The Cretaceous spore Stoverisporites and its taxonomy and stratigraphy

The Cretaceous palynomorph commonly designated Kuylisporites lunaris is correctly identified taxonomically as Stoverisporites lunaris, while Kuylisporties should apply to “Hemitelia type” spores. Stoverisporites lunaris is a useful stratigraphic indicator of Aptian – Albian sediments in the Canadian Arctic.


INTRODUCTION
In investigations of Arctic microfloras, it has been expedient to locate entities with stratigraphic utility that might prove valuable as zonal markers. These are often of limited geographical distribution or quantitatively scarce, but still of sufficient abundance to be valuable zonal tools. An example of such an entity, but with questionable taxonomy, is the spore Stoverisporites lunaris (Cookson & Dettmann) Burger, 1975. Cited frequently in palynological literature as Kuylisporites lunaris, it is an example of confused taxonomy, with both generic and specific assignments being questionable. We have recorded it in a wide range of study areas, both in the Canadian Archipelago and the Mackenzie District of northern Canada (Fig. 1). Other investigators active in these areas have also noted it in both surface and subsurface, often in considerable numbers. Geographically it is widespread with reported occurrences also in Australia, South American, and numerous records in the USSR. S. lunaris is a species of short stratigraphic distribution in the Arctic, and it has value as a palynology zonal marker. Doerenkamp et al. (1976) did a comprehensive Arctic palynology study and established a number of palynomorph zones, which they considered of significance in stratigraphic studies. Their palynomorph zones included six Cretaceous and two Tertiary, with their work being ecological and stratigraphic in scope and limited to Cretaceous-Tertiary assemblages primarily on Banks Island and adjacent Arctic areas.

TAXONOMY
The spore is also an entity whose nomenclature has been emended by Burger (1975) but is still unknown or ignored by most recent investigators in their recording its presence in Cretaceous age rocks. Burger (1975) considered the taxon described as Kuylisporites lunaris to be more appropriately assigned to the genus Stoverisporites. In doing so it would appear that he considered Kuylisporites (sensu Potonie) the proper repository for Hemitelia type spores, but with morphologically dissimilar spores as represented by K . lunaris more properly assigned to the genus Stoverisporites.
The genus Kuylisporites was originally established by Potonie (1956) for a lower Tertiary spore with Kuylisporites waterbolki being designated the genotype. The type had previously been recognised by Kuyl et al. (1955) but not validly described and was referred to only as Hemitelia type. However, Krutzsch (1959) considered Kuylisporites being best typified by K . mirabilis (Bolkhovitina) Potonie, 1958, an opinion later rejected by Dettmann (1963, p. 39). These assignments all referred to a spore characterised by three oval depressions on the proximal surface, each depression being situated half the distance between the spore angles. It does display a distinct resemblance to extant Cyatheaceae. Muller (1959, pl. 1, fig. 5) figured such a configuration for Hemitelia, and Selling (1946) described similar depressions for Cibotium.
Other workers were also preoccupied with a possible affinity with the extant fern Hemitelia of the Cyatheaceae. Among these were Chlonova (1961, pl. 2, fig. 13) and Kuyl et al. (1955, pl. 1, fig. 7). Admittedly the spore's morphology is suggestive of Hemitelia, but superficial comparisons to extant pteridophyta are commonplace in the fossil record, and it would appear that this implied relationship is the basis for the subsequent transfer of Hemitelia mirabilis to Kuylisporites by Potonie (1958). Chlonova (1961) recorded Hemitelia separata, which Dettmann (1963) included in Kuylisporites. Such implied relationships to extant Cyatheaceae must be suspect, for despite morphological similarities, the genotype of Hemitelia is lower Tertiary in age and a Cretaceous age has been assigned to most specimens attributed to Kuylisporites. However, there is admittedly a definite morphological similarity between fossil and extant Hemitelia. Also, Hemitelia is by no means restricted to the Tertiary as Komarova (1973) figured it in the Cretaceous of the Turgay Plain. It appears that it was the intention of Burger (1975) to differentiate from the Hemitelia type with the three oval depressions located mid-way between the radial positions and permit Kuylisporites to represent such a taxon. inasmuch as the original diagnosis referred to this Hemitelia type, a separate designation for forms with shallow pits on the spore surface is justified. Therefore, Kuyhporites should include only those specimens of Cyatheaceae characters with the three depressions between the radial positions. Accordingly, those entities with the numerous, scattered exinal pits and not of the Hemitelia type deserve separate taxonomic rank and Stoverisporites provides such a repository. The designation of S. microverrucutus as the type species of Stoverisporites might be questioned, and we would have suggested K . lunaris as a more appropriate specific epithet for the genotype. However. Burger's (1975) assignment of the type is taxonomically correct and acceptable.
It would appear that only Kemp & Harris (1977) have recognised the legitimacy of Stoverisporites and of S. microverrucatus as the type. However, they did not encounter S. lunaris in their studies, which were Tertiary.

PALYNOMORPH MORPHOLOGY
The taxon which Cookson & Dettmann (1958) established as Kuylisporites lunuris has a diagnostic feature of prominent ridges or elevations, which enclosed shallow lumina. These are the "scutula" of Potonik (1956), and they are referred to as crescenticshaped due to their elliptical outline in most instances. The crescent-shaped scutula are distinctly different from the oval depressions of Hemitelia. Dettmann (1963) also referred to the scutula as crescentic-shaped and partially enclosing shallow lumina in K . funaris. i n their description of the holotype of K . funaris Cookson & Dettmann (1958) described the scutula as half-moon shaped, However, their illustrations clearly depicted some completely circular scutula on the polar surface, with crescent-shaped thickenings situated marginally. They also noted the half-moon shaped scutula as occurring on both the proximal and distal surfaces in the holotype description. However, Dettmann (1963) in the description of K . funaris from the Neocomian of Australia, noted the scutula as confined to the distal surface. The illustrations of K . funaris by Brideaux & McIntyre (1975) showed eight crescentic scutula on the distal surface and apparently none proximally. However, Shakhmoundes (1971) figured K. funaris with six entire circular pits and reported all on the proximal surface. Although these various reports exist of scutula on the proximal surface, most investigators have noted them only on the spore's distal surface. Of the hundreds of specimens observed in this study, all have had the scutula restricted to the distal surface. It would appear that the reported proximal occurrences are the result of misinterpretation.
Although considerable morphological variation exists, especially as regard the type and number of the distal scutula, these differences seem insufficient for delineation of additional species. Numerically, a wide range occurs in our material from rare to numerous scutula, and varying combinations of hemi or circular scutula. Some spores bear only hemi-, others both forms; however, no entity displayed only circular scutula without hemi-being represented. Even the distribution of the scutula on the distal surface is inconsistent. They may have a peripheral orientation, with or without the depression in the mid-spore body, along with varying occurrences of hemior circular scutula peripherally or on the midbody.
The only instance of possible new speciation was a form differentiated by the scutula, which were uniformly oval in shape (PI. 1, fig. 4) and without circular or hemi-scutula being present. In all other aspects it compared closely with S. funaris. This variant did not display any stratigraphic utility, being observed in surface samples of Hassel Formation from Ellef Ringnes Island and Isachsen Formation from Melville Island. These samples contained numerous populations of spores with circular and hemi-scutula. The variant was rare, only five specimens having been observed during this study, and this is considered insufficient material to justify a new species.

COLLECTING LOCALITIES
The palynomorph is widespread and common in the Arctic sediments. but the major material for this study involved surface localities specifically collected by the senior author as reference standards for Arctic investigations. Four of these reference standards are listed (Table 1) with the geological formations in which Sroverisporites specimens were studied. A number of subsurface occurrences were a!so utilised. consisting of both well cuttings and core samples (Table 1).

STRATIGRAPHY
Three distinctive formations of Lower Cretaceous Rocks are of stratigraphic interest in the distribution of this spore. These are the Isachsen, Christopher, and Hassel Formations, Across the northern Arctic there is a wide variation in Cretaceous deposition based upon deposition, topography, erosion, and environment. Plauchut & Jutard (1976) discussed the complexities of the stratigraphy and the difficulty in recognising equivalent facies. Although there will be variations in the Sverdrup Basin in the formations of interest, the spore Stoverisporites is likely restricted to the Isachsen-Christopher-Hassel as depicted in Fig. 2.
Probably the most thorough palynological studies of the stratigraphic section in which S. lunaris occurs have been the investigations of Plauchut & Jutard (1976) and Doerenkamp e f al. (1976) in which they made comprehensive studies of the palynology of the Hassel, Christopher, and Isachsen formations on Banks and Eglinton Islands, as well as the Anderson Plain of the Canadian mainland. Although their microfloras compared remarkably well with those of our Sverdrup Basin studies in the same stratigraphic intervals, S. funaris was not reported in any of their assemblages.
Eglinton Island is not strictly part of the Sverdrup Basin proper, but it seems reasonable to consider it since the island is a minor horst with a major graben stretching southward from Prince Patrick Island towards Banks Island, providing a link from the Sverdrup Basin to the Western Canadian Sedimentary Basin. During the Cretaceous, sediments overlapped the basin confines in the Eglinton Graben and Banks extension. In view of the geographic and stratigraphic extent of the studies by Plauchut & Jutard (1976) and Doerenk-amp et al. (1976), the apparent absence of S. lunaris is somewhat unexpected. The spore could have simply been overlooked, inasmuch as it is a minor element quantitatively. However, it has been recorded often in the Mackenzie Delta and has a widespread occurrence in the giant Sverdrup Basin, and its absence here was unexpected.
Numerous references exist of K . lunaris being cited and not figured, but it is likely that one can assume that the entity cited is the spore featured by the hemiscutula ( K . lunaris sensu Cookson & Dettmann 1958). Burger (1973) recorded it from the Neocomian of the great Artesian Basin, Queensland. Archangelsky & Gamerro (1967) referred to it in their Lower Cretaceous study in Patagonia. Robbins et al. (1975) noted it in their Cretaceous work on the Atlantic Coastal Plain.
The only significant reference of its occurrence in other than Cretaceous deposits is an assignment to Norwegian Jurassic rocks by Vigran & Thusu (1975), Thusu & Vigran (1976, and Birkelund et a[. (1978). In these three instances they referred to Kuylisporires lunaris, and we assume their reference is to Sfoverisporites lunaris. However, only Vigran & Thusu (1975) illustrate the spore referred to (pl. 3, figs. 16-17), and it is clearly not S . lunaris. Accordingly, the other Norwegian assignments to the Jurassic are suspect, and a Cretaceous range only for the spore appears most likely.

Explanation of Plate 1
Sroverisporites lunaris (Cookson & Dettmann) Burger, 1975 All figures are ~5 0 0 .     The total reported stratigraphic range for the taxon, exclusive of the Norweigian Jurassic occurrences, extends from the Hauterivian report of Shakhmoundes (1971) to a Cenomanian age for the Bathurst Island K3a Unit occurrences of Burger (1975). Burger (1973), Kemp (1976) and Dettmann (1963) had the lower limits in the Neocomian. However, it appears that the majority of assignments, especially in the northern hemisphere, suggest an Aptian-Albian range. In the Arctic area of interest, primarily the Sverdrup Basin, the taxon occurs in the three formations, again suggesting Aptian-Albian. However, only in the Christopher Formation is it abundant and extends through the entire formation. Its occurrence at present does also seem to extend into the lower Hassel and the upper Isachsen formations.
There are problem areas in identifying the formations of interest. The Christopher is a widespread sequence of marine shales in the Sverdrup Basin and of variable thickness. At various localities the Christopher shale rests conformably on the Isachsen sandstone, but on Ellef Ringnes Island a transition zone some 350 ft. thick exists with more of a Christopher appearance. The Isachsen usually consists primarily of porous, quartzose sandstone, sands, and interbedded carbonaceous shales and coals. Often it is difficult to distinguish from consolidated portions of the Tertiary age Eureka Sound sandstones. Similarly, a transition from Christopher shale to Hassel sandstone is often characterised by a gradual lithology change. In view of the still questionable stratigraphy, and the seeming dominance of S. lunaris in Christopher sediments, an Albian age appears justifiable. Certainly our Arctic studies have indicated no wider range than Aptian-Albian.

Manuscript received August 1987
Revised manuscript accepted February 1988