MORPHOLOGICAL VARIATION IN THE XIPHOSURID SPECIES LIMULITELLA BRONNII

Horseshoe crabs (Xiphosura) are aquatic chelicerates that inhabit modern marine environments. Xiphosurans have an evolutionary history extending to the Early Ordovician, 480 million years ago, and have occupied a variety of marine and non-marine habitats on every continent. Although over 70 species of horseshoe crab are now known, only a handful are represented by more than a few specimens, making analysis of ontogenetic changes and phenotypic variation within populations challenging. Limulitella bronnii, found in the Triassic Grès à Voltzia formation in France, is known from multiple individuals. Therefore, it provides an excellent opportunity to study variation within an early Mesozoic horseshoe crab. Limulitella bronnii is a member of Austrolimulidae, one of two clades of extinct horseshoe crabs that occupied non-marine environments, for which ontogenetic data is generally lacking. Here, we present the first detailed analysis of the Limulitella bronni specimens and diagnose recognizable morphotypes within the species.

Eleven specimens were analyzed using the measuring software ImageJ. Thirteen prosoma measurements and nine thoracetron measurements were obtained, totaling 22 sections measured. Measurements included prosoma length and width, opisthosoma dimensions, length of movable spines, and a variety of other morphological characteristics. Two Principal Component Analyses (PCA) were performed using the R function prcomp. Data were logged in order to reduce the effect of size on subsequent analyses. Measured data input into Excel was converted into a .csv file, then separated by prosoma and thoracetron measurements. Data visualization was performed using the R package ggplot2. Comparison of the PCA plots indicates four clusters of distinct morphotypes labeled as A, B, C, and D. High rates of body size variance are indicated on PC1; morphotypes A and B in particular show what might be a linear growth trajectory indicating that ontogeny may be responsible for at least some of these morphotypes. The resulting PCA provides a framework of ontogenetic research into Limulitella bronnii and further advances our knowledge of sexual dimorphism as well as possible morphological variance between freshwater and brackish ecologies.