Skull-shaped Puffball
Calvatia craniiformis
Also called brain puffball, skull puffball
A large, smooth, pale tan puffball shaped like an inverted skull, with a strongly wrinkled tapering base and an olive-brown mature spore mass. The upper wall flakes away in papery sheets.
Overview
Skull-shaped Puffball (Calvatia craniiformis) takes both its scientific epithet and its common name from its outline: a broad rounded head narrowing abruptly into a furrowed, stalk-like base, so that the whole fruit body resembles an inverted cranium. It is one of the largest puffballs of eastern North American woodlands.
The skin is thin, smooth to slightly felted, and pale — whitish, greyish or a soft tan — with deep wrinkles and folds concentrated on the sterile base. Unlike its purple-spored relative, the surface does not break into distinct polygonal plates, and the mature spore powder ripens through yellow to a dull olive-brown or yellow-brown.
At maturity the papery upper wall flakes and peels away in irregular sheets, exposing the spore mass to the wind and eventually leaving the persistent, wrinkled base.
How to identify it
Fruiting body
Large, 6–20 cm across and up to 20 cm tall, obovoid to skull-shaped, with a rounded broad head and a distinctly narrowed, deeply wrinkled sterile base.
Surface
Smooth to finely felted or minutely areolate, whitish to greyish tan, becoming buff to brownish with age. The base is conspicuously folded and furrowed. The skin is thin and papery, flaking away in sheets when mature rather than cracking into regular polygons.
Interior and spore mass
White and firm when young, then yellowing, becoming olive and finally a powdery yellow-brown to olive-brown. The sterile base is spongy and chambered, remaining as a solid mass after the spores have dispersed.
Spore print
Not applicable in the usual sense; the mature spore mass is olive-brown to yellow-brown, never purple.
Where and when it grows
It grows on the ground in open deciduous woods, along woodland edges, in grassy clearings, parks and gardens, often on rich soil and among leaf litter. It occurs singly, in scattered groups or in loose rings.
The season is summer through autumn, following warm wet weather. It is common in eastern North America and also recorded from East Asia, with related species elsewhere.
Ecological role
Calvatia craniiformis is a saprophyte of soil and leaf litter, decomposing humus and dead plant material at woodland margins and in grassy openings. Its mycelium is long-lived and can produce fruit bodies in the same spot for years.
As with other Calvatia species, spore release is passive and wind-driven: the fragile upper wall breaks down over days, and the vast olive-brown spore mass is gradually blown away, leaving the tough wrinkled base behind on the ground.
Similar species
Calvatia cyathiformis is the closest match in size and shape, but its mature spore mass is purple to purple-brown and its surface cracks into distinct polygonal patches; its base persists as a deeper vase-shaped cup. Calvatia gigantea is enormous, nearly spherical and almost without a narrowed base. Calvatia utriformis has a coarsely mosaic-scaled surface and is a grassland and dune species. Lycoperdon puffballs are far smaller and release spores through a small apical pore instead of shedding the whole upper wall.
A large smooth pale puffball with a strongly wrinkled tapering base and olive-brown spore powder is the skull-shaped puffball.