Rounded Earthstar Identification Guide

Where You Meet It

Rounded Earthstar is one of the most frequently encountered small earthstars, and it usually shows up in numbers. The classic setting is a shaded patch of hardwood litter, a bed of decaying wood chips, the base of an old stump, or the humus-rich ground under a mature broadleaf canopy. You look down and see a scatter of small pale stars, often a dozen or more, pressed flat into the debris.

Weathered fruit bodies persist for months, so the population you find in January may be the remains of an autumn flush. Fresh material is firmer, paler and cleaner, and it is what you want for identification. The unopened stage is a small onion-shaped ball 1.5 to 3 cm across with a pointed beak, whitish-buff, half-buried and with grit and litter glued to its surface.

The Rays

As the fruit body matures, the thick outer wall splits into 4 to 9 rays, most often 5 to 8. These bend backwards and downwards, and in doing so they push the spore sac upwards. The expanded star is usually 2 to 5 cm across, so this is a modest earthstar rather than a large one.

The habit of the rays gives the fungus its name. When hydrated they typically remain bowl-shaped, cupping the spore sac like a saucer or a small sack, rather than flattening out into a plane or curling right under. That saccate posture is worth noting in the field, though it varies with moisture and age.

The inner face of the rays is pale buff to tan when fresh, darkening to brown and cracking radially as it dries. Importantly, there is no pink or reddish flush in the fleshy layer, either on handling or on cutting. The outer face carries a buff mycelial layer studded with adhering soil, wood fragments and litter, which can peel away in old specimens.

The Spore Sac

The spore sac is globose, 0.8 to 2 cm across, and pale ochre-brown to greyish-tan, with a smooth to minutely felted surface that has a slightly matt, papery finish.

It is sessile: it sits directly on the rays with no stalk beneath it and no apophysis, the small collar or disc that some relatives develop at the sac base. Lift the sac gently with a fingertip and look at the junction from the side to confirm there is nothing between it and the ray tissue.

The Mouth: the Key Character

Everything in small-earthstar identification comes down to the peristome, so get a hand lens on it and look straight down.

In Rounded Earthstar the mouth is a low silky-fibrillose cone, and it sits inside a distinctly delimited zone: a pale, often slightly sunken circular area with a clean, definite edge where it meets the darker surface of the sac. Mycologists call that zone a court. Seen from above it reads as a neat disc with a rim, not as a gradual fade. That sharp boundary is the character to record, and it is the single most efficient way to separate this species from its two most confusable relatives.

Section and Interior

If you can spare a young specimen, cut it vertically. A firm white gleba fills the sac, and a slender columella runs up through the middle from the base. The thick outer wall shows its distinct layers on the cut face, and you should watch it for a minute: in this species the exposed tissue does not develop a pink or wine-coloured flush.

As the fungus ripens, the gleba dries into a powdery mid-brown to dark brown spore mass laced with capillitium threads, and the sac becomes a thin papery bag that puffs when compressed or struck by rain.

Assessing the Spores

There is no gill surface to print from, so use the field equivalent. Hold a mature fruit body a few centimetres above white paper and tap the sac. A cloud of fine brown powder settles out. Colour it against the paper: mid to dark brown, without olive or purple tones.

Under a microscope the spores are globose and covered in low warts, roughly 3 to 4.5 micrometres in diameter, brown in mass. Capillitium threads are thick-walled, brownish, and largely unbranched.

Substrate, Season and Ecology

Rounded Earthstar is a litter and woody-debris saprotroph. It fruits in accumulated hardwood leaf litter, on rotting wood fragments and buried twigs, in wood-chip mulch and along woodland edges, and it is one of the earthstars most likely to appear in landscaped and disturbed ground. It has an extremely wide reported distribution across temperate and tropical regions.

The main season is summer through autumn, with fruit bodies continuing to appear through mild spells and old stars persisting long afterwards. Because it depends on decaying plant material rather than on a tree partner, the presence of deep, damp, woody litter predicts it better than any particular tree species.

Separating the Look-alikes

Geastrum fimbriatum is the closest problem. It is a similar size with a similarly sessile spore sac, but its mouth is fibrillose and undelimited, blending gradually into the sac with no pale court and no defined edge. Get the lens on the mouth and the two separate cleanly.

Geastrum rufescens is larger overall, typically 4 to 10 cm expanded with thicker, more substantial rays, and its fleshy layer flushes pink to reddish on handling and drying. Its mouth is also undelimited. Two characters therefore differ, so confusion should be brief.

Geastrum triplex is bigger, commonly 5 to 12 cm, and in typical form the thick fleshy layer cracks around the base of the spore sac and separates to form a raised collar or bowl beneath it. That collar is unmistakable when present. Its mouth is delimited too, so rely on size and the collar rather than the mouth.

Geastrum lageniforme is taller and narrower in the unopened stage, expands larger, and usually has more numerous, narrower rays that often split further at the tips.

Geastrum coronatum has a distinctly darker, greyish spore sac, generally shows a short stalk with a small collar at the sac base, and is a more robust fungus.

Geastrum pectinatum and Geastrum striatum are excluded at a glance: both carry the spore sac on an obvious stalk, and their beaks are grooved and pleated rather than silky and fibrous.

Astraeus hygrometricus has thick, leathery, hygroscopic rays that fold shut over the sac in dry weather and reopen when wet, with an upper ray surface broken into small plates, and its mouth is an irregular tear with no defined structure at all.

Field Checklist

1. Hardwood litter, wood chip or rotting debris, often many together.
2. Star 2 to 5 cm expanded, rays commonly staying bowl-like.
3. No pink flush in the fleshy layer, on handling or cutting.
4. Spore sac sessile, no stalk and no collar beneath it.
5. Mouth silky-fibrous and clearly ringed by a pale, sharp-edged circular court.
6. Spore dust brown; spores globose and warted, 3 to 4.5 micrometres.