New Mexico Geological Society Annual Spring Meeting — Abstracts

We document two body fossils of the xiphosurid (horseshoe crab) Euproops danae Meek and Worthen from lacustrine black shale of the Missourian interval of the Beeman Formation near Alamogordo, New Mexico. These specimens, although incomplete (they only preserve the opisthosoma), show diagnostic features that justify assignment to E. danae, including a median tubercle on the third ring of the axial column, a tubercle on the posterior lobe of the axial column and pleural ridges that end in marginal spines. This is the first report of E. danae from the western USA and the first documentation of xiphosurid body fossils in the Pennsylvanian of New Mexico. Euproops danae is a characteristic Carboniferous coal swamp form that is usually associated with freshwater habitats. The paleoecology of the site that yielded the xiphosurid fossils is most likely a freshwater lake deposit. The shale and siltstone layers from which the xiphosurid fossils are derived comprise a 10-meter thick coarsening upward sequence. At the base, a dark, organic-rich shale rests directly on medium-grained, ripple laminated sandstone. The upper parts of the basal sandstone contain foliage of the coniferophyte Cordaites, and there are thin layers enriched in fine-grained sediment that are also mudcracked. The dark shale is clay rich and contains a well preserved flora dominated by cordaitalean foliage, with a moderately diverse assemblage of subsidiary taxa, possibly washed into the deposits, the most common of which mirror Cordaites in being typical of seasonally dry environments: Walchia, Sphenopteridium, Charliea and Taeniopteris. This flora includes a small mixture of more typically wetland plants, such as the pteridosperms Neuropteris, Macroneuropteris, and Alethopteris, calamitalean foliage, and the tree fern foliage, Pecopteris.The flora changes little upward, becoming increasingly fragmentary and allochthonous in character as the sediment becomes both coarser and less organic rich. The deposit terminates in a thin, 3-cm-thick coaly layer that is overlain by 25 cm of ripple-bedded sandstone. The thinly laminated shale (green to black in color)  also contains a low-diversity fauna composed of bivalves, microconchids (spirorbids) and conchostracans, in addition to the xiphosurids. This fauna is not diagnostic of a particular salinity level, but it suggests that conditions were certainly not marine, though the presence of xiphosurids suggests a connection to a marine environment. Conchostracans, in particular, are consistent with conditions of intermittent dryness in the surrounding landscape. There is insufficient lateral exposure to characterize the geometry of this deposit, but the general features – a basal shallow-water, ripple laminated sandstone with evidence of subaerial exposure--suggests a channel with intermittent periods of low water. This channel was abandoned and flooded and may have become a lake, indicated by the organic shales at the base. Its proximity to marine strata (above and below) and the presence of xiphosurids suggests this was a coastal lake. Subsequent filling ensued and a swampy habitat developed at this site as the former channel filled.