Platysulcus tardus (from Latin platy 'wide' and sulcus 'furrow') is an eukaryotic microorganism that was recently discovered to be the earliest diverging lineage of the Heterokont phylogenetic tree.

The basal body and the transitional region of the flagella lack ring-shaped or helical structures.

They contain extrusomes and a large, flat vesicle surrounding the cytoplasm where the nucleus, mitochondria, and microbodies are found.

[1] Phylogenetic analyses recover Platysulcea as the earliest diverging lineage of Stramenopiles, sister to a clade containing Bigyra and Gyrista.

[3] Ochrophyta Pseudofungi Bigyromonadea Sagenista Opalozoa Platysulcus tardus Despite the phylogenetic results, Platysulcea has been classified as part of the phylum Bigyra.