A SYSTEMATIC NEOICHNOLOGIC LABORATORY STUDY OF THE LIFE STAGES AND AGES OF LIMULUS POLYPHEMUS

A frequent problem confronting paleoichnologists is the inability to associate a particular ichnofossil with a specific organism. Neoichnological studies have attempted to resolve this problem with varying degrees of success. Even when the animal is known, important biological data (e.g. sex, age, size) may not be discernible from the trace. In order to maximize information retrieval from ichnofossils, systematic neoichnological studies covering all life stages of a given organism and their traces need to be undertaken. This paper reports on one such study utilizing laboratory-reared Limulus polyphemus juveniles, young adults and adults.

Laboratory-reared Limulus polyphemus juveniles and young adults of known ages and stages were measured (width, length, telson length, weight) and classed according to width. Individuals were placed on a substrate (sand or muddy sand) under 3 different inundation levels and allowed to move across the substrate for 30 minutes. All movements observed were categorized (e.g. burrowing, walking, gliding) and recorded. At the end of 30 minutes, the crabs were removed and the substrate was allowed to air dry for 48-96 hours. Once dried, a plaster cast was made of the tracks; track features (e.g. width, length, complexity) were identified, photographed, and measured. A second set of trials was conducted wherein 2–8 animals were allowed to move over a substrate at the same time. Additional measurements from these runs include ones that reflect animal interactions (e.g. burrows, intersecting trackways).

The data reveal an exponential relationship between juvenile crab width and track width (r²= .944) for sand tracks with the track casts showing consistent reduction in width relative to the live animal. A more complex relationship between crab width and track width exists for mud tracks. In general, sand tracks preserved unique identifying characteristics more clearly than mud tracks. Burrow depth and width were affected by substrate, animal size, and inundation level. Track complexity varied with inundation level and crab and ranged from short simple straight tracks to elaborate spirals and extensive meandering patterns. This work supports and extends that of Martin and Rindsberg (2007) and allows for enhanced retrieval of biological data from fossil Limulid trackways.