To assess the teratology, we focused on the aberrant spore data for two morphological groups that have relatively simple morphologies and sufficient exine thickness to enable distinction between true malformations and preservational artifacts. The first morphogroup, LTT-spores, comprises laevigate, triangular, trilete spores, which, during the Rhaetian and Early Jurassic in NW Europe, are known to have been produced by ferns of the Dipteridaceae, Dicksoniaceae, or Matoniaceae (46). The second morphogroup, the LCT-spores, comprises laevigate, circular, trilete spores that are known to have been produced by ferns assigned to the Osmundaceae or Marattiales (46). Thus, these two morphogroups are both most likely derived from heterogeneous mother plants that may have had different ecological preferences. The LTT-morphogroup includes spores that could, if found dispersed, be assigned to several different form taxa, including Cyathidites, Concavisporites, Deltoidospora, and Dictyophyllidites. This also includes some form taxa that are herein regarded as aberrant forms but that have been described as species of their own in published literature (Supplementary Text).

Rather than subdividing these fern spores according to form taxonomy on the species level, we focused on morphological traits related to function, i.e., viability (see below). The teratology may reflect different types of disturbance during spore formation, which may then indicate various forms of environmental stress. For this purpose, the teratologies were categorized on a five-step severity scale during the counts (Table 1 and fig. S3). In some cases, spores exhibited several types of teratogeny, in which case they were classified after the severest form. Each sample was categorized according to the maximum severity of the teratology in that sample.

Mild teratology. Usually, dwarfed forms could be distinguished from normal spores by their denser exine and underdeveloped character, but occasionally, the spores were merely unusually small in size. In the latter case, the spores may still have been viable. Dwarfed forms and unexpanded forms were considered to represent mild teratology and were primarily considered to have formed because of premature shedding from the sporangia (Fig. 2, G to K). Unexpanded forms were most likely nonviable, as they never matured.

Mild to moderate teratology. Uneven trilete rays, aberrant folding or cracking of the exine, and thickened labra or labra with deformations (i.e., growths, commonly in the form of verrucae or baculae) were considered mild to moderate teratology (Table 1 and Fig. 2, H to V). Uneven trilete rays may have developed if one or more of the spores in a spore tetrad had not matured when the spores are shed (Fig. 2, L to N, and fig. S1, F and G). Spores with thickened labra, exemplified by Fig. 2 (T and U) have previously been described as a separate for taxon (Supplementary Text) but were herein considered malformed. Aberrant folding and cracking of the exine are enigmatic features, which may indicate genetic disturbance that affected the concentration of sporopollenin in the spore wall (Fig. 2, N to P, V, DD, and II, and fig. S1O). Specimens similar to these were often assigned to specific form taxa in the literature (Supplementary Text). Cracking of the exine was often also associated with more severe teratology. They were placed here within the mild to moderate teratology category, as these features may not necessarily have rendered the spores nonviable. Similarly, abnormally wide labra or labra with growths were also considered to have formed because of a minor genetic disturbance, but since the outline and size of the spores often appeared normal, the spores may still have been viable.

Moderate teratology. Mono- or multilete marks and deformed outline were classified as moderate aberrancies (Fig. 2, W to CC). Spores with monolete marks instead of trilete marks were either formed in a tetragonal instead of a tetrahedral tetrad or by unbalanced cytokinesis (Fig. 2, X to AA, and fig. S1K). A multilete mark also demonstrated malfunctioning meiosis where the spore mother cell was divided into more than four spores (Fig. 2, W, EE, and HH). A deformed outline was usually expressed as an invagination of the spore wall. In LTT-spores, this invagination was often located near one of the radial apices (Fig. 2, AA to DD and FF). This invagination could possibly be a remnant of malfunctioning or incomplete cytokinesis.

Moderate to severe teratology. This included weakly deformed proximal area (i.e., laesura still discernable) and deformed tetrads (including fused tetrads) (Fig. 2, DD to GG). The weakly deformed proximal area was expressed as thickening and wrinkling of the proximal area, which often partially affected or distorted the trilete mark (Fig. 2, DD to FF, and fig. S1, L and M). Deformed tetrads included fused tetrads, tetrads with various sized spores, and also fused dyads (Fig. 2GG and fig. S1N) and was characterized by at least one of the individual spores being morphologically deformed. Both these teratologies were interpreted as reflecting genetic disturbance resulting in nonviable spores.

Severe teratology. This included only spores with severely deformed proximal areas, where the laesurae were no longer discernable (Fig. 2, HH and II, and fig. S1, O and P). These spores were nonviable.

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