Early fossil record of Cirripedia
Cirripedes have a comparably good fossil record, at least concerning their adults. Rhamphoverritor reduncus from the Silurian (420 mya) is exceptional as only a possible cypris larva and a juvenile are known (Briggs et al., 2005). The species most likely represents the sister group to all other cirripedes (Høeg et al., 2009). There is generally a distinction of three groups within Cirripedia: Acrothoracica, Thoracica and Rhizocephala, with the latter two groups representing sister groups. The monophyly of each of the three groups is generally well supported. Yet, Thoracica is not as well characterized by morphological characters. It is therefore possible that any pedunculated fossil barnacle older than the presumed split between Rhizocephala and Thoracica (see below) might be situated phylogenetically below this point.
Representatives of Acrothoracica have been reported as trace fossils from the Devonian (380 mya; Glenner et al., 1995). Molecular analyses give support for the origin of Thoracica in the Early Carboniferous (340 mya; Pérez-Losada et al., 2008). Based on the reconstruction of a co-evolution between rhizocephalans and anomalan crabs and molecular reconstructions of thoracican barnacles, representatives of Rhizocephala have been estimated to be present also since the Carboniferous (Walker, 2001; Boyko and Williams, 2009). As a consequence, all pedunculated fossil thoracicans older than 340 million years could be considered as representatives of the unnamed sister group to Acrothoracica. The first more direct fossil indications of rhizocephalans are feminized male crabs from the Miocene (4–23 mya; Feldmann, 1998). Also important to mention in this aspect: fossils of cirripedes are well known to occur in the lithographic limestones of southern Germany (Barthel et al., 1990; Nagler et al., 2017).
With this fossil record an interpretation of the here described fossil as the nauplius of a cirripede and even as a possible relative of Rhizocephala seems reasonable; at least it is not contradicted. The fossil, therefore, most likely represents the first fossil record of a cirripede nauplius. It also follows the general pattern that we seem to be more likely able to find especially giant larval forms as fossils.