The family Cycloposthiidae (Entodiniomorphida) comprises ciliated protists that are symbionts of the gastrointestinal tract of several herbivore mammals, such as rodents, elephants, equids, primates, hippopotamus, marsupials, rhinoceros and tapir, where they contribute to the digestion of their host's plant-based diet. Despite the significance of these ciliates to the evolution of their hosts, many characters used in the taxonomy of the group are homoplastic and most of the valid species do not have molecular data available. For these reasons the systematics of this family is poorly understood. Here, we sequenced the 18S-rDNA of ten cycloposthiids, including nine Cycloposthium spp. and Monoposthium cynodontum, all of them isolated from the cecum of a Brazilian capybara. Our phylogenetic analyses indicate that the family Cycloposthiidae might be polyphyletic, while M. cynodontum and Cycloposthium spp. constitute a single monophyletic group. Given the great morphological and molecular similarities between members of M. cynodontum and Cycloposthium ciliates, it is possible that this species, although it has been described in the genus Monoposthium, is actually a Cycloposthium ciliate.

Cycloposthiidae (Entodiniomorphida) 包括纤毛原生生物，它们是几种食草动物哺乳动物胃肠道的共生体，如啮齿动物、大象、马科动物、灵长类动物、河马、有袋动物、犀牛和貘，它们有助于消化宿主植物 -基于饮食。尽管这些纤毛虫对其宿主的进化具有重要意义，但该组分类学中使用的许多特征是同质的，并且大多数有效物种没有可用的分子数据。由于这些原因，人们对这个家庭的系统学知之甚少。在这里，我们对 10种Cycloposthiids的 18S-rDNA 进行了测序，其中包括 9种Cycloposthium spp。和Monoposthium cynodontum，所有这些都是从巴西水豚的盲肠中分离出来的。我们的系统发育分析表明 Cycloposthiidae 家族可能是多系的，而M. cynodontum和Cycloposthium spp。构成一个单一的单系群。鉴于M. cynodontum和Cycloposthium ciliates成员之间在形态和分子上的巨大相似性，尽管该物种已被描述在Monoposthium属中，但它实际上可能是一种Cycloposthium ciliate。