ESTABLISHING CRITERIA FOR THE RECOGNITION OF CAMBRIAN CRYPTOSPORES

Spores of probable non-marine affinity occur throughout the Conasauga Group in Tennessee. Many of these palynomorphs are similar to ones recovered from other sites along an inner clastic belt that flanks much of the Cambrian craton in the United States. The palyniferous Conasauga sequence spans three trilobite biozones (Glossopleura - Ehmaniella - Bolaspidella), covering the latter half of the Middle Cambrian (Sundberg). The assemblages at first appear to be dominated by sphaeromorphs (acritarchs), but upon closer inspection, it is evident that many of these simple spore-like microfossils have more in common with alete spores of land-plants than with algal cysts. The Maryville Ls contains leiospheres and small acanthomorphic acritarchs (Asteridium), enabling comparison of acritarchs, leiospheres and cryptospores on the same slide. These direct observations help establish unambiguous criteria for distinguishing between cryptospores and leiospheres. Such criteria are needed, since the presence of cryptospores is used to establish the first occurrence of land plants, i.e., their provenance has far-reaching implications for both evolution and Earth systems history.

There is debate about the definition of the term cryptospore, which Steemans limits to alete spores of known embryophytic derivation. In part, this notion of embryophytic affinity is based on an overemphasis on the tetrahedral configuration of dispersed cryptospore tetrads which are accepted as post-algal in derivation. But most extant bryophytes do not disperse their spores in this manner, so a non-tetrahedral meiospore configuration does not rule out embryophytic derivation. In mixed assemblages, cryptospores are distinguished from leiospheres and acritarchs by their thicker walls, darker color, and contact features. In addition, some cryptospores show symmetries and topologies that are not shared with the planktonic algae. A reëxamination of lower Silurian non-marine samples from the Tetrahedraletes type section shows a range of variation (in cryptospore wall thickness, shape and configuration) that encompasses many of the problematic Cambrian forms. This comparison with younger accepted plant remains strengthens the assertion that land plants were established by Middle Cambrian time.