PYWACKIA IS NOT A CAMBRIAN BRYOZOAN: EVIDENCE FROM SKELETAL MICROSTRUCTURE AND TAPHONOMY

Bryozoa remains the only major phylum without a fossil record in the Cambrian (FAD lower-Tremadocian, lower Ordovician). This well-known feature of the fossil record, that all other bilaterians have FADs in the Cambrian or earlier, has led to a long search for a Cambrian representative of the Bryozoa. The most recent candidate Pywackia baileyi Landing, 2010 (Tiñu Formation, Mexico) has the exterior morphology consistent with modular colonial organisms, but because of phosphatic preservation, it was originally assigned to Bryozoa without consideration of the skeletal microstructure. Taylor et al. (2013) argued for a pennatulacean affinity of Pywackia based on overall morphology.

In this study, four topotypes were sectioned and examined with secondary and backscatter electron imaging and elemental composition was analyzed using EDS. All specimens exhibit regions of calcium phosphate overgrowth and the primary skeleton is either completely replaced by calcium phosphate or was originally phosphatic. Specimens exhibit varying degrees of replacement by massive calcium phosphate, silica and iron oxides, that in places completely obscure primary structure. The least altered parts of colonies have an outer rind of massive calcium phosphate (0–10 µm thick) which wraps around the growing tip and is present in the interior of some walls, indicating the position of the former growing edge. The inner skeleton is porous (20-60 µm thick), and in the least altered regions, this region resolves into structure of cancellate layers that parallel the form of the growing tip, with no chemical difference between the massive and porous regions. The variable thickness of the massive exterior and impurities present when the region is replaced by silica, suggest that the rim might be controlled by diagenesis rather than primary structure.

A bryozoan interpretation for Pywackia requires that all specimens represent broken/abraded endozones of specimens, yet many of the walls display terminal growth (wall termini) and some specimens have additional “modular layers” that are stacked in episodes of discrete growth. An accurate phylum-level assignment of Pywackia must account for both the skeletal microstructure, growth and overall morphology. Candidates based on microstructure include some algae, chordates and octocorals.