Testograptus testis ( Barrande, 1850 )

Lenz, Alfred C. & Melchin, Michael J., 2008, Convergent evolution of two Silurian graptolites, Acta Palaeontologica Polonica 53 (3), pp. 449-460 : 454

Testograptus testis ( Barrande, 1850)

Figs. 2C–F , 3A–F , 4A, B , 5C .

1850 Graptolithus testis sp. nov.; Barrande 1850: 52, pl. 3: 19–21.

Abridged synonymy follows:

1913 Monograptus testis inornatus Elles ; Elles and Wood 1913: 446, pl. 44: 7a, b; text−fig. 307a, b.

1974 Monograptus (Testograptus) testis (Barrande) ; Urbanek and Teller 1974: 237, text−figs. 1–5; pl. 22: 1–3; pl. 23: 1, 2; pl. 24: 1.

1990 Monograptus testis (Barrande) ; Lenz 1990: 177, fig. 1: 1–12; fig. 2: 1–10.

1991 Monograptus testis testis (Barrande) ; Lenz and Melchin 1991: 231, fig. 8a–c.

1991 Monograptus testis incomptus ssp. nov.; Lenz and Melchin 1991: 231, figs. 7a, b, 17D, E, I, J.

1999 Testograptus testis (Barrande) ; Kozłowska−Dawidziuk 1999: 355, fig. 2C.

2001 Testograptus testis incomptus Lenz and Melchin ; Lenz and Kozłowska−Dawidziuk 2001: 11, pl. 3: 1, 10, 15.

2006 Testograptus testis (Barrande) ; Lenz and Kozłowska 2006: figs. 13.1, 13.4, 13.7.

Material. —More than 30 immature, isolated specimens from the Cyrtograptus lundgreni Biozone , upper Wenlock, of Rookery Creek (RC01−2; 75 ° 22.5’N, 95 ° 41’W), and several from Snowblind Creek (SB−E; 75 ° 11.6’N, 93 ° 51’W), northeastern Cornwallis Island, two of which show a well−developed pseudovirgular growth mode. Associated with Cyrtograptus lundgreni Tullberg, 1883 , the index species for this biozone, the very common retiolitid Eisenackograptus eisenacki Kozłowska−Dawidziuk, 1990 , and Cyrtograptus hamatus ( Bailly, 1861) , C. radians Törnquist, 1887 , and Testograptus cubitus Lenz and Kozłowska, 2006, the last three taxa restricted to the Cyrtograptus lundgreni Biozone.

Remarks.—For this paper, and following Loydell and Cave (1994) and Lenz and Kozłowska−Dawidziuk (2001), the name “ Testograptus ” is used in the full generic sense, rather than as a subgenus as originally proposed by Příbyl (1967). This species has been earlier described by Urbanek and Teller (1974) and Lenz (1990), both studies being based on isolated, undeformed specimens. Beyond the details in these descriptions, further important morphological features include the abrupt ventral deflection in the post−sicular region of the rhabdosome (already noted by Urbanek and Teller 1974), a positioning of the proximal rim of the porus 0.06–0.08 mm proximal of the sicula aperture ( Fig. 2E 2 ), and the very rare occurrence of development by means of a pseudovirgula ( Fig. 4A, B ), the last feature paralleling that seen in rare specimens of Cochlograptus veles . Additional features include thecal overlap ranging from about 50% proximally to about 75% distally, and proportion of rhabdosomal width occupied by free metathecal portions consistently about 25–30%. In addition, the angle formed between the interthecal septum and the dorsal wall of the rhabdosome decreases from 70–80 ° to only 40–50 ° distally, as the degree of thecal overlap increases. Relevant comparative morphological details for Testograptus testis and Cochlograptus veles are detailed in the next section. 2TRD from second to fourth thecae 1.1–1.4 mm; width at theca 1 about 0.7 mm, and 0.8 mm at theca 5.

The range of variation in proximal rhabdosomal curvature seen in the present collections suggests that the supposedly distinct subspecies T. testis testis , T. testis incomptus , and T. testis inornatus may be part of a single, variable population, rather than being separate subspecies, as earlier accepted by Lenz and Melchin (1991).

A recently described species of Testograptus , T. cubitus Lenz and Kozłowska, 2006, from the Cyrtograptus lundgreni Biozone of Arctic Canada ( Lenz and Kozłowska 2006), but distinctly differing from T. testis , also demonstrates the abrupt post−sicular ventral deflection with a prominent, keel−like protuberance, exactly as seen in the typical form.

Figures

Fig. 2. Silurian graptolites Cochlograptus veles (Richter, 1871) (A, B) and Testograptus testis (Barrande, 1850) (C–F) in direct comparison. Chemically cleared and infrared views of specimens from collection SJF−02, talus nodule 2A, Cape Sir John Franklin, Devon Island. A. Variously oriented views of specimens with two developed thecae and common canal of theca 3, GSC34908; A1, profile view; A2, enlargement showing normal porus; A3, dorso−lateral view; A4, ventro−lateral view. B. Ventral and lateral views of specimen, GSC34909; B1, ventral view showing tip of sicula (clear region) and well marked thickened interthecal septum (arrow); B2, profile showing well−developed nema. (C–F) Testograptus testis (Barrande, 1850). Chemically cleared and infrared images of specimens from collection RC01−2, 2.5 m, Rookery Creek, Cornwallis Island. C. Largest specimen profile showing deflexed nema distal of sicula tip, and hollow, keel−like protuberance distal of sicula tip (arrow), GSC34910. D. Profile of specimen with only gentle ventral flexure and moderately curved sicula, GSC34911. E. Near−profile view of specimen and enlargement of distal end of sicular region, GSC34913; E1, profile view of cleared specimen; E2, enlargement showing normal porus. F. Specimen with well−preserved two proximal thecae, strong ventral flexure, and hollow protuberance (arrow) distal of tip of sicula, GSC34913. Scale bars 100 µm.

Fig. 3. Silurian graptolite Testograptus testis (Barrande, 1850). All collections from section RC01−2, 2.5 m, Rookery Creek, Cornwallis Island. A. Largest specimen, stereopair profile and ventro−lateral view, GSC34914; A1, profile view; A2, ventro−lateral view showing long, laterally projecting thecal spines. B. Stereopair of specimen with very strong dorsal flexure and prominent keel−like protuberance at point of flexure, GSC34915. C. Lateral and ventro−distal view of specimen, GSC34916; C1, with strongly dorsally flexed nema; C2, nema strongly ventrally flexed.D. Various views of specimen GSC34917; D1, dorso−lateral view stereopair; D2, ventro−lateral view stereopair showing prominent keel−like protuberance at point of flexure; D3, ventral view; D4, dorso−lateral view, V−shaped protuberance at point of flexure. E. Steropair of specimen with strong ventral flexure and prominent protuberance distal of sicular tip, GSC34918. F. Dorso−lateral and lateral views of specimen, GSC34919; F1, dorso−lateral view showing prominent flexure; F2, profile view. Scale bars 500 µm.

Fig. 4. Silurian graptolites Testograptus testis (Barrande, 1850) (A, B) and Cochlograptus veles (Richter, 1871) (C–F) in direct comparison. Specimens from SB−E, 49 m, Snowblind Creek, Cornwallis Island. A. Fragment of specimen with pseudovirgula development, GSC34920; A1, stereopair showing isolated nema; A2, enlargement to show nema area, and infilling between pseudovirgula and nema. B. Stereopair of dorso−lateral view of specimen with free nema and subsequent development of rhabdosome via a pseudovirgula, GSC34921. (C–F) Cochlograptus veles (Richter, 1871); from CP98, 6.4 m, Cape Phillips, Cornwallis Island (C, F) and from CM3, 6.1 m, Cape Manning Cornwallis Island (D, E). C. Extremely well−preserved specimen, showing blunt−ended sicula and flexure; note well−developed rutellum, GSC34922. D. Ventro−lateral view of well−preserved specimen showing four complete thecae, GSC34923. E. Infrared image showing strongly curved sicula, GSC34924. F. Views of a specimen showing a pseudovirgula (arrows) and exceptionally long rutellum, GSC34925; F1, distal view enlargement; F2, SEM image; F3, infrared image.

Fig. 5. Line drawings of four monograptid species to compare thecal characteristics. A. Monograptus? cf. ayagusensis (Obut and Sobolevskaya, 1966) compressed mature specimen from Twilight Creek (TC), Bathurst Island, TC 27.5 m (Melchin 1989) showing thecae very similar to those seen in Cochlograptus veles, GSC34926. B. Cochlograptus veles (Richter, 1871), compressed mature specimen from Twilight Creek, Bathurst Island, TC 27 m (Melchin 1989), ROM46027. C. Testograptus testis incomptus (Lenz and Melchin, 1991) holotype, compressed specimen from Rookery Creek (RC), Cornwallis Island, RC 284 m (Lenz and Melchin 1991: figs. 7B, 17I), GSC95423. D. Monograptus flemingii (Salter, 1852), compressed specimen from Prairie Creek (PC), Northwest Territories, PC 210 m (Lenz 1988: pl. 1: A), GSC91494.