when do bryophytes (including mosses liverworts and hornworts) first appear in the fossil record
Timeline of plant evolution The evidence of plant evolution changes dramatically in the Ordovician with the first extensive appearance of spores in the fossil record (Cambrian spores have been found, also). The first terrestrial plants were probably in the form of tiny plants resembling liverworts when, around the Middle Ordovician, evidence for the beginning of the terrestrialization of the land is found in the form of tetrads of spores with resistant polymers in their outer walls. These early plants did not have conducting tissues, severely limiting their size. They were, in effect, tied to wet terrestrial environments by their inability to conduct water, like extant liverworts, hornworts, and mosses, although they reproduced with spores, important dispersal units that have hard protective outer coatings, allowing for their preservation in the fossil record, in addition to protecting the future offspring against the desiccating environment of life on land. With spores, plants on land could have sent out large numbers of spores that could grow into an adult plant when sufficient environmental moisture was present.
Meadowcroft Rockshelter Radiocarbon dating of the site indicated occupancy beginning 16,000 years ago and possibly as early as 19,000 years ago. The dates are still controversial. A recent survey carried out by the Society for American Archaeology reported support from 38% of archaeologists, with 20% rejecting the early dates.[11] Criticism of these early radiocarbon dates has focused on the potential for contamination by ancient carbon from coal-bearing strata in the watershed.[12] The samples, tested by an independent third party geomorphologist, concluded that the samples showed no evidence of groundwater activity. Tests performed via accelerator mass spectrometry also support the earlier dates.[13] If authentic, these dates would indicate that Meadowcroft was used in the pre-Clovis era and, as such, provides evidence for very early human habitation of the Americas.[14][15] Meadowcroft Rockshelter may be the oldest known site of human habitation in North America, providing a unique glimpse into the lives of prehistoric hunters and gatherers. Paleoindian, Archaic, and Woodland remains have all been found at the site.