Tackling species variation
Where is the ‘boundary’ between Falcisporites and Alisporites in a continuously variable space?
In palynology, one of the ways to correlate distant sections in different palaeophytogeographic provinces is to use ‘bridge’ taxa – the few taxa which appear to be common to the provinces, despite their other differences. Bridge taxa become very important when there are no other ways to correlate, for example in non-marine sequences with no other fossils, or in sequences that have no dateable ash horizons. The problem is that bridge taxa may not be well described, or that conceptions of them in different palaeophytogeographic provinces may be different where different ‘schools’ of palynology have grown up. Traditional and formal (‘legal’) documentation in scientific journals with a description, diagnosis and types can help us understand taxa, but not their full variability, their ‘taxonomic distance’ from their nearest neighbours or the way that they ‘grade’ into their taxonomic neighbours. We should use the internet more to create galleries of images of key bridge taxa and not be constrained by journal page and plate limits.
Gallery of images
Categorization of any collection of objects involves seeing the similarities and differences between things where there might be an amount of continuous variation. In palynology, palaeontology, botany and zoology we call this taxonomy - a hierarchical system which can be used to organise and index knowledge.
For a palynologist dealing with objects that may or may not have a biological expression in morphology, taxonomy can be difficult. A palynologist is faced with a slide with possibly thousands of palynomorphs. If there are no taxon names for some of the palynomorphs, she is faced with the task of looking at the palynomorphs and grouping them into individuals that have similar characteristics using her judgement and knowledge. She may want to then give the group a name and ally it with other groups or place it inside other groups higher in the hierarchy – again using her judgement and knowledge.
If she decides that the entity she has ‘defined’ is ‘new’ to science, she may want to give the entity a taxon name. She will choose a type specimen of some kind which is an embodiment of the category or taxon that she has envisaged.
All this is good. It’s a way or organising and indexing knowledge. If she has chosen a good characteristic type and written a good enough diagnosis and description, other palynologists can come along and make a judgement as to whether a specimen that they have found is also a member of the group. The taxon name and the type has to be published and peer reviewed so that the taxon has some validity and so that the concept of the taxon is accessible to others. Palynologists use journals to publish this information. Journals are tight on space, so publication may take place in the journal but only a few photographs may be published - and text is required to be succinct.
As a practising taxonomist I’ve always been frustrated by the fact that the brevity of descriptions and paucity of photography have meant that it is sometimes difficult to understand the variation within a taxon, where its limits are, and how far it is from the nearest similar taxon - in other words the ‘taxonomic space’ between one taxon and the next. In the case of what appears to be continuous variation between one taxon and another, how (and where) do you ‘draw the line’ that separates the taxa? You might have thought that the internet would have come along to provide extra photos dealing with variation within a taxon and continuous variation between taxa, but in the majority of cases this has not happened. Taxonomy continues as it always has: based on and limited by the strictures of journal publishing.
Only a few publications and websites stand out, as least in Permian palynology, to deal with variation within a taxon and continuous variation between taxa. One is the lavishly illustrated monograph by JM Anderson on the Permian and Triassic palynology of South Africa (Anderson 1977). The paper uses some unorthodox taxonomic names but in a way that does not matter because most taxa are so well illustrated. Each taxon is illustrated as a population. It is possible at a stroke to understand a bit more about the natural variation of a taxon and its neighbours in taxonomic space just by flicking through the pages. Andersons’s paper was used heavily by Backhouse (1991) to make correlations using ‘bridge’ taxa between quite distant parts of the Gondwana continent in the Permian. Using the many plates of Anderson’s paper, Backhouse was able to reassure himself that specimens that he thought belonged to bridge taxa that existed in both the Australian and South African parts of Gondwana were indeed one and the same. A case in point is the spore Verrucosisporites pseudoreticulatus, an important biozonal index in Australia. The taxon was well known in Australia and the series of photographs published by Anderson allowed some assurance that it was indeed present in South Africa. Using a series of such bridge taxa, Backhouse (1991) was able to make a close correlation between the Collie Basin and the northern Karoo Basin.
Fig. 1. Variation in Reduviasporonites catenulatus illustrated in the BGS Taxonomy online Reduviasporonites gallery
The internet is of course the best place to illustrate variation within a taxon and continuous variation between taxa. A great source of data on two enigmatic taxa which is rich in imagery is the BGS’ Taxonomy online Reduviasporonites database or gallery (Figs. 1, 2). This gallery features almost 200 light microscope and SEM pictures of the variation within, and variation between, two species of the genus Reduviasporonites. This genus was of great interest and importance a number of years ago because of its disputed connection with the Permian-Triassic mass extinction.
Fig. 2. Details of some Reduviasporonites catenulatus specimens in the BGS Taxonomy online Reduviasporonites gallery
In particular two of the species R. chalastus and R. catenulatus were hard to separate and there is a lot of variation within each species. Also the affinities of Reduviasporonites were ambiguous and disputed – algae or fungus? The huge gallery of specimens helps other palynologists decide on the taxonomy of their specimens, and helped mycologists and phycologists think about the affinity of these important species without having to look at small pictures in journals that allow only a few illustrations and small amounts of description.
Useful resource for groups of palynological taxonomists making decisions about important taxa
At the moment, as a practising palynologist I am interested in a few problems that are taxonomic at heart. One is the use of the bisaccate, bitaeniate pollen Lueckisporites virkkiae as a marker for the Middle and Upper Permian; the other is the continuous variation between the genera Falcisporites and Alisporites.
The first problem is important, I think, because Lueckisporites virkkiae is one of the few taxa that could be could be regarded as a bridge between different parts of the highly phytogeographically differentiated Permian world. Lueckisporites virkkiae has long been considered useful for correlation in the Permian phytogeographical province of Euramerica because it is widespread in the province (Stephenson 2016), and because the taxon is very distinctive with a diploxylonoid outline, a thin corpus intexine and a prominent cappa formed chiefly by two reniform exoexinal taeniae (see Klaus 1963; fig. 27). The biostratigraphic value of Lueckisporites virkkiae also stems from its well-established first occurrence in the lower part of the Kazanian (Roadian) in its type area in the Russian Platform (Stephenson 2016), and therefore was useful for correlating to the then international scale of the Upper Permian before the Guadalupian Epoch was established in the United States. Early confirmation of a Guadalupian first occurrence for Lueckisporites virkkiae in the Gondwana phytogeographic province came from radioisotopic dating of the Argentinian Striatites Biozone (Archangelsky and Vergel, 1996), at the base of which that taxon makes its first appearance, though other identifications of Lueckisporites virkkiae in the Gondwana province may use a wider conception of the species (for example that used by Clark 1965) than was perhaps intended by Klaus (1963). Lueckisporites virkkiae therefore seems to me to a prime candidate as a bridge taxon which needs investigation and a clearer (or tighter) circumscription – if Gondwana/Euramerica correlation is to be achieved in palynology.
The difference between Falcisporites and Alisporites is important because these taxa, which both occur in the Late Permian Umm Irna Formation in Jordan, were produced by plants which in the last few years have been under intense study by a team from Munster University in Germany. The Munster team believe that the Umm Irna Formation was a kind of evolutionary cradle for a number of plant types that appeared early there (e.g. Blomenkemper et al. 2018). It is a kind of evolutionary hotspot, that for example harboured the seed fern Dicroidium that was once thought to be a Triassic-only species. Dicroidium produced the pollen Falcisporites stabilis which is extremely common pollen in the Umm Irna Formation. However Falcisporites stabilis is known to grade continuously into Alisporites nuthallensis. What is the relationship between the two and where is the ‘boundary’ between Falcisporites stabilis and Alisporites nuthallensis in a continuously variable space? Again to foster discussion and good taxonomic decision making it seems useful to be able to display these variations via photographs and maybe short videos that indicate the character of change between species. This and a gallery of Lueckisporites virkkiae specimens from many locations could form a useful resource for groups of palynological taxonomists making decisions about important taxa.
References
Anderson, J. M. 1977. The biostratigraphy of the Permian and Triassic. Part 3. A review of Gondwana Permian palynology with particular reference to the northern Karroo Basin of South Africa. Memoirs of the Botanical Survey of South Africa., 41, 1-133
Archangelsky, S. & Vergel, M. 1996. Paleontología, bioestratigrafía y paleoecología. In El sistema Permico en la Republica Argentina y en la Republica Oriental del Uruguay, Academia Nacional de Ciencias, Cordoba, Rep. Argentina, 1996, 40-44.
Backhouse, J. 1991. Permian Palynostratigraphy of the Collie Basin, Western Australia. Rev. Palaeobot. Palynol., 67, 237-314.
Blomenkemper, Patrick, Hans Kerp, Abdalla Abu Hamad, William A. DiMichele and Benjamin Bomfleur 2018. A hidden cradle of plant evolution in Permian tropical lowlands. Science 362 (6421), 1414-1416. DOI: 10.1126/science.aau4061
Clarke, R. F. A. 1965. British Permian saccate and monosulcate miospores. Palaeontology, 8, 322-354.
Klaus, W. 1963. Sporen aus dem südalpinen Perm. Jb. Geol. Bundesanst., Wien, 106, 229-363.
Stephenson, M H. 2016. Permian palynostratigraphy: a global overview. In: Lucas, S G, and Shen, S Z. (eds) The Permian Timescale. Geological Society, London, Special Publications, 450; 321-347.
Prof M H Stephenson is available for consultancy. Contact mikepalyno@me.com