A couple of Arched Earthstars. Geastrum fornicatum, 4th February 2024.

I’ve seen quite a few different earthstars and I’ve written about some of them on this blog – the Vaulted Earthstar (Geastrum britannicum), Collared Earthstar (Geastrum triplex) and the very localised coastal Dwarf and Tiny Earthstars (Geastrum schmidelii and Geastrum minimum). All are basiodiomycete fungi and members of the family Geastraceae.

Arched Earthstars in Norfolk

One species I hadn’t seen until recently was the Arched Earthstar, Geastrum fornicatum. But fungi can be like buses – you wait ages to see one and then two come along at once. Since the start of this month I’ve seen Arched Earthstars in two different places (note 2).

Arched Earthstars aren’t common in Norfolk and up until the end of 2023 were only known from 14 different sites.

My first sighting was on 4th February. Returning from a trip to a site just outside Norwich with our friends Sarah and Ian, Sarah stopped the car by our Vaulted Earthstar (Geastrum britannicum) site. I spotted an earthstar on the road verge and Vanna was about to say ‘Arched Earthstar’ but Ian got there a couple of seconds earlier. We then looked under the nearby Yew tree and Vanna counted 31 earthstars. They were all Arched Earthstars, Geastrum fornicatum, and we didn’t see any Geastrum britannicum.

Six days later Vanna and I cycled down to South Norfolk. It was a successful day – we had found a new site for Sandy Stiltball (Battarrea phalloides) on a road verge on our way south. We had lunch in a churchyard and found a couple of dozen Arched Earthstars beneath a Yew tree. There were several Yews but just one with earthstars underneath.

Identifying Arched Earthstars

The genus name for earthstars, Geastrum, comes from geo meaning ‘earth’ and aster meaning ‘star’. The fruitbody consists of an inner and outer wall (peridium): the inner peridium is the spore sac and the outer peridium splits to form rays in a star shape (note 2).

The specific name for the Arched Earthstar, fornicatum, means ‘arched’. Its red-brown rays are upright and push the spore sac upwards, giving the earthstar an upright (arched) appearance. The spore sac has a swelling on its underside, known as the apophysis.

The rays of an Arched Earthstar are attached to a basal cup of mycelium, a feature shared with the Vaulted Earthstar (Geastrum britannicum) and Rayed Earthstar (Geastrum quadrifidum) (note 3). Arched Earthstars are 4 – 8cm across when the fruitbody is expanded, with four to five rays. The peristome, the hole in the top of the spore sac, is not delimited, in contrast to that of G. britannicum (note 4).

If you find a group of earthstars, it is worth examining as many as possible, especially when they are old and weathered. The spore sac on the one pictured below is a bit damaged but its basal cup is intact; other fruitbodies had a spore sac in better condition but had become detached from their basal cups.

Arched Earthstar. Geastrum fornicatum

Distribution

Geastrum fornicatum is found in southern England and Wales and there is a single record on the NBN Atlas for Ireland. it is widespread elsewhere in Europe but uncommon (note 5). It also occurs in the United States, growing under Monterey Cypress in California and in Australia, “in litter under trees in dry woodlands and mallee scrub”.

‘Reminiscent of ballet dancers’

In the United States Geastrum fornicatum is sometimes known as the Acrobatic Earthstar and Pat O’Reilly on his First Nature website notes how “these earthstars do have silhouettes reminiscent of ballet dancers”.

In 1799 the English naturalist James Sowerby (1757 – 1822) wrote a book entitled “Coloured Figures of English Fungi or Mushrooms“. In it, he described how Geastrum fornicatum (then known as Lycoperdon fornicatum) resembled the human figure:

“So strange a vegetable has surprised many; and in the year 1695 it was published under the name of Fungus Anthropomorphus, and figured with human faces on the head. It is at first roundish; in ripening the head bursts through the two coats or wrappers; the inner wrapper, detaching itself from the outer, becomes inverted, connected only by the edges; the coats most constantly split into four parts.”

There are good photos of Geastrum fornicatum on the First Nature website.

If you’re interested in identifying more earthstars, the guide “How to identify British earthstars” by Phil Gates on the Discover Wildlife website describes some of the other species.

I also thoroughly recommend “Mushrooms and Toadstools of Britain & Europe – Volume 1” (2017) by Geoffrey Kibby, which has two pages of earthstar illustrations and shows the three species which have basal cups side by side. Kibby describes the habitat of Arched Earthstars as being on soil under broadleaved trees.

Finally, we found the Vaulted Earthstars we’d missed on 4th February. Last Saturday we cycled past our Vaulted Earthstar (Geastrum britannicum) site and looked a little further along the road under a different Yew tree. We found a few more Arched Earthstars and, happily, at least half a dozen Vaulted Earthstars too (note 6).

One of the Vaulted Earthstars, Geastrum britannicum. 17th February 2024.

Notes

Note 1 – The phenomenon of buses arriving together is explored on Jason Cole’s blog and in this New Scientist article from 2009.

Note 2 – It is estimated that there are up to 120 species of Geastrum throughout the world.

Note 3 – I haven’t seen the Rayed Earthstar (Geastrum quadrifidum). There are two records for Norfolk, but from the days before the Vaulted Earthstar (Geastrum britannicum) was recognised as a separate species. Rayed Earthstar is small (just 1.3 – 3.7cm across), rather uncommon and is usually found under Beech trees (Fagus sylvatica) on calcareous soils in southern England.

Note 4 – There is a good diagram of what a delimited peristome looks like in the 2009 publication “The distribution and identification of earthstars (Fungi: Geastraceae) in Norfolk” by Tony Leech, Trevor Dove & Jonathan Revett. See figure 4. (Note that this paper was written before Geastrum britannicum was recognised as a separate species.)

Note 5 – Geastrum fornicatum is featured on page 1251 in Volume 2 of Laessoe and Petersen’s two volume “Fungi of Temperate Europe”. They describe it as “widespread in the nemoral zones, very rare”. (“Nemoral” refers to the vegetation zone of temperate forests in Eurasia.)

Note 6 – I also checked the spores from the Arched and Vaulted Earthstars I found on each occasion. Both have globose, warted spores but those of Arched Earthstar (Geastrum fornicatum) are 3.5 – 4.2 µm across and those of the Vaulted Earthstar (G. britannicum) are smaller, 3 – 3.8 µm across. Both measurements exclude the warts.