Thomasaria simplex ( Phillips, 1841 )

Halamski, Adam T. & Baliński, Andrzej, 2019, Thomasaria vs. Pyramidalia conundrum in Devonian brachiopod systematics solved: An argument to formalize epitypification under the ICZN, Acta Palaeontologica Polonica 64 (2), pp. 399-408 : 404-405

Thomasaria simplex ( Phillips, 1841)

Figs. 2 , 3 .

1841 Spirifera simplex ; Phillips 1841: 71; pl. 29: 124αa–d; pl. 60: 124α.

1864 Spirifera simplex, Phillips ; Davidson 1864: 46; pl. 6: 18–22.

1893 Spirifera simplex, Phillips ; Whidborne 1893: 109.

1913 Spirifera simplex Phill. ; Ussher 1913: 22.

non 1930 Reticularia (Eoreticularia) simplex Phillips ; Nalivkin 1930: 133–134; pl. 10: 5 [= Thomasaria rotunda Oleneva, 2006 ].

non 1947 Pyramidalia simplex Phillips, 1841 ; Nalivkin 1947: 124–125; pl. 31: 4 [= Thomasaria rotunda Oleneva, 2006 ].

1966 Pyramidalia simplex (Phillips) ; House and Selwood 1966: 54; pl. 2: 8–9.

Lectotype (selected herein): Articulated shell GSM 6915 (coll. Austen),

illustrated by Phillips (1841: pl. 60: 124α), re-illustrated by House and

Selwood (1966: pl. 2: 8, 9) and herein ( Fig. 2D ).

Type locality: Wolborough quarry (SX 8522 7047), on the east side of the A381, east of Wolborough church, on the southwest outskirts of

Newton Abbot, Devon, England, UK ( House 2002: 281).

Type horizon: East Ogwell Limestone; Givetian, Maenioceras molarium Zone ( House 2002: 288–289) .

Material. — Wolborough : GSM 6916 , GSM 50930 , GSM 50931 (possible paralectotypes; coll. Austen) , NHMUK PI B 2807 (coll. Lee), B 22089 (coll. Vicary). Lummaton : NHMUK PI B 9144 (coll. Pengelly) , SM H2521–2525 (coll. Whidborne) . Torquay : NHMUK PI B 7320 (coll. Davidson) . Port Stephens : TCD 16991 a–e (collector unknown). Probably all Givetian, England, UK .

Description. —Shell pyramidal, (thickness-to-length ratio usually about 0.9–1.1), strongly ventribiconvex (ventral valve ca. 2.5–4 times as thick as the dorsal one), variably transverse (width-to-length ratio 1.15 in the lectotype, but 1.79 in the topotypic specimen GSM 6916; Table 1). Maximal observed width ca. 44 mm (estimated on the basis of the subcomplete ventral valve GSM 6916). Hinge line straight. Cardinal extremities most often feebly angular forming a wide angle with rounded lateral margins. Anterior margin straight to indented. Maximal width usually slightly anteriorly to the mid-length of the dorsal valve. Dorsal valve weakly to distinctly convex; fold none to distinct; interarea very low, approximately orthocline. Ventral valve strongly convex, subpyramidal, rarely with slightly twisted umbo; interarea very high, laterally bordered by delicate but sharp extremities, narrower than the total shell width, attaining about three quarters of the latter, most often procline, more seldom apsacline, concave, flat or weakly convex ( Fig. 2C 3 ), with growth striation parallel to the hinge line; delthyrium well developed, with delthyrial angle 15–19°, its width up to about one fifth of that of the shell; four of six investigated shells show delthyrium closed for most of its height by a convex pseudodeltidium ( Fig. 2A 5, B 5, C 5, F). Anterior commissure uniplicate to weakly parasulcate. Ornamentation either none or consisting of low, rounded, often hardly discernible costae, 1–2(–3) per valve. Micro-ornamentation poorly preserved, consisting of regularly spaced faint concentric growth lines with density 10 lines per 1 mm.

Interior of the shell was studied by serial sections of a single, slightly incomplete and deformed topotypic shell NHMUK PI B 2807 shown in Fig. 2E .

Ventral valve ( Fig. 3A ): dental plates strongly divergent at about 80° toward the floor of the valve and in contact with the valve floor on a length of 2.8 mm, then extending anteriorly as high dental ridges on the inner edges of the delthyrium; in the umbo bases of the dental plates fused and thus occluding the apical region of the delthyrium; other features of the internal structure of the valve not preserved.

Dorsal valve ( Fig. 3B ): cardinal process not preserved in the sectioned specimen; sockets bordered by inner socket ridges coalescent with crural bases and thus forming concave, subvertical plates; crural blades slightly concave ventrally and subparallel to the valve floor; crural plates not developed; spiralium broken and displaced, so only some of the whorls are visible on the serial sections.

Shell substance impunctate (see SEM micrograph on Fig. 2E 6 taken from a fragment of the shell NHMUK PI B 2807). Remarks. —The type locality and horizon of Spirifera simplex were originally given as “[i]n South Devon: Plymouth; Newton” ( Phillips 1841: 71) and “ Plymouth Group” ( Phillips 1841: 147), this unit being more inclusive than the Plymouth Limestone sensu House and Selwood (1966: 54, fig. 2). However, according to the Art. 76.2 of the ICZN, “the place of origin of the lectotype becomes the type locality of the nominal species-group taxon, despite any previously published statement of the type locality”, so the type locality and horizon of the discussed species are given as above as a consequence of the lectotype designation effectuated herein.

According to the original description, “the specimens, both at Plymouth and Newton, are entirely devoid of lateral radiating plaits” ( Phillips 1841: 71). A similar observation was made by Whidborne (1893: 109): “there is no indication [of ribs] in any English specimens”. However, three faint rounded costae may be observed in NHMUK PI B 9144 and one or two faint costae in NHMUK PI B 7320. Moreover, smooth and costate individuals occur within a single sample (compare TCD 16991b, Fig. 2B and TCD 16991a, Fig. 2C ). Variability in dorsal valve convexity, presence or absence of the fold, and the form and position of the interarea has been noted in the description.

Detailed comparison of Thomasaria simplex with other representatives of the genus is out of the scope of the present paper, but it may be briefly noted that T. altumbona (the type species of the genus) is much smaller in size and its dorsal fold is more distinct ( Stainbrook 1945).

Stratigraphic and geographic range. —The synonymy of Spirifera simplex given above is limited to the type area sensu lato (south-western England) and to the works related to the problem of the genus Pyramidalia . The following discussion of localities is also limited to south-western England. Both Phillips (1841) and Davidson (1864) quote this species solely from Newton Bushel (= Wolborough) and Plymouth. According to museum labels, other localities include Torquay, Lummaton near Torquay, and “Port Stephens”. As there is no such place name as Port Stephens in south-western England, one may suppose it refers to St Stephens by Saltash situated just opposite Plymouth on the western side of the Tamar; St Stephen-in-Brannel near St Austell, and St Stephen-by- Launceston are less likely. Probably the age of all the above-mentioned localities is Givetian. Whidborne (1893) seems right in qualifying this species as “rather rare”.

Figures

Fig. 2. Spiriferide brachiopod Thomasaria simplex (Phillips, 1841) from the Middle Devonian of south-western England, UK (type region).A–C. Articulated shells (TCD 16991c, A; TCD 16991b, B; TCD 16991a, C) from Port Stephens, in dorsal (A1–C1), ventral (A2–C2), lateral (A3–C3), anterior (A4–C4), and posterior (A5–C5) views. D. Lectotype, articulated shell GSM 6915 from Wolborough, in dorsal (D1), ventral (D2), lateral (D3), anterior (D4), and posterior (D5) views. Reproduced with the permission of the British Geological Survey ©UKRI. All rights reserved. E. Topotype, articulated shell NHMUK PI B 2807 (specimen serially sectioned, see Fig. 3) from Wolborough, in dorsal (E1), ventral (E2), lateral (E3), anterior (E4), and posterior (E5) views, enlargement (SEM micrograph) of the secondary shell layer showing impunctate condition (E6). F. Articulated shell TCD 16991d from Port Stephens, fragmentary posterior view showing the delthyrium partially closed by a convex pseudodeltidium.

Fig.3. Transverse serial sections of spiriferide brachiopod Thomasaria simplex (Phillips, 1841) through shell NHMUK PI B 2807 (topotype) from the Givetian of Wolborough, England, UK. Sections through ventral (A) and dorsal (B) valves. The schematic drawings on the left of each section describe the direction of sectioning (see text). Distances measured in millimetres from the tip of the ventral and dorsal umbos, respectively. Gray areas denote local silicification.