Despite extensive and comprehensive research on the origin, phylogeny and astogeny of the linograptids, mostly by Jaeger, 1959, Jaeger, 1960 and Jaeger, 1969 and Urbanek, 1963, Urbanek, 1970, Urbanek, 1997a and Urbanek, 1997b, the issue of the origin of the linograptids remains unsolved. The ancestor of the Linograptinae probably lived as early as the Late Wenlock (Urbanek, 1970). Lobograptus? sherrardae Sherwin, 1974, from the praedeubeli Biozone, was proposed as the common ancestral species for the three major Ludlow lineages: the Linograptinae, the Cucullograptinae and the Neocucullograptinae (see Fig. 2, p. 30 from Urbanek and Teller, 1997). Rickards et al. (1995) on the other hand remain sceptical about this idea, because there are too many crucial differences between L.? sherrardae and Neodiversograptus nilssoni Lapworth, 1876 sensu Urbanek, 1954, considered the earliest known representative of the subfamily Linograptinae.

Although the origin of Linograptinae is not clear, the relationships within three older genera of this subfamily (Neodiversograptus, Linograptus and Abiesgraptus) were exhaustively described by Urbanek, 1963 and Urbanek, 1997a. They were all considered chronospecies (Urbanek, 1997a). Temporal species represented by Neodiversograptus, Linograptus and Abiesgraptus display a transformation of the one-stiped monograptid into a multiramous one and replace each other in time.

Describing the species of the subfamily Linograptinae as chronospecies Urbanek (1997a) did not take P. orangensis into consideration. He supposed that the Australian species came from the border of nilssoni- progenitor or bottom of the scanicus Biozone and represents a stage of development comparable to N. beklemishevi. Urbanek's paper of 1997 was actually written in 1995 (Urbanek, 1997a, p. 233), before Rickards and Wright (1997) identified Prolinograptus and described another species of this genus, the P. packhami. Moreover, as shown by Rickards et al. (1995) and Rickards and Wright (1997), Prolinograptus overlaps with the range of Linograptus, which means they cannot be chronospecies.

Regarding the Australian material, Rickards and Wright (1997) suggested a division of the genus Linograptus into two groups, both originating probably from Neodiversograptus (Fig. 3). This means that linograptids would not be a single line of ancestral-descendant succession but at least two different directions of evolution. One group is represented by Linograptus and Neodiversograptus species, except Neodiversograptus palmeri Rickards, Packham, Wright and Williamson 1995. This line corresponds to the chronospecies described by Urbanek (1997a).

These forms evolved increasingly introverted thecae, from straight apertures to the rhabdosomal axis as in N. nilssoni (Fig. 4), to slightly turned inwards or even strongly introverted thecae as in Linograptus posthumus introversus Rickards and Wright, 1997 (Fig. 4A, B). The second line is represented by much less examined species: N. palmeri, P. orangensis and P. packhami. This is a group of very diminutive specimens, with slender stipes and thecal apertures everted to varying degrees (Rickards and Wright, 1997) (Fig. 4E). We agree with Rickards and Wright's (1997) suggestion of the existence of two lineages of linograptids.

Size reduction might have been caused by the continuation of a lilliput effect that followed the lundgreni Event (Urbanek, 1993). But in contrast to the most monograptid genera, the Prolinograptus retained small and long rhabdosomes. After the crisis it may have had to compete with larger graptolites that spread to more areas. Prolinograptus was far less frequent than the other genera of the subfamily Linograptinae. In spite of this, the genus was present in Baltica (Poland), Laurentia (Arctic Canada) and Australia (Fig. 1).

Order: GRAPTOLOIDEA Lapworth, 1873

Family: MONOGRAPTIDAE Lapworth, 1873

Subfamily: LINOGRAPTINAE Obut, 1957

Genus Prolinograptus Rickards and Wright, 1997

Type species: P. packhami Rickards and Wright, 1997.

Species included: P. orangensis Rickards, Packham, Wright and Williamson 1995 and P. packhami Rickards and Wright, 1997.

P. packhami Rickards and Wright, 1997

Fig. 5 and Fig. 6.

1997 P. packhami Rickards and Wright; Rickards and Wright, 1997: 226, text-figs. 9K, L, N, O, 10A.

2004 P. packhami Rickards and Wright?; Lenz and Kozłowska-Dawidziuk, 2004: 37, pl. 2: 1–7.

Material examined: Three siculae with the first thecae (th¹), well-preserved parts of rhabdosomes: one fragment with six thecae, four fragments with three thecae, 20 fragments with two thecae, and 50 fragments with one theca.

Diagnosis: After Rickards and Wright (1997).

Description: All the studied fragments are very slender and mostly straight; only two of them display a slightly, ventral curvature (Fig. 6D, E). The thecae are 0.15–0.20 mm wide at the aperture and 0.09–0.1 mm at the thinnest portion. Thecae are 1.3–1.5 mm long; the distance between the lips is 1.0–1.2 mm. Thecae number 3.5–4 in 5 mm. Thecal apertures are strongly everted with thick rims (Fig. 5E–G). The shape of thecal apertures is characteristic with its large distal indentations (Fig. 4 and Fig. 6). Siculae are straight, 0.9–1.15 mm long, with apices reaching 2/3 along the length of the first theca (th¹). Diameter of the sicular aperture 0.22 mm. One of the siculae has an additional fuselli on the dorsal side of the sicular aperture (Fig. 6A–C). The other two siculae do not have dorsal apertural processes or sicular cladium. Virgella directed proximally. The whole thecae are covered with pustular micro-ornamentation that is densely and irregularly spaced (Fig. 5C–G). One short fragment of the rhabdosome with an unidentified growth about 0.3 mm long perhaps represents some remnant of a parasite or an abnormally developed thecal aperture (Fig. 6G).

Remarks: Compared with the material described by Rickards and Wright (1997) there are differences in stratigraphic occurrence, the size of the sicula, and the large apertural distal indentations (Fig. 4 and Fig. 6). The Australian P. packhami occurs in the inexpectus/kozlowskii biozonal interval, while the form from the Mielnik IG-1 core appears earlier, in the praecornutus Biozone (Fig. 3). The siculae of the Mielnik IG-1 specimens are bigger than the Australian ones. Their minimum length corresponds to the maximum of the Australian siculae. The apertural indentations in P. packhami described by Rickards and Wright (1997) are only slight, while the Mielnik IG-1 material displays a very clear one (Fig. 4E). Rickards et al. (1995) described these indentations as “apertures (…) geniculated above thecal aperture”. No cladia were found in the Mielnik IG-1 material. The micro-ornamentation on the described specimens is recorded for the first time for the monograptids.

The sicular cladia are one of the most important diagnostic features of the genus Prolinograptus and subfamily Linograptinae (Rickards and Wright, 1997), but not every individual in the population must have them; for example, the specimens of P. packhami (Rickards and Wright, 1997, Fig. 9K, O, p. 223) or the specimen of P. orangensis described by Rickards et al. (1995, Fig. 35G, p. 60) formed at least 15 thecae and no cladium. Only one of the siculae of Prolinograptus from Poland has an additional fuselli that might be the beginning of the first cladium (Fig. 6A–C). The lack of a well-developed cladium in the Mielnik IG-1 specimens does not question its affinity with Prolinograptus. The Polish material presents only a portion of a very rare population of this genus, and that might be the reason why no cladium was found.

According to SEM observations the specimens display numerous pustules, irregularly spaced on the surface of the rhabdosome, including the apertural lips (Fig. 5 and Fig. 6). This is an extraordinary morphological feature for monograptids, which usually have smooth bandages. Until now a similar micro-ornamentation has been known only in retiolitids (Retiolitidae), but unlike P. packhami from Poland, the retiolitids usually display a regular spacing of the ornamentation. Two types of micro-ornamentation have been considered one of the most important characters to distinguish the retiolitids into the two subfamilies Retiolitinae and Plectograptinae (Bates et al., 2005, Lenz and Kozłowska, 2007 and Lenz and Melchin, 1987).

Although some of the P. packhami described by Rickards and Wright (1997) display a gently ventral curvature (i.e. Rickards and Wright, 1997, Fig. 9N, p. 223), Rickards and Wright (1997) did not say anything about it in the diagnosis of the genus or the species. The Mielnik IG-1 material also contains two slightly, ventrally curved specimens (Fig. 6D, E). But because such forms in the population are few, it probably cannot be a diagnostic feature of the species.

Parasitism on graptolites is a rare phenomenon, but when it happens, it may be manifested as outgrowths of periderm (Bates and Loydell, 2000). Perhaps the growth on one of the examined fragments of P. packhami (Fig. 6G) is a blister formed as a result of the activities of some parasite. However, because the specimen is fragmentary, accurate identification is difficult.

As one of the major features of this genus, Rickards and Wright (1997) mentioned little or no increase in dorso-ventral width throughout the colony. Our material as well as measurements of specimens from Australia (Rickards and Wright, 1997) do not agree with this view. In extreme cases, differences in the width of the thecae reach up to 100% in the Australian material (Table 1). Fragmentary material from the Mielnik IG-1 core also displays a distinction among the thecae, but the differences are smaller. This may be because only the proximal and medial thecae are present in the examined material.

The present study suggests that P. packhami Rickards and Wright, 1997, one of the members of the subfamily Linograptinae, was not geographically restricted to Australia as previously thought (Rickards and Wright, 1997 and Rickards and Wright, 1999). Apart from Australia the genus Prolinograptus appeared in Poland, Baltica as well as in Arctic Canada, Laurentia (Fig. 1).

These are also new data on the stratigraphic occurrence of P. packhami, that show it ranged through at least five zones from the praecornutus (present paper) to the inexpectatus- ozlowskii Biozones (Rickards and Wright, 1997). SEM observations of isolated specimens enabled detailed studies on morphology and description of the micro-ornamentation unprecedented in other monograptids.