It forms flat, dark green, rosette-shaped patches measuring 10–20 mm in diameter on bare soil and rock surfaces.

While it often behaves as an annual plant in temporary habitats such as arable fields, it can persist longer in continuously moist environments and survive dry periods by forming tuber-like structures.

The species is not only important ecologically but has also been developed as a model organism for genetic research due to its simple morphology and ease of cultivation.

The genus Phaeoceros, which was erected by Johannes Max Proskauer in 1951 and currently includes about 40 accepted species, is characterised by solid, smooth thalli, antheridial chambers typically containing 1–8 antheridia, and capsules without a distinct line of dehiscence.

[6] However, by 1958, after observing considerable variability in specimens from many countries, Proskauer temporarily merged them as subspecies, treating the taxon as P. laevis subsp.

In 1987, Hässel de Menéndez determined these taxa should be treated as separate species based on detailed studies of spore morphology.

On its underside, the thallus produces two types of root-like structures called rhizoids: smooth, transparent ones and pale brown tuberculate (warty) ones.

[11] These spore characteristics are important diagnostic features that help distinguish P. carolinianus from the closely related P. laevis, which has densely papillate proximal surfaces.

In Croatia, it has been documented growing with Anthoceros agrestis and Notothylas orbicularis, forming part of the plant community Riccio glaucae-Anthocerotetum crispuli, which typically develops on temporary dry and loamy soils.

In Europe, it is found from the Mediterranean to northern regions, in Asia it occurs across temperate and tropical areas, and in North America it ranges from Canada to Mexico and the Antilles.

In the Southern Hemisphere, it has been documented in Australia, New Zealand, various parts of Africa, and South America including Colombia, Peru, Brazil, Chile, Bolivia, and Argentina.

[18] Despite considerable morphological variation across its range, the species maintains consistent defining characteristics, particularly its monoicous condition and distinctive spore ornamentation.

[5] It shows a preference for fresh to moist, sandy-loamy or sandy soils that are neutral to slightly acidic, and can grow in conditions ranging from full light to shade.

[19][12] In Switzerland, it occurs from colline to montane elevations (200–1,080 m (660–3,540 ft)), primarily in agricultural areas of the Central Plateau, Jura, and Southern Alps, where despite abundant spore production, populations typically remain relatively small and stable compared to other hornwort species.

[15] In Central Europe, it typically behaves as an annual species, being frost-sensitive and developing during summer to autumn months, though it may persist longer in sites that maintain sufficient moisture for continuous growth.

[13] The main threats include intensification of farming methods, particularly earlier and more frequent tillage operations that reduce the time available for the species to complete its life cycle.

[15] The abandonment of traditional agriculture and subsequent succession of arable land to woodland represents an additional threat to populations in some regions.

The species requires temporarily open, disturbed ground with sufficient moisture, typically found in traditional agricultural settings.

Modern intensive farming practices often do not provide suitable conditions for the completion of its life cycle, which requires several weeks from spore germination to mature sporophyte development.

[9] Conservation efforts are hindered by limited knowledge of its distribution and population dynamics in many regions, highlighting the need for comprehensive surveys and monitoring programs.

Its single chloroplast per cell (monoplastidic condition) with a pyrenoid-based carbon-concentrating mechanism is unique among land plants, offering insights into the evolution of photosynthetic systems.

The species also forms symbiotic relationships with cyanobacteria, providing opportunities to study plant-microbe interactions that are relatively rare among land plants.

The development of genetic transformation techniques for P. carolinianus has enabled the use of fluorescent proteins to study cellular structures and processes, including the visualisation of organelles such as mitochondria, chloroplasts, and the endoplasmic reticulum.