Sigillariostrobus barkeri, Thomas & Seyfullah, 2016

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), pp. 1-9 : 4-6

Sigillariostrobus barkeri sp. nov

( Figure 4 )

Holotype: Specimen number 1971.I28 held in the Barker collection, Sheffield City Museum.

Type locality: Brierly Colliery, Yorkshire, Great Britain.

Type horizon: Shale above the Shafton Coal, top of the Similis-Pulchra Zone, Bolsovian.

Etymology: Named after the collection where the specimen had been deposited, the Barker collection held at the Sheffield Museum, Great Britain.

Diagnosis: Cone at least 13 mm in diameter without sporophyll laminae; axis 3 mm in diameter. Sporangia ca. 5.5 mm long and 2 mm high, megasporangiate. Megaspores trilete, probably originally plano-convex with greatest diameters of 1400–1800 µm in their equatorial planes. Lesurae one-third to one-half the spore radius, ca. 15 µm broad. Spore walls are about 20 µm thick, covered with numerous conical apiculi. Apiculi on distal walls ca. 10 µm high, varying in basal diameter and in numbers from 300 apiculi, 40–45 µm broad at the base, to 550 apiculi, 25–30 µm broad at the base. Apiculi on the contact faces are about 15 µm broad at the base and less than 5 µm high.

Description: The specimen is a fragment of a cone with little remaining except the megaspores arranged in the positions they must have occupied in the sporangia ( Fig. 4a– b ). This is a single Sigillariostrobus cone 13 mm in diameter that lacks sporophyll laminae ( Fig. 4a ), with an axis 3 mm in diameter. The sporangial remains are ca. 5.5 mm long and 2 mm high, containing in situ spores, referable to the dispersed spore genus Tuberculatisporites (Ibrahim) Potonié and Kremp. Megaspores recovered from the specimen have a maximum diameter of 1400–1800 µm in their equatorial planes, with lesurae one-third to one-half of the spore radius, and are clearly trilete ( Fig. 4c ) and tuberculate ( Fig. 4d ). The spore walls are about 20 µm thick and are covered with numerous conical apiculi ( Fig. 4d–e ). The size and distribution of the apiculi vary over the surface of the megaspores, with the apiculi on the contact faces being smaller than those elsewhere on the surface. On the distal walls of the megaspores, the apiculi are ca. 10 µm high, varying in basal diameter and in numbers from 300 apiculi, 40–45 µm broad at the base to 550 apiculi, 25–30 µm broad at the base. On the contact faces of the megaspores, the apiculi are about 15 µm broad at the base and less than 5 µm high.

Discussion: This Sigillariostrobus cone was originally described by Thomas (1980: Figs. 3 –6). Although the megaspores were closely compared with Tuberculatisporites brevispiculus (Schopf) Potonié and Kremp , which was the first record of this spore, the cone was not given a specific name. Spores belonging to the genus Tuberculatisporites , like those referred to the smooth-walled Laevigatisporites (Zerndt) Potonié and Kremp (1954) , have been recovered several times from sigillarian cones but from no others. Spores referable to Tuberculatisporites mamillarius have been previously described from Sigillariostrobus rhombibracteatus by Bochenski (1936), Chaloner (1953) and Wood (1957), but this is the only record of in situ spores referable to T. brevispiculus . As explained above, Tuberculatisporites brevispiculus has been discussed many times in relation to its possible affinity with T. mamillarius (Bartlett) Potonié and Kremp (1955) . We are following Spinner (1968), who kept them separate. As Thomas (1980) regarded the present cone as sigillarian and referred it to Sigillariostrobus , we now consider that it would be preferable to make it the basis of a new species.

The age of the new cone is also of some importance since Spinner (1968) gives the stratigraphical range of T. brevispiculus as generally Asturian and early Stephanian. In contrast, T. mamillarius and Sigillariostrobus rhombibracteatus have been only described from the Langsettian and Duckmantian. The present cone, in coming from the early Bolsovian, seems to fall between the two zones mentioned by Spinner (1968). Nevertheless, no real problem is posed by this new record as this extended stratigraphical range of T. brevispiculus still does not overlap with the range of T. mamillarius .

Figures

Figure 3. Second cone of Sigillariostrobus saltwellensis sp. nov. Specimen Pb. 707, Hunterian Museum, Glasgow. (a) The cone (scale bar 20 mm). (b) Detail of the upper part of the cone (scale bar 5 mm). (c) Detail of the mid-region of the cone, showing the megasporophyll impressions at the edge of the cone, often without the cuticle preserved (scale bar 5 mm). (d) Detail of (c), showing the megasporophylls with fissile compression and scattered megaspores (scale bar 5 mm). (e) Detail of (d), showing details of megaspores within the cone (scale bar 2 mm). (f) Megaspore referable to Laevigatisporites glabratus macerated from the spore mass shown in (e) (scale bar 2000 µm).

Figure 4. Sigillariostrobus barkeri sp. nov. No. 1971.I28, Barker collection, Sheffield City Museum, from the shales above the Shafton Coal, Brierly Colliery, Yorkshire (top of the Similis-Pulchra Zone, Bolsovian, Moscovian). (a) The remains of the cone showing the arrangement of the megaspores (scale bar 10 mm). (b) Camera lucida drawing of the cone in (a) (scale bar 10 mm). (c) Macerated in situ megaspore assigned to Tuberculatisporites mamillarius (Bartlett) Potonié and Kremp (scale bar 500 µm). (d) Contact face of the in situ megaspore (scale bar 150 µm). (e) Apiculi on the distal surface of the in situ megaspore (scale bar 10 µm). Figure previously published by Thomas (1980).