CLASSIFICATION OF SHRIMP FOSSILS (DECAPODA: DENDROBRANCHIATA, CARIDEA): PROBLEMS AND PROGRESS

The fossil record of shrimp is well-known, due in part to beautiful specimens from plattenkalks of the Jurassic of Germany and Cretaceous of Lebanon. Higher classification of extant shrimp depends heavily on mouthparts and reproductive structures, rarely preserved in fossils. Chelae type and number are also important, but terminations of chelipeds are often missing in fossil specimens. Superficially complete specimens often reveal partial preservation of important features such as pereiopods, antennae, rostra, and telsons upon close examination. Carapace ornamentation is often missing or crushed; in addition, the carapace may be compressed over the internal skeleton and other internal structures, creating phantom features. Fracturing and twisting of the carapace and pleon, which are poorly calcified in shrimp, further obscures morphology.

Examination of diagnostic characters for extant shrimp indicates that many caridean families can be eliminated from consideration for fossils due to the unusual and distinctive shapes of their antennae and pereiopods. Thus, most fossil carideans are placed within Palaemonidae, one of the largest extant families and one that lacks distinct specializations other than well-developed first and second chelae. Within the dendrobranch shrimp, most fossils are placed within Penaeoidea, which like palaeomonids, are characterized by few specializations. Notable exceptions include a fossil member of Luciferidae, a highly derived dendrobranch shrimp, and Carpopenaeus, which exhibits well-preserved multi-articulate pereiopods not otherwise seen in dendrobranchs.

Experiments involving burial and rotting of recent shrimp in simulated freshwater and marine environments indicate that pereiopod terminations (claws) degrade relatively quickly and are easily broken from the specimens. Rostra often break at the tip. Decapods buried in simulated freshwater environments disappear completely, except for robust chelae when present, after a few weeks. Presence of calcium in cuticle is greatly reduced after extended burial as compared to control specimens, suggesting that calcite in the cuticle breaks down quickly. Thus, preservation of shrimp must occur very quickly after death to preserve important morphological details useful for classification.