Permineralized Alethopteris ambigua (Lesquereux) White: A medullosan with relatively long-lived leaves, adapted for sunny habitats in mires and floodplains
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Permineralized Alethopteris ambigua (Lesquereux) White occurs in the Kalo Formation (Moscovian, Pennsylvanian) of Iowa. The following characteristics distinguish A. ambigua from other permineralized Alethopteris species: narrow pinnule width, a deeply sunken midrib having a round cross-section, deep midline depth, and a thick lamina that is not enrolled. Both the new permineralized Alethoptheris from the Kalo Formation and adpressed A. ambigua (Lesquereux) White have 'pecopteroid' pinnules with lamina that are linked at the base. Because the midrib of A. ambigua is round in cross-section, the widest part of the midrib lies below the midrib-lamina junction. In adpression this configuration would produce the 'midrib flanges' found in adpressed A. ambigua from Missouri. Three Alethopteris species with narrow pinnules and pecopteriod insertion have been extensively synonomized by previous workers: A. ambigua (Lesquereux) White, Alethopteris friedelii P. Bertrand, and Alethopteris lesquereuxii Wagner. Together with Alethopteris leonensis Wagner, these four species form a morphological continuum similar to other alethoperid morphological continua (e.g. the Alethopteris densinervosa, Alethopteris ingbertensis, Alethopteris lonchitifolica, Alethopteris missouriensis, Alethopteris westphalensis continuum). Nonetheless, because their permineralized forms can be readily distinguished, we favor retention of both A. ambigua (Lesquereux) White and A. lesquereuxii Wagner.Most permineralized mire alethopterids, with the exception of Alethopteris sp. from the Lewis Creek deposit (Kentucky, U.S.A.), have anatomical features consistent with foliage that grew in sunny, wet or humid habitats, including thick photosynthetic lamina, thick cuticle and enrolled edges. Most permineralized mire neuropterids, with the exception of Laveineopteris rarinervis from the Illinois Basin, have anatomical features consistent with foliage that grew in shady, wet or humid habitats, including thin photosynthetic lamina, and thin cuticle. In seed-ferns with broadly attached pinnules and compound leaves, lamina thickness/pinnule width (LT/MaxW) is a proxy for leaf mass per area (LMA), and indicates the relative economic cost and longevity of medullosan foliage. Measures of LT/MaxW for mire medullosans suggest that most mire alethopterids produced metabolically-expensive, long-lived fronds, whereas most mire neuropterids produced metabolically-cheap, short-lived fronds. 2013 Elsevier B.V.
