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

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Description of the specimen

The specimen has a maximum length of 4.7 mm. The main preserved part is an oval-shaped shield-like structure, with smaller structures protruding from it. This shield represents the maximum length of the specimen and has a maximum width of 3.1 mm.

The texture, color and fluorescence capacities of the shield (and partly the protruding structures) resemble that of crustacean cuticle from the same Lagerstätte (which is different from most remains of fish, insects, echinoderms or molluscs). Different regions of the shield can be differentiated. A very central region is apparent in the color images as a darker area (Fig. 1A–B). This same area is also elevated in relief (Fig. 1C) and shows a stronger fluorescence (Fig. 1D). This region most likely represents the main body, partly compressed through the shield.

FIG2

Fig. 1. Different photographic methods applied to the fossil specimen (SMNS 70409). A) Macro-photography under cross-polarized light. B) Like A, but with optimized histogram. C) Stereo-photography, please use red-cyan glasses. D) Highlighted version of C. E) Fluorescence photography. F) Highlighted version of E. G) Detail of appendage. H) Detail of horn. Abbreviations: ant? = antenna (orange); atl? = antennula (orange); b = body under the shield (dark blue); fc? = floating collar (green); fh = fronto-lateral horn (blue); s = shield rim (light blue).

The central region extends latero-posterior and posterior into a thinner-appearing region. It is almost transparent under cross-polarized light; the matrix is visible (Fig. 1A–B). It appears to lack relief (Fig. 1C) and also shows a weaker fluorescence (Fig. 1D). Central region and extended region together are about 3.7 mm long and 2.5 mm wide.

Around the central region and the extended part of the shield a well set-off, ring-like region is apparent. It is set off from the central shield, i.e. in the anterior region by a distinct edge. In the posterior region the differentiation against the extended region is apparent due to a dark color of the ring (Fig. 1A–B), a slight positive relief and stronger fluorescence capacities.

The central shield bears a pair of spine-like protrusions. These spines originate antero-lateral from the edge between the central shield region and the ring. They are oriented mostly lateral, slightly anterior. They curve slightly backwards. The protrusions are slightly bellied proximally, but taper distally (Fig. 1H). The tip appears blunt; it is unclear whether this is the original condition or due to preservation. The protrusions reach slightly beyond the ring (Fig. 1A–D).

Three additional structures protrude from under the shield. The first is far anterior, also anterior to the spine-like protrusions. It is a short structure, more or less rectangular in outline. Originally, this was most likely a tube-shaped part of an appendage. It can be differentiated into two similar-appearing elements, most likely representing ringlets. Each of them bears a seta pointing antero-median. The structure can only be observed under cross-polarized light, it does not possess recognizable relief (Fig 1C) nor does it show fluorescence (Fig. 1D). It therefore differs from the preservation of the shield. The color is more orange and less glossy. Most likely it is not phosphatized (lack of fluorescence).

The second structure protrudes from under the shield laterally towards the latero-posterior (Fig. 1). The structure is preserved in different ways. Some areas resemble the preservation of the first protruding structure, show no fluorescence and appear orange. Other areas appear to be phosphatized (certain glossiness) and show fluorescence. Lastly, some areas are not at all apparent under cross-polarized light, but only under fluorescence. The central part of the structure appears elongate, originally tube-like, composed of several elements (at least eight), originally ringlets (Fig. 1G). More distal elements are narrower than more proximal ones. Also more distal ones are slightly oblique towards the main axis of the structure, as the anterior (originally median?) dimension of each ringlet appears to be slightly longer than the posterior (originally lateral?) one. The supposed median sides of each ringlet appear drawn out into setae. The more proximal ones appear to bear a pair of setae, while the more distal ones appear to bear only a single seta. Ringlets are preserved more pronounced; their edges appear to be also phosphatized. Setae are only visible under fluorescence, especially the more distal regions of the setae (Fig. 1F–G). The overall organization of the structure resembles a multi-annulated exopod.

The third structure is preserved at an area, where apparently a part of the shield is broken off, with this revealing the structure, which would have been otherwise concealed. The preservation is rather weak, the structure only being apparent under fluorescence (Fig. 1E–F). It is elongate, most likely originally tube-shaped, tapering distally. It is curved, or partly folded or kinked. Proximal and distal region are both concealed by the shield. The surface appears to some degree granulose, with weakly outlined rings. Possibly the structure was rather weakly sclerotized originally, not being subdivided into discrete sclerites.