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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Other possible interpretations:

1) Malacostracan affinity:

Most fossil larvae from the Solnhofen limestone have been identified as malacostracan larvae (see below). The fossil specimen described herein resembles in certain aspects a supposed malacostracan larva from the Solnhofen limestone (Haug et al., 2011a; 2014b, fig. 32.2K). The specimen has been suggested to represent the remains of a shield of a decapod zoea. Could this interpretation also apply to the specimen described here? This is unlikely. The supposed fronto-lateral horns could be interpreted as lateral spines for example of a brachyuran zoea. In such a case we would expect additional spines, especially a rostral spine and a postero-dorsal spine (Wear, 1968; Martin, 1984; Haug et al., 2011a; Martin, 2014a). Also in other decapod zoeas especially a pronounced rostral spine should be expected. No breakage indicators are apparent that could indicate an absence due to preservation. Also the shape of the spines and their blunt tips would be unusual for a zoea larva. Therefore an interpretation of the new fossil as a zoea appears unlikely to us. Notably, already Haug et al. (2014b, p. 176) stated that the “systematic affinities remain uncertain until better-preserved specimens are found”. The specimen from Haug et al. (2014b) could in the light of the new fossil described here also represent the conspecific cypris larva. The specimen should be reinvestigated for this aspect.

2) Branchiopod affinity:

There is also a certain resemblance of the fossil to the nauplius larva of representatives of Laevicaudata, an ingroup of Branchiopoda. These have a kind of spine-like extensions that represent the still immobile antennulae (Olesen, 2005). In contrast to larval representatives of Laevicaudata, in which these horns protrude from the ventral side of the head (Olesen, 2005; 2007), it seems that the horns in the fossil specimen described herein protrude from the dorsal side of the head shield, indicated by the relative position of the appendages (Fig. 1). Additionally, laevicaudatan nauplii have a distinct triangular shape of the anterior head which should be expected to be seen in the fossil if present. Yet, this is not the case. Also other characteristic features, such as a large, rounded labrum or caudal lobes, which are spine-like extensions posterior from the shield (Olesen, 2005; 2007) are not present in the fossil specimens. Yet, these could be more difficult to be visible, as the labrum is a soft ventral structure and the caudal lobes are comparably small. Lastly, most branhciopods are fresh water forms, only few groups of raptorial cladocerans have re-entered the marine realm, yet the original lagoons of the Solnhofen lithographic limestones must have represented a marine environment. Thus, a laevicaudatan or even a branchiopod affinity is very unlikely.

Summarizing: From the morphological point of view it seems likely that the here described fossil indeed represents a cirripede nauplius. It appears to possess a kind of floating collar that may point to a closer relationship to rhizocephalan cirripedes. The “main” shield would then measure about 3 mm and could molt into a cypris larva of the size as it is known for the fossil Rhamphoverritor reduncus with 4 mm length (Briggs et al., 2005). While the new larva is well in a possible size range for cirripede larvae, it clearly represents a giant form.