First Report of a Pre-Molt Isopod (Crustacea) from the Cretaceous of Lebanon

A single specimen, part and counterpart, of a cirolanid isopod preserved in lithographic limestone of Cenomanian age from the Sahel Alma quarry, Lebanon, preserves remarkable features documenting the late pre-molt stage in the life cycle of the animal. Isopods molt in two separate intervals referred to as biphasic molting. The posterior part of the pereon and the entire pleon and pleotelson are cast off first, usually as a single unit which is commonly preserved in the fossil record. Subsequently, the head and anterior part of the pereon are shed, typically fragmented so that preservation potential is slight. Entire fossil isopod fossils support the interpretation that they are remains of a corpse, not a molt. The Lebanese specimen is unique for several reasons. It is entire and is preserved exposing the venter, rather than the dorsum. Although the prehensile, anterior pereiopods are not preserved, the posterior, ambulatory pereiopods are well exposed. Three pairs of dark, opaque, calcareous dermoliths lie on either side of the midline of pereonites 2-4. These structures, best known from terrestrial isopods, form during the pre-molt stage by resorption of calcium from the cuticle, storing calcium to mineralize the newly-molted posterior regions. They are less well known from extant marine isopods and have never been reported from the fossil record. The large-faceted compound eyes are visible from the ventral surface, indicating that the carapace was decalcified to the point of being transparent. To our knowledge, this represents the first report of a fossil pre-molt isopod and the first report of fossil dermoliths, suggesting that the molting behavior of Cretaceous isopods was similar to that of extant forms.