Geastrum austrominimum

For general comments about the genus see https://canberra.naturemapr.org/species/5724.

When mature the star-like base may measure up to about 3 centimetres across and the spore sac up to about 1 centimetre. The spore sac sits atop a short stalk and is coated with a dense layer of whiteish calcium oxalate crystals. The underlying surface is creamy to brownish. The mouth of the spore sac is surrounded by conical and densely fibrillose structure (technically a peristome). The peristome is surrounded by a circular zone that is usually well-differentiated from the rest of the spore sac.

The star-like arms do not fold inwards when the earthstar is dry. The undersides of the arms are densely encrusted with tightly bound soil/debris.

This species is known from Australia and it may also be present in New Zealand.  

Look-alikes

Geastrum tenuipes is another earthstar in which the spore sac sits atop a short stalk and is a common earthstar in Canberra and the surrounding region. In that species (1) the spore sac has a strongly pleated, conical mouth (not so in austrominimum) and (2) the base of the spore sac typically has radial grooves (as you see in these sightings: https://canberra.naturemapr.org/sightings/4412321, https://canberra.naturemapr.org/sightings/3346639, https://canberra.naturemapr.org/sightings/4344334). Geastrum austrominimum has neither of those features

Technical note

The name Geastrum minimum was long used for certain small earthstars from many parts of the world and the species was supposedly widespread in Australia.  The authors of the paper listed below found that the long-prevailing concept of ‘minimum’ actually contained a number of species which the authors differentiated (see pages 147-152) and one of their new species was austrominimum (based on material collected in New South Wales in 2001).   

Reference

Zamora JC, Calonge FD & Martín MP, (2015). Integrative taxonomy reveals an unexpected diversity in Geastrum section Geastrum (Geastrales, Basidiomycota), Persoonia, 34, 130–165.