Data from: The evolution of annelids reveals two adaptive routes to the interstitial realm

Many animals permanently inhabit the marine interstitium, the space
between sand grains [ 1, 2 ]. Different evolutionary scenarios may explain
the existence of interstitial animals [ 3, 4 ]. These scenarios include
(1) that the interstitial realm is the ancestral habitat of bilaterians [
5, 6 ], (2) that interstitial taxa evolved from larger ancestors by
miniaturization, or (3) progenesis [ 3 ]. The first view mirrors the
former hypothesis that interstitial annelids, called archiannelids, were
at the base of the annelid radiation [ 7 ]. Based on morphological data,
however, progenesis is generally favored for interstitial annelids today [
3, 4, 8 ]. Herein, our phylogenomic approach revealed that interstitial
archiannelids are robustly placed into two groups nested within the
annelid tree. Evolution of the first group comprising among others
Dinophilidae is best explained by progenesis. In contrast, the second
group comprising Protodrilida and Polygordiidae appears to have evolved by
stepwise miniaturization adapting from coarser to finer sediments. Thus,
in addition to progenesis [ 3, 4 ], miniaturization, thought to be too
slow for an adaptation to the interstitium [ 3 ], is an important second
route allowing adaptation to interstitial environments. Both progenesis
and miniaturization should be considered when investigating evolution of
interstitial taxa [ 1, 3 ].