The paper focuses on patterns of the evolution of the simplest and longest-ranging (approximately 18 Ma) Silurian graptolite Pristiograptus dubius. The Pristiograptus dubius species group consists of the P. dubius stem lineage represented by a sequence of a number of subspecies displaying only small morphological changes as well as derivative species produced from the stem lineage by means of iterative speciation. This long raging graptolite lineage is the only one, apart of one retiolitid, which survived the most severe environmental event for graptolites, the Cyrtograptus lundgreni Event. Based on three-dimensional, isolated material two P. dubius groups taxa are distinguished. One group has an obtuse angle between the thecal lip and the succeeding thecal wall, the second group has a right or acute angle. Other characters differentiating P. dubius forms are: the shape of the apertural lips, differences in rhabdosome shape and size, and a different number of sicular rings. Sixteen species and subspecies of Pristiograptus from the East European Platform, Poland and Lithuania are discussed. Five new subspecies P. dubius magnus, P. dubius paezerensis, P. dubius praelodenicensis, P. dubius postfrequens; and P. dubius postmagnus are proposed.
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.
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).
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.
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.
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).
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).
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.
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.
Order Graptoloidea Lapworth, 1873
Family Monograptidae Lapworth, 1873
Subfamily Pristiograptinae Gürich, 1908
Genus Pristiograptus Jaekel, 1889
Type species: Pristiograptus frequens Jaekel, 1889, erratic boulders, North Germany, Lobograptus scanicus Biozone, Lower Ludlow.
Diagnosis.—Rhabdosome of variable length and curvature but usually almost straight; thecae are straight, simple tubes throughout length of rhabdosome, thecae have varying degrees of overlap and inclination; sicula small to large.
Remarks.—Pristiograptus rhabdosomes have a large variation of characters, that consequently gives a large “number” of species and subspecies.
Pristiograptus dubius dubius (Suess, 1851)
Figs. 5A, 7A.
1851 Graptolithus dubius sp. nov.; Suess 1851: 115, pl. 9: 5a, b.
1913 Monograptus dubius (Suess, 1850); Elles and Wood 1913: 376–378, text-fig. 247a, b.
1943 Pristiograptus dubius dubius (Suess, 1850); Přibyl 1943: 3–4, pl. 1: 4–6.
1952 Pristiograptus dubius dubius (Suess, 1850); Münch 1952: 85, pl. 18: 1.
1958 Pristiograptus dubius dubius (Suess, 1850); Přibyl 1958: 117.
2000 Pristiograptus dubius dubius (Suess, 1850); Radzevičius and Paškevičius 2000: 91–92, pl. 1: 2.
Material.—Ten fragments of rhabdosomes of different preservation from the Kurtuvėnai-161 (1420.5-1396.9 m) and Parovėja-9 boreholes (651.3–638 m), Lithuania.
Description.—The ß angle is obtuse. The thecal lips are distinct, not extending onto the ventral wall of the succeeding thecae and ending in the beginning of the ventral wall (Figs. 5A, 7A). No sicular rings are observed.
Discussion.—The type material of the P. dubius (Suess, 1851) is flattened and poorly illustrated. It has never been redescribed and refigured. Details of the P. dubius dubius rhabdosome structures are not visible at the type material. The P. dubius morphotype is considered to be P. dubius dubius subspecies, which thecal apertural lip not extending onto the ventral wall of succeeding thecae and ending at the beginning of the ventral wall and without sicular rings.
Stratigraphic and geographic range.—Lower Wenlock Monograptus riccartonensis—M. antennularius Biozones. P. dubius dubius is widespread, having been recorded from all continents.
Pristiograptus dubius pseudodubius (Bouček, 1932)
Figs. 5B, 6A, 7C, D.
Material.—More then 200 rhabdosomes and fragments from Kurtuvėnai-161 (1351.9–1328.1 m), Likënai-369 (630.0–582.1 m), and Parovėja-9 (621.5–588.6 m) boreholes, Lithuania and Zawada-1 borehole (1590.65–1584.9 m), Poland.
Discussion.—The type material of P. dubius pseudodubius has been described from flattened material and detailed account of the structures of its rhabdosome are not visible. In this work P. dubius pseudodubius is considered to be as dubius morphotype with ß angle obtuse, which thecal apertural lip reaches only the middle of the free ventral wall of the succeeding theca.
Stratigraphic and geographic range.—Wenlock Monograptus belophorus-Cyrtograptus lundgreni Biozones; P. dubius pseudodubius is widespread, known from all continents. Pristiograptus dubius parvus Ulst, 1974 Figs. 5J, 6H, 10C, D.
1974 Pristiograptus parvus sp. nov.; Ulst 1974: 111-113, pl. 12: 43, pl. 11 : 5.
1996 Pristiograptus dubius “parvus” Ulst, 1974; Gutierrez-Marco et al. 1996: 660, pl. 1: 1–3.
Material.—About 100 rhabdosomes and fragments from the Kurtuvėnai-161 (1323.2–1321.1 m), Parovėja-9538 m), Šiupyliai-69 (1004–1000.8 m) boreholes, Lithuania and Bartoszyce IG-1 borehole (1660–1657.8 m), Poland.
Description.—The rhabdosome is straight, sometimes slightly curved to the ventral side, the ß angle is obtuse (Fig. 5J). The first theca begins at the level of the sicula aperture (Fig. 6H). P. parvus is like a small version of P. dubius pseudodubius.
Discussion.—The type of P. parvus Ulst, 1974 was described using non-isolated, flattened material. This species is specific having very small rhabdosome.
Stratigraphic and geographic range.—P. parvus Biozone, East European Platform (Poland, Latvia, Lithuania, Sweden), Czech Republic, Germany, Spain, Canada.
Pristiograptus dubius frequens Jaekel, 1889
Figs. 5M, 11A–C, F–H.
Material.—More then 50 rhabdosomes and fragments from erratic boulder (S36), Poland and Šiupyliai-69 (994.2–905.5 m), Kurtuvėnai-161 (1297.1–127.2 m) boreholes, Lithuania.
Description.—The rhabdosome is narrow and graceful. The proximal end is slightly curved ventrally as far th5. The ß angle is obtuse. The thecal apertural lip is distinct, not extending onto the ventral wall of the succeeding theca and ending in the beginning of the ventral wall. The number of sicular rings varies from 3 to 8 (Fig. 11A, B).
Remarks.—The genus Pristiograptus was erected by Jaekel (1889), designating P. frequens as its type species. Recent studies reveal that the differences between P. frequens and P. dubius (Suess, 1851) are so small, that the former species should be considered as a subspecies of the last mentioned, and earlier established, species by Suess (1851) [compare Jaeger 1991, who used the name Monograptus dubius frequens (Jaekel, 1889)]. The difference between P. dubius dubius and P. dubius frequens is in absence of sicular rings in P. dubius dubius and their presence in P. dubius frequens.
Stratigraphic and geographic range.—Colonograptus praedeubeli—Cucullograptus hemiaversus Biozone. P. dubius frequens having been recorded from: Latvia, Lithuania, Poland, Czech Republic, and Germany.
Etymology: From Latin post, after; frequens, frequent.
Holotype: ZPAL G./24, isolated rhabdosome with sicula and thee thecae illustrated in Urbanek 1997: fig. 43D.
Type locality: Mielnik-1 borehole, depth 786.5 m, Poland.
Type horizon: Upper Ludlow, Monograptus (Uncinatograptus) acer Biozone.
Material.—About 50 rhabdosomes and some fragments from Mielnik-1 borehole (915–690 m), Poland and Milaičiai-103 borehole (1255–1210 m), Lithuania.
Diagnosis.—Thecal lip ends in the middle or above the free ventral wall of the succeeding theca. Sicula with three rings.
Description.—The rhabdosome is narrow and slightly curved ventrally at the proximal end. The rhabdosome widens gradually from 0.6 mm at th1, to 0.7 mm at th2, 0.8 mm at th3. Interapertural width is 0.5 mm at th1, 0.6 mm at th2, 0.7 mm at th3. There are 10–12 thecae in 10 mm. The ß angle is obtuse. Th1 is 0.9 mm long and 0.3 mm wide. The free part of each theca is half of its whole length. The end of the thecal lip may continue up to the half length of the free part of succeeding thecae or above. The λ angle is 30°. Sicula length is 2.2 mm and width is 0.3 mm. The apex of the sicula reaches the base of th3. There are three sicular rings.
Discussion.—P. dubius postfrequens is similar to P. dubius frequens. The difference is in the number of sicula rings and in the position of the thecal apertural end. P. dubius postfrequens has fixed 3 sicular rings whereas in P. dubius frequents the number of sicular rings vary from 3 to 8. The end of the thecal lip of P. dubius postfrequens may continue up to the half length of the free part of succeeding theca or above, whereas the thecal apertural lip of P. dubius frequens is ending at the beginning of the ventral wall of succeeding theca.
Stratigraphic and geographic range.—Saetograptus leintwardinensis—Monograptus (Uncinatograptus) spineus Biozone, Poland and Lithuania.
Etymology: From Latin magnus, great, grand.
Holotype: VU-9-220, isolated rhabdosome with with sicula and 11 thecae illustrated in Radzevicius 2003: fig. 5A.
Type locality: Paroveja-9 borehole, depth 605.25 m, Lithuania.
Type horizon: Lower Wenlock, C. perneri Biozone.
Material.—About 25 rhabdosomes and additional fragments from the Paroveja-9 borehole (605.25–590.5 m), Lithuania, depth and Zawada-1 borehole (1577.5–1582.5 m), Poland.
Diagnosis.—Thecal lips distinct and connected with those of succeeding thecae. No sicular rings.
Description.—The rhabdosome is robust and straight, although some specimens show ventral curvature proximally. L is >11.5 mm. The rhabdosome widens gradually from 0.8 mm at th1, to 0.9 mm at th2, 1 mm at th3, and 1.2 mm at th4. Wmax is 1.7 mm. The interapertural W is 0.55 mm at th1, 0.65 mm at th2, 0.85 mm at th3, 0.95 mm at th4, and 1.05 mm at th5. There are 10–11 thecae in 10 mm. The ß angle is obtuse. Th1 is 1 mm long. The free part of each theca is half of its whole length. The thecal lips are distinct and connected with that of the succeeding thecae, except th1 and th2 although it may have been obscured in these thecae by overgrowth of cortical tissue. The lip connection is particularly distinctive in the medial and distal parts of the rhabdosome. The λ angle is 30°. The sicula is short and narrow, L is 1.8 mm and apertural W is 0.3 mm. The apex of the sicula reaches the base of th3. The free part of the sicula is 0.3 mm long. The virgella is short, up to 0.6 mm. In one specimen there is a thickened protrusion in the middle part of the virgella. No sicular rings are observed.
Stratigraphic and geographic range.—Lower Wenlock, C. perneri Biozone, Poland and Lithuania.
Pristiograptus dubius paezerensis subsp. nov.
Figs. 5F, G, 6C, D, E, 8F–I, 9A, B.
Etymology: After Paežeriai village and borehole in Lithuania.
Type locality: Paežeriai-222 borehole, depth 730 m, Lithuania.
Type horizon: Upper Wenlock, C. lundgreni Biozone.
Material.—About 20 rhabdosomes and many fragments from boreholes: Paežeriai-222 (734.5–730 m), Likënai-369 (577–605.9 m), Šiupyliai-69 (1007 m) boreholes, Lithuania and Zawada-1 (1546.5–1552.7 m), Bartoszyce IG-1 (1665.3–1663.2 m) boreholes, Poland.
Diagnosis.—Thecal lip distinct, extending onto the ventral wall of the succeeding theca up to the middle of the ventral wall, not reaching its apertural lip. First theca starts 0.3 mm above the sicular aperture. Long virgella. No sicular rings.
Description.—The rhabdosome is straight and narrow, L is >12.5 mm, W is 0.7 mm at th1, 0.75 mm at th2, 0.85 mm at th3, 0.95 mm at th4, and 1mm at th5. Wmax is 1.1 mm on th6. The interapertural W is 0.5 mm at th1, 0.6 mm at th2, 0.75 mm at th3, 0.8 mm at th4, and 0.9 mm at th5. There are 10–11 thecae in 10 mm. Thecae are cylindrical. The angle ß is obtuse. The free part of each theca is half of its entire length. The thecal lip is distinct, the end of thecal lip may continue up to the half length of the free part of succeeding thecae. The λ angle is 30°. The sicula L is 2 mm, apertural W is 0.35 mm. The apex of the sicula is at approximately at the level of the apertural lip of th2. Th1 arises 0.3 mm above the sicular aperture. The virgella is thin, L to 1.35 mm.
Remarks.—The thecal apertural lips are thick and distinctive and interthecal septa of astogenetically old rhabdosomes are very thick (Fig. 10A, B).
Stratigraphic and geographic range.—Upper Wenlock, C. lundgreni Biozone; Poland and Lithuania.
Pristiograptus sp. sensu Radzevičius, 2003
Figs. 5D, 7B.
2003 Pristiograptus sp.; Radzevičius 2003: 8–9, fig. 10.
Material.—One specimen, VU-9-219, from the Parovėja-9 borehole (595.4 m), Lithuania; rhabdosome with first 8 thecae and sicula.
Remarks.—The thecal lip is distinct, extending over the free part of the succeeding theca and additionally its ends are connected (Fig. 5D). This connection is clearly visible in the medial and distal parts of the rhabdosome. Th1 and th2 either do not have such thecal apertural lip connections or they are covered by cortical tissue. The lips of the distal thecae are bifurcated. An intergrowth arch is well developed especially in the proximal part of the rhabdosome. The distance between the thecal apertural lip connection on the succeeding theca and the origin of its bifurcation is 0.25mmfor th1 and th2, and 0.35 mm for th3 to th5. There are no sicular rings.
Stratigraphic and geographic range.—Wenlock, C. perneri Biozone, Lithuania.
Pristiograptus sardous (Gortani, 1922)
Figs. 5E, 6B, 8B, D.
Material.-About 5 rhabdosomes from Zawada-1 borehole (1590.6-1597.7 m), Poland.
Description.—The apertural lips of symmetrically flattened thecae form an acute or right ß angle. Overall, thecal apertures are perpendicular to the rhabdosome axis in the medial and distal parts of the rhabdosome. The λ angle is 30°. There are no sicular rings.
Discussion.—The type material of P. sardous is flattened. P. sardous is P. dubius type Pristiograptus with wide proximal end of rhabdosome.
Stratigraphic and geographic range.—Wenlock Cyrtograptus rigidus, Cyrtograptus perneri biozones; Poland, Czech Republic, Italy.
Etymology: From Latin prae, before; supposedly P. dubius praelodenicensis is the ancestor of P. lodenicensis.
Holotype: ZPAL G.44/5, isolated rhabdosome with sicula and 6 thecae, Fig. 8A.
Type locality: Zawada IG-1 borehole, depth 1546.5-1552.7 m, Poland.
Type horizon: Upper Wenlock, Cyrtograptus lundgreni Biozone.
Material.-About 25 rhabdosomes and many fragments from Zawada IG-1 borehole (1546.5–1552.7 m), Poland.
Diagnosis.— ß angle acute. Thecal lips straight and perpendicular to the rhabdosome axis. No sicula rings.
Description.-The rhabdosome is narrow, the proximal end is slightly curved ventrally up to th4 or th5. L is more than 30 mm.Wis 0.7 mm at th1, 0.85 mm at th2, 1 mm at th3, 1.2 mm at th4, and 1.2 mm at th5 level. Wmax is usually 1.5 mm. The interaperturalWis 0.65 mmat th1, 0.7 mmat th2, 0.8.5 mmat th3, 0.9 mm at th4, and 1 mm at th5. The ß angle is acute. The thecae are uniform. Thecal lips are perpendicular to the rhabdosomal axis. The th1 L is 0.9-1 mm, W is 0.3 mm and overlaps th2 for 1/3 to 1/2 of its length. The λ angle is 29–32°. The sicula is slightly curved ventrally, L is 1.5–2.0 mm, and W 0.25–0.3 mm at apertural level. The sicular apex is on a level between the middle and the top of th2. No sicula rings.
Discussion.—P. dubius praelodenicensis subsp. nov. is similar to P. lodenicensis Přibyl, 1943 in having an acute to right β angle.
Stratigraphic and geographic range.—Upper Wenlock, Cyrtograptus lundgreni Biozone, Poland.
Pristiograptus lodenicensis Přibyl, 1943
Figs. 4B, 5H, 6G, 10.
1943 Pristiograptus lodenicensis sp. nov.; Přibyl 1943: 23–24, textfigs. C, D, J, K; pl. 3: 8–11.
2000 Pristiograptus lodenicensis Přibyl, 1943; Radzevičius and Paškevičius 2000: 97–98, pl. 4: 1a–c, 2a–c, pl. 6D.
Remarks.—The thecae are slightly curved, and widen towards their apertures. The ß angle is acute, and the λ angle is right. The thecal lips are symmetrically curved (Fig. 9A, E). The depressions and curvature of lips are variable in proximal and distal thecae. The lips maybe slightly or much curved, they may widen towards their ends, and may have small or large ventral incisions and curvatures close to their ends. In some specimens there are large symmetrical “stingray wing” structures located at the ends of lips (Fig. 9F). These structures may occur in a few thecae or all the thecae (Radzevičius 2006: fig. 4c–f). The thecal apertures are right angles to the rhabdosome axis. Similar thecae occur in P. d. labiatus, thus both forms are not typical members of P. dubius group. This type of theca is similar to those of Istrograptus Tsegelniuk, 1976 and Neocolonograptus Urbanek, 1997. This species is separated from the lodenicencis group (Radzevičius and Paškevičius 2000).
P. praelodenicensis is very similar to P. lodenicensis with thecae slightly curved, widening towards the aperture, which shows a small ventral depression. Thecae of P. praelodenicensis do not have such kind of depression.
Stratigraphic and geographic range.—Upper Wenlock Cyrtograptus lundgreni Biozone; Latvia, Lithuania, Poland, Czech Republic.
Pristiograptus tumescens (Wood, 1900)
Figs. 5N, 12A–F.
Material.—More then 50 rhabdosomes and rhabdosomes fragments from Mielnik-1 borehole (976–896 m), Poland and Milaičiai-103 borehole (1269–1259 m), Lithuania.
Description.—The ß angle is obtuse or right. The thecal apertural lip reaches the middle part or above of the free ventral wall of the succeeding theca. The sicula has rings.
Stratigraphic and geographic range.—Ludlow Cucullograptus hemiaversus, Saetograptus leintwardinensis Biozones, East European Platform (Latvia, Lithuania, Poland), Czech Republic, Great Britain.
1965 Pristiograptus welchae sp. nov.; Rickards 1965: 261, pl. 30: 6; text-fig. 2f.
Material.—About 20 rhabdosomes and rhabdosomes fragments from Mielnik-1 borehole (836.6 m), Poland.
Description.-The rhabdosome is very small, up to 5 mm long, narrow 0.7-1.2 mm, and slightly curved to the ventral side, sometimes straight. The ß angle is obtuse or right. The thecal lip is distinct, ending at the beginning of the free ventral wall of succeeding theca. The sicula has three rings.
Remark.—The P. dubius welchae is very similar to the Wenlock species P. dubius parvus, although P. dubius parvus does not have sicular rings.
Stratigraphic and geographic range.—Saetograptus leintwardinensis—Neocucullograptus kozlowskii Biozone, Poland; Neodiversograptus nilssoni—Lobograptus scanicus and S. leintwardinensis biozones of North England (Rickards 1965).
Pristiograptus dubius labiatus Urbanek, 1997 Figs. 5R, 6T, 13D–F.
1997 Pristiograptus dubius labiatus subsp. nov.; Urbanek 1997: 160–161, pl. 16: 1–11, text-fig. 44A–D.
Material.—20 rhabdosomes and many from Mielnik borehole (761.75–727.7 m), Poland.
Remarks.—The ß angle is acute and the thecal apertural lip is at a right angle to rhabdosomal axis. The thecal lip is distinct with an incision in its ventral side, similar to P. lodenicensis (Fig. 5H). The sicula has three rings.
Stratigraphic and geographic range.—Monograptus (Slovinograptus) balticus—M. (Uncinatograptus) spineus Biozone, Upper Ludlow, Poland.
Etymology: From Latin post, after; magnus, great, large.
Holotype: ZPAL G.44/48, rhabdosome with sicula and seven thecae, Fig. 13.
Type locality: Mielnik-1 borehole, depth 780.5 m, Poland.
Type horizon: Upper Ludlow, M. (U.) acer Biozone.
Material.—15 rhabdosomes and many fragments from Mielnik borehole (780.5 m), Poland.
Diagnosis.—Distinct thecal lips connecting with those of the succeeding thecae. Sicula with three rings.
Description.—The rhabdosome is robust and the proximal end is slightly curved ventrally up to th5. The longest rhabdosome L is more than 15 mm long. The rhabdosome W from 0.65 mm at th1, to 0.8 mm at th2, 9 mm at th3, 1 mm at th4 , 1.1 mm at th5, 1.2 mm at th6. The Wmax is 1.3 mm. The interapertural width is 0.55 mm at th1, 0.65 mm at th2, 0.75 mm at th3, 0.8 mm at th4, and 0.9 mm at th5. There are 10–11 thecae in 10 mm. The ß angle is obtuse. Th1 is 1mm long and 0.3 mm wide. The free part of each theca is half of its whole length. The thecal lips are distinct and connected with the lip of the succeeding thecae (Figs. 5O, 13A–C). The lip connection is particularly well developed in the medial and distal parts of the rhabdosome. The λ angle is 30°. The sicula is short and narrow. Its L is 1.8 mm and apertural W is 0.3 mm. The apex of the sicula reaches the base of th3. The sicula has three rings. The virgella is short, up to 0.5 mm.
Remarks.—P. dubius postmagnus subsp. nov. is similar to P. dubius magnus subsp. nov., in the connection of the lip with the succeeding thecal lip. In contrast to P. dubius magnus subsp. nov. it has sicular rings.
Stratigraphic and geographic range.—M. (U.) acer Biozone, Upper Ludlow, Poland.
Pristiograptus dubius shearsbyi Rickards and Wright, 1999
Fig. 11D, E, I, K.
Material.—10 rhabdosomes, Mielnik-1 borehole (688.8 m), Poland.
Description.—The rhabdosome is narrow and proximally is either slightly curved ventrally or straight. The ß angle is obtuse. The thecal lips are distinct, not extending onto the free ventral wall of the succeeding thecae. The λ angle is 25°. The sicula L is 2 mm and W is 0.3 mm. The sicula has three rings.
Stratigraphic and geographic range.—M. (U.) spineus—Neocolonograptus parultimus Interzone, Australia and Poland.
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.
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.
This paper is dedicated to the memory of Professor Barrie Rickards, a distinguished graptolite worker. We thank paper reviewer Jan Zalasiewicz (University of Leicester, Leicester, UK) for his valuable comments. SR thanks the Science Council of Lithuania (No. MIP-034/2012), Synthesis and DK-TAF-2966. This paper is a contribution to the International Geoscience Programme (IGCP) Project 591-The Early to Middle Paleozoic Revolution.