The architecture of primary and secondary pinnae of the fossil fern Weichselia reticulata (C. Stokes et Webb) Fontaine is studied based on 28 large specimens from the upper Barremian La Huérguina Formation of the fossil locality Las Hoyas, Cuenca, Spain. The study of the specimens is performed through a morphometric analysis consisting in a reformulation of the Branching Algorithms method of shape description including measurements (insertion angle, distance between pinnae, first segment length and rachis width) and ratios (interval ratio, branching ratio and tapering ratio). A protocol to relocate isolated fragments of fossil pinnae is also established using the interval ratio (distance between pinnae/ previous distance between pinnae) and insertion angle of the pinnae. All specimens show a similar architecture, having elliptic primary pinnae with a sinuous apically tapering primary rachis and triangular secondary pinnae with pinnules of different morphologies. The analysis of the architecture allows to propose that the position of the frond was plagiotropic and that the frond growth was basiplastic for the petiole head and acroplastic for the primary pinnae. The metric method is applied to explore the architecture of four living fern species ( Angiopteris evecta (Forst.) Hoffm., Matonia pectinata R.Br., Sphaeropteris cooperi (F.Muell.) R.M.Tryon, and Woodwardia unigemmata (Makino) Nakai). Weichselia architecture results extremely ordered and regular in comparison with the primary pinnae variation of the living species.

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The sequence of decay in fern pinnules was tracked using the species Davallia canariensis. Taphonomic alterations in the sediment–water interface (control tanks) and in subaqueous conditions with microbial mats were compared. The decay sequences were similar in control and mat tanks; in both cases, pinnules preserved the shape throughout the four-month experience. However, the quality and integrity of tissues were greater in mats. In control tanks, in which we detected anoxic and neutral acid conditions, the appearance of a fungal–bacterial biofilm promoted mechanical (cell breakage and tissue distortions) and geochemical changes (infrequent mineralizations) on the external and internal pinnule tissues. In mats, characterized by stable dissolved oxygen and basic pH, pinnules became progressively entombed. These settings, together with the products derived from mat metabolisms (exopolymeric substances, proteins, and rich-Ca nucleation), promoted the integrity of external and internal tissues, and favored massive and diverse mineralization processes. The experience validates that the patterns of taphonomic alterations may be applied in fossil plants.

Coelacanths are rare, mostly marine fi shes, but the species from the Lower Cretaceous Spanish locality of Las Hoyas (Barremian) is a freshwater form and we know almost nothing about it. The Las Hoyas specimens are very rare and relatively incomplete, but there are still many things we can learn from the isolated skeletons and scales. First, the coelacanth scales were distinguished from other superfi cially similar scales (i.e., other "amioid" scales). Coelacanth scales are distinguished by the presence of a smooth central surface, a particular pattern of arrangement of concentric growth cessation marks, and mainly a relatively short posterior fi eld with thick elongated ridges. Only a few articulated coelacanth specimens have been recovered from Las Hoyas to date, and only 7.3% (n = 11) of the total isolated scales are coelacanth. The Las Hoyas coelacanth scales represent relatively large individuals. This suggests that a natural population of the coelacanth may have not inhabited permanently the freshwater pool represented RESUMEN

The Mesozoic is a key period in fern evolution, with the rise of most modern families. Weichselia reticulata is a widely distributed Jurassic-Cretaceous fern that has been suggested to belong to the Matoniaceae or possibly the Marattiaceae. The most accepted classic whole-plant reconstruction for this species is based on stem and foliage material from the Barremian-Aptian locality of N egresse (Bernissart, Mons Basin, Belgium). In this work, two of these stems are revised and analysed using computed tomography, providing new information on their internal anatomy and external morphology. Weichselia reticulata has a unique anatomy, distinct from all other extant or fossil ferns. Stem external morphology suggests adaptations to stressful environments, due to the scale insertions, and a thick cortex. Especially noteworthy is the presence of structures interpreted as aerophores or nectaries at the base of putative roots and petioles that could suggest a need for extra ventilation of the frond or interactions with animals. Although these new results do not settle the question of the systematic affinities of Weichselia, they provide new information about its autoecology and allow us to test the validity of previous whole-plant reconstructions.