Fossil Lycopsid Research Articles

Functional diversity and convergence in the evolution of plant reproductive structures

Structures that simultaneously perform many functional roles are likely to show a variety of morphological solutions to these demands, and thus probably exhibit high morphological disparity. In contrast, specialization for a few simple functions should result in a more limited suite of morphologies. We explore this idea using lycopsid reproductive structures, which, throughout their history, have performed a limited set of functional roles compared with the reproductive structures of other plant groups such as seed plants. We scored living and fossil lycopsid taxa for 18 discrete character measurements and several continuous traits, including sporangium size, supporting axis diameter, and strobilus length and width. We used the discrete characters to construct a multivariate morphospace for lycopsid reproductive morphology through time, and the continuous characters to test whether fossil and extant lycopsids show similar patterns of tissue allocation within reproductive structures. Lycopsids occupy similar areas of reproductive morphospace and show similar patterns of tissue allocation over most of their history, alternating between diffuse fertile zones with leaf-like sporophylls and compact strobili with specialized sporophylls that allow sporangia to be closely packed while also protected during their development. Growth habit also plays an important role in lycopsid reproductive evolution, broadly influencing the size and shape of reproductive structures. Lycopsid reproductive structures are primarily specialized for densely packaging sporangia, and are consistent with the idea that performing limited functional roles is associated with reduced morphological disparity. Morphologies similar to lycopsid strobili are also found in other groups with simple, wind-dispersed propagules, suggesting that the same processes occur across plant lineages.

Naming the chaos: generic redelimitation in Schismatoglottideae (Araceae)

Phylogenetic relationships within Araceae tribe Schismatoglottideae are elucidated based on nuclear ITS and plastid matK. Thirty genera are recognised, composed of 13 pre-existing genera (Apoballis, Aridarum, Bakoa, Bucephalandra, Fenestratarum, Galantharum, Ooia, Phymatarum, Pichinia, Piptospatha, Schismatoglottis, Schottariella and Schottarum), five resurrected genera (Colobogynium, Gamogyne, Heteroaridarum, Hottarum and Rhynchopyle) and 11 new genera: Bakoaella, Bidayuha, Burttianthus, Gosong, Hera, Kiewia, Nabalu, Pursegloveia, Naiadia, Tawaia and Toga. Significant recircumscription is proposed for the pre-existing genera Aridarum, Bakoa and Piptospatha. Schismatoglottis remains imperfectly delineated despite resolution of a well-supported corpus defined by hapaxanthic stems and containing the nomenclature type. An updated taxonomic treatment is presented and all necessary nomenclatural changes are made including a new generic name, Vesta to replace Hestia, which has been discovered to be preoccupied by Hestia, a fossil lycopsid. All genera, including representatives of each clade pertinent to Schismatoglottis sens. str. are illustrated from living plants. 2017 © 2018 Dipartimento di Biologia, Universita di Firenze

Periodicity of reproductive growth in lycopsids: An example from the Upper Devonian of Zhejiang Province, China

Monilistrobus Wang et Berry is a fossil lycopsid reported from the Upper Devonian Wutong Formation of Jiangsu Province, China, and includes only the type species Monilistrobus yixingensis Wang et Berry. Along a single fertile axis, Monilistrobus has cone-like structures separated by and alternated with a length of vegetative region, which is an important character here termed as periodicity of reproductive growth. The alternation of fertile and vegetative growth occurs regularly and produces repeated intercalary (non-terminal) fertile zones. New specimens, identified as Monilistrobus cf. yixingensis, from the same formation but of Zhejiang Province extend the geographic distribution of this genus. These new specimens demonstrate morphological variations within this genus, such as the lack of spiny appendages on the leaves, different measurements of leaf bases, and different shapes and positions of cone-like structures. The distribution of intercalary fertile zones, as summarized for the first time in this paper, occurs in the Drepanophycales, the Protolepidodendrales, the Isoëtales, the Lycopodiales, the Selaginellales, and Wuxia Berry, Wang et Cai and Monilistrobus of uncertain affinities. Thus, this peculiar trait might have evolved in the very early evolutionary stage of the lycopsids, as indicated in the basal group Drepanophycales. However, the presence of repeated, two or more intercalary fertile zones along a single fertile axis is demonstrated only in Monilistrobus and the extant Huperzia Bernhardi.

Do eligulate herbaceous lycopsids occur in Carboniferous strata? Hestia eremosa gen. et sp. nov. from the Mississippian of Oxroad Bay, East Lothian, Scotland

A new lycopsid, Hestia eremosa R.M. Bateman, Kenrick and Rothwell gen. et sp. nov., is described based on three fragmentary permineralised axes from separate horizons of the Lower Carboniferous (Tn3) sequence of tuffs and lacustrine deposits at Oxroad Bay, East Lothian, Scotland. These stem fragments are used to test the feasibility of employing pre-existing cladistic matrices to determine the range of credible phylogenetic placements of poorly known fossils. Hestia is unusual for a post-Devonian fossil lycopsid in being putatively herbaceous and phylogenetically primitive. As currently understood, this stem-genus lacks autapomorphies, but can be diagnosed by the combination of a stellate stele, scalariform xylem pits and perforate sheets of wall material partially infilling the pits. Review in the context of previous morphological and molecular phylogenetic analyses of Lycopsida suggests that Hestia bears a suite of vegetative anatomical characters largely consistent with Huperzia, the most plesiomorphic extant genus of Lycopodiaceae; this observation is particularly noteworthy as Lycopodiales appear surprisingly under-represented in the fossil record. However, the limited number of characters that can be scored from these fragmentary fossil axes cannot preclude placement of Hestia within the mono- or paraphyletic Drepanophycales s.l., which are generally viewed as being exclusively Devonian. Taphonomic inference suggests scorching (presumably volcanically induced) followed by medium-distance transport. Nonetheless, Hestia persisted on this volcanigenic landscape throughout a series of substantial environmental perturbations.

A new species of the Devonian lycopod genus, Leclercqia, from the Emsian of New Brunswick, Canada

A new species of the fossil lycopsid genus, Leclercqia, is reported from the Early Devonian (Emsian) of northern New Brunswick, Canada. The plant remains consist of dochotomizing axes covered with numerous, small, closely spaced leaves. The leaves are laminar and divided distally into five segments which curve upwards. An ellipsoidal sporangium attaches to the adaxial surface, proximal to divisions, of many leaves. Sporangia contain abundant and well-preserved spores assigned to the sporae dispersae genus Acinosporites. The habit of the plant is proposed to be that of an herbaceous lycopsid. Leclercqia andrewsii differs from the type species, L. complexa, in the organization of its leaf segments: all five segments curve or bend upward after the point of forking. The genus has previously been described from mostly Middle Devonian sediments of North America, South America (Venezuela), Australia (Queensland), and Europe, and also from the Early Devonian of North America. This account adds another Emsian occurrence of the genus Leclercqia in North America, documents a new species, and presents evidence of novel variation in early Devonian leclercqioid lycopsids relative to those present in the Middle Devonian.

Role of evaporite withdrawal in the preservation of a unique coal-bearing succession: Pennsylvanian Joggins Formation, Nova Scotia

The Pennsylvanian Joggins Formation, in the Cumberland Group of Nova Scotia, is widely regarded as the world's best exposure of coal-bearing Carboniferous strata. This 1.5-km-thick coal-bearing unit is famous for upright fossil lycopsid trees, and is preferentially preserved in the Athol syncline in the western Cumberland basin. New seismic profiles show that the Athol syncline is atop a salt weld and that the Joggins Formation thins on the flanks of adjacent evaporite-cored anticlines. These observations indicate that during deposition of the Joggins Formation, at least 1 km of syndepositional subsidence was facilitated by flow of underlying salt into the adjacent anticlines. In contrast, halo-kinesis was mainly active during the Mississippian in the eastern Cumberland basin (Tatamagouche syncline); minibasins were filled by Mabou Group sediments, whereas the Cumberland Group is thin and lacks significant coals. This basinwide comparison shows that much of the subsidence responsible for the preservation of the coal-bearing Joggins Formation was the result of salt withdrawal at depth.

Low atmospheric CO(2) levels during the Permo- Carboniferous glaciation inferred from fossil lycopsids.

Earth history was punctuated during the Permo-Carboniferous [300-250 million years (Myr) ago] by the longest and most severe glaciation of the entire Phanerozoic Eon. But significant uncertainty surrounds the concentration of CO(2) in the atmosphere through this time interval and therefore its role in the evolution of this major prePleistocene glaciation. Here, I derive 24 Late Paleozoic CO(2) estimates from the fossil cuticle record of arborsecent lycopsids of the equatorial Carboniferous and Permian swamp communities. Quantitative calibration of Late Carboniferous (330-300 Myr ago) and Permian (270-260 Myr ago) lycopsid stomatal indices yield average atmospheric CO(2) concentrations of 344 ppm and 313 ppm, respectively. The reconstructions show a high degree of self-consistency and a degree of precision an order of magnitude greater than other approaches. Low CO(2) levels during the Permo-Carboniferous glaciation are in agreement with glaciological evidence for the presence of continental ice and coupled models of climate and ice-sheet growth on Pangea. Moreover, the Permian data indicate atmospheric CO(2) levels were low 260 Myr ago, by which time continental deglaciation was already underway. Positive biotic feedbacks on climate, and geotectonic events, therefore are implicated as mechanisms underlying deglaciation.

Morphology and Wall Ultrastructure in Devonian Spores with Bifurcate‐Tipped Processes

A detailed investigation has been undertaken on two dispersed spore taxa, characterized by their zonate structure and an ornament of bifurcate‐tipped processes, that were recovered from Middle Devonian deposits from Scotland: Ancyrospora grandispinosa Richardson 1960 and Ancyrospora (Triletes) ancyrea (Eisenack) Richardson 1962. Both taxa have been comprehensively examined using light, scanning electron, and transmission electron microscopy, and a detailed understanding of their morphology, gross structure, and wall ultrastructure has emerged. Both have an inner body, enclosed within an outer layer that is extended into a pseudozona, and bifurcate‐tipped processes of similar construction. On the whole, morphology and wall ultrastructure are strikingly similar in the two species, although subtle differences lead to slight reservations when attempting to identify homologous wall layers. Affinity of the spores is assessed based on detailed comparisons with the spores of plant groups believed to have comprised the Middle Devonian vegetation. I conclude that the taxa are closely related and have lycopsid affinities, the most compelling evidence for the latter being similarities in wall ultrastructure with extant and fossil lycopsid spores. Mode of spore wall formation is interpreted in light of the new data. Because of the absence of ontogenetic information, I acknowledge the speculative nature of my interpretation. However, it is clear that in both taxa, the bifurcate‐tipped processes are constructed in a similar manner, and this involves a relatively simple developmental process. Spores with bifurcate‐tipped processes are a conspicuous, but enigmatic, feature of Middle and Upper Devonian spore assemblages. Their reproductive strategies, functional morphology, and paleoecology are discussed in light of the new data.

Recognition and characterization of inner exospore wall layers in modern and fossil lycopsids — The mesospore

Abstract Differing interpretations of sporoderm development in megaspores of Selaginella have resulted in confusion surrounding use of the term mesospore. Different ontogenetic events result in one of two distinct structural types in the innermost exospore. The first type consists of a thin, highly compressed layer (referred to as an inner separable layer; ISL) which separates, along with the endospore, from the remainder of the overlying exospore. This layer forms if a viable protoplast persists in the megaspore. Growth of the viable protoplast compresses the ISL tightly to the inside of the overlying exospore although a line of separation can still be detected even in fully mature megaspores. The second structural type, which may be termed a “mesospore proper”, develops if the protoplast aborts. In the latter case, there is evidence to suggest that the aborting protoplast adds additional material (if only residues of cellular autolysis) to the inside of the mesospore. This protoplast abortion and deposi...

SPECIAL PAPER: MERISTEMS AND EVOLUTION: DEVELOPMENTAL CORRESPONDENCE AMONG THE RHIZOMORPHS OF THE LYCOPSIDS

This paper deals with a comparison of the rooted axes (rhizomorphs) of Isoetes with those of Stigmaria, Pleuromeia and other fossil lycopsids from the standpoint of developmental correspondence. The objective is to present a mechanistic approach to the problem of evaluating possible interrelationships among the genera. The results complement those from other comtemporary approaches to the same problem. These main points are advocated: 1) The rhizomorphs of Isoetes and Stylites correspond. Because neither of these contains an apical meristem, neither corresponds to Stigmaria with respect to organization of the axis or root production. 2) Rooted portions of Isoetes and Pleuromeia correspond, although external lobing in Pleuromeia parallels lobing of the stele, whereas in Isoetes external lobing occurs between the lobes of the stele. In both cases, lobing of the stele is associated with root production. 3) Paurodendron rhizomorphs appear to correspond to Stigmaria developmentally with regard to the presence of an apical meristem and the arrangement of roots, but Paurodendron rhizomorphs are shorter. If several lines of root production were added to the vascular cambium of the kind of rhizomorph seen in Paurodendron, a plant part like the rooted portion of Pleuromeia could result. Further extrapolations of the Paurodendron form of growth are explored for their bearing on comparison of rhizomorphs among the lycopsids.

Meristems and Evolution: Developmental Correspondence Among the Rhizomorphs of the Lycopsids

This paper deals with a comparison of the rooted axes (rhizomorphs) of Isoetes with those of Stigmaria, Pleuromeia and other fossil lycopsids from the standpoint of developmental correspondence. The objective is to present a mechanistic approach to the problem of evaluating possible interrelationships among the genera. The results complement those from other comtemporary approaches to the same problem. These main points are advocated: 1) The rhizomorphs of Isoetes and Stylites correspond. Because neither of these contains an apical meristem, neither corresponds to Stigmaria with respect to organization of the axis or root production. 2) Rooted portions of Isoetes and Pleuromeia correspond, although external lobing in Pleuromeia parallels lobing of the stele, whereas in Isoetes external lobing occurs between the lobes of the stele. In both cases, lobing of the stele is associated with root production. 3) Paurodendron rhizomorphs appear to correspond to Stigmaria developmentally with regard to the presence of an apical meristem and the arrangement of roots, but Paurodendron rhizomorphs are shorter. If several lines of root production were added to the vascular cambium of the kind of rhizomorph seen in Paurodendron, a plant part like the rooted portion of Pleuromeia could result. Further extrapolations of the Paurodendron form of growth are explored for their bearing on comparison of rhizomorphs among the lycopsids.

Embryogeny and Reproductive Biology of Bothrodendrostrobus mundus (Lycopsida)

Permineralized lycopsid megagametophytes and embryos from Upper Carboniferous strata (Westphalian A) at Burnley, England have been found within isolated megaspores assignable to Setosisporites. The specimens illustrate for the first time the reproductive biology and embryogeny of a free-sporing, bisporangiate, Paleozoic lycopod, and permit reinterpretation of the megagametophyte of Bothrodendrostrobus. Megagametophyte development is entirely endosporal. Embryogeny is comparable to that of the extant genus Isoetes, and is fundamentally different from that of the fossil lycopsid, Lepidocarpon—Lepidophloios. This further illustrates the diversity among Paleozoic lycopods and helps to clarify relationships among both fossil and extant lycopsid taxa.

EMBRYOGENY AND REPRODUCTIVE BIOLOGY OF BOTHRODENDROSTROBUS MUNDUS (LYCOPSIDA)

Permineralized lycopsid megagametophytes and embryos from Upper Carboniferous strata (Westphalian A) at Burnley, England have been found within isolated megaspores assignable to Setosisporites. The specimens illustrate for the first time the reproductive biology and embryogeny of a free‐sporing, bisporangiate, Paleozoic lycopod, and permit reinterpretation of the megagametophyte of Bothrodendrostrobus. Megagametophyte development is entirely endosporal. Embryogeny is comparable to that of the extant genus Isoetes, and is fundamentally different from that of the fossil lycopsid, Lepidocarpon—Lepidophloios. This further illustrates the diversity among Paleozoic lycopods and helps to clarify relationships among both fossil and extant lycopsid taxa.