Reconstruction of the proximal ends of retiolitid rhabdosomes (Graptolithina) from the Upper Wenlock and the Lower Ludlow
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Detailed study of the ultrastructures on the cortical lists of retiolitids occurring after the lundgreni event has been undertaken. The extremely well preserved material from the Gołdap borehole of Poland (Baltica) enables classification of clathrial lists into two types—with single and double seams. Significantly different seam development is observed between two groups of retiolitids occurring before and after the lundgreni event. The study of seams and increments on the lists enables reconstruction of the membrane arrangement of the rhabdosomes. Reconstruction of the proximal ends, the arrangement of the ancora umbrella, and the layout of the proximal ventral and lateral orifices of five genera, Neogothograptus, Holoretiolites, Plectograptus, Spinograptus, and Quattuorgraptus n. gen., is presented. The new genus Quattuorgraptus is separated from Spinograptus because of the different development of the ancora umbrella and the width of the lateral walls of the rhabdosome.
KeywordsAncora umbrella Membrane Retiolitidae Graptolites Proximal end Silurian
Detailierte Untersuchungen der Feinstrukturen der kortikalen Leisten der Retioliten aus dem Zeitbereich nach dem lundgreni Ereignis (Oberes Wenlock-Ludlow) wurden vorgenommen. Sehr gut erhaltetes Material aus der Gołdap Bohrung in Polen (Teil von Baltica im Paläozoikum) erlaubt die Unterscheidung der klathrialen Leisten aufgrund ihrer Bauweise in zwei Typen: Leisten mit Einzel- und mit Doppelnähten. Diese wichtige Differenzierung in der Nahtentwicklung ist zwischen den Retioliten vor und nach dem lundgreni Ereignis zu beobachten. Die Untersuchungen der Leisten und der Anwachsmarkierungen der Fuselli an den Nähten erlauben die Rekonstruktion der nicht erhaltenen Membranen des Rabdosomes. Die Rekonstruktion der Proximalentwicklung, die Entwicklung der Ankorakuppel, sowie die Anordnung der ventralen und lateralen Öffnungen der fünf Gatungen Neogothograptus, Holoretiolites, Plectograptus, Spinograptus, and Quattuorgraptus werden diskutiert. Die neue Gattung Quattuorgraptus wird aus der Gattung Spinograptus ausgegliedert, da sich die Entwicklung der Ankorakuppel und die Breite der lateralen Wänden des Rabdosomes von Spinograptus deutlich unterscheidet.
SchlüsselwörterAncore Umbrella Membran Retiolitidae Graptolithen Proximale Ende Silur
The early Paleozoic Graptolithina were of significant importance as plankton in the marine biocenosis. Their Silurian family Retiolitidae evolved rapidly from the Aeronian to the Ludfordian. The retiolitids were a peculiar group of colonial graptolitids characterized by the unique structure of their rhabdosome. All graptolites had a thecal rhabdosome. However, for the retiolitids it was internal, because outside they had an additional layer: the ancora sleeve, i.e. a structure developed from the bifurcation and subsequent distal development of the virgella. Another specificity was the construction of rhabdosomal walls. The walls of graptolite rhabdosomes consisted of an internal fusellar layer and an external cortical layer, the latter consisting of bandages, whereas the walls of retiolitids consisted of cortical lists and membranes which were stretched between them. The membranes of the ancora sleeve and thecal walls were extremely thin. In contrast, the lists, which were much thicker, were built of multilayered cortical bandages and are usually well preserved (Kozłowska-Dawidziuk 2004). Usually, the rhabdosome looks like a regular network of lists, although sometimes the whole rhabdosome (lists and membranes) is preserved (Lenz 1994a, b; Lenz and Thorsteinsson 1997; Kozłowska-Dawidziuk 1997; Lenz and Kozłowska-Dawidziuk 2002). For a long time, the arrangement of the membranes was a mystery. However, if scanning electron microscopy (SEM) is used to view specimens of retiolitids, traces of the membranes on the cortical lists can be seen as prominent seams (Bates 1987). Nowadays, it is usually possible to reconstruct the membranes on the basis of seams occurring on the lists. Bates and Kirk (1978, 1992, 1997) described in some detail development of the ancora umbrella in Stomatograptus, Pseudoretiolites, Gothograptus nassa, Holoretiolites mancki, and Retiolites. Reconstruction of the membranes’ position on the basis of the seam arrangement of the ancora umbrella of the genus Plectograptus was presented by Bates et al. (2006), without detailed studies of seams around the proximal ventral and lateral orifices.
It is commonly accepted that the mode of development and proximal structures are the best features for recognition of high-level taxonomic categories within the Graptoloidea (Mitchell 1987; Kozłowska-Dawidziuk et al. 2003). For the retiolitids, the mode of development and, particularly, the development of the ancora umbrella with location of the proximal lateral and ventral orifices are characteristic features of each genus. This is probably because the ancora umbrella started its development during the early stages of the astogeny of the colony.
To better understand changes of astogeny, evolution, and retiolitid behavior connected with the construction of rhabdosomes it is very important to know the structure of the entire rhabdosome including details of the membrane arrangement. The purpose of this paper is threefold:
to present a classification for clathrial seams of retiolitids occurring after the lundgreni event;
to reconstruct membranes of the proximal end of rhabdosomes for four genera: Neogothograptus, Holoretiolites, Plectograptus, Spinograptus; and
to present a new genus, Quattuorgraptus, separated from Spinograptus on the basis of differences in ancora umbrella development.
Materials and methods
Ultrastructure of the retiolitid rhabdosomes
Bates and Kirk (1992, 1997) described the ultrastructures of Llandovery ancorate diplograptids and retiolitids occurring before the lundgreni event with remarkable accuracy. They observed enwrapping and insertion seams. The insertion seam can be equivalent to a single seam on lists of the young retiolitids occurring after the lundgreni event. The double seam described in this paper can comprise an enwrapping seam on one side of the list and an insertion seam on the other. However, on the basis of observations of the post-lundgreni material from the Gołdap borehole and the pre-lundgreni material from the Bates and Kirk (1992, 1997) papers, it is difficult to compare the seams of the earlier and later retiolitids because they differ in appearance. The seams of the earlier are distinct and deep, those of the later retiolitids are less pronounced and shallow, and it is easy to confuse them with bandages. The increments of old retiolitids are more often preserved than those of young retiolitids.
Bates et al. (2006) described the primary lists of the ancora sleeve in Plectograptus. Two different types of primary list (A and B) are found in this genus (Bates et al. 2006, Fig. 5). The type A lists include the ancora sleeve lists without pleural lists, the type B lists build pleural lists. The initial list of the type A lists has a proximally-facing insertion seam (Bates et al. 2006, Fig. 5a) which marks the extension of the initial fusellum of the ancora sleeve. The type B lists have an initial core (Bates et al. 2006, Fig. 5b) which is similar to the fusellar core of the spines, and is surrounded by concentric asymmetric cortical layers.
Membrane reconstruction of the proximal ends of retiolitid rhabdosomes occurring after the lundgreni event
The proximal lateral orifices belonging to the lateral walls are bounded proximally by part of the ancora umbrella rim, and laterally and distally by the ancora sleeve lists in Neogothograptus, Holoretiolites, and Spinograptus (Fig. 9b–c). In Plectograptus, these orifices are bounded proximally by membranes of the ancora umbrella or, in rare cases, by part of the ancora umbrella rim and distally by the ancora sleeve membranes (Figs. 7, 9a). In Quattuorgraptus, the lateral orifices are surrounded proximally by the ancora umbrella rim, distally by the ancora sleeve lists, and, at the theca 12 side of the rhabdosome, by pleural lists below the first pair of thecae (Fig. 9d).
Here, the ultrastructures appearing on lists in the proximal end of the retiolitids from the Upper Wenlock and the Lower Ludlow from the Gołdap borehole are analyzed in detail. On the basis of the classification of the observed seams, the development of the ancora umbrella can be reconstructed, with the location of the proximal ventral and lateral orifices and the course of the membranes of the proximal ends of the rhabdosomes, for five genera of retiolitids. Because of detailed analysis of seams of the species Sp. munchi, it was also possible to erect a new genus, Quattuorgraptus. For each diagnosis of the described genera, a detailed description of the proximal end of the rhabdosome is given on the basis of the ultrastructures occurring on lists of the ancora umbrella and on lists surrounding the proximal lateral and ventral orifices.
Neogothograptus purus Kozłowska-Dawidziuk, 1995
Diagnosis (emended from Maletz 2008)
Ancora umbrella thicker than the rest of rhabdosomal lists in mature forms, consisting of three short branches and one long branch divided twice, usually at theca 12 side of the rhabdosome; ancora umbrella has six meshes and fully developed ancora umbrella rim; outer ancora usually present in mature specimens; nema located centrally; proximal ventral orifices and proximal lateral orifices may be overgrown by reticulum in mature specimens. Ventral wall consists of lips, genicular lists, lateral apertural rod, and mid-ventral lists; genicular processes present in most species; reticulum development highly variable; rhabdosome ended by appendix.
Retiolites balticus Eisenack, 1951; Neogothograptus purus Kozłowska-Dawidziuk, 1995; Neogothograptus purus labiatus Lenz and Kozłowska-Dawidziuk, 2004; Neogothograptus romani Kozłowska-Dawidziuk, 1995; Neogothograptus alatiformis Lenz and Kozłowska-Dawidziuk, 2004; Neogothograptus melchini Lenz and Kozłowska-Dawidziuk, 2004; Neogothograptus thorsteinssoni Lenz and Kozłowska-Dawidziuk, 2004; Neogothograptus eximinassa Maletz, 2008; Neogothograptus ornatus Maletz, 2008; Neogothograptus reticulatus Kozłowska, Lenz and Melchin, 2009.
Diagnosis (emended from Kozłowska-Dawidziuk 1995)
Ancora umbrella consists of three short branches and one long branch divided twice located at the theca 12 side of the rhabdosome; branches of ancora umbrella form six meshes; ancora umbrella rim is fully developed; outer ancora developed only in H. erraticus; nema located centrally. Ventral wall consists of lips, genicular lists, and fully developed mid-ventral lists in H. mancki, H. manckoides, and H. erraticus, reduced in H. helenaewitoldi and H. atrabecularis; no genicular processes; reticulum present only in H. erraticus; rhabdosome ended by appendix.
Retiolites mancki Münch, 1931; Retiolites erraticus Eisenack, 1951; Holoretiolites manckoides Kozłowska-Dawidziuk, 1995; Holoretiolites atrabecularis Kozłowska-Dawidziuk, 1995; Holoretiolites helenaewitoldi Kozłowska-Dawidziuk, 2004.
Diagnosis (emended from Maletz 2010)
Simplified shallow ancora umbrella consisting of two short branches and two long branches: one of these long branches divides once at the theca 12 side of the rhabdosome; ancora umbrella has five meshes and fragmentary outlined ancora umbrella rim; no outer ancora; nema not attached to lateral wall, only in Plectograptus robustus is it attached to the nematularium in the distal part of the rhabdosome; proximal ventral orifices are of different sizes, at the theca 12 side of the rhabdosome usually larger; proximal lateral orifices big. Ventral wall consists of lips, genicular lists, lateral apertural rods, and mid-ventral lists; mid-ventral lists much thinner than other lists in rhabdosome; genicular processes can be observed in some species; reticulum present only in Pl. wimani; rhabdosomes usually open-ended, only in Pl. robustus is it ended by the nematularium.
Retiolites macilentus Törnquist, 1887; Agastograptus robustus Obut and Zaslavskaya, 1983; Retiolites wimani Eisenack, 1951; Plectograptus toernquisti Bates Kozłowska, Maletz, Kirk and Lenz, 2006; Plectograptus mobergi Bates Kozłowska, Maletz, Kirk and Lenz, 2006; Plectograptus trijunctus Bates Kozłowska, Maletz, Kirk and Lenz, 2006.
Diagnosis (emended from Kozłowska et al. 2012, http://dx.doi.org/10.4202/app.2011.0020)
Ancora umbrella consists of two short branches and two long branches, the branch at the 11 side of the rhabdosome is divided once, the branch at 12 side of the rhabdosome is divided twice; ancora umbrella has seven meshes and full rim; outer ancora present in some species; nema centrally located, in two species in distal part of the rhabdosome is attached to obverse side of the rhabdosome below the appendix; proximal ventral orifices can sometimes have paired genicular processes, in one species Sp. tubothecalis they can be overgrown by reticulum in mature specimens; proximal lateral orifices medium sized and shifted toward the 12 side of the rhabdosome. Ventral wall is composed of lips, genicular lists, lateral apertural rods and rudimentary mid-ventral lists, in some species with rudimentary transverse rods; ancora sleeve consists of parietal lists forming distinct zigzag pattern; paired reticulo-fusellar apertural processes present; reticulum can be present; rhabdosomes may be open-ended or finite with short appendix; finite rhabdosomes may taper distally or, exceptionally, may be parallel-sided with two distal isolated thecae with openings directed distally. In some species the membrane of sicula, thecae, or ancora sleeve can be present.
Retiolites spinosus Wood, 1900; Retiolites clathrospinosus Eisenack, 1951; Holoretiolites (Balticograptus) lawsoni Holland Rickards and Warren, 1969; Agastograptus quadratus Lenz, 1993; Spinograptus reticulolawsoni Kozłowska-Dawidziuk, 1997; Spinograptus latespinosus Kozłowska-Dawidziuk, 1997; Spinograptus praerobustus Lenz and Kozłowska-Dawidziuk, 2002; Spinograptus tubothecalis Kozlowska Dobrowolska and Bates, 2012 (http://dx.doi.org/10.4202/app.2011.0020).
Genus Quattuorgraptus n. gen.
Origin of name: quattuor Latin—four, due to the simple development of the ancora umbrella with only four meshes.
Type species: Retiolites munchi Eisenack, 1951
Simple ancora umbrella consists of four undivided branches, four meshes, and fully developed rim; outer ancora absent; nema not connected to lateral wall of the rhabdosome; proximal ventral orifices can sometimes have paired, short, reticulo-fusellar processes; proximal lateral orifices large and shifted toward the 12 side of the rhabdosome. Ventral wall composed of lips, genicular lists, lateral apertural rods, and sometimes rudimentary mid-ventral lists; parietal lists of ancora sleeve form distinct zigzag pattern; paired, reticulo-fusellar genicular processes present, usually split in their medial part, sometimes straight or more complicated; reticulum absent; rhabdosome open-ended.
Retiolites munchi Eisenack, 1951
1951 Retiolites munchi Eisenack: 138, pl. 22: 9–12, pl. 23: 3–5, pl. 24: 1.
1983 Agastograptus munchi Obut and Zaslavskaya: 111–112, pl. 22: 4, pl. 26: 1–4.
1995 Spinograptus munchi Kozłowska-Dawidziuk: 316–317, Fig. 31 a, 32 a–b.
2001Spinograptus munchi Kozłowska-Dawidziuk, Lenz and Štorch: 155, Fig. 5.4–5.6, 5.10, 5.14, 5.16.
2010 Spinograptus munchi Maletz: 511–512, Fig. 2 o, 5c–d, 7a–h.
Seven specimens with a minimum three pairs of thecae and about 120 smaller fragments of rhabdosomes. Depths: 1,255.0 m, 1,257.0 m—ludensis zone; 1,264.0 m, 1,267.0 m—deubeli zone. ZPAL G. 48/2; 48/16–22.
As for the genus.
The longest rhabdosome is 3.91 mm long with three pairs of thecae (Fig. 13a3). The width of the lateral wall (832–871 μm) was measured between the pleural lists at the height of the lips of the first pair of thecae, on the level of the second and the third pairs of thecae at 1.07 and 1.08 mm, respectively. The ventral wall slightly extends through the distal end of the rhabdosome. The ancora umbrella is from 588 to 770 μm wide and approximately 121 μm deep. The proximal ventral orifices are almost at the same level and usually without genicular processes (Fig. 13b). Only one specimen from 1,257.0 m of the Gołdap borehole has paired reticulo-fusellar processes over the orifice of the 11 side of the rhabdosome (Fig. 13a). In all specimens, the orifice at the 11 side of the rhabdosome is smaller than the orifice at the 12 side (Fig. 13b). The distance from the ancora umbrella to thecal lip at the 11 side of the rhabdosome is 804–838 μm and that to the thecal lip at the 12 side of the rhabdosome is 1.15–1.22 mm. Usually, there are no mid-ventral lists, although, in some specimens, short rudimentary mid-ventral lists occur. There are no transverse rods, although, in a few specimens, the denticles are visible in the place where transverse rods usually occur. The thecal apertures are rectangular; the genicular list is slightly longer than the lip. Apertures have a width of ca. 400–500 μm and a height of ca. 200–250 μm. The successive lips are distanced from each other as follows: the first from the second thecae, 616–714 μm; the second from the third, 619–711 μm; the third from the fourth, 651 μm. The rhabdosomes have paired reticulo-fusellar genicular processes which usually bifurcate in their medial parts (Fig. 13d). Sometimes there are unbifurcating straight genicular processes or ones of a more complex form (Fig. 13e). These latter occur in the shallowest sample at a depth of 1,255.0 m. The length of shorter arm of a typical process is 400 μm and that of the longer arm is 776 μm. Processes over the proximal ventral orifices are 340 μm long. Membranes are not preserved.
On the basis of material from a glacial erratic boulder from Rügen Island, Maletz (2010) reported that Spinograptus munchi has a shallow ancora umbrella formed by four central meshes and a number of smaller outer meshes, which are often poorly and incompletely outlined by a thin ancora umbrella rim. According to him, the ancora umbrella rim is difficult to trace. In this paper, on the basis of detailed analysis of seams of the proximal end of Q. munchi rhabdosomes, it is shown above that the ancora umbrella of Q. munchi consisted of four meshes only. It is also suggested that the material from Rügen Island is of younger astogenetic stages than the material from the Gołdap borehole, because the latter contains specimens with thicker lists and a fully developed ancora umbrella rim. The material from Rügen Island contains rhabdosomes with poorly preserved proximal ends and thin lists. Therefore, the description of this part of the rhabdosomes could not be unequivocal. Quattuorgraptus munchi is constructed of thick lists and does not have a reticulum and an outer ancora. In this regard and by similarity of shape of rhabdosomes (the walls are almost parallel) it resembles the genus Plectograptus, especially Pl. robustus (this species has paired genicular processes similar to Q. munchi). These two species differ in the development of the ancora umbrella. This difference is already visible in specimens of young astogenetic stages of the colony when the first pair of thecae are not yet developed (Fig. 13f, h). The ancora umbrella of Q. munchi consists of four arms of equal length surrounded by the ancora umbrella rim (Fig. 13f), whereas the ancora umbrella in Pl. robustus consists of two short branches and two long branches and one of these branches is divided once and the acora umbrella rim is not developed (Fig. 13h). Q. munchi does not have mid-ventral lists, which are well developed in Plectograptus.
Exceptionally well preserved post-lundgreni event retiolitids from the Gołdap borehole of Poland (Baltica) were studied under the scanning electron microscope. As a result, two types of lists with single and double seams are distinguished. On the basis of detailed analysis of seams on lists, reconstructions of the proximal ends of rhabdosomes with the arrangements of membranes of five retiolitid genera are presented.
The development of the proximal ends of the described genera differs substantially (except for Neogothograptus and Holoretiolites). It seems that the ancora umbrella of Spinograptus is the most complex among the post-lundgreni retiolitid genera: it has seven meshes and has a fully developed rim. The ancora umbrella of Quattuorgraptus n. gen. has only four meshes and a full rim. The ancora umbrella of Neogothograptus and Holoretiolites has six meshes and a full rim. Although the two genera have an identical construction of the ancora umbrella, they differ from each other by the structure of the ancora sleeve and ventral walls. The Plectograptus ancora umbrella has five meshes without a rim; in rare cases only is a small fragment of rim developed. After tracking the proximal seams in Plectograptus it seems that its large proximal lateral orifices were probably much smaller than described, and they were of slit-shape rather than oval.
Such large differences in development of the proximal end of the rhabdosome may indicate a change in behavior of the first zooid.
On the basis of analysis of the ancora umbrella seams, Spinograptus munchi can be assigned to the new genus Quattuorgraptus.
Significant differences in seam development are observed between the two groups of retiolitids, occurring before and after the lundgreni event. The earlier retiolitids have deep, distinct, and clearly visible seams; the later ones have less pronounced, shallow seams which are easy to confuse with bandages. The increments of the older retiolitids are more often preserved than those of younger ones.
I would like to thank Anna Kozłowska (Institute of Paleobiology Polish Academy of Sciences, Poland) for helpful discussions, constructive comments, and for kindly reading the manuscript. I am very grateful to Denis Bates for correcting the English text and to Jörg Maletz for correcting the German text. I thank the reviewers Jörg Maletz from State University (Buffalo, USA) and Denis Bates from University of Wales (Aberystwyth, UK) who made useful and thoughtful comments. Financial support was given by The Minister of Science and Higher Education, decision 499/N-NSERC/2009/0. The SEM pictures were made in Institute of Paleobiology Polish Academy of Sciences on Philips XL.
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