Protodriliformia is a clade of small marine

polychaete Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are ...

s, comprised by the groups of

meiofauna Meiobenthos, also called meiofauna, are small benthic invertebrates that live in both marine and fresh water environments. The term ''meiofauna'' loosely defines a group of organisms by their size, larger than microfauna but smaller than macrof ...

l interstitial worms

Protodrilida Protodrilida is an order of polychaetes belonging to the class Polychaeta. Families: * Protodrilidae Czerniavsky, 1881 * Protodriloididae ''Protodriloides'' is a genus of polychaetes belonging to the monotypic family Protodriloididae. The spec ...

and Polygordiidae, formerly considered " archiannelids". It is the most basal clade of

Errantia Errantia is a diverse group of marine polychaete worms in the phylum Annelida. Traditionally a subclass of the paraphyletic class "Polychaeta", it is currently regarded as a monophyletic group within the larger Pleistoannelida, composed of Erra ...

Evolutionary history

Phylogenetic analysis In biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that pro ...

of annelids has found Protodriliformia to be the earliest diverging clade of

. At the same time, the other half of "

archiannelid Haplodrili, or Archiannelida, is an order of primitive polychaete worms. Zoologist Ray Lankester gave it the name haplodrili, while zoologist Berthold Hatschek later named it Archiannelida. Once considered to be a class under Annelida, and eve ...

" worms,

Orbiniida Orbiniida is an order of small polychaete worms in the phylum Annelida. It is the earliest diverging clade in Sedentaria. Along with Protodriliformia (in Errantia), this order is composed of meiofaunal marine worms formerly known as " archian ...

, was found to be the earliest diverging clade of

Sedentaria Sedentaria is a diverse clade of annelid worms. It is traditionally treated as a subclass of the paraphyletic class Polychaeta, but it is also a monophyletic group uniting several polychaetes and the monophyletic class Clitellata. It is the sist ...

. The

convergence Convergence may refer to: Arts and media Literature *''Convergence'' (book series), edited by Ruth Nanda Anshen * "Convergence" (comics), two separate story lines published by DC Comics: **A four-part crossover storyline that united the four Weir ...

between these two groups occurred through

progenesis Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the physiological, or somatic, development of an organism, typically an animal. Neoteny is found in modern humans compared ...

and

miniaturization Miniaturization ( Br.Eng.: ''Miniaturisation'') is the trend to manufacture ever smaller mechanical, optical and electronic products and devices. Examples include miniaturization of mobile phones, computers and vehicle engine downsizing. In e ...

, as a way to adapt to the marine interstitial ecosystem between sand grains (i.e. interstitium). This means that the

larval A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. T ...

or juvenile stages of a larger pleistoannelid ancestor temporarily inhabiting the interstitium stopped their somatic growth, became

sexually mature Sexual maturity is the capability of an organism to reproduce. In humans it might be considered synonymous with adulthood, but here puberty is the name for the process of biological sexual maturation, while adulthood is based on cultural defini ...

and stayed in the interstitium permanently.

References

Annelid unranked clades Polychaetes {{Annelid-stub