DOI: 10.15468/6wxbxx
title: Two new species of megasporangiate Sigillariostrobus Schimper (Sigillariostrobaceae) fructifications from the British Coal Measures

description: This dataset contains the digitized treatments in Plazi based on the original journal article Thomas, B. A., Seyfullah, L. J. (2016): Two new species of megasporangiate Sigillariostrobus Schimper (Sigillariostrobaceae) fructifications from the British Coal Measures. Fossil Record 19 (1): 1-9, DOI: 10.5194/fr-19-1-2016, URL: http://dx.doi.org/10.5194/fr-19-1-2016Abstract. The arborescent lycophyte group known as the sigillarians bore pedunculate fructifications, including Sigillariostrobus Schimper, which has a known propensity to disaggregate, making comparisons of cones and in situ spores often difficult if not impossible. Sigillariostrobus is monosporangiate, containing either megaspores or microspores. Two new species of Sigillariostrobus with megaspores are described from the British Coal Measures. Two cones of Sigillariostrobus saltwellensis sp. nov. are described from Langsettian–Duckmantian strata of Great Britain, containing in situ Laevigatisporites glabratus (Zerndt) Potonié and Kremp spores, making this the first British Sigillariostrobus species described containing such spores. Sigillariostrobus barkeri sp. nov. is given here to the previously described cone with in situ Tuberculatisporites brevispiculus (Schopf) Potonié and Kremp spores. This is the first Sigillariostrobus cone with this megaspore species in situ, and thus shows that T. brevispiculus and T. mamillarius (Bartlett) Potonié and Kremp are not conspecific as others have previously suggested.1 IntroductionSigillarian trees are one of the distinctive groups of Carboniferous arborescent lycophytes that are not as tall, or as branched, as Lepidodendron Sternberg. Arnold (1947) mentions Sigillaria reniformis to be 18 ft (ca. 6 m) tall and 6 ft (ca. 2 m) in diameter at the base, tapering to 1 foot (30.5 cm) at the top. This description suggests that the stem was incomplete. Balfour (1872) states that they can be 40–50 ft (12– 15 m) tall and 5 ft (ca. 1.5 m) in diameter. Balfour also gives measurements of 20 stems as 10–20 ft (3–6 m) high and 1– 3 ft (30.5 cm– 1 m) in diameter, standing erect in alternating strata of shales and sandstones, near Morpeth in Northumberland (UK). They bear cones (fructifications) on peduncles off the main stem or branches. The first figures of a fructification belonging to Sigillaria Brongniart were given by Goldenberg (1855, pl. 4, Fig. 3; 1857, pl. 10, Figs. 1, 2), but he gave no name to them. Schimper (1870, pl. 47, Figs. 12– 14) refigured some of Goldenberg’s illustrations and gave them the name Sigillariostrobus but did not propose a holotype. Andrews (1970) included Sigillariostrobus (Schimper) Geinitz 1873 in the index that he published because the first specific name S. bifidus was given by Geinitz. However, Seward (1910) and Chaloner (1967) believed S. bifidus to be a conifer cone belonging to the genus Gomphostrobus Marion, 1890. For this reason Chaloner (1967) took S. goldenbergi of Feistmantel (1876) as the type species and gave the generic authorship as Sigillariostrobus (Schimper 1870) Feistmantel, 1876, non Geinitz. Feistmantel (1876) had merely given the specific name to specimens figured by Goldenberg (1855) and Schimper (1870), leaving it to Zeiller (1884) to give the first clear description and figure of S. goldenbergi. For this reason Chaloner (1967) proposed the type species authorship as Sigillariostrobus goldenbergi (Feistmantel) Zeiller, 1884. Feistmantel’s other species, S. feistmanteli, was first regarded to be not a cone but a fertile zone and, therefore, is the type of Sporangiostrobus Bode – a genus which has since been synonymised with Omphalophloios White (Opluštil et al., 2010; Bek et al., 2015).In situ spores have become an integral part of describing fructifications, e.g. Chaloner (1953), Thomas (1970), Brack-Hanes and Thomas (1983), Bek and Opluštil (1998, 2004), Thomas and Bek (2014). This leads onto a comparison of the in situ spores with named genera and species of dispersed spores and then to using this information for palaeoecological interpretations.Although the spores of the type specimens of Sigillariostrobus (S. goldenbergi) are unknown, Sen (1958) described the spores of a specimen he identified to be S. goldenbergi as Laevigatisporites mamillarius. Examination of the spore contents of other species has shown the genus to be made up of monosporangiate cones that contain either megaspores or microspores. Of the megasporangiate cones, the megaspores spore walls are generally about 20–30 µm with triradiate marks that are approximately half the radius of the spore. If the entire surface of the spore is smooth and usually with prominent curvaturae, it is of the type referred to the dispersed spore genus Laevigatisporites (Ibrahim) Potonié and Kremp. If the spore wall is ornamented with tubercles on the dorsal surface it would be referred to the dispersed spore genus Tuberculatisporites (Ibrahim) Potonié and Kremp. The number of dispersed megaspores species belonging to these two genera far exceeds the number of known cones, which must be partially due to the fact that most, if not all, Sigillariostrobus cones fall to pieces by losing their sporophylls and can even be reduced to a bare axis (Fig. 1).The rarity of finding two more or less intact cones allows us here to describe a new Sigillariostrobus species from the British Coal Measures, with spores referable to the smooth-surfaced dispersed spore genus Laevigatisporites. We also give a name to another Sigillariostrobus cone from the British Coal Measures (Bolsovian) that has previously been described (Thomas, 1980) with spores referable to the tuberculate-surfaced dispersed spore genus Tuberculatisporites.Figure 1. Sigillariostrobus fructification with only a few basal sporophylls (lower arrow), having shed most of its sporophylls exposing the upper part of the axis (upper arrow) as illustrated by Kidston (1897) (scale bar 50 mm). From the Kidston Collection at the British Geological Survey. Catalogue number: P686530. Reproduced with permission of the British Geological Survey ©NERC. All rights reserved.Megaspores that are trilete and circular in outline, with an originally concavo-convex shape, and having a complex exine ornamentation of tubercles, would clearly, if found isolated, be included within the genus Tuberculatisporites (Ibrahim) Potonié and Kremp (1954).There are two species of Tuberculatisporites that concern us in this study: T. mamillarius (Bartlet) Potonié and Kremp (1955) and T. brevispiculus (Schopf) Potonié and Kremp (1955). Some authors, including Zerndt (1932), Dijkstra (1946, 1955a, 1955b), Arnold (1950) and Winslow (1959), believed the two species to be conspecific, while Potonié and Kremp (1955), Bhardwaj (1957a, b), Arnold (1961) and Selosse (1953) kept them separate. Spinner (1964) reexamined and redefined the genus Tuberculatisporites and included a list of species in which he distinguished T. mamillarius from T. brevispiculus. Following Spinner (1968) the two differ in their ornamentation. The tubercles are larger in T. mamillarius, being up to 130 µm tall and 94 µm in basal diameter on the distal surface, with them up to 28 µm tall and 25 µm in basal diameter on the proximal contact faces. In T. brevispiculus they are only 10 µm tall and vary from 25 to 30 and 40 to 45 µm in basal diameter on the distal surface, while the proximal contact faces are covered in apiculi that are 15 µm in diameter and less than 5 µm tall.