The Early Cambrian age for orthoquartzite in the Glen Falls Formation, in the lower part of the Saint John Group, is also fairly well-constrained by dated ash beds, macrofossils, and acritarch
assemblages in overlying and underlying formations (e.g., Tanoli and Pickerill 1988; Landing et al.
The Doushantuo formation shales and phosphorites, dated to 570 Ma, elegantly preserve acritarchs
, multicellular algae, and significantly, animal embryos of uncertain affinity.
"One or a few species of phytoplankton monopolizes nutrients at the expense of others," said Porter, explaining the die-off of diverse acritarchs
Other acid-resistant microfossils encountered during this study include acritarchs
, foraminiferans, melanoscleritoids, chitinous hydroids, and graptolite siculae, but these were not studied quantitatively.
Marine oil shales are composed of lipid-rich organic matter derived from marine algae, acritarchs
(unicellular organisms of questionable origin), and marine dinoflagellates (one-celled organisms with a flagellum).
In addition, palynologic analysis reveals the presence of land spores, acritarchs
, chitinozoans, and scolecodonts.
A good fossil record exists for the dinophytes, proof of the group's antiquity; the remains of dinophytes similar to contemporary species are known from the Silurian and similar forms, such as some acritarchs
, are even older, dating from the Cambrian period or before (see volume 1, p.
Paleontologists call these acritarchs
, a catch-all category of unicellular eukaryotic fossils.
Other biotic components are rare but include calcareous sponge spicules, bryozoans, acritarchs
, conodonts, scolecodonts, chitinozoans and various problematica (approximately listed in order of abundance).
The high percentages of [C.sub.29] steranes detected from the Silurian and Cambrian are attributed to algae such as benthic brown algae or macro algae, acritarchs
, cryptospores, and arthropods, rather than a terrestrial origin ( and references therein).
Sarjeant (1970) explained the acritarchs
and Tasmanitids from the Chhidru Formation.