MORPHOLOGY AND ONTOGENY OF THE PENNSYLVANIAN HORSESHOE CRAB PALEOLIMULUS SIGNATUS

Horseshoe crabs (Xiphosura), have long been a textbook example of the disputed living fossil phenomena, as they appear in the Ordovician and have not significantly changed in their appearance. Numerous Xiphosuran specimens have been reported from Pennsylvanian and Permian rocks of Kansas, resulting in multiple insights on the diversity of these organisms. Recently, remarkably preserved specimens of Paleolimulus signatus (Beecher) have been retrieved from the uppermost Pennsylvanian Pony Creek Shale Member (known as the Pony Creek Shale Lagerstätte) of the Wood Siding Formation in Wabaunsee County, Kansas. These specimens have been interpreted to have lived on a tidal flat or estuarine setting; nearby Permian specimens are interpreted to be from freshwater localities. This locality has supplied more than 140 specimens, comprising internal and external molds. These largest is an isolated prosoma 89 mm wide and the smallest prosoma is 19 mm wide. Due to the large number of well-preserved specimens we recovered a near linear, putatively ontogenetic trend (that unfortunately lacks larval stages). This trend differs from the growth of the genus Euproops which portrayed drastic growth between stages. The linear relationship in Paleolimulus can provide insight into the evolutionary and taxonomic relationships of the extant suborder Limulina. 35 specimens happened to have well-preserved appendages and/or book gills suitable for measurement, which is typically not found in fossil xiphosurans. Our measurements show that the length and width of the appendages have similar rates of growth throughout the various ontogenetic stages, and thus change little in appearance from juvenile to adult specimens. Understanding the differences in ontogenetic variation of appendages in various xiphosurans can offer important insight into the taxonomic classification of these animals and also how their ontogeny evolved through time.