Morphology and reconstruction of the retiolitines: Silurian graptolites of the Paraplectograptus lineage (Graptolithina)

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INTRODUCTION
One of the most fascinating planktic groups of the graptolites is the retiolitines. They differ from the other graptolites in having the tubarium (rhabdosome) comprising fuselli, which are mostly thin and only rarely preserved, and an extrathecal ancora sleeve forming a layer outside the lateral thecal walls. The bandages are developed as lists, forming a mesh-like framework to make a tubarium unlike that of any other retiolitids (sensu Melchin et al. 2011). Detailed scanning electron microscope (SEM) studies on isolated three-dimensionally preserved material (e.g. Bates & Kirk 1978, 1984, 1992, 1997Bates 1987Bates , 1990Lenz & Melchin 1987;Lenz 1993Lenz , 1994aKozłowska-Dawidziuk 1997;Lenz & Thorsteinsson 1997), have helped in understanding this type of rhabdosome.
This current study is based mostly on well preserved retiolitines from Arctic Canada donated by Alf Lenz, and from Poland. In this paper, we focus attention on the overall morphological characteristics, and the reconstruction of the tubaria of the genera Paraplectograptus Pribyl, 1948 andPseudoplectograptus Obut &Zaslavskaya, 1983, comprising the Paraplectograptus lineage. Comparison of their construction is made with that of Retiolites Barrande, 1850 andStomatograptus Tullberg, 1883, which belong to the oldest group of retiolitines.
The aim of this paper is to present the detailed morphology of the Paraplectograptus rhabdosome and development of the colony based on SEM pictures.

MATERIAL AND METHODS
The majority of the material derives from Arctic Canada; two specimens come from Poland. The specimens studied for the paper have been isolated from the rock following slow dissolution of the host carbonate in 5-10% HCl. A fine hairbrush was used to pick and transfer specimens to a box with glycerin or onto SEM stubs. The material is stored in glycerin in plastic containers, as well as on the SEM stubs, in the Institute of Paleobiology, Polish Academy of Sciences, number collection ZPAL G. The 3D retiolitine models were made by Nancy Kirk based on detailed observation and measurement of many specimens under the SEM. The material used for making models is sheathed wire and paper tape painted in different colours corresponding to certain morphological features.

MOTS CLÉS
The complete retiolitine morphological terminology is clarified and explained in Bates et al. 2005 andLenz et al. 2018. The classification of the graptolites follows Melchin et al. 2011.  tubarium with almost no reticulum, and a nema connected by the connecting rods to the obverse thecal wall (Fig. 1B) Bouček & Münch (1952) described the new species Para plectograptus hemmanni, and included Retiolites tenuis Eisenack, 1951 in the genus. Both species come from the T. testis Biozone. Later these species were questionably placed in Paraplectograptus by Lenz & Melchin (1987) and Kozłowska-Dawidziuk (1995). Plectograptus? lejskoviensis, described by Bouček (1931), and Plectograptus praemacilentus described by Bouček & Münch (1952), both from the T. testis Biozone, were included by Lenz & Melchin (1987) in Paraplectograptus, based on the development of the main characters, e.g. the connecting rods linking the transverse rods of the thecal wall with the nema.

AbbreviAtions
The material of Paraplectograptus eiseli described by Manck (1917) was flattened on the rock surface. The breakthrough in research came thanks to observations under the SEM of three-dimensional specimens of Paraplectograptus, started by Lenz & Melchin (1987). Further research gradually revealed the diversity of species within Paraplectograptus. Lenz et al. (2012) described P. senarius Lenz, Senior, Kozłowska &Melchin, 2012 andP. reticulum Lenz, Senior, Kozłowska &Melchin, 2012, occurring in Arctic Canada, from the Mono climacis flumendosae to Cyrtograptus perneri biozones. They differ in the development of the ancora sleeve lists, which in P. senarius have hexagonal meshes with a thin reticulum, which are present mostly at the proximal end, while in P. reticulum the reticulum is mainly developed in the middle part of the ancora sleeve.
The oldest form, questionably assigned to Paraplectograptus, is described from the lower Telychian Spirograptus guerichi Biozone and possibly the Aeronian Demirastrites convolutus Biozone of the Llandovery (Melchin et al. 2017). It shows  Fig. 3), stub 1F, ZPAL G.47/9; C, growth stage in obverse view, with incomplete ancora umbrella, and lists joining ancora to th1 1 complete; those to th1 2 partial, Cape Phillips Formation, Arctic Canada, uppermost Llandovery, stub 1258A ZPAL G.47/10; D, reverse view, with complete ancora umbrella, and four lists (arrowed) leading towards the first two thecae, stub 1257A, ZPAL G.47/11; E, growth stage with two first thecae, reverse view, lists of the lateral panels of the ancora sleeve (arrowed) tapering proximally, are developed in addition to the four initial lists as seen in D, in addition, pleural sleeve lists (arrowed), tapering distally, are now being developed, and project forwards from the thecal lips of the first two thecae, stub 1B, 201901, ZPAL G.47/12. Scale bars: 100 µm. a similarity of the tubarium construction to Paraplectograptus, but differs in having a simple ancora umbrella consisting of four meshes and a short sicula, 0.4-0.49 mm long, whereas the ancora umbrella of Paraplectograptus eiseli has eight meshes and a longer sicula, about 0.9-1.15 mm long. The micro-ornamentation of its tubarium bandages consists of fine parallel striae, in contrast to the pustular ornamentation of Paraplectograptus. The fact that this taxon occurs in the Llandovery is important since it may represent both a temporal and phylogenetic link between typical Aeronian retiolitines and the more derived taxa in later Telychian and younger strata.
In conclusion, the Paraplectograptus lineage is a relatively diverse group of retiolitines, comprising two genera, Paraplecto graptus and Pseudoplectograptus, and seven species (Kozłowska-Dawidziuk 1995, 2004Lenz et al. 2012). It is a long ranging group, occurring from the lower Telychian, S. turriculatus/ Str. crispus Biozone up to the upper part of the C. lundgreni Biozone in the Homerian.

MorPHoLoGY
General characteristics Paraplectograptus and Pseudoplectograptus differ mostly in the size of tubaria and arrangement of the lateral walls of ancora sleeve and ventral walls (Kozłowska-Dawidziuk 1995;Lenz et al. 2012).
They have distally open-ended tubaria, which may contain about 20 pairs of thecae in the longest specimens. Pseudoplectograptus has a more robust and wider rhabdosome whereas that of Paraplectograptus is slender and parallel-sided. Their thecal frameworks include: connecting rods linking transverse rods with the nema, lateral apertural rods, and thecal lips (Fig. 1). Their ancora umbrellas are simple, orderly, with well-defined rims, sometimes very thin and not preserved (Figs 1-4). The ancora sleeves of Para plectograptus and Pseudoplectograptus are variably developed, from more or less regular simple lists, to a reticulated wall (Lenz et al. 2012).
The ancora sleeve reticulum is better developed in Pseudo plectograptus species. The lists of the ventral wall have seams inside with well-developed pustules on their outer sides, as opposed to ancora sleeve lists with seams outside and inner smooth surfaces.

Sicula and nema
As with most retiolitines, the earliest part of the Paraplecto graptus tubarium to survive is the composite list which comprises the proximal part of the nema, the virga, which forms part of the prosicula; and, with the development of the metasicula, the virgella. The length of the prosicula, based on the position of the prosicular apex and prosicular rim, is about 0.4 mm. The sicular length is about 0.9-1.15 mm.
The preservation of the prosicula is commonly incomplete, generally limited to the prosicular rim (Fig. 2). This rim is very variable, ranging from a complete, circular rim (Fig. 2E, F) to scarcely visible vestiges on the inner face of the virga-virgella (Fig. 2G). Where the rim is more fully preserved, traces of longitudinal rods can occasionally be seen extending from the adapical edge of the prosicular rim (Fig. 2D).
The structure of the prosicula and longitudinal rods is similar to that of Ordovician diplograptid species (Bates 1996), suggesting that their construction is a very conservative feature. The wall is formed of randomly oriented fibrils; the longitudinal rods are formed of parallel fibrils making a keel which fades towards the prosicular aperture; and the prosicular rim has parallel fibrils, probably forming a bandage (Fig. 2D).
The virga, the unique structure for retiolitines, is formed on the outer face of the prosicula by the deposition of bandages, and has a concave internal face. On this there may be traces of the prosicular wall, including fibrils which may be parallel to the spiral line ( Fig. 2A). The ab-apertural end of the virga, and the transition to the nema, is marked by the end of the concave face; it becomes "internal" at the apex of the prosicula (Figs 2A, B; 5C). It should be noted that it has not been possible to establish the presence of a cauda (Bates 1996) apically from the conus.
Distal to the virga, the nema is characterised by having a concentric structure, around a central, largely hollow, axis. The thickening of the nema was by the deposition of bandages, with a pustular ornament (Fig. 2B).
The virgella of Paraplectograptus is very similar to that of most retiolitines, including those of both Retiolites and Stomatograptus. It is formed of a fusellar core, thickened both internally and externally by bandages running along its length (Fig. 5A, E). Traces of metasicular fusellar closures are usually only preserved in the slots on the obverse and reverse faces of the virgella nearer to the metasicular aperture (Fig. 5E). They sweep forwards into the slots, indicating the forwards sweep of the fuselli themselves.

Ancora umbrella
In Paraplectograptus the ancora umbrella commences, as in other ancorate retiolitines, by forking of the virgella to form the first two lists (Figs 3; 4A, B). These are inclined at an angle of about 40°, anticlockwise (viewed looking from outside the specimen) from the plane of symmetry of the two thecal series. A second forking then results in four secondary lists separating the four central meshes of the umbrella. Two of these lists extend to the rim of the ancora umbrella adjacent to the obverse and reverse proximal lateral orifices. The other two lists fork again to give rise to lists which define a minor mesh proximal to th1 1 , and three minor meshes proximal to th1 2 (Fig. 3). The resulting ancora umbrella is consequently oval in plan and asymmetrical about the virgella, being more elongated where it arches over the sicular aperture and ventral wall of th1 2 . In some other tubaria, and possibly in certain Paraplecto graptus species, the number of minor meshes in the umbrella reticulum became greater, but a similar asymmetrical oval form seems to have been retained.
The ancora umbrella of Paraplectograptus is completed with a definite rim. This is similar to that in most retiolitines like Pseudoplegmatograptus (Bates & Kirk 1992: figs 212, 213) but is in contrast to the umbrella in both Retiolites and Stoma tograptus, in which there is no distinct rim (Bates et al. 2005: fig. 6B; Lenz & Kozłowska 2007: fig. 4).
The rim is a somewhat lobate oval list closure, with a centripetally facing insertion slot continuous with the attachment grooves over the absicular faces of the adjoining radiating lists of the umbrella reticulum. Distinctive shards of possible fuselli are associated with the list/rim junctions (Fig. 3A-G).
The ancora umbrella is connected to the thecal framework by four main pleural lists, which mark the "corners" of the tubarium between the ventral and lateral sides (Fig. 4D, E).

Growth of tubarium walls
In the growth of any graptolite, bandages are always laid down on a pre-existing surface, not produced "in space". So, in the growth stages illustrated, all of the lists, as preserved, are represented by the bandaging which was laid down on a fusellum: this can be either a wall, as in the sicula and ancora umbrella, or a spine, as in the nema, the pleural lists and the four initial lists joining the ancora umbrella to the first two thecae (Fig. 4D, E). Therefore, each growth stage illustrated is actually a record of the appearance of the secondary bandaging, not of the actual growth.
The earliest growth stage is the production of the prosicula and the associated nema (Fig. 4A). Both the nema and the virga are preserved because they were thickly bandaged. The succeeding walls of the metasicula are only preserved as the deep seams which have within them the traces of the fuselli (Fig. 5E). These are angled forwards in the direction of growth, as is seen in the growth of most siculae.
The early growth stages show the development of the ancora umbrella; in Figure 4A the radial lists do not extend to the rim, in Figure 4B and C forkings of some of these lists show the position of the ancora umbrella rim. This forking is also seen in Figure 3, in a mature ancora umbrella.
The appearance of the lists of th1 1 and th1 2 is shown in Figure 4C, those of th1 1 being more complete than those of th1 2 (which lacks the apertural lip). Together with these, the first pleural lists of the ancora sleeve have already joined the ancora rim to the lateral apertural lists of th1 1 . At this stage, however, the lists on th1 2 are not complete: they give the appearance of pairs of spines projecting towards one another.
In Figure 4D the umbrella rim is bandaged sufficiently to be preserved as a complete rim. Both initial thecae are complete, and the next four pleural lists are projecting forwards. The four pleural lists are now completely present proximal to the first thecae, marking the presence of the four proximal orifices (Fig. 4D).
The development of the ancora sleeve lists is shown in Figure 4E. Two lists are projecting proximally towards the ancora umbrella rim, and other lists of the sleeve are starting to form the sleeve growing distally. As further thecae are generated, this pattern of growth is continued. fig. 7. -Stereopairs of two Pseudoplectograptus models made by Nancy Kirk: A model with one pair of thecae, ZPAL G.47/13, obverse view: red, nema, virga, connecting rods; gray, ancora umbrella; green, transverse rod; yellow, lateral apertural list; orange, thecal lip; white, pleural list and other ancora sleeve lists; B model with two pairs of thecae and postulated thecal walls, ZPAL G.47/14, reverse view, colours indicate: pink, th1 1 ; green, th2 1 ; yellow, th1 2 ; red, th2 2 . Scale bars: 10 cm. Thecal framework Paraplectograptus has a thecal framework which differs from that of Retiolites and Stomatograptus principally in having no dorsal or zigzag list (Fig. 1), a structural element which is also noticeably attenuated in Stomatograptus (Bates & Kirk 1997). The thecal framework is again supplemented by ancora sleeve lists extending from the proximal ancora umbrella. Examination at the ultrastructural level shows the strongly notched ventral margins of the Paraplectograptus rhabdosome, like those of Stomatograptus, to be due to an alternation of lateral apertural thecal lists (yellow) and pleural ancora sleeve lists (gray) bordering the proximal ventral orifices (Figs 1; 4; 5). There are also obverse and reverse orifices adjacent to the rim of the ancora umbrella, but no stomata.
Thecae 1 1 and 1 2 are defined by lists with seams arising from the region of the prosicula (Figs 1; 5). Th1 1 probably had its origin in a porus on the sicula, growing proximally before turning distally towards its aperture, defined by the first transverse rod, lateral apertural rods and thecal lip (Fig. 6A). The aperture of th1 2 is similarly marked by the equivalent lists on the other side (Fig. 6C). Subsequent thecae have their apertures also formed of transverse rods, lateral apertural rods, and thecal lips (Figs 1;  6). Details of the fusellar seams of Paraplectograptus eiseli can be seen in all four photographs on Figure 6B-E: note that the seams are rotated as they pass from proximal-facing on the transverse rods to inward-facing on the lateral apertural rods.
The transverse rod marks the initiation of the ventral thecal wall of successive thecae (Figs 5B; 6A). It is probably homologous with the ab-apertural list in diplograptids and monograptids. In these the list forms the base of the interthecal septum and forms initially as a spine (perhaps a pair of spines extending towards one-another) extending from side to side of the tubarium. It is also found in Retiolites and Stoma tograptus, with the thecal wall extending distally from it to the thecal lip. In Rotaretiolites (Bates & Kirk 1992: fig. 114) the transverse rod also has a strong seam on its proximal side.
On the obverse side of the tubarium the thecal walls include the nema and connecting rods, in a continuous lateral wall, inside the lists of the ancora sleeve (Figs 1A-C; 6A). The same arrangement is found in both Retiolites (Fig. 1E) and Stomatograptus. Note that the sleeve lists are secreted from the inside, implying that there was a space between the thecal wall and the ancora sleeve, the obverse external common canal (Fig. 1E). On the reverse side there is no zigzag or dorsal list, as is found in Retiolites and Stomatograptus. However, the ancora sleeve lists here, in Paraplectograptus, are also secreted from the inside, and it is probable that the reverse external wall lay inside the ancora sleeve, with an external common canal between them.
The walls of successive thecae would have overlapped one another (Fig. 7B), but there was no thickening along their junctions, which would have generated the zigzag list (cf. the generation of the zigzag list in Retiolites (Bates & Kirk 1978: pl. 12)).

Ancora sleeve
The ancora sleeve of Paraplectograptus is formed of lists which have outward-facing seams (Fig. 5A, B), as have those in Retiolites and Stomatograptus, in which they form a continuation of the lists of the ancora umbrella. This indicates that the bandages were laid down on the inner side of the sleeve, in spaces between the sleeve and the thecal framework, the external common canals. In the absence of a preserved dorsal or zigzag list on the reverse side of Paraplectograptus, it is not possible to be certain of the presence of an external common canal on this side. However, the bandages of the sleeve lists here were also laid down on the inside, so there was probably a space internal to the sleeve, but outside the thecal framework -the reverse external common canal.
In Retiolites and Stomatograptus the dorsal list marks the proliferation of successive thecae (thn 2 succeeding thn 1 ), its bandaging being laid down before the wall of thn 2 is deposited (Bates & Kirk 1997). It is presumed to have been present here also, but, without any bandaging to thicken and so form the lists, it is not preserved; its likely path is indicated in Figure 1D. Pleural lists mark the ventral limits of the ancora sleeve, and occupy similar positions to those of both Retiolites and Stomatograptus.
The sleeve bandages have a pustular ornament (Fig. 5A, B), unlike those of Retiolites and Stomatograptus, which are simply striated. The pustular ornamentation is an advanced feature characteristic of all post-lundgreni period retiolitines. Para plectograptus is one of a few pre-lundgreni retiolitines having pustules on its bandages (Kozłowska-Dawidziuk 2004: 510).

Proximal lateral orifices
Proximal lateral orifices form a prominent feature of many retiolitines, and, together with the proximal ventral orifices, form a circlet of four proximal orifices. In young specimens of Paraplectograptus they are separated only by four single lists (Fig. 4D, E). However, as growth continues, other lists of the ancora sleeve are developed, and they constrain the shape of the proximal lateral orifice. This, in common with that of a number of other genera of retiolitines, has a kidney-shaped appearance (Figs 1A; 7-9A, B). The rim of the ancora umbrella bends outwards and upwards to form a re-entrant shape when viewed laterally, while the other section of the rim is convex. In flattened specimens this feature can be accentuated, as the ancora rim section is bent upwards.

CONCLUSIONS
The morphology and reconstruction of the Paraplecto graptus lineage, containing the two genera Paraplectograptus and Pseudoplectograptus, is described herein based on wellpreserved three-dimensional material from Arctic Canada and Poland. Based on these data, comparison is made with the Llandovery retiolitines Retiolites and Stomatograptus. The similarities of their tubaria are shown in the main construction of the thecal wall and ancora sleeve.
The development of the transverse rods and their connection to the nema in Paraplectograptus and Retiolites make for a strong rhabdosome (tubarium) construction in both the Paraplectograptus lineage and Retiolites. The construction is made by the well-developed transverse rods (the basal part of thecae) connected to the nema, as well as the lateral apertural rods and lips. The lengths and angles between these lists are different in both groups of retiolitine.
There is a similar arrangement of the ancora sleeve in all the genera, which defines external common canals outside the internal common canal. The ancora sleeve of Paraplectograptus and Pseudoplectograptus is formed by more irregular and thinner lists. In both groups the ancora sleeve is secreted from the inside, which is typical for the older retiolitines.
In spite of the early appearance of Paraplectograptus, in the lower Telychian, its tubarium shows some advanced features compared with Retiolites. It is one of the first retiolitines having the later feature of pustular bandages, characteristic of all post-lundgreni period retiolitines. The bandages of Retiolites are smooth, typical of the older retiolitines.
Another important feature, used in retiolitine classification, is the development of the ancora umbrella. The ancora umbrella of Paraplectograptus and Pseudoplectograptus has a well-defined rim, in contrast to those of Retiolites and Stomatograptus.
The proximal lateral orifices are similar in shape in both the Paraplectograptus lineage and in Retiolites and Stomatograptus. Their patterns are widespread in the retiolitines.
One other difference is in size of tubaria, which are strong and large in Retiolites and Stomatograptus, as opposed to the small and slender Paraplectograptus tubaria, which are similar to most post-lundgreni forms.
To conclude, the Paraplectograptus lineage has features typical of the older Llandovery retiolitines, as well as advanced ones characteristic of all post-lundgreni forms. It may be regarded as an evolutionary link between the two groups.