DETERMINING THE CHARACTERISTICS OF PALEOLIMULIDS FROM PRESERVED TRACKWAYS

In a previous study, laboratory-reared Limulus polyphemus juveniles and young adults of known ages and stages were measured (width, length, telson length, weight), placed on sand or muddy sand and allowed to freely move about for 30 minutes after which they were removed from the sediment and placed back in their tanks. The tracks and burrows resulting from these movements were measured (length, width, telson depth, complexity etc.) in both fresh sediments and casts and related back to the morphometrics of the individuals that made them. The results of this study demonstrated the existence of an exponential relationship between juvenile crab width and track width in sand and a more mathematically complex relationship between crab width and track width in mud. Unique identifying characteristics of the tracks were more clearly preserved in sand relative to mud and it was noted that both the depth and width of burrows were affected by substrate, animal size, and inundation level. The complexity of tracks was observed to vary with inundation level, as well as with individual organisms.

The study reported on here extends the aforementioned work into the fossil record. Fossil limulid trackways and trails obtained from the collections within the Yale Peabody Museum and the American Museum of Natural History were examined and measurements were taken on trace fossil width, half-width, telson trace depth, length and complexity. This data was then directly compared to the data previously obtained from the extant traces in order to derive biological date (i.e. relative age, size, weight, behavior) and geological data (state of inundation) about the original organism and the paleoenvironment at the time of trace formation.

The results of the present study confirm that modeling the relationship between limulid trace morphometrics and animal morphometrics (using a modern analog species) is effective for deriving and assigning biological characteristics to the missing organism that made the preserved tracks. Additionally, it should be noted that the determined size of all of the animals from the tracks falls below the sexually mature adult size of modern limulids.