The new retiolitid species, Spinograptus tubothecalis, is described from the Colonograptus praedeubeli and C. deubeli biozones from two localities in Poland: a borehole on the East European Platform and the Holy Cross Mountains. This was a recovery phase after the severe Silurian Cyrtograptus lundgreni Event. The new species has a unique, previously undescribed form of finite rhabdosome. Unlike the species Spinograptus reticulolawsoni and S. lawsoni, in which the finite rhabdosomes taper distally, its rhabdosome is parallel-sided with the two distal thecae developed as isolated tubes without genicular processes, with a small appendix between them. The new species also has preserved membranes of the sicula, thecae and ancora sleeve, similar to a few species of Spinograptus from the lower Homerian. Spinograptus tubothecalis, like Spinograptus clathrospinosus and S. spinosus, has paired reticulofusellar genicular processes on the pre-thecal ventral orifices, similar to but shorter than thecal processes. Transverse rods, a rare character in post-Cyrtograptus lundgreni Event retiolitids occur in the new species in rudimentary form.
The last 25 years of study of retiolitid graptolites has brought to light more information about the evolution, anatomy, astogeny, and mode of life of these colonial organisms (Bates and Kirk 1984, 1992, 1997; Lenz and Melchin 1987; Lenz 1993; Kozłowska-Dawidziuk 1995, 1997, 2004) than the previous one hundred years. This is largely due to the use of isolated specimens and the scanning electron microscope (SEM).
Spinograptus spinosus was one of the first retiolitids to be described, about 110 years ago, by Wood (1900). It was illustrated in three small line drawings of flattened specimens. Today we know of eight species of Spinograptus, and the ultrastructural details of three dimensional specimens. They vary in many characters, such as the development of the paired reticulofusellar processes on the geniculum, and the development of the reticulum of the ancora sleeve and thecae (Kozłowska-Dawidziuk 1997; Kozłowska-Dawidziuk et al. 2001). These species appeared during a diversification phase of the plectograptinid retiolitids (Porbska et al. 2003), which followed the Cyrtograptus lundgreni Event, one of the most severe extinction events experienced by graptolites. The youngest species of Spinograptus known is from the Neodiversograptus nilssoni Biozone (Fig. 1), except in Arctic Canada where two species reach the Saetograptus linearis--Monograptus ceratus Biozone (Lenz and Kozłowska-Dawidziuk 2004).
The paper describes the new species Spinograptus tubothecalis with its new, unique type of finite rhabdosome.
Institutional abbreviations.—ZPAL, Institute of Paleobiology Polish Academy of Sciences, Warsaw, Poland.
New observations on retiolitid colony development
Many retiolitids, as with most other graptoloids, grew rhabdosomes along linear stipes that are open-ended. They have almost parallel walls or widen distally, such as Stomatograptus Tullberg, 1883 and the youngest and smallest, Plectodinemagraptus gracilis Kozłowska-Dawidziuk, 1995 (Kozłowska-Dawidziuk 2004: fig. 1).
There are some retiolitids, as well as some other graptolites (e.g., Corynites wyszogrodensis Kozłowski, 1956), that have finite ends to their colonies. Many retiolitids have rhabdosomes that taper distally and are ended by an appendix. They belong to the Gothograptus lineage, appearing in the late Sheinwoodian, (e.g., Gothograptus Frech, 1897 and Eisenackograptus Kozłowska-Dawidziuk, 1990), and continue to the end of the Gorstian (Neogothograptus Kozłowska-Dawidziuk, 1995 and Holoretiolites Eisenack, 1951). The rhabdosomes vary from 23 pairs of thecae in Gothograptus nassa Holm, 1890 to two pairs of thecae in Neogothograptus alatiformis Lenz and Kozłowska-Dawidziuk, 2004 (Kozłowska-Dawidziuk 2004: fig. 1).
The genus Spinograptus has quite a large variation of forms with strong differences in the development of characters, such as reticulum density and the shape of the reticulofusellar genicular processes (Fig. 1). The most striking and variable character is the end of the colony. There are three different types of endings of Spinograptus rhabdosomes: open ended (e.g., S. spinosus Wood, 1900), finite tapering distally ended by an appendix (S. reticulolawsoni Kozłowska-Dawidziuk, 1997, S. praerobustus Lenz and Kozłowska-Dawidziuk, 2002, and S. lawsoni Holland, Rickards, and Warren, 1969), and the new type described here. It is finite and parallel-sided distally, with a short appendix (Figs. 2–5), isolated from the distal thecae.
The finite rhabdosome with parallel walls to the end of the rhabdosome represents a newly discovered type of colony, found only in the new species Spinograptus tubothecalis (Figs. 2–5). Its rhabdosome is short, having three to four pairs of thecae with a diminutive appendix between the last two thecae. In this new type the nema is connected to the wall just below the appendix (Figs. 2A1, A3,4A, B3), whereas in Gothograptus and Eisenackograptus it is connected in the proximal and distal part of the rhabdosome respectively, and runs along the wall of a usually long appendix. The two distal thecae are developed differently from the others, being partly isolated tubes without characteristic paired genicular processes (Figs. 2A1, 3C, 4, 5). Their orifices are surrounded by thecal lip, genicular and pleural lists (Fig. 3B, C).
The graptolites were recovered following slow dissolution of the host carbonate in acid (1–10% HCl). A fine hairbrush was used to pick up and transfer specimens. The material is stored in plastic containers in glycerin and on SEM stubs (10b, 18b, 19b, 324) at ZPAL.
Order Graptoloidea Lapworth, 1873
Family Retiolitidae Lapworth, 1873
Subfamily Plectograptinae Bouček and Münch, 1952
Genus Spinograptus Bouček and Münch, 1952
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.
Spinograptus tubothecalis sp. nov.
Figs. 2A, B, D, E, 3A–D, 4.
Etymology: From Latin tubo, tube and theca, theca; due to the shape of the two last thecae.
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.
There is a large variation in the development of retiolitid rhabdosomes, e.g., open ended with many thecae to finite with few thecae. Typical for some finite rhabdosomes is the appendix, possibly a modified last theca ending the rhabdosome. Usually one type of rhabdosome is characteristic of each retiolitid genus. However, in Spinograptus we observe some variation of rhabdosome development, both open ended and finite. S. tubothecalis sp. nov. represents a new type of finite rhabdosome, never observed before. Its short rhabdosome has two tube-shaped, isolated distal thecae, with rounded openings built by thecal lip, genicular and pleural lists, without processes. The thecae are directed upwards and between them there is the small appendix.
The transverse rods found in S. tubothecalis sp. nov., a character present in pre-Cyrtograptus lundgreni Event, shows a combination of primitive and advanced characters in Spinograptus, implying a mosaic pattern of its evolution. The new material gives us new data about graptolite evolution.
Aleksandra Hołda-Michalska (ZPAL) is thanked for the improvement of the figures and Grazyna Matriba (ZPAL) for technical assistance. AK and KD acknowledge financial support from the Ministry of Sciences and Higher Education, Poland, decision number 499/N-NSERC/2009/0.