Fossil Angiosperm Woods from the Jose Creek Member of the McRae Formation
MetadataShow full metadata
The Jose Creek Member of the McRae Formation, south-central New Mexico, preserves a diverse mixed angiosperm and conifer flora of Late Campanian (76.5–72.5 Ma) age. The Jose Creek Member is of special interest because it provides an abundance of fossil evidence in the form of leaves, reproductive structures, and silicified woods found in situ and as float. The wood flora, as currently understood, is one of the three most diverse Cretaceous wood floras in the world, and the only one where the majority of wood types represent mature wood. The focus of the present study is non-monocot angiosperms. To date, approximately thirty-five species have been discovered, representing both members of the magnoliid clade and eudicots. Many of the McRae xylotypes are common elements in other Cretaceous wood assemblages. Most magnoliids represent Lauraceae (~seven wood types), a dominant element in modern Asian tropical and subtropical vegetation. Assignment of these woods to the family Lauraceae is supported by the presence of idioblasts containing dark contents presumed to represent oil cells characteristic of modern Lauraceae. The McRae lauraceous woods represent multiple genera and include xylotypes differentiated by axial parenchyma distributions ranging from scarce to confluent. The remaining xylotypes represent eudicots. Two genera (Agujoxylon and Metcalfeoxylon) with exclusively scalariform perforation plates co-occur in assemblages in the southwest of North America. Four xylotypes have a combination of exceptionally wide (>10 cells wide) and nearly homogeneous rays, wood anatomical features that characterize extant Platanus. The exclusively solitary vessels and scalariform perforation plates are considered ancestral to extant Platanus and suggest the woods represent a stem lineage of modern Platanaceae. The Platanus-like woods fall outside the generic limits for Platanoxylon, warranting a new genus. While extant Platanaceae is represented by a single genus with few species, an extensive record of fossil leaves and reproductive structures documents a major radiation and diversification of the family Platanaceae during the mid- to Late Cretaceous and supports recognition of a new fossil genus for early Platanus–like woods. The Forest of Giants is an assemblage of exceptionally large angiosperm in situ stumps and logs that represent a riparian forest preserved in a sequence of fluvial sandstones. Four wood xylotypes have been recognized. The site is dominated by one species of Paraphyllanthoxylon-like wood with anatomical features that, in a wood where oil cells are not present, suggest affinity with the extant clade [Kirkiaceae [Burseraceae + Anacardiaceae]] within Sapindales. The presence of ray cells interpreted as gum cells in at least some specimens supports Kirkiaceae as the probable modern affinity for some, if not all the Paraphyllanthoxyon-like woods. One McRae Paraphyllanthoxylon-like stump is the largest Cretaceous angiosperm stump yet recorded worldwide (2 m in stem diameter). Three individual stumps represent probable Lauraceae, Sapotaceae and Urticales (Rosales). The Forest of Giants xylotypes have features that are common in tropical woods, including the absence of well-defined growth rings and presence of large vessels vulnerable to freeze-induced embolisms. Woods from the Forest of Giants add to a growing body of evidence from in situ assemblages for large angiosperms as the dominant tree form in some localities during the Late Cretaceous, especially in regions of warmer climate such as the southern Western Interior of North America. Affinities of most woods with simple perforation plates are, as yet, unidentified. The McRae Formation flora will provide unique insight into the stature and diversification of angiosperms at a critical period in their radiation.