THE ORIGIN OF HORSESHOE CRABS: DISCOVERY OF THREE-DIMENSIONALLY PRESERVED XIPHOSURAN SPECIES IN THE LOWER TREMADOCIAN (LOWER ORDOVICIAN) OF THE GREAT BASIN, WESTERN USA

Two new species of stem-group xiphosurans (horseshoe crabs), together comprising a new genus, represent the earliest xiphosurans, and earliest crown group euchelicerate body fossils, yet found. The species occur in separate stratigraphic intervals of the late Skullrockian (Early Ordovician; early Tremadocian) Red Canyon Member of the House Formation in the southern House Range, Ibex area, western Utah. Remarkably, sclerites of the xiphosurans are preserved in three dimensions in limestones and can be freed via digestion of the rock with hydrochloric acid. This is due to replacement of the original organic cuticle by a combination of clay minerals and silicon oxide. Morphologically, the new genus is of strikingly modern aspect and compares closely with Lunataspis from Manitoba and Ontario, Canada, despite the much younger (Sandbian–Katian; Late Ordovician) age of that taxon. The two new species are represented by multiple well preserved prosomal shields and associated mesosomal thoracetrons comprising seven fused tergites. By comparison to Lunataspis, it may be expected that posterior to the mesosoma there were three articulated metasomal segments and a telson, but these elements have not yet been found. The xiphosurans occur along with rich and well preserved silicified trilobite faunas. These trilobite associations turn over very rapidly through the section and permit a highly resolved species level biostratigraphy. Xiphosurans have been found at seven stratigraphic horizons assigned to four trilobite zones, but the two new species are stratigraphically distinct, with no overlapping occurrence. The inferred environment of deposition is shallow subtidal (within fairweather wave base) open marine, and the rocks are predominantly lime mudstones, calcisiltites, and inter-rudites (flat-pebble conglomerates). Less well preserved xiphosuran sclerites with similar preservation have also been recovered from coeval (dated by shared trilobite species) strata of the Garden City Formation in southeastern Idaho, some 500 km distant, suggesting that this mode of preservation may be widespread along the northern Laurentian margin in the earliest Ordovician.