Patricia A. Sims

The evolution of the diatoms (Bacillariophyta). I. Origin of the group and assessment of the monophyly of its major divisions

The diatoms are one of the best characterised algal groups. Despite this, little is known of the evolution of the group from the earliest cell to the myriad of taxa known today. Relationships among taxa at the family or generic level have been recognised in some diatoms. However, relationships at higher taxonomic levels are poorly understood and have often been strongly influenced by the first appearances of key taxa in the fossil record. An independent assessment of relationships among the diatoms at these higher taxonomic levels has been made using rRNA sequence data to infer phylogenetic relationships. In this paper we present an analysis of 18S rRNA data from several chosen centric, araphid and raphid pennate taxa. The phylogenetic inferences from these 18S rRNA sequences are supported by evidence from the fossil record and evidence from ontogenetic data. Ribosomal RNA data indicate that both the centric and araphid pennate lineages may not be monophyletic.

The fine structure of the frustule in centric diatoms: A suggested terminology

The scanning electron microscope has provided much information about the structure of the valves of centric diatoms that supplements or, at times, corrects the accounts based on transmission electron microscopy. In particular, the morphology of the special features that have been called processes, spines, ocelli, tubules, etc., has been elucidated. This paper reviews the results so far obtained, discusses the way in which the various structures found should be grouped, and proposes a terminology for the description of these. As far as has been possible, existing terms have been retained and used in their current sense, but precision has not always permitted this; definitions of all the terms adopted are given. The concept that diatom frustules could be divided into those with a one-layered structure and those with a two-layered structure is rejected.

The diatom genera Stellarima and Symbolophora with comments on the genus Actinoptychus

Diatom species formerly referred to the genus Symbolophora Ehrenb. are herein assigned partly to Symbolophora sensu Hasle & Sims and partly to Stellarima nom. nov. The generitype for Symbolophora is established as S. trinitatis Ehrenb. and that for Stellarima as S. microtrias (Ehrenb.) comb. nov. Symbolophora has been recorded so far from deposits of middle Eocene to middle Miocene age, whereas Stellarima contains extant as well as extinct species. Symbolophora includes triangular forms hitherto placed in the genera Actinoptychus Ehrenb., Schuettia De-Toni or Cymatogonia Grun.; some circular and hexagonal species currently in the genus Actinoptychus may require transfer to this genus.

Two recent Stellarima species: S. microtrias and S. stellaris (Bacillariophyceae)

The identity of the two Recent, marine, centric diatoms, Stellarima microtrias (Ehrenb.) Hasle et Sims and S. stellaris (Roper) Hasle et Sims as separate species is established by differences in biology, habitat and geographical distribution. Stellarima microtrias forms resting spores (and probably also resting cells) and is confined to the Antarctic being particularly common on or in shelf-ice or in surrounding plankton whereas S. stellaris has not been found to form resting spores or resting cells, and is planktonic in temperate and warm waters. Light microscope examination shows the areolation of the vegetative valves of S. microtrias to be furcate and that of S. stellaris and the resting stages of 5. microtrias fasciculate. Stellarima stellaris differs from 5. microtrias in having smaller areolae, wider sectors, a narrow hyaline margin and a valve centre almost filled by areolae. SEM studies also show differences in the structure of the vela of the ordinary as well as specialized areolae.

The diatom genus Coscinodiscus Ehrenb.: C. argus Ehrenb. and C. radiatus Ehrenb.

Specimens of Coscinodiscus argus Ehrenb., the proposed generitype, and the closely related C. radiatus Ehrenb. have been examined using Ehrenbergs own slides and material from the type localities. A defmition of Coscinodiscus sensu stricto is given based on the structure of both valve and girdle. A single ring of marginal labiate processes, including two larger ones, the absence of processes in the central area and three bands per theca are the chief diagnostic features.

BENETORUS, GLADIOPSIS AND RELATED GENERA FROM THE CRETACEOUS

Species hitherto identified as Pomphodiscus morenoensis (Long, Fuge & Smith) Barker & Meakin (Craspedodiscus morenoensis Long, Fuge & Smith) and Coscinodiscus morenoensis var. sensu Long, Fuge & Smith are transferred to the genus Benetorus Hanna; Coscinodiscus morenoensis Hanna is assigned to a new genus Azpeitiopsis. It has recently been suggested that the above species may be related to Gladius antarcticus Harwood. This investigation proves that they are not so related but belong to two different families, Benetoraceae, fam. nov. and Thalassiosiropsidaceae Nikolaev.

SKELETONEMOPSIS, A NEW GENUS BASED ON THE FOSSIL SPECIES OF THE GENUS SKELETONEMA GREV.

The fossil species Skeletonema barbadense Grev. and S. simbirskianum A. Schmidt, together with four new fossil species, have been examined by LM and SEM and found to be unrelated to the living members of the genus. A new genus, Skeletonemopsis P. A. Sims, has been created for them and assigned to the family Trochosiraceae; descriptions of the new species are given and relationships with other genera established.

The morphology and taxonomy of the centric diatom genus Paralia. I, Paralia siberica comb. nov.

Paralia sulcata (Ehrenb.) Cleve is confirmed as the correct name for the type species of Paralia. This species is distinguished from another fossil species, Paralia siberica (A. Schmidt) Crawford & Sims, comb. nov. which has two varieties, P. siberica var. siberica and P. siberica var. laevis Crawford, nom.nov. This species and its two varieties are described.

The typification of Achnanthes Bory based on Echinella stipitata Lyngbye, with an account of the morphology and fine structure of Lyngbye's species

Achnanthes was described by Bory to include three species, A. adnata, A. dubia and A. baccillarioides, formerly included under Echinella stipitata Lyngbye. Boyer typified the genus Achnanthes using A. adnata and this typification has been accepted by subsequent authors. However no extant material of A. adnata is known. Since this species was based on E. stipitata, Lyngbye material was sought and examined. This material contains specimens of Achnanthes sensu stricto and can be used as the generitype of that genus. Light and electron microscopical observations of this material are presented. C. A. Agardh created the name Achnanthes brevipes as an avowed substitution for A. adnata and this is the name that has generally been used. There is little doubt that E. stipitata, A. adnata and A. brevipes refer to the same species. Since A. brevipes has been used most extensively in the literature there is a case for conserving this name. Based on our observations we propose that Lyngbyes material be accepted as the type for Achnanthes but that the name A. brevipes be considered for conservation against E. stipitata and A. adnata.

THE EARLY HISTORY OF THE BIDDULPHIALES. I. THE GENUS MEDLINIA GEN. NOV.

A brief history of the Biddulphiales is given, from the first appearance, in the early Cretaceous, of Bilingua in the late Aptian to early Albian (115–110 m.y.a.) up to the end of the Turonian (90 m.y.a.), by which time the genera Hemiaulus, Trinacria, Biddulphia, Briggera and a Eunotogramma-like species were established. A new genus, Medlinia, is described, which ranges from the late Cretaceous (early Campanian) to the Oligocene. A generic description is given and twelve species transferred to Medlinia from Sheshukovia and Triceratium; one new species, M. pseudoweissei P.A. Sims, and one new variety, M. venusta var. capitata, are described. Evolutionary trends within the Biddulphiales and within the genus Medlinia are discussed.