leaved plants, particularly Acacia species. Their larvae feed upon roots. When ready to pupate, they construct a cell in the ground by rotating their bodies to compact the surrounding dirt or sand particles, and they may use body fluids to stabilize the walls of their pupal chambers or cocoons. It is these cocoon walls that may subsequently become hardened by infiltration of calcium-rich water or fossilised within bauxite. When these structures are found, they are often considered fossils of unknown origin. When mature and ready to emerge, the adult weevil, with the aid of elongated cusps on the mandibles, breaks through the walls of the pupal chamber and digs upward to emerge from the soil. The mandibular cusps are then shed; their retention would interfere with leafeating by the adult weevil. The pupal case remains in the soil where it may disintegrate or be hardened by mineralization if conditions permit. The exit holes in the fossil pupal cases vary 4

according to the sizes of the weevils and how much of the pupal case wall is removed. Some of the weevils die before maturing; thus cases are found that have no exit holes. The fossils may accumulate in large numbers over time as long as the hostplants grow in an area, and they may be found in places where the plants have not grown for many years. In South Australia they are found commonly in places where the wind has blown sand away and exposed them to accumulate on the surface. For coloured illustrations of some species of Leptopius, see Zimmerman (1991), Plates 193-196. The dating of ovate compound pisoliths is problematic. Several hollow ovate compound pisoliths were broken open, however no insect remains appear to have survived the oxidisation and severe leaching conditions associated with bauxitisation. Consequently, radiocarbon dating could not be considered even if the pupal cases are Late Quaternary in age. The existence of only a very thin layer of bauxite material around individual ovate compound pisoliths suggests they were constructed by weevils during or after the last bauxitisation episode. Dammer et al. (1994) investigated weathering-related cryptomelane ((K,Ba)MnO16.xH2O) from nearby Groote Eylandt, Northern Territory and concluded that three major periods of intense chemical weathering, corresponding to more humid conditionsk, were associated with cryptomelane precipitation at 6-20 Ma (Middle to Late Miocene), 30-40 Ma (Oligocene) and 46-54 Ma (Early to Middle Eocene). Since bauxite formation requires weathering conditions similar to that necessary for the formation of cryptomelane, ovate compound pisoliths can be no older than 30 Ma otherwise a second bauxite layer would be evident, similar to that often seen in common compound pisoliths.

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Acknowledgement Comalco Aluminium Limited provided financial support for this paper through funding of the Ph.D project entitled "Models of bauxitic pisolith genesis: data from Weipa, Queensland".

References DAMMER, D., CHIVAS, A.R. & McDOUGALL, I., 1994. K-Ar dating of weathering-related cryptomelane and alunite from western and northern Australia. Abstracts of the 8th. International Conference on Geochronology, Cosmochronology and Isotope Geology;

U.S. Geological Survey Circular 1107, p. 74.

LEA, A. M., 1925. Notes on some calcareous insect puparia. Records of the South Australian Museum v. 3, p. 35-36, Plate I. TAYLOR, J. C. and CLAPP, R. A., 1992. New features and advanced applications of SIROQUANT: a personal computer XRD full profile quantitative analysis software package. Advances in X-ray Analysis v. 35, p. 49-55. TILLEY, D. B., MORGAN, C. M., and EGGLETON, R. A., 1995. The bauxitisation of a ferruginous-kaolinite mottled horizon: an explanation for the evolution of the Weipa bauxite deposit. Extended Abstracts of the 17th International Geochemical Exploration

Symposium p. 305.

ZIMMERMAN, E. C., 1991. Australian Weevils (Coleoptera : Curculionidae): CSIRO Publications v. 5, p. 1-633, Plates 1-303.

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______________________________________________ length (l)

width (w)

(mm)

(mm)

l/w

______________________________________________ Minimum

44

27

1.50

Maximum

59

32

1.84

Mean

49.3

29.9

1.65

Standard Deviation

3.8

1.8

0.09

______________________________________________

Table 1.

Dimensions of 17 ovate compound pisoliths from the Weipa area, northern Queensland.

______________________________________________ length (l)

width (w)

(mm)

(mm)

l/w

______________________________________________ Minimum

49

28

1.73

Maximum

60

31

1.94

Mean

54.2

30.0

1.81

Standard Deviation

3.6

1.1

0.09

______________________________________________

Table 2.

Dimensions of 6 Leptopius duponti pupal cases from the Elliston area, South Australia. 7

______________________________________________________________ OVATE COMPOUND PISOLITHS LEPTOPIUS PUPAL CASES Bauxite horizon mineral

shell

matrix*

Soil shell

matrix

shell

matrix

______________________________________________________________ Gibbsite

90

84

8

7

Boehmite

4

11

39

37

Hematite

2

2

2

3

Anatase

3

3

2

2

Kaolin 1

29

27

Quartz

20

24

Calcite Halite

5

4

90

90 5

6

______________________________________________________________ * The composition of the material which envelops the shells, termed the matrix, is included for comparison.

Table 3.

The mineralogical composition (weight %) of ovate compound pisolith shells from the bauxite horizon and the overlying soil, compared with fossilised Leptopius duponti pupal cases.

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Fig. 1. Schematic diagram of four different types of bauxitic compound pisolith identified in the Weipa bauxite deposit of northern Australia.

Fig. 2. Specimens A-D are ovate compound pisoliths from Weipa. Those shells which are empty (A and B) have no apparent opening whereas those containing ordinary bauxitic pisoliths (C and D) have an opening into their chambers. Specimens B and D were mounted in polyester resin prior to their sectioning. Specimens E-H are fossilised Leptopius duponti pupal cases from the Elliston area, South Australia.

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