Type species: Retiolites spinosus Wood, 1900. Lectotype BU 1366, figured Wood 1900: pl. 25: 29A, selected and refigured Elles and Wood 1908: pl. 34: 16a (Strachan 1971: 48). Lower Ludlow, Neodiversograptus nilssoni Biozone, West borderland, United Kingdom.

Species included.—Retiolites spinosus Wood, 1900; Retiolites clathrospinosus Eisenack, 1951; R. münchi Eisenack, 1951; Holoretiolites (Balticograptus) lawsoni Holland, Rickards, and Warren, 1969; Agastograptus quadratus Lenz, 1993; Spinograptus reticulolawsoni Kozłowska-Dawidziuk, 1997; S. latespinosus Kozłowska-Dawidziuk, 1997; S. praerobustus Lenz and Kozłowska-Dawidziuk, 2002; S. tubothecalis sp. nov.

Emended diagnosis (modified from Maletz 2010).—Nema free, exceptionally in two species attached distally, to obverse wall of short appendix; shallow ancora umbrella; outer ancora may be present; pre-thecal ventral orifices sometimes with paired processes, sometimes overgrown by reticulum; proximal lateral orifices above ancora umbrella medium sized; ventral wall formed of lip, genicular and lateral apertural lists; rudimentary mid-ventral lists and transverse rods sometimes present; ancora sleeve formed of oblique lists arranged in zigzag pattern; reticulum may be present on ventral and lateral walls; paired, reticulofusellar genicular processes. Rhabdosomes may be open-ended or finite with short appendices, the finite rhabdosomes may taper distally or exceptionally be parallel-sided with two distal isolated thecae with openings directed distally.

Remarks.—Our observations of the new species Spinograptus tubothecalis show a new character within Spinograptus, which is also unique among post Cyrtograptus lundgreni retiolitids: the nema attached distally to the obverse wall. This character is also possible in S. reticulolawsoni, but is not so clearly seen because of the presence of membranes in a flattened specimen (Kozłowska-Dawidziuk 1997: fig. 10A, B). An outer ancora, not mentioned in Maletz‘s (2010) diagnosis, is present in three species: S. reticulolawsoni, S. clathrospinosus, and S. tubothecalis sp. nov. Maletz (2010) described parietal lists of the lateral walls often arranged in an irregular zigzag pattern, but in the referred material the zigzag pattern is rather regular.

Etymology: From Latin tubo, tube and theca, theca; due to the shape of the two last thecae.

Type material: Holotype ZPAL G. 47/1, stub 18b, Figs. 2A, 3C; paratype ZPAL G. 47/6, stub 324, Fig. 4A.

Type locality: Goldap IG-1 borehole, depth 1267.0 m, Poland, East European Platform.

Type horizon: Colonograptus deubeli Biozone, Homerian, Wenlock, Silurian.

Referred material.—The well preserved isolated, slightly flattened specimens come from two localities: the Goldap IG-1 borehole, depth 1267.0 m and a nodule from Prgowiec 2, Holy Cross Mountains, Poland. Goldap IG-1 contains one finite rhabdosome and 65 fragments, mostly proximal parts of young and mature rhabdosomes of S. tubothecalis sp. nov.; Plectograptus robustus, and monograptids: Colonograptus deubeli, Pristiograptus dubius ludlowensis, Pristiograptus ludensis, representing the Colonograptus deubeli Biozone. From the second location, Prgowiec 2, are 800 specimens, mostly of the medial part of rhabdosomes, several proximal and distal ones, together with Neogothograptus reticulatus Kozłowska, Lenz, and Melchin, 2009; Plectograptus? and Colonograptus colonus, representing the Colonograptus praedeubeli Biozone.

Diagnosis.—Parallel-sided, densely reticulated rhabdosome terminated by small appendix located between two distal thecae. Rhabdosomes contain three to four pairs of thecae. Pre-thecal ventral orifices with paired short, apertural processes; in mature rhabdosomes orifices may be overgrown by reticulum. Paired reticulofusellar apertural processes similar to Spinograptus clathrospinosus. Two distal thecae tube-shaped, and isolated, having rounded openings built by lip, lateral apertural rod and geniculum without processes. Reticulated outer ancora present in some specimens. Nema free through rhabdosome except species with finite rhabdosomes where most distal part of nema is included in the appendix wall.

Description.—The holotype ZPAL G. 47/1, a well preserved finite rhabdosome, is 2.64 mm long, and bears three pairs of thecae (Figs. 2A₁, 3C). There are some fragments, especially from Prgowiec 2, indicating rhabdosomes with four pairs of thecae. It is about 1 mm wide between the pre-thecal ventral orifices, and about 0.93 mm between the second pair of thecae. The ancora umbrella is asymmetric, typical for Spinograptus, with proximal lateral orifices located on th1² side (Fig. 2A₁, A₂). The reticulum on the ancora umbrella, as well as on the outer ancora is developed only on the th1¹ side, mainly in mature rhabdosomes in specimens from Goldap (Fig. 2A₄, B₁, D). The reticulum is denser in specimens from Goldap than from Prgowiec 2 (see Figs. 2A, B, D, 4). The pre-thecal ventral orifices have paired processes half the size of the thecal processes. The orifices are sometimes overgrown by reticulum (Fig. 2B₁, D). The apex of the well preserved sicula reaches to the end of second pair of thecae; the length of sicula is 1.1 mm. The two first pairs of thecae have paired reticulofusellar processes about 0.7 mm long. The last theca in the holotype is developed as a partly isolated tube with rounded openings made by the genicular, pleural and thecal lip lists without paired apertural processes (Fig. 2A₁). The specimens from Prgowiec 2 have two distal thecae (Fig. 4) similarly developed as the last theca in the holotype. The isolated parts of the distal thecae are about 0.5 mm long. Between the distal thecae is a small appendix of about 60 µm diameter (Figs. 2A₁, 4A). In a young specimen with one theca developed the outer ancora has already started to grow (Fig. 2E). Mid-ventral lists are short, developed only at the first pair of thecae (Fig. 2A₁). The rudimentary transverse rods are at first pair of thecae (Fig. 2B₂). The dense reticulum of the ventral walls makes it difficult to check the presence of transverse rods in the most distal thecae. The nema is free up to the appendix, where it is attached to the wall on the obverse side of the rhabdosome (Figs. 2A₁, A₃, 4A, B₃). Very dense reticulum occurs in mature rhabdosomes, developed in the thecal and ancora sleeve walls (Fig. 2A₁). Among specimens from both localities there are several with the sicular and thecal membranes preserved (Fig. 3A–D).

Discussion.—The distal end of the new species differs from that in other species of Spinograptus. The last two thecae, partly isolated, have unusual rounded openings without genicular processes (Figs. 2A₁, A3, 3B, C, 4A, B₁, B₂). Such openings are unique among all retiolitids. The rudimentary appendix in the new form is similar to that in other species of Spinograptus with appendixes, but does not extend more distally than the last thecae.

Spinograptus tubothecalis sp. nov. belongs to the older group of Spinograptus species, occurring in Colonograptus praedeubeli and C. deubeli biozones (Fig. 1). Most of them have, unusually for the retiolitids, well preserved peridermal membranes, both thecal and ancora sleeve (S. lawsoni, S. latespinosus, S. reticulolawsoni, S. praerobustus, and S. tubothecalis sp. nov.). This kind of preservation does not usually occur in other retiolitids (Lenz 1994a, b; Kozłowska-Dawidziuk 1997; Lenz and Kozłowska-Dawidziuk 2002). The new species probably had an ancora sleeve membrane (Fig. 5), which was too thin to be preserved.

Finite rhabdosomes occur in four species of Spinograptus together with the new one; the new species representing at the same time the new type of the finite rhabdosome. The new species is similar to S. reticulolawsoni Kozłowska-Dawidziuk, 1997 in having a finite colony ended by a short, rudimentary appendix. The most significant difference is in the sides of the finite rhabdosomes, which taper distally in S. reticulolawsoni and are parallel in the new species (Fig. 1).

The older species of Spinograptus: S. reticulolawsoni, S. latespinosus, and S. clathrospinosus have a dense reticulum. The new species S. tubothecalis has the densest reticulum of all the Spinograptus species, not only on the ancora umbrella and the thecal and ancora sleeve walls, but the reticulum of the outer ancora is also much denser than in other species, e.g., S. clathrospinosus, and is located at the th1¹ side of rhabdosome (Fig. 2C₂). The pre-thecal ventral orifices are sometimes overgrown by dense reticulum. Among Spinograptus species the paired genicular processes, present in the new species, occur also in S. spinosus and S. clathrospinosus. Another feature of the new species is the presence of a rudimentary transverse rod, which is characteristic in pre-Cyrtograptus lundgreni retiolitids.

Geographic and stratigraphic range.—Goldap IG-1 borehole, East European Platform, and Prgowiec 2, Holy Cross Mountains, Poland; Colonograptus deubeli Biozone and Colonograptus praedeubeli Biozone respectively.