Contributions to Zoology, 86 (3) – 2017Christina Nagler; Jens T. Høeg; Carolin Haug; Joachim T. Haug: A possible 150 million years old cirripede crustacean nauplius and the phenomenon of giant larvae
Discussion

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A possible interpretation of the fossil

Although the specimen is small in comparison to other fossil larvae, at least from this Lagerstätte, and may not appear to bear many details, some of these details that are present allow a well-founded interpretation on the identity of the specimen. Texture and fluorescence capacities of the fossil resemble crustacean remains from the same deposits. Also from a structural point of view many interpretations that could come into mind, such as a fish scale, can be easily discarded. Specimens distantly resembling the fossil have been interpreted as possible remains of crustacean larvae (Haug et al., 2011a; 2014b). This seems also a likely interpretation of the new fossil.

When comparing the specimen to small-sized eucrustaceans it shows similarities to larval forms of barnacles and their relatives (Cirripedia). The pelagic larvae of cirripedes (nauplius larvae) are characterized by a pair of spine-like extensions of the anterior shield region, generally termed fronto-lateral horns (Høeg, 1987; Walker, 1992, Høeg and Møller, 2006; Pérez-Losada et al., 2009; 2012; Høeg et al., 2015). Historically, these fronto-lateral horns are an important character that was first recognized by Thompson (1830). For a long time these structures were the only argument for the monophyly of Cirripedia (Høeg et al., 2015). Shape and relative position of the two spine-like extensions of the fossil (Figs. 1D, F, H, 2A) strongly resemble these fronto-lateral horns (Fig. 2B–E).

FIG2

Fig. 2. Fossil and modern cirripede nauplii. A) Reconstruction of the fossil nauplius (center) and size comparison to modern counterparts (in circles). B) Model of a modern cirripede nauplius, not to scale. C) Macro-photography under cross-polarized light of modern lepadomorph nauplius (MNHN IU-2014-5478), lateral and dorsal view. D) Scanning electron microscopic photography of a modern rhizocephalan nauplius, please note the floating collar, lateral and ventral view. E) Fluorescence photography of modern balanomorph nauplius, stereo-projected (left, please use red-cyan glasses) and colour-marked version (right), dorsal view. Abbreviations: atl = antennula; ant = antenna; fc = floating collar; fh = fronto-lateral horn; md = mandible; tr = (initial) trunk.

The preserved presumed appendage remains of the fossil would also well fit into this interpretation. Cirripede nauplii have three functional pairs of appendages: antennulae, antennae and mandibles (Fig. 2B–E; Chan et al., 2014; Høeg et al., 2014a; b; Kolbasov et al., 2014).

The second structure protruding from underneath the shield of the fossil specimen (Fig. 1D, F) strongly resembles the setose swimming exopods of antennae or mandibles of modern cirripede nauplii (Fig. 2B–E; e.g. Walossek et al., 1996). Due to the number of ringlets and setae, the structure on the fossil could represent an antenna, although an interpretation as a mandible cannot entirely be excluded.

The appendage remain on the other side of the fossil specimen (third structure; Fig. 1D) could represent the less well preserved antenna of the other body side, although it remains unclear whether it could then represent the endopod or the exopod. The further anterior, very incomplete appendage (first structure, Fig. 1D) is more difficult to interpret. The distinct ringlets could be understood as another exopod. The position would argue more for an interpretation as an antennula, yet, an antennula would not be organized into such discrete ringlets. In conclusion, the observed structures are compatible with the interpretation of the fossil as a cirripede nauplius.