DO MORPHOLOGICAL AND SYSTEMATIC TRENDS IN ARCHAEOCYATHS RECORD CHANGING CLIMATE IN THE EARLY CAMBRIAN?

Examination of archaeocyathan morphology and systematics from their peak reef building through their decline (Botomian –Toyonian) from western Laurentia (Poleta and Harkless formations of Nevada) and from the Yangtze Platform of China (Xiannudong and Tianheban formations) reveal two trends. The most distinct trend is the rapid faunal changeover from predominately regular archaeocyaths to predominantly irregular archaeocyaths by the end of the Early Cambrian (Toyonian). This faunal changeover is observed within the Xiannudong Formation; where archaeocyath-bearing float-rudstone present between microbial reefs changes from dominantly regular archaeocyaths lower in the section to dominantly irregular archaeocyaths in the upper part of the section. This same trend occurs in Nevada from the Poleta to the Harkless formations.

Another distinctive trend involves the thickening of irregular intervallar elements from the Botomian to the Toyonian. Neither the outer nor inner walls of the irregulars vary in thickness through time, but a statistically significant thickening is observed in the taeniae. Testing whether the intervallar elements of regulars vary is difficult due their almost complete absence in Toyonian reefs.

We hypothesize that both these trends were driven by climatic changes, in particular rising sea surface temperatures. Carbon mass-balance models indicate that atmospheric pCO₂ was rapidly rising in the Early Cambrian; therefore, sea-surface temperatures were also rising. In general, calcification is physiologically easier as seawater temperatures rise. However, as pCO₂ rises, the saturation state of Ca decreases, inhibiting calcification.

The skeletal thickening of the intervallar elements in irregular archaeocyaths must be explained in the light of undersaturation of calcium. Many researchers have suggested that archaeocyaths, like most modern sponges, had a symbiotic relationship with microbes. The presence of photosymbionts in archaeocyaths would have caused the Ca concentration state to increase due to CO₂ uptake by the symbionts during photosynthesis. This allows for an increase in CaCO₃ precipitation of internal elements in irregulars, which are inherently denser with intervallar elements and are physiologically more robust than regulars.