ONTOGENY OF THE PENNSYLVANIAN HORSESHOE CRAB PALEOLIMULUS SIGNATUS (BEECHER) FROM THE PONY CREEK SHALE KONSERVAT-LAGERSTÄTTE, EASTERN KANSAS

Xiphosurans (horseshoe crabs) are a temporally long-ranging group of aquatic arthropods, originating by, the early Ordovician and reaching peak diversity in the Late Paleozoic. Xiphosurids are known from several deposits in eastern Kansas and have yielded important information about the diversity and ecology of these animals. The Uppermost Pennsylvanian Pony Creek Shale Member of the Wabaunsee Formation, south of Maple Hill (Wabaunsee County, Kansas), preserves some of the most exceptionally preserved specimens of Paleolimulus signatus. Paleolimulus signatus represents one of the earliest species of the suborder Limulina (the group that includes extant horseshoe crabs) and ranges from the Uppermost Pennsylvanian to the Permian. The specimens at the Maple Hill locality are interpreted to have lived on a tidal flat or in an estuary setting, but Permian specimens found in nearby localities are known from freshwater deposits.

Recent collections increased the number of known specimens from 24 to 122, one of the largest collections of horseshoe crabs from a single outcrop. The mostly complete specimens (most do not preserve the telson spine), range in size from 20 to 61 mm in length. However, the largest known specimen (89 mm wide and 60 mm long), an isolated prosoma, suggests that P. signatus could reach a length of at least 94 mm without the telson spine. Most specimens represent mostly complete internal or external moulds and several of the specimens preserve book gills and appendages. The excellent preservation provides insights into morphological changes throughout the growth of the species. Analyses of 48 specimens with complete prosoma, opisthosoma or both show a linear trend. The width to length ratio of the prosoma increases over time, while that of the opisthosoma decreases. The size differences are interpreted as ontogenetic stages rather than sexual dimorphism and represent a near complete growth series, with exception of the larval stages. Both the prosoma and opisthosoma show a considerable ontogenetic variation, this differs from the ontogeny of Euproops, which shows major changes in the shape of the opisthosoma through its growth series. Understanding the ontogenetic variation in xiphosurans offers important insights in the taxonomic classification of these animals and their development through time.