A Carboniferous Synziphosurine (Xiphosura) From the Bear Gulch Limestone, Montana, Usa

Abstract

A new synziphosurine, Anderella parva gen. et sp. nov., extends the known range of this group from the Silurian to the Carboniferous and is the youngest known so far from the fossil record. Previously the youngest synziphosurine, Kasibelinurus, was from the Devonian of North America. Anderella parva has a semi-oval carapace with pointed genal regions, nine freely articulating opisthosomal segments and a long styliform tail spine. It is the third xiphosuran genus to be described from the Bear Gulch Limestone and its discovery highlights this deposit as containing an unusually diverse xiphosuran biota. It is also the only known locality where synziphosurines occur alongside the more derived xiphosurids. Xiphosurans reached their greatest diversity in the Carboniferous when the xiphosurids began to occupy brackish and freshwater habitats and became dominant over the synziphosurines. The discovery of the only known Carboniferous synziphosurine in marine sediments may indicate their inability to exploit these same environments.

An unusual euchelicerate linking horseshoe crabs and eurypterids, from the Lower Devonian (Lochkovian) of Yunnan, China

“…), while that of their supposed stem lineage ranges from Ordovician to Carboniferous (Moore et al . ; Van Roy et al . ).…”

Revised systematics of Palaeozoic ‘horseshoe crabs’ and the myth of monophyletic Xiphosura

“…This congruence between stratigraphic and phylogenetic sequence was upset, however, by the discovery of an unequivocal xiphosurid from the Upper Ordovician of Manitoba, Canada (Rudkin, Young & Nowlan, ), followed by further xiphosurid reports from the Lower Ordovician of Morocco (Van Roy et al ., ). The stratigraphic range of the synziphosurines has also been expanded, with the youngest representative now known from the Carboniferous (Moore, McKenzie & Lieberman, ) while the oldest occurs in strata slightly younger than that of the undescribed xiphosurid in Morocco (Van Roy et al ., ). It is clear that the synziphosurines described to date are at best offshoots of the xiphosurid stem and not the direct ancestors of xiphosurids as inferred by Anderson & Selden () for a few taxa.…”

Silurian horseshoe crab illuminates the evolution of arthropod limbs

“…Synziphosurines are a paraphyletic group low on the stem of Xiphosura with an unfused opisthosoma composed of nine to 11 segments (4). Synziphosurines also first appear as an undescribed taxon in the Lower Ordovician of Morocco, although they likely originated even earlier (5), and range into the Lower Carboniferous (Namurian) (6). The interrelationships of synziphosurines are uncertain: limbs are only known in the Silurian Offacolus kingi from Herefordshire, England (7,8), here shown to be a synziphosurine, and Venustulus waukeshaensis from Waukesha, Wisconsin, USA (9), and the Devonian Weinbergina opitzi from Hunsrück, Germany (10).…”