EXCEPTIONALLY PRESERVED MICKWITZIA FROM THE INDIAN SPRINGS LAGERSTÄTTE (CAMBRIAN STAGE 3), NEVADA AND IMPLICATIONS FOR EARLY CAMBRIAN BRACHIOPOD EVOLUTION

The origins and early evolution of Brachiopoda can be traced back to the plethora of scleritome and brachiopod-like taxa of the early Cambrian. Most conspicuous have been members belonging to classic Lagerstätten localities such as the Burgess Shale and Chengjiang. Exceptionally preserved specimens of the Cambrian stem-group brachiopod Mickwitzia occidens Walcott, 1908 are described in detail from the Indian Springs Lagerstätte of the Poleta Formation (Cambrian Stage 3–Stage 4) northern Montezuma Range, Nevada, USA. Shell structure and preserved mantle setae from these specimens reveal a variable diagenetic (taphonomic) history in alternating clastic and carbonate depositional intervals and provides insight into the phylogenetic position of mickwitziids.

Morphologic and morphometric comparison to M. monilifera (Linnarsson, 1869) from Sweden and M. muralensis Walcott, 1913 from British Columbia, Canada reveals clear species-level distinctions facilitating taxonomic revision of the genus Mickwitzia based on shell microstructure data from scanning electron microscopic analysis. The Mickwitzia shell is characterized by the presence of inward pointing phosphatic cones and tangential setae-bearing tubes. These inward pointing cone structures are not consistent with setal bearing tubes as was previously interpreted, but rather represent endopunctae-like structures. A possible link to heliomedusids from China is also strengthened in light of new data on mantle setae from the Nevada material and presence of the inward pointing cones in Heliomedusa from the Chengjiang locality. Acrotretid-like shell structures and shell-penetrating setae in M. occidens strengthen the previously proposed close relationship between stem-group phosphatic brachiopods and tommotiids, a group of small shelly fossils, in particular the genera Micrina, Setatella and Tannuolina. Thus shedding light on the assembly of the brachiopod body plan during the Cambrian explosion.