Introduction

Pristiograptus dubius (Suess, 1851) ranges from the Monograptus riccartonensis Biozone of the Wenlock to the Monograptus perneri Biozone of the Přidoli and thus is one of the longest-ranging graptolite taxa known (Fig. 1). Both thecae and rhabdosome are simple. The Pristiograptus stem lineage represents a type of evolution that shows numerous new lineages arising by means of iterative speciation (Urbanek 1998).

Pribyl (1943) first distinguished more than a dozen Pristiograptus species and subspecies, based on flattened material from the Czech Republic. The first detailed studies of isolated, three-dimensional Pristiograptus dubius forms were made by Urbanek (1953) who also reviewed some earlier papers and then followed by Urbanek (1959, 1997) and Walker (1953) based on Ludlow material from Poland and northern Germany, respectively. These studies noted the presence of thickening rings in the sicula. Urbanek's (1997, 1998) studies, additionally, gave a general view of the extraordinary evolution of the P. dubius group.

Jaeger (1959, 1991) also studied the Pristiograptus in Germany using isolated material. Rickards (1965) investigated pristiograptids from Northern England and from Australia (Rickards and Wright 1999), and an attempt at understanding the evolution of the P. dubius group was presented by Rickards and Wright (2003).

Holland et al. (1969) described a new species P. jaegeri Holland, Rickards, and Warren, 1969 from the upper Wenlock and lowermost Ludlow of Great Britain. More detailed studies of that important appearance and succession of Pristiograptus species after the Cyrtograptus lundgreni extinction Event were made by Ulst (in Gailite et al. 1967; Ulst 1974), based on material from Latvia. She described the diminutive Pristiograptus parvus Ulst, 1975, the only monograptid taxon to appear immediately after the most severe extinction event in graptolite history.

Pristiograptus taxa from the Llandovery to Ludlow interval of Lithuania were studied by Paskevicius (1974, 1979). The most recent revision of P. dubius morphotypes, based on isolated Wenlock material from Lithuania, was made by Radzevičius and Paškevičius (2000), and Radzevičius (2003, 2006, 2007).

The Cyrtograptus lundgreni Extinction Event was probably the only one to severely affect the evolution of P. dubius lineage. It survived other less severe events that extinguished many co-existing taxa. This exceptional survivability of P. dubius may probably be explained by eurybiotic nature of its adaptation and lack of specialization. This is why Jaeger (1991) called it “a generalist”. The long period of existence of P. dubius is doubtlessly related to its ability to adapt to environmental changes, without responding by any considerable morphological change.

Fig. 1.

Graptolite phylogenetic relationships between the iterative of Pristiograptus dubius group taxa species and subspecies from Poland and Lithuania and their stratigraphical ranges. For more data concerning iterative clades that branched off the stem lineage see Fig. 4. Generalized graptolites biozones (Koren' et al. 1996) correlated with biozones of Poland and Lithuania (Urbanek and Teller 1997; Radzevičius 2007). Abbreviations: B., Bohemograptus; Col., Colonograptus; Cucullo., Cucullograptus; Cyrto., Cyrtograptus; d., dubius; G., Gothograptus; I., Istrograptus; L., Lobograptus; M., Monograptus; Neocol., Neocolonograptus; Neocucul., Neocucullograptus; Neodiver., Neodiversograptus; Neolob., Neolobograptus; P., Pristiograptus; S., Saetograptus; Slov., Slovinograptus; t., transgrediens; U., Uncinatograptus.

The purpose of this paper is an attempt to understand the evolution of P. dubius group and show its iterative character, extraordinary among graptolites. The paper describes the morphological changes and stratigraphic distribution of isolated P. dubius forms from Wenlock and Ludlow strata of Poland and Lithuania (Fig. 2).

This paper employs terminology (Fig. 3) from Urbanek (1997) and Radzevičius (2003).

Institutional abbreviations.—ZPAL G., Institute of Paleobiology, Polish Academy of Sciences; VU, collection of Vilnius University, Department of Geology and Mineralogy, Geological Museum.

Other abbreviations.—L, length; W, width; ß, the angle between the thecal lip and the free ventral wall of succeeding theca; λ, the angle between interthecal septum (thecal axis) and virgula; th, theca; th¹, first theca (Fig. 3); SEM, Scanning Electron Microscope; K, borehole Kybartai; S, borehole Siupyliai; P, borehole Paežeriai.

Fig. 2.

Map of western margin of the East European Platform. A. Facies across the studied region (modified after Porębska et al. 2004). B. Location of the investigated boreholes in Lithuania and Poland.

Evolution in the Silurian Pristiograptus dubius (Suess, 1851) species group

The Pristiograptus dubius species group consists of (i) the proper P. dubius stem lineage represented by an unbroken succession of P. dubius populations (chronodemes), displaying essentially a morphological near-stasis, and therefore showing only limited change through time, (ii) representatives of a numbers of phyletic lines which appeared as the result of early divergence immediately after the P. dubius/Gothograptus nassa survival interval, as well as of a number (iii) of derivative species produced by means of iterative speciation from the stem lineage parental species, and displaying usually minor differences from it.

Phyletic evolution.—The concept of the stem lineage developed gradually. Urbanek (1997, 1998) and Rickards and Wright (2003) considered that a stem lineage is represented by a single species or even subspecies. Other observations suggested, however, a certain amount of variation between successive populations. Urbanek (1997) recognized a fluctuating variation expressed in the degree of robustness of the rhabdosome. A gracile morphotype is present in the Gorstian, represented by P. dubius frequens Jaekel, 1889, substituted at higher horizons (Pseudomonoclimacis latilobus— Monograptus [Slovinograptus] balticus biozones) by a much more robust morphotype, with taxa having strong virgula and thick virgella. A gracile morphotype, however, reappears again in higher horizons of the Ludfordian (P. dubius welchae).

Besides fluctuating morphological changes, Urbanek (1997) recognized a directional (anagenetic) trend in the number and placement of sicular rings (annuli). In the Gorstian, sicular rings vary in number (up to 8) and in position, while in the Ludfordian there are always 3 rings occupying the same position on the sicula (Fig. 1). Urbanek (1997) provisionally classified this latter form as P. dubius cf. frequens, relying on general similarity. In this paper, this form is distinguished as new subspecies P. dubius postfrequens subsp. nov.

Fig. 3.

Morphology of graptolite Pristiograptus rhabdosomes. A. Drawing of rhabdosome showing its characters (modified from Radzevičius 2003). B-E. Morphological features on flattened specimens. B. Pristiograptus lodenicensis Přibyl, 1948; VU-813, Likënai-396 borehole, Lithuania, depth 589.5 m; Cyrtograptus lundgreni Biozone, Riga Formation. C. Pristiograptus dubius magnus subsp. nov., VU-835, Parovėja-9 borehole, Lithuania, depth 599.8 m; Cyrtograptus perneri Biozone, whole specimen (C₁), enlargement showing thecal apertural lip (C₂). D. Pristiograptus dubius pseudodubius (Bouček, 1932), fragment of rhabdosome, VU-A3-0047a; C. lundgreni Biozone, Żdanów outcrop, Sudetes, Poland. E. Pristiograptus jaegeri Holland, Rickards, and Warren, 1969; Vilkaviškis-131 borehole, depth 1073.2 m, VU-8004, Colonograptus deubeli Biozone. Abbreviations: ß angle between thecal lip and wall of succeeding theca; λ, angle between interthecal septum (thecal axis) and virgule; r, sicula ring; th1, first theca; tal, thecal apertural lip.

Taking into account the stratigraphic range of the P. dubius lineage (from the Monograptus crispus Biozone of the Llandovery to the Cyrtograptus perneri Biozone of the Přidoli) and its duration (estimated by Jaeger 1991, as about 20 Ma, but in the light of currently available radiometric data the time span in question might be some 12 Ma), certain microevolution within the stem species seemed almost inevitable. This is also recently indicated by the studies of Radzevičius (2003) on chemically isolated material from deep boreholes of Lithuania and the Polish Lowland. He has suggested the following succession of subspecific variants within the initial portion of the stem lineage: Pristiograptus dubius dubius—P. dubius pseudodubius—P. dubius parvus—P. dubius frequens—P. dubius postfrequens subsp. nov. The succession of the remaining part of the stem lineage is still not recognized, the first of the mentioned subspecies being probably preceded by the diminutive and little known Upper Llandovery P. dubius initialis (Kirste, 1919) (compare Jaeger 1991). As the study of Pristiograptus species has been proceeding piecemeal, particular taxa were differently ranked (as species or subspecies), but in the above list all are treated as subspecies of P. dubius, taking into account limited morphological differences and the presence of transient forms during its long evolution.

In Europe immediately following the Cyrtograptus lundgreni extinction event, the P. dubius lineage is represented solely by the dwarf subspecies P. dubius parvus (see above), while pristiograptids from the P. dubius group with a normal size of rhabdosome reappear above the P. parvus horizon and are represented by P. dubius frequens (see Jaeger 1991). However, in other regions of the world the situation is different. In Central Asia Koren' and Urbanek (1994) were unable to recognize P. parvus in their carefully measured sections. It is likely that immediately above the C. lundgreni extinction horizon there, the retiolitid Gothograptus nassa (Holm, 1890) is associated with P. dubius of normal size. The same seems to be true for Arctic Canada (Lenz and Kozłowska-Dawidziuk 2002). It seems that in different, parts of the world the P. dubius lineage was represented in the early post-C. lundgreni time by coeval, vicarious subspecies, namely either by a dwarf variety P. dubius parvus or by normal-sized P. dubius. At present, these early post-C. lundgreni normal-sized representatives are insufficiently known morphologically and cannot be assigned taxonomically to any subspecies.

A special role in stratigraphy and phylogeny was played by the diminutive P. dubius parvus. However, the origin as well as the nature of this subspecies is disputed. According to Jaeger (1991) the dwarf variety was “always” present in P. dubius populations but because it was rare it remained unnoticed. However, under the stress conditions caused by environmental changes related to the C. lundgreni Event, this variety (probably a mutation) proved to be better adapted, in some parts of the world, than the normal forms with long rhabdosomes and underwent a rapid numerical increase, marking a distinct horizon (P. parvus Biozone). After the recovery of environmental conditions the dwarf mutation was replaced by populations showing a normal size of rhabdosome that had survived “somewhere”.

Urbanek (1993) emphasized that P. parvus populations display characteristics that he has called the post-event syndrome, namely a monospecific mass occurrence as well as reduced size of rhabdosome, that he called the Lilliput effect. Dwarfism in particular could be interpreted as an ecophenetic reaction on general impoverishment of environmental conditions and at the same time the main cause of mass extinction. Biological causation of the post-Event syndrome is discussed in Urbanek (1993), while Twitchett (2007) discussed a wide occurrence of the Lilliput effect close to horizons of mass extinction at different stratigraphic settings. All authors agree that in Europe the dwarf variety P. dubius parvus was later replaced by P. dubius frequens representing the mainstream evolution and being soon followed by one of the greatest population and adaptive radiation bursts among graptolites (Koren' and Urbanek 1994). The status of P. dubius parvus is difficult to evaluate: it differs from standard iterative taxa of the P. dubius group, as it completely replaced the normalsized form on a continental scale, making its first appearance immediately after a major biotic crisis. If we interpret the subnormal size of P. parvus as a result of the Lilliput effect, the range of its distribution might be interpreted as the area characterized by extreme environmental conditions. For these reasons we consider P. dubius parvus provisionally as a link in the evolution of the stem lineage taxa.

One of the products of the post-extinction big radiation was again the conservative P. dubius stem lineage, accompanied by the ancestors of a number of progressive lineages which split from the main stock of P. dubius as a result of early divergence. They exhibited a great evolutionary potential producing on the one hand cucullograptids, bohemograptids including the horned bohemograptids and, related to them, neo-cucullograptids as well as multiramous linograptids and on the other hand spinose monograptids (Colonograptus, Saetograptus) and specialized ones such as Colonograptus deubeli (Jaeger, 1959). At the same time the Pristiograptus dubius stem lineage emerged from an early divergences without greater morphological changes, representing a conservative morphotype, capable of further near-stasis. Great amounts of variation, needed as raw material for this diversification, were probably generated during the population burst, recorded taphonomically as mass abundances of graptolite remains (Koren'and Urbanek 1994) in the Pristiograptus dubius/Gothograptus nassa Biozone.

Iterative and parapatric speciation.—Therefore one should distinguish taxa produced during the early divergence, from Pristiograptus species repeatedly produced from the stem lineage and termed by Rickards and Wright (2003) as an iterative species. This term was also applied by Koren' and Urbanek (1994: 150) and Urbanek (1998: 561–563). These species are derivatives of stem lineage taxa, and were produced at different times, either before or after the P. dubius/G. nassa Event. As a rule they are usually of short duration, differing only in a few characters from the parental stem species. In their careful revision of iterative species derived from the stem lineage Rickards and Wright (2003) have distinguished some 5 “robust” and 8 “gracile” ones.

A classical example of an iterative speciation process may be the appearance of incipient bilateral apertural lobes in Neocolonograptus parultimus (Jaeger, 1975) which were developed from the persistent stem lineage of Pristiograptus dubius in the early Pridoli (Urbanek 1997). Similar structures developed many times, heterochronously and independently from the stem lineage of P. dubius with straight apertural margins (Fig. 3). According to Urbanek (1998), within the early Ludlow to late Přidoli time span, these iterations occurred at least five times, producing in each case a short lateral clade which branched off the P. dubius stem lineage, namely: Colonograptus praedeubeli—Saetograptus chimaera clade A, Pseudomonoclimacis latilobus clade B, P. d. labiatus clade C, Neocolonograptus parultimus—N. lochkovensis clade D, and Istrograptus transgrediens clade E displaying isochronous parallelism to clade D. Apertural elaborations attain in each case a different degree of expressivity (Fig. 4), from gentle elevations of the apertural rim (weak expressivity), through distinct lateral lobes (moderate expressivity) to strongly developed lobes overhanging the aperture (strong expressivity). In fact, the first of these clades (Fig. 1) branched off so early that it may be considered also as one of the early divergent phyletic lines (compare Koren' and Urbanek 1994). Morphological effects of changes are also somewhat greater: lateral lappets being provided in terminal forms with an additional structure in the form of the spines (Fig. 4). In most cases the early representatives of particular clades reveal low expressivity of the newly introduced character, later being subject to gradual increase. But these are exceptions to the rule and some iterative species display strong expressivity of new characters from the very beginning (Urbanek 1997).

Fig. 4.

The distribution and degree of expression of paired lateral apertural elevations or lobes within iterative clades that branched off the graptolite Pristiograptus dubius stem lineage (modified after Urbanek 1998).

Some iterative species reveal remarkable homeomorphy, as in the case of Pristiograptus lodenicensis Přibyl, 1948 from the upper Wenlock and P. dubius labiatus Urbanek, 1997 from the upper part of Ludfordian, which form almost identical apertural elaborations (Fig. 4). Therefore as the P. dubius stem lineage shows no significant alteration of its evolutionary potential, producing remarkably similar iterative species separated by a few Ma, it seems that iterative speciation in each case involved only a small fraction of the existing parental gene system the vast majority remaining intact. The survival of parental species along with daughter species provides evidence that the evolutionary potential of the stem species, remains essentially unchanged (compare Koren' and Urbanek 1994; Urbanek 1998).

Fig. 5.

SEM micrographs showing thecal lip variations in proximal thecae in graptolites Pristiograptus and Colonograptus praedeubeli (Jaeger, 1991). A, D, H. Parovėja-9 borehole, Lithuania. A. Pristiograptus dubius dubius (Suess, 1851), VU S.P9-31, depth 644.8 m; Monograptus riccartonensis Biozone. D. Pristiograptus sp. sensu Radzevičius, 2003, VU S.P9-219, depth 595.4 m; Cyrtograptus perneri Biozone. H. Pristiograptus lodenicensis Přibyl, 1943, VU P.P9-4b, depth 561.9 m; Cyrtograptus lundgreni Biozone. B, C, E–G, I. Zawada borehole, Poland. B. Pristiograptus dubius pseudodubius (Bouček, 1932), ZPAL G.44/2, depth 1584.9-1590.6 m; Cyrtograptus perneri Biozone. C. Pristiograptus dubius magnus subsp. nov., ZPAL G.44/3, depth 1577.5-1582.5 m; Cyrtograptus perneri Biozone. E. Pristiograptus sardous (Gortani, 1922), ZPAL G..44/8, depth 1590.6-1597.7 m; Cyrtograptus rigidus Biozone. F,G. Pristiograptus dubius paezerensis subsp. nov.; Cyrtograptus lundgreni Biozone. F. ZPAL G.44/10, Zawada borehole, Poland, depth 1546.5-1552.7 m. G. ZPAL G.44/17, Bartoszyce borehole, Poland, depth 1663.2 m, mature rhabdosome. I. Pristiograptus dubius praelodenicensis subsp. nov., ZPAL G.44/6, depth 1546.5-1552.7 m; Cyrtograptus lundgreni Biozone. J. Pristiograptus parvus Ulst, 1974, VU P.K14-108, Kybartai-14 borehole, Lithuania, depth 1071.7 m; Pristiograptus parvus Biozone. K. Pristiograptus virbalensis Paškevièius, 1974, VU P.S87-71, Sutkai-87 borehole, Lithuania, depth 768.9 m; Colonograptus praedeubeli Biozone. L. Colonograptus praedeubeli (Jaeger, 1991), ZPAL G.44/23, Bartoszyce IG-1borehole, Poland, depth 1643.9 m; Colonograptus praedeubeli Biozone.M. Pristiograptus dubius frequens Jaekel, 1889, ZPAL G.44/30, Baltic erratic boulder S36, Gdañsk, Poland; Lobograptus scanicus Biozone. N-R. Mielnik-1 borehole, Poland. N. Pristiograptus dubius tumescens (Wood, 1900), ZPAL G.44/41, depth 892.2 m; Lobograptus scanicus Biozone. O. Pristiograptus dubius postmagnus subsp. nov., ZPAL G.44/50, depth 780.5 m; Monograptus (Uncinatograptus) acer Biozone. P. Pristiograptus dubius welchae Rickards, 1965, ZPAL G.44/47, depth 836.6 m; Saetograptus leintwardinensis Biozone. Q. Pristiograptus dubius labiatus Urbanek, 1997, ZPAL G.44/55, depth 753.3 m; Monograptus (Uncinatograptus) spineus-Nocolonograptus parultimus Interzone.

Therefore application of the “nodal principle” of cladistic taxonomy demanding that after each speciation the ancestral species should be given different binominals seems unjustified and undesirable in view of parsimony of nomenclature (see Urbanek 1996 for detailed discussion). An application of non-cladistic understanding of the relations between P. dubius stem species and iteratively derived species was suggested by Urbanek (1996, 1998). In this practice, the surviving ancestral species retains its previous name even after iterative speciation of a new species. This is justified by the mode of speciation used by an iterative species that does not break the cohesion of the ancestral lineage and involves only peripheral populations that later became isolated or semi-isolated as in the case of parapatric speciation.

Rickards and Wright (2003) mention as a remarkable fact that the stem taxon P. dubius always occurs in the same strata as species derived by iteration. They explain this co-occurrence by adaptation to different depth zones or hydrodynamic regime. The distribution of both species in question may be also related to a parapatric mode of speciation, considered by some zoogeographers as a common phenomenon in marine macrozooplankton (van der Spoel 1983). Iterative species, branching off the stem lineage, may be considered as probable examples of the parapatric mode of speciation.

An exceptional role of the P. dubius stem lineage, in the phylogeny of monograptids, relies on it being a persistent source of non-specialized morphologies, capable of easy specialization under suitable conditions. P. dubius as the stem species and all iterative species derived from it form a monophyletic group which may be called a genealogical main. As defined by phylogenetic systematics, such domains are characterized by a proximity of their common ancestry and by similar apomorphic tendencies within its taxa or clades. This is shown by abundant (rampant) parallelism which involves repeatedly evolving characters in our case e.g., bilateral apertural lobes. The capacity to develop synapomorphies (derived shared characters) called the underlying synapomorphy by Saether (1979, 1983) and Sluys (1989) may be considered one of the most characteristic features of evolution within a genealogical domain. The P. dubius group provides a graphic example of such capacity.

Morphological variation in Pristiograptus dubius group

There are two important characters for distinguishing forms of Pristiograptus dubius group (Fig. 3). One is the angle ß between the thecal apertural lip and the succeeding metathecal wall (ß-angle), in which the variations in the angle permits splitting Pristiograptus into two main gonujroups. Another important feature is the shape of thecal apertural lip and its location to the ventral wall of the succeeding theca (the end of thecal apertural lip). The thecal apertural lip is a thickening around the aperture and may be linked to the development of the ectocortex. The end of the thecal apertural lip is regarded herein as a lateral margin of the thecal aperture. Its location is a very important taxonomic feature in distinguishing forms of the P. dubius group.

Our descriptions of species and subspecies are based on isolated material. Some species of P. dubius morphotypes are described fromisolatedmaterial for the first time. The specific features e.g., the position of the thecal aperture end in P. dubius morphotype species and subspecies have never been documented, except someWenlock taxa of P. dubius morphotype (Radzevičius and Paškevičius 2000; Radzevičius 2003). Width of 3D rhabdosomes is different than in the flattened ones. The problem is that the description of majority species and subspecies of P. dubius morphotypes is based on flattened material what sometimes makes a difference comparing the description of 3D specimens. Thus the assignment of those isolated specimens to P. dubius morphotype species and subspecies is relative.

Fig. 6.

SEM micrographs showing variations of proximal end in some graptolites Pristiograptus and Colonograptus forms. A, B. Zawada borehole, Poland. A. Pristiograptus dubius pseudodubius (Bouček, 1932), ZPAL G.44/2, depth 1584.9-1590.6 m; Cyrtograptus perneri Biozone. B. Pristiograptus sardous (Gortani, 1922), ZPAL G.44/8, depth 1590.6-1597.7 m; Cyrtograptus rigidus Biozone. C–E. Pristiograptus dubius paezerensis subsp. nov.; Cyrtograptus lundgreni Biozone. C. ZPAL G.44/17, Bartoszyce borehole, Poland, depth 1663.2 m. D. ZPAL G.44/10, Zawada borehole, Poland, depth 1546.5–1552.7 m. E. VU P.P222-216, Paežeriai-222 borehole, Lithuania, depth 734.5 m. F. Pristiograptus dubius praelodenicensis subsp. nov., ZPAL G.44/6, depth 1546.5-1552.7 m; Cyrtograptus lundgreni Biozone. G, H. Pristiograptus lodenicensis Přibyl, 1943, Zawada borehole, Poland; Cyrtograptus lundgreni Biozone. G. ZPAL G.44/14, depth 1546.5-1552.7 m. H. ZPAL G.44/19, depth 1555.9-1562 m. I–M. Pristiograptus virbalensis group; Colonograptus praedeubeli-C. deubeli biozones. I-L. Pristiograptus virbalensis Paškevičius, 1974. I-K. Sudkai-87 borehole. I. VU P.S87-121a, depth 768.2 m. J. VU P.S87-121b, depth 768.2 m. K. VU P.S87-382, abnormal virgella, depth 877 m. L. ZPAL G.44/27, Bartoszyce borehole, depth 1642.4 m. M. Pristiograptus auctus Rickards, 1965, VU P.S87-362, Sutkai-87 borehole, 768.2 m depth; Colonograptus deubeli Biozone. N. Pristiograptus dubius parvus Ulst, 1974, VU P.K14-109, Kybartai-14 borehole, Lithuania, depth 1088.4 m, Pristiograptus parvus Biozone; th1 with sicula (N1), enlargement of sicula aperture (N2). O, P. Colonograptus praedeubeli (Jaeger, 1991); Colonograptus praedeubeli Biozone. O. ZPAL G.44/21, Bartoszyce IG-1 borehole, Poland, depth 1646 m. P. VU K.PS87-134, Sutkai-87 borehole, Lithuania, depth 776 m. Q. Colonograptus deubeli (Jaeger, 1959), VU S.S69-61, Šiupyliai-69 borehole, Lithuania, depth 983 m; Colonograptus deubeli Biozone. R. Pristiograptus dubius postfrequens subsp. nov., ZPAL G.44/37, Mielnik-1 borehole, Poland, depth 716.3 m; Monograptus (Uncinatograptus) spineus-Nocolonograptus parultimus Interzone. S. Pristiograptus dubius labiatus Urbanek, 1997, ZPAL G.44/52, Mielnik-1 borehole, Poland, depth 753.3 m; M. (U.) spineus-N. parultimus Interzone. T. Pristiograptus dubius welchae Rickards, 1965, ZPAL G.44/44, Mielnik-1 borehole, Poland, depth 836.6 m; Saetograptus leintwardinensis Biozone.

General features.—The widths of P. dubius morphotypes rhabdosomes vary, and the proximal end up to level of the fifth theca varies from straight to slightly ventrally curved. The rhabdosome widens gradually from 0.5 mm (e.g., P. dubius shearsbyi Rickards and Wright, 1999) to 0.8 mm (e.g., P. sardous [Gortani, 1922]), to 1.2 mm (e.g., P. tumescens [Wood, 1900]) at th⁵.

Thecae are simple and cylindrical (Fig. 5). Thecal spacing ranges from 9–12 in 10 mm, the numbers varying whether measured in the proximal, medial and distal parts of rhabdosome. The thecae are inclined at angle λ varying between 25–35° (Fig. 4). The first theca is 1–1.2 mm long and 0.3 mm wide.

The siculamay be straight or weakly ventrally curved (Fig. 6), and its length varies from 1.3 mm in P. dubius parvus to 2.2 mm in P. dubius postmagnus subsp. nov. The width of the sicular aperture ranges from 0.2 mm (e.g., P. dubius parvus) to, most commonly, 0.3 mm. The aperture of the sicula is concave in lateral view. The apex of the sicula reaches the base of th² or th³. The prosicula is 0.38–0.46 mm long. The metasicular length ranges from 1.41–1.48 mm. The width of the free part of sicula could be above 0.2 mm e.g., P. dubius paezerensis subsp. nov. (Fig. 6C, D, E).

The inclination of the thecal aperture to the sicular axis (angle ß) ranges from oblique to a right angle e.g., P. dubius praelodenicensis subsp. nov. In most of the P. dubius group species, angle ß is obtuse but may be a right angle in some forms e.g., P. sardous, P. dubius praelodenicensis subsp. nov., P. dubius labiatus Urbanek, 1997 and others (Fig. 5). The apertural lip can be narrow, or sometimes very thick as in P. lodenicensis Přibyl, 1943 (Fig. 5H).

The rim of the thecal lip may reach only as far as the middle part of the free ventral wall of the succeeding theca e.g., P. dubius pseudodubius (Bouček, 1932) (Fig. 5B), P. dubius postfrequens subsp. nov., or may be connected to the lip of the succeeding theca e.g., P. dubius magnus subsp. nov. (Fig. 5C), P. dubius postmagnus subsp. nov. (Fig. 5O). The thecal lips can be simple e.g., P. dubius dubius (Fig. 5A), can have some depression e.g., P. lodenicensis (Fig. 5H) and made some intergrowth arch e.g., Pristiograptus sp. sensu Radze- vičius 2003 (Fig. 5D).

Variation groups.—The investigation of all mentioned characters in P. dubius group permits the recognition of two main groups that differ in the angle ß (Fig. 3) between the apertural margin of the theca and the succeeding thecal wall.

The first, and main, P. dubius group contains formswith an obtuse angle ß. This group consists of P. dubius dubius (Suess, 1851) (Fig. 7A), P. dubius pseudodubius (Bouček, 1932) (Fig. 7C, D), P. dubius magnus subsp. nov. (Fig. 7E, F), P. dubius paezerensis subsp. nov. (Figs. 8F–I, 9A, B), Pristiograptus sp. sensu Radzevičius 2003 (Fig. 7B), P. dubius Ulst, 1974 (Fig. 10C, D), P. dubius frequens Jaekel, 1889 (Fig. 11A–C, F–H), P. dubius aff. welshae Rickards, 1965 (Fig. 12H–K), P. dubius postmagnus subsp. nov. (Fig. 13A–C), P. dubius postfrequens subsp. nov. (Fig. 11J, L), P. dubius shearsbyi Rickards and Wright, 1999 (Fig. 11D, E, I, K).

The second P. dubius group is characterized by possessing a right or acute angle ß. The species diversity is smaller, comprising only five forms: P. sardous (Gortani, 1922) (Fig. 8B, D), P. dubius praelodenicensis subsp. nov. (Fig. 8A, C, E), P. tumescens (Wood, 1900) (Fig. 12A–F), P. dubius labiatus Urbanek, 1997 (Fig. 13D–F), and P. lodenicensis Přibyl, 1943 (Fig. 10).

Other differences between the two main groups include the shape of the apertural lips, differences in rhabdosome shape and size, and a different number of sicular rings (annuli). Ludfordian species display sicula with a fixed 3-ring pattern (Urbanek 1997).

Material and methods

The Wenlock and Ludlow material described herein was derived from the East European Platform of Poland and Lithuania (Fig. 2). Our study is based on 3D specimens studied on light microscope and scanning electron microscope (SEM). Some characters are recognisable on well preserved flatten material (Fig. 3B–E). The Wenlock pristiograptids were recovered from boreholes Zawada IG-1 and Bartoszyce IG-1 of Poland, and Kurtuvėnai-161, Šiupyliai-69, Paežeriai-222, Likënai-396, and Parovėja-9 boreholes of Lithuania. The Ludlow material was collected from the Mielnik IG-1 borehole, Baltic erratic boulders from Jarosławiec of Poland, and the Milaičiai-103 borehole of Lithuania. The rocks were dissolved in 5–10% hydrochloric acid using standard techniques for graptolites. A fine hairbrush was used to pick and transfer specimens. The recovered material is stored in glycerine or on the SEM stubs in the Institute of Palaeobiology Polish Academy of Sciences, Warsaw, Poland (ZPAL G.), and in Vilnius University, Geological Museum, Lithuania. Photographs were made utilizing the SEM in the Geological Museum of Denmark, Copenhagen, and ZPAL.

Fig. 7.

SEM micrographs of Sheinwoodian (Wenlock) graptolite Pristiograptus forms. A, B. Parovėja-9 borehole, Lithuania. A. Pristiograptus dubius dubius (Suess, 1851), VU S.P9-31, depth 644.8 m; Monograptus riccartonensis Biozone, general view of rhabdosome (A₁), th⁴ apertural lip (A₂), apex of sicula and first theca apertural lip (A₃), mouth of sicula and virgella (A₄). B. Pristiograptus sp. sensu Radzevičius, 2003, VU S.P9-219, depth 595.4 m; Cyrtograptus perneri Biozone, general view of rhabdosome (B₁), connection of thecal apertural lip with succeeding theca apertural lip (B₂), intergrowth arch of th1 (B₃), and intergrowth arch of th3 (B₄), thick virgella and sicular apertural lip (B₅). C, D. Pristiograptus dubius pseudodubius (Bouček, 1932), Zawada borehole, Poland, depth 1584.9-1590.6 m; Cyrtograptus perneri Biozone. C. ZPAL G.44/1, general view of rhabdosome. D. ZPAL G.44/2, general view of rhabdosome (D₁), th5 apertural lip (D₂), and th¹ with sicula (D₃). E, F. Pristiograptus dubius magnus subsp. nov., Zawada borehole, Poland. E. ZPAL G.44/3, depth 1577.5-1582.5 m; Cyrtograptus perneri Biozone, general view of rhabdosome (E₁), th6 apertural lip (E₂), and th¹ with sicula (E₃). F. ZPAL G.44/4, depth 1590.6-1597.7 m, Cyrtograptus rigidus Biozone, general view of rhabdosome (F₁), th4 apertural lip (F₂), and th¹ with sicula (F₃).

Systematic palaeontology

Fig. 8.

SEM micrographs of Sheinwoodian and Homerian (Wenlock) graptolite Pristiograptus forms. A, C, E. Pristiograptus dubius praelodenicensis subsp. nov., Zawada borehole, Poland, depth 1546.5-1552.7 m; Cyrtograptus lundgreni Biozone. A. Holotype, ZPAL G.44/5, general view of rhabdosome (A₁), proximal part of rhabdosome (A₂), the end of th 2 apertural lip (A₃), and aperture of sicula (A₄). C. ZPAL G.44/6, general view of rhabdosome (C₁), th3 and th4 apertures (C₂), and sicula with th² (C₃). E. ZPAL G.44/7, young rhabdosome. B, D. Pristiograptus sardous (Gortani, 1922), Zawada borehole, Poland, depth 1590.6–1597.7 m; Cyrtograptus rigidus Biozone. B. ZPAL G.44/8, general view of rhabdosome (B₁) and th⁶ lip (B₂). D. ZPAL G.44/9, general view of rhabdosome. F–I. Pristiograptus dubius paezerensis subsp. nov.; Cyrtograptus lundgreni Biozone. F, G, I. Zawada borehole, Poland. F, G. Depth 1546.5–1552.7 m. F. ZPAL G.44/10, general view or rhabdosome (F₁), enlargement with the end of th² lip (F₂). G. ZPAL G.44/11, general view of rhabdosome (G₁) and sicula with long virgella (G₂). I. ZPAL G.44/12, depth 1555.9–1562 m, general view of rhabdosome. H. VU P.P222-216, Paežeriai-222 borehole, Lithuania, depth 734.5 m, general view of proximal part of a rhabdosome (H₁), th⁵ lip (H₂), end of the th² lip (H₃), and sicular aperture with dorsal process and long virgella (H₄).

Fig. 9.

SEM micrographs of the graptolite Pristiograptus lodenicensis Přibyl, 1943; Cyrtograptus lundgreni Biozone, Lower Homerian, Wenlock. A–D, F. Zawada borehole, Poland. A–C. Depth 1546.5–1552.7 m. A. ZPAL G. 44/13, general view of rhabdosome (A₁), lips of th³ and th⁴ (A₂), and th¹ with sicula (A₃). B. ZPAL G.44/14, general view of dorsal side of rhabdosome (B₁), proximal end (B₂). C. ZPAL G.44/15, general view of rhabdosome. D, F. Depth 1540–1546.5 m. D. ZPAL G.44/16, proximal end of rhabdosome. F. ZPAL G.44/17, proximal end of rhabdosome (F₁) and th² lip with cortical tissue “script” (“manta ray wing”) (F₂). E. VU P.P9–4b, Parovėja-9 borehole, Lithuania, depth 561.9 m, general view of rhabdosome (E₁), depressions of thecal apertural lip (E₂).

Fig. 10.

SEM micrographs of two Homerian (Wenlock) graptolite Pristiograptus forms. A, B. Pristiograptus dubius paezerensis subsp. nov., mature rhabdosomes; Cyrtograptus lundgreni Biozone. A. ZPAL G.44/17, Bartoszyce IG-1 borehole, Poland, depth 1663.2 m. B. ZPAL G.44/19, Zawada borehole, Poland, depth 1555.9–1562 m. C, D. Pristiograptus dubius parvus Ulst, 1974 growing rhabdosomes, Bartoszyce IG-1 borehole, Poland, depth 1660.7 m; Pristiograptus parvus Biozone. C. ZPAL G.44/25. D. ZPAL G.44/26.

Fig. 11.

SEM micrographs showing variations of proximal end of rhabdosomes in three Pristiograptus dubius subspecies of graptolites.A–C, F–H. Pristiograptus dubius frequens Jaekel, 1889, Baltic erratic boulder S36, Gdańsk, Poland; Lobograptus scanicus Biozone. A. ZPAL G.44/28, general view of rhabdosome (A1) and proximal end (A2). B. ZPAL G.44/29, lateral view of rhabdosome. C. ZPAL G.44/30, general view of rhabdosome. F. ZPAL G.44/31, proximal end of rhabdosome with sicular rings. G. ZPAL G.44/32, general view of rhabdosome (G1) and proximal end (G2). H. ZPAL G.44/33, ventral view of young rhabdosome. D, E, I, K. Pristiograptus dubius shearsbyi Rickards and Wright, 1999, Mielnik borehole, Poland, depth 688.8 m; Neocolonograptus parultimus Interzone. D. ZPAL G.44/27, proximal end of rhabdosome (D2) and general view of rhabdosome (D1). E. ZPAL G.44/34, general view of rhabdosome (E1) and proximal end of rhabdosome (E2). I. ZPAL G.44/35, lateral view of medial part of rhabdosome (I1) ventral view (I2). K. ZPAL G.44/36, general view of rhabdosome (K1) and sicula with two first thecae (K2). J, L. Pristiograptus dubius postfrequens subsp. nov., Mielnik borehole, Poland; Monograptus (Uncinatograptus) spineus-Neocolonograptus parultimus Interzone. J. ZPAL G.44/37, depth 716.3 m, general view of rhabdosome (J1) and proximal end of rhabdosome (J2). L. ZPAL G.44/38, depth 715.6 m, lateral view of young rhabdosome (L1), and proximal end (L2).

Fig. 12.

SEM micrographs of graptolites Pristiograptus tumescens (Wood, 1900) and P. dubius welchae Rickards, 1965. A–F. Pristiograptus tumescens (Wood, 1900). A–C, F. Mielnik borehole, Poland. A–C. Depth 892.2 m; Lobograptus scanicus Biozone. A. ZPAL G.44/39, lateral view of rhabdosome. B. ZPAL G.44/40, whole rhabdosome (B1), enlargement of th3 lip (B2), sicula with th1 (B3). C. ZPAL G.44/41, medial part of rhabdosome. F. ZPAL G.44/42, general view of rhabdosome. D, E. Milaičiai-103 borehole, Lithuania, depth 11267.5 m. D. VU S.M103-21X, lateral view of rhabdosome. E. VU S.M103-22X, lateral view of rhabdosome. G–K. Pristiograptus dubius welchae Rickards, 1965, Mielnik borehole, Poland, depth 836.6 m; Saetograptus leintwardinensis Biozone. G. ZPAL G.44/43, lateral view of rhabdosome (G1) and sicula with th1 (G2). H. ZPAL G.44/44, young rhabdosome (H1), sicula (H2). I. ZPAL G.44/45, lateral view of rhabdosome (I1), distal part (I2), and aperture of sicula (I2). J. ZPAL G.44/46, ventral view of distal end of rhabdosome. K. ZPAL G.44/47, lateral view of rhabdosome (K1), th3 (K2), and sicula with th1 (K3).

Conclusions

The detailed investigation in SEM of three-dimensionally preserved Silurian graptolites chemically isolated from host rocks provides numerous new morphological information. This throws a new light on pattern of evolution of the longest ranging monograptid Pristiograptus dubius (Suess, 1851) and a number of closely related taxa, forming the P. dubius group taxa. The studied fauna comes mainly from boreholes of Poland and Lithuania which permits to trace their changes in a stratigraphie interval from Monograptus riccartonensis to Neocolonograptus ultimus Biozone.

The P. dubius group taxa is divided into two morphological groups differing in the angle between the thecal lip and the succeeding thecal wall, namely into the group with obtuse and acute angle. The P. dubius stem lineage taxa forming an unbroken sequence of closely related subspecies and displaying only minor morphological changes through time all belong to the first and main morphological group. These taxa represent a graphic example of phyletic evolution. The second morphological group is composed of iterative P. dubius group taxa derived from the stem lineage by means of iterative speciation.

Fig. 13.

SEM micrographs of Ludfordian graptolites Pristiograptus dubius postmagnus subsp. nov. and Pristiograptus dubius labiatus Urbanek, 1997. A–C. Pristiograptus dubius postmagnus subsp. nov., Mielnik-1 borehole, Poland, depth 780.5 m; Monograptus (Uncinatograptus) acer Biozone. A. ZPAL G.44/48, lateral view of rhabdosome (A1), the thecal lip with connection to the succeeding theca (A2), and proximal end of rhabdosome (A3). B. ZPAL G.44/49, lateral view of rhabdosome sicula with rings (B1) and proximal end of rhabdosome (B2). C. ZPAL G.44/50, medial part of rhabdosome (C2), enlargements showing connections to the succeeding thecae (C1, C3). D–F. Pristiograptus dubius labiatus Urbanek, 1997, Mielnik-1 borehole, Poland, depth 753.3 m; Monograptus (Uncinatograptus) spineus-Neocolonograptus parultimus Interzone. D. ZPAL G.44/51, general view of rhabdosome (D1), sicula with two first thecae and depression on th1 lip (D2). E. ZPAL G.44/52, lateral view of rhabdosome (E1) and proximal end of rhabdosome (E2). F. ZPAL G.44/53, lateral view of rhabdosome (F₁) enlargement of proximal end (F₂).

Examined phylogenetic relationship between fifteen species and subspecies derived from P. dubius reveals many examples of heterochronic parallelism which may indicate the preservation of evolutionary potential capable of multiple production of similar forms. The taxa give the stem lineage continuing at successive stratigraphic levels during the extremely long time span, approximately 18 Ma.

Significance of the presence of dwarfed form P. dubius parvus, the only monograptid taxon to appear immediately after the most severe extinction event in graptolite history, the C. lundgreni Event is discussed. We advanced the hypothesis that this taxon in question appeared as result of the so-called Lilliput effect (defined by Urbanek 1993), and probably represents a geographically vicarious form of P. dubius with the normal rhabdosome size. Thus we consider P. dubius parvus provisionally as a link in the evolution of the stem lineage taxa.