Thecofilosea

Thecofilosea is a clade or monophyletic group. It belongs to the subphylum Monadofilosa, a group of cercozoan classes that evolved after the early divergence of Reticulofilosa. Thecofilosea is the only group of testate cercozoan amoebae with ancestrally organic scale-lacking shells, while all the Imbricatea (to which the scale-bearing Kraken belongs) have non-organic silica scaled tests. The Thecofilosea appear to have evolved from the Imbricatea, a group previously considered monophyletic but based on new analyses seems paraphyletic to Thecofilosea. Their common ancestor carried scales in its theca and no organic cement, a trait lost in Thecofilosea in favour of an organic theca without scales.^[3][4][5]

An earlier analysis that recovers the Kraken within Sarcomonadea, a group of naked amoebae, proposes the unlikely possibility that not only the common ancestor of Imbricatea+Thecofilosea, but also the common ancestor of all Filosa, already bore inorganic silica scales in its test. In this scenario, both Thecofilosea and Sarcomonadea would have lost their scales independently.^[6]

The class Thecofilosea was created in 2003 by protozoologist Cavalier-Smith to accommodate the testate orders Cryomonadida and Tectofilosida. They were first described as uninucleate cells surrounded by an organic flexible tectum (in the case of Tectofilosida) or a rigid test (in Cryomonadida) with one or two apertures for filopodia, either two flagella or none, and tubular mitochondrial cristae.^[7] It was later emended in 2011 by the same author, to include Ventricleftida, Ebriida and Phaeodaria.^[2] Finally, the elusive Mataza was added to the class in its own order Matazida.^[8]

Two different classification schemes are in place, despite containing the same orders.