Microcarpolithes hexagonalis Vangerow, 1954

McLoughlin, Stephen, Santos, Artai A., Donaldson, Sherri, Pott, Christian & McCurry, Matthew, 2024, Termite activity in the mid-Cretaceous of Australia, Palaeontologia Electronica (a 48) 27 (3), pp. 1-30 : 5-7

Microcarpolithes hexagonalis Vangerow, 1954

Figure 2A–J

Description. The specimen consists of an aggregation of several hundred hexagonally prismatic opaline pellets; the block is 27 mm long, 16 mm wide, and 10 mm thick ( Figure 2A–D ). Individual pellets are oblong, 0.97–(1.33)– 1.62 mm long (n=40), 0.59–(0.76)–1.00 mm in transverse diameter (n=40), and conspicuously hexagonal in cross-section ( Figure 2E–G ), with smoothly faceted surfaces ( Figure 2H–J ), and truncate to broadly rounded ends. Tomographic analysis reveals that individual coprolites have either a uniform finely granular internal composition ( Figure 2E ) or have an equivalent uniform outer rind and an internal cavity or porous region constituting one- to twothirds of the pellet diameter ( Figure 2F–G ). The cavities locally contain irregularly arranged minute flecks of detritus. The coprolites lack any obvious regular organization or common orientation ( Figure 2C ). They are bound together by porous interstitial clays or opaline silica ( Figures 2D, I , 3 ).

Remarks. The structures described in this study differ from replacements of euhedral quartz crystals by their bluntly rounded termini (at both ends of the pellets) and irregular arrangement. These mid-Cretaceous structures are interpreted to be termite coprolites based on their equivalency in shape and cross-section to termite coprolites assigned to Microcarpolithes hexagonalis Vangerow , from the Late Cretaceous of the Netherlands (Vangerow, 1954; Colin et al., 2011). Microcarpolithes hexagonalis was originally considered to represent angiosperm seeds but later reinterpreted by Kovach and Dilcher (1988) as termite coprolites (Colin et al., 2011; Moreau et al., 2019). Here we follow the criteria of Colin et al. (2011) who proposed that Microcarpolithes hexagonalis “must be used to uniquely designate termite coprolites that are cylindrical in shape and with a conspicuous hexagonal section, making it clearly distinct from some coprolites of curculionid weevil larvae that occasionally produce subhexagonal pellets.” Although the Australian Cretaceous coprolites are slightly longer than the type specimen of M. hexagonalis from Europe, we do not regard this small difference (less than 1 mm) to be taxonomically significant for a trace fossil, which is in line with the proposal that size should not be considered primary criteria for differentiating ichnotaxa (Bertling et al., 2022).

Figures

FIGURE 2. Illustrations of opalized termite coprolites from the Griman Creek Formation, Lightning Ridge (all images of specimen AM F 128064). A, Overview of specimen. B, Details of irregularly arranged pellets. C, Details of hexagonally prismatic faecal pellets. D, Details of faecal pellets and remnants of intervening clays and opaline silica. E, Greyscale tomographic (μCT) reconstruction of fossil frass showing a mix of pellets with uniform and hollow interiors; hexagonal shape is evident in the cross-section in the coprolite on the right. F, Greyscale tomographic reconstruction of fossil frass showing pellets in longitudinal (oblong) and transverse (hexagonal) section. G, Greyscale tomographic (µCT) reconstruction of fossil frass showing pellets held together by porous clays or opaline silica. H, Scanning electron micrograph of hexagonally prismatic coprolites. I, Scanning electron micrograph of hexagonally prismatic coprolites with blunt termini. J, Scanning electron micrograph of coprolites showing details of truncate termini. Scale bars equal 10 mm for A; 1 mm for B–G; 500 µm for H–J.

FIGURE 3. Energy-Dispersive X-ray Spectroscopy (EDS) plot for coprolites showing overwhelming dominance of silicon and oxygen X-ray counts denoting opaline composition of the fossils.