Sciency Thoughts: Fossil Bee's nests from the Taung Child Hominin locality in South Africa.

In 1925 Raymond Dart announced the discovery of the first pre-Modern Human Hominin remains in Africa, the Taung Child skull, from which the first Australopithicine species, Australopithecus africanus, was described. Since this time a wealth of Hominin species have been described from Africa, and South Africa in particular, greatly enhancing our understanding of the origin of our species, but no further Hominin material was ever recovered from the Buxton Limeworks site that produced the Taung Child. This site was originally interpreted as a limestone cave system (early Man was expected to live in caves, an expectation that has subsequently been vindicated with many of South Africa's most productive Hominin sites found in cave systems), with the skull recovered from a slitstone deposit thought to have accumulated within the caves, but more recent analysis of the limestone sediments has suggested that the presence of root mats, Insect burrows and fragments of Bird's eggs in the sediment, all of which suggest an in situ soil rather than a cave deposit, as well as a microcalcitic crystalline structure to the limestone matrix, which would be typical of a limestone concretion formed within a soil.

In a paper published in the journal PloS One on 28 September 2016, Jennifer Parker of the Institute of Archaeology at University CollegeLondon and the Department of Earth and Planetary Sciences at Birkbeck College, University of London, Philip Hopley, also of the Department of Earth and Planetary Sciences at Birkbeck College, and Brian Kuhn of the Department of Geology at the University of Johannesburg, describe an aggregation of fossil Bee's nests from the Buxton Limeworks locality.

The majority of Bee species are both solitary and ground nesting. They favour fairly dry soils, as they seal their eggs into closed chambers with a supply of food (pollen or nectar), and do not re-enter or move the nests, which means that excess moisture can lead to Fungal infections, destruction of the food source or even drowning of the immature Bee (Bees living in particularly humid environments generally do not nest on the ground). These burrows are typically lined with a waxy substance secreted from an organ possessed by the female Bees, called the Dufour’s gland, which helps to protect the nest and juvenile from excess moisture.

The aggregation examined by Parker et al. comprises about 25 visible cells on a block of sediment 115 cm long and 50 cm high. This block had been broken into a number of fragments previously, enabling Parker et al. to confirm the nests ran throughout the block. Previous studies of similar material have relied on cutting serial thin sections of the rock in order to determine the internal structure, a process which destroys the samples, however Parker et al. were able to examine the specimen using micro-CT (computed tomography) facilities at the Natural History Museum in London, enabling the construction of a three-dimensional model of the interior of the block without destroying the specimen.

CT Images of slice of Block 5 in shrink-wrapped form (above) and solid form (below). Note the complex matrix of porosity, Cells (white arrows) and tunnels made by unknown organism (blue arrows). Parker et al. (2016).

This revealed that the cells were not connected by any form of tunnel network, as would be expected in a nest built by communal Bees, indicating this is an aggregation of solitary Bee's nests rather than a single structure. This may be caused by the activity of a single female Bee (some species construct over 30 individual nests close together, each containing a single egg plus a food source), or a group of Bees that nested in close proximity where favourable conditions were found.

CT Image of Block 6D in shrink-wrapped form. Note the complex matrix of porosity, Bee cell (white arrow) and tunnels made by unknown organism (blue arrows). Parker et al. (2016).

The individual cells are approximately 14 mm in length and 7 mm in width, and approximately flask shapes. All are at least partially infilled with sediment. The cells were apparently lined, though this lining has been replaced by calcite (limestone), making its exact nature hard to judge, though as the cells themselves are infilled with sediment rather than cacite, it is likely that the shape and thickness of this lining has been preserved.

Three Different Individual Cells. (A) and (B) have been extracted from the nest, and (C) (although broken in half laterally) remains in the matrix. (A) displays a proximal view of a cell that has been broken at the neck (the entrance); note the cell lining (arrow). (B) displays a dorsal view of a cell, note the smooth cell wall; the proximal end (at the left of the image) has been broken at the neck. (C) shows a dorsal view of a cell that has been broken laterally; the calcite lining (arrow) is clear, as is the cell shape (narrower and blunter at the proximal (left) end, more rounded at the distal (right)). Scale bar is 3mm. Parker et al. (2016).

The material that made up the cell lining has been replaced by calcite (limestone), but this calcite still contains some trace of the original material. Notably a large number of fibrous structures 160 to 350 μm, either unbranching of bifurcating (spiting in two) are present within this matrix, these being apparently random in their positioning and orientation, which Parker et al. interpret as non-glandular plant trichomes (leaf hairs). The selection of leaf hairs on their own as inclusions in a nest lining seems unlikely, but several types of Bee do use leaf fragments or pulped leaves to line their nests, and it is possible that the hairs could have survived the destruction and diagenetic replacement of the rest of the leaf material.

Petrographic Microscope Images of sections of a thin section of a Cell. (A) shows the position of images (a), (b), and (c) in the whole thin section. (a), (b), and (c) display sections of the cell lining at a higher magnification to show the trichomes (arrows) and organic lining structure (c; darker grey). (a), (b), and (c) are 1.5mm in width (left to right). Parker et al. (2016).

If the lining of the nest had comprised leaf fragments it is likely that the leaf hairs would be clumped into groups with similar orientation. Since this is not the case, Parker et al. conclude that the lining of the nest was almost certainly originally a leaf pulp. Only one type of Bee makes such a nest today, Wood Carver Bees, Anthidiini, and this is unlikely to have changed notably since the Plio-Pleistocene deposits at Taung were laid down (about 2.8 million years ago). Wood Carder Bees are numerous and diverse on every continent except Australia, and can be expected to have been present in South Africa in the Plio-Pleistocene.

These Bees favour dry soils in exposed sunny locations for nesting sites, and are not known to nest in caves. This strongly supporting the idea that Taung child was originally buried in soil deposits that became calcified later rather than in a cave. The most likely location for such a soil, given the Insect fauna, implied flora (Carder Bees pollinate Herbaceous Flowering Plants) soil type (silt particles) and later mineralization, would be a broad river floodplain. The original (and subsequent) diggings at Taung found no further Hominin remains, but these proceeded on the basis that the site was an infilled cave, and that any remains would be found within a very localized area. However a calcified palaeosol laid down on a broad floodplain could potentially extend much further, forming beds from hundreds of metres to tens of kilometres in extent, greatly widening the area in which further Taung-associated material could be found.

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Friday, 7 October 2016

Fossil Bee's nests from the Taung Child Hominin locality in South Africa.