Fonticula is a genus of cellular slime mold which forms a fruiting body in a volcano shape. As long ago as 1979 it has been known to not have a close relationship with either the Dictyosteliida or the Acrasidae, the two well-established groups of cellular slime molds. In 1979, Fonticula was made a new genus of its own due to the unique characteristics of its fruiting body, with only one species: Fonticula alba.

The life cycle of Fonticula alba alternates between an amoeboid vegetative stage and aggregative fruiting stage. The fruiting body of the genus has a unique shape, as its sorocarp resembles a volcano and sorus looks like a ball of hot lava emerging from that volcano.

Molecular phylogenies have found alignments in genes of Fonticula alba to subgroups in Opisthokonta. A 2009 study has found that Fonticula is the sister taxa to Nuclearia, thus making it related to the kingdom Fungi.

Fonticula, Nuclearia, and Fungi have been united into the Holomycota, which is sister to the Holozoa.

While working at the University of Wisconsin in 1979, Ann Worley, Kenneth Raper and Marianne Hohl discovered an organism that fit no recognized genus of the slime mold taxon Acrasiomycetes. At the time Acrasiomycetes could be divided into two classes: Acrasidae and Dictyostelidae. This classification was based on morphological characteristics, but it is now known that the two subgroups are not closely related. Moreover, F. alba did not truly fit into either of these subclasses but shared a few characteristics of both. Although F. alba shared characteristics of subclasses within Acrasiomycetes, Worley et al. (1979) were convinced that its best fit taxonomically would be in a new, undescribed family designated as Fonticulaceae, which would then contain the genus Fonticula. The new genus name Fonticula is a reference to the fruiting-body morphology: Fonti- from the Latin word Fons (fountain, "shape, form") and -cula, from Latin diminutive culus (little, "size").

Opisthokonta is an exceptionally diverse eukaryotic group, containing shared ancestry between fungi, animals and even a few protists (Brown et al., 2009). In 2009 it was concluded that the genus Fonticula is part of the unranked group Opisthokonta. Brown et al. (2009) sequenced nuclear encoded genes of Fonticula alba for phylogenetic analysis and concluded that the genus was a sister group to the filose amoebae in the genus Nuclearia and that the Fonticula and Nuclearia clade are sister groups to fungi.^{[citation needed]}

The morphological characteristics of genus Fonticula are unlike those seen in slime mold subgroups Acrasidae or Dictyostelidae. Several studies have found that in its vegetative state, myxamoebae of F. alba are generally small and irregular in form, ranging from 8–12 x 6–10 μm in size. The myxamoebae have finger-like projections deemed as filose pseudopodia, which extent at the posterior or lateral ends of the cell. Worley et al. (1979) found that the myxamoebae had a distinguishable ectoplasm and endoplasm. The clear ectoplasm is on the outer edges, while the inner endoplasm is more granular. Vacuoles are also found in numerous digestion stages in actively feeding F. abla. These small vacuoles contain bacteria. In active feeding stages, there is a slime coat surrounding the myxamoeba to which bacteria sink onto. The ultrastructure of Fonticula also includes small contractile vacuoles, which are mainly deposited towards the posterior end of the cell. An ultrastructural feature that is shared between Fonticula and certain Acrasidae is mitochondria with discoid cristae. The golgi apparatus lends a helping hand in the fruiting stage in Fonticula as numerous dictyosomes are involved in the sorogenesis process. Cells in the genus are generally uninucleate, however there have been cases of some cells containing two or even three nuclei. The nucleus of F. alba cells have an inconspicuous nucleolus under the light microscope. The fruiting body of F. alba contains an unbranched sorocarp, which is composed of upright tapered stalks which apically bear a round source containing spores. Stalks range from 200–500 μm in length. The sori on the fruiting body are white, and roughly 200–350 μm in diameter. Spores are cystic in shape, and roughly 5.0–6.0 μm in diameter.

Multigene phylogenetic analysis was conducted in F. alba in 2009 which allowed it to be placed into Opisthokonta. The five nuclear encoded genes that were sequenced were: small subunit ribosomal RNA (SSU rRNA), actin, beta-tubulin, elongation factor 1-alpha (EF1-a) and heat shock protein 70 (HSP70). The study's results depicted that from 42 eukaryotic taxa, many alignments were made with a total of 2802 aligned sequences (Brown et al., 2009). Phylogenetic trees based on the molecular genetics sequenced in this study showed that the genus Fonticula was a sister taxon to the genus Nuclearia. Those two sister taxa as a clade are in turn a sister taxa to Fungi. Fonticula therefore represents the first evolution of an organism with a cellular slime mold-like morphology within the broad-group Opisthokonta (Brown et al., 2009); (Brown, 2010).