Abstract
Dictyastrum chestriensis n. gen. and sp. is a seed fern that occurs in the Chester Series (Upper Mississippian) of the Illinois Basin, and is based on compressions, impressions, and pyritemarcasite petrifactions of the stem and frond. The stem has a mixed prostele with indistinct protoxylem, and the larger specimens have a thin zone of secondary xylem that contains many small rays. The inner cortex contains numerous transversely-oriented sclerotic nests, whereas the outer cortex contains prominent anastomosing fiber bundles that produce the Dictyoxylon-type pattern that is frequently observable on compressions and impressions. The petiole is massive, has an expanded base, and forks distally into unequal subdivisions. The distal frond has small pinnules with rounded lobes and corresponds to the form species Sphenopteris stricta (=S. bermudensiformis), which is used for isolated compressions and impressions. Dictyastrum chestriensis belongs to the Lyginopteridales and is intermediate morphologically between Lyginopteris and Heterangium.
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