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Evolutionary history of the Gothograptus lineage of the Retiolitidae (Graptolithina).

INTRODUCTION

The evolutionary pattern of graptolites is one of the best recognized patterns due to their well-documented fossil record of flattened and isolated material. Evolutionary trends through time have been recognized in graptolites as a whole, including the reduction of the rhabdosome (tubarium) size and number of branches and an increase in the degree of thecal complexity, e.g., Bulman (1970) and Mitchell (1990).

Some good examples of the simplification and reduction of graptolite colonies are shown in the retiolitids (Retiolitidae) evolving from the Llandovery to the mid-Ludlow, Silurian (Kozlowska-Dawidziuk 2004). The group is unlike all other graptolites in mostly possessing double lateral walls: an inner one which is the homologue of the 'normal' diplograptoid lateral walls (thecal walls) and an outer wall (ancora sleeve wall) derived entirely from the distal extension of the ancora (e.g. Bates & Kirk 1984; Bates et al. 2005). These walls are rarely completely preserved (Lenz & Melchin 1987; Kozlowska-Dawidziuk 1997; Lenz & Kozlowska-Dawidziuk 2002), being built from very thin and incrementally deposited fusellar layers supported by strong lists formed from bandages. Normally, therefore, only the rod-like bandaged lists of the retiolitid tubarium are preserved.

The phenomenon of limited growth of the colony (ending with an appendix) in retiolitids may be regarded as gradual loss of coloniality (Kirk 1978; Kozlowska-Dawidziuk 2004). It means that during their evolution, the siculozooid became larger, while zooids were small in size and reduced in numbers (Kozlowska-Dawidziuk 2004, fig. 7). As a consequence, the large siculozooid could have had a dominant role for the colony of a few small zooids. There are large colonies of earlier Silurian retiolitids with no appendix, containing about 80 pairs of thecae with small siculae, whereas some of the youngest retiolitids possess a finite tubarium with a large sicula and as few as three thecae (Kozlowska-Dawidziuk 2004, fig. 7). Similar relationships between the siculozooid and zooids occur exceptionally in Ordovician graptolites, such as Corynites Kozlowski, 1956 (Kozlowska-Dawidziuk 2004). Some significantly reduced rhabdosomes are also observed among Llandovery retiolitids, such as Rotaretiolites Bates & Kirk, 1992, but its sicula is short as in other Llandovery forms and the appendix is not known.

Limited colony growth is clearly recognized in the Gothograptus lineage, identified by the presence of the finite tubarium terminated by an appendix (Kozlowska-Dawidziuk 2004). The number of thecae in species of this lineage varies from 18 in older taxa to three pairs in the youngest forms. The lineage is regarded herein as a group that includes the genera Gothograptus, Baculograptus, Neogothograptus and Holoretiolites, which evolved during the Homerian Cyrtograptus lundgreni Biozone to the lower Ludfordian, to the Saetograptus leintwardinensis Biozone (Kozlowska-Dawidziuk 1995; Dobrowolska 2013).

The fossil record shows that among retiolitids only gothograptids survived the C. lundgreni event (e.g. Koren' 1991; Kozlowska-Dawidziuk et al. 2001), making it the ideal form to be ancestral to all subsequent plectograptines (Bates et al. 2005, fig. 9). The lundgreni event is regarded as one of the most profound, global graptolite extinction events (Jaeger 1991; Koren' & Urbanek 1994; Lenz 1995).

The purpose of this paper is a new synthesis of hypotheses about relationships between the species of the Gothograptus lineage, in light of new observations and the new insights derived from the cladistic studies of Bates et al. (2005) and Kozlowska et al. (2009).

The specimens figured are housed at the Institute of Paleobiology, Polish Academy of Sciences (abbreviation ZPAL).

HISTORY OF THE GOTHOGRAPTUS LINEAGE

Finite growth of tubaria is characteristic of some of the upper Wenlock and Ludlow retiolitids. According to the fossil record, Eisenackograptus from the Cyrtograptus perneri Biozone is the earliest retiolitid of limited growth of colony ending with an appendix (Lenz & Melchin 1987). The genus was regarded as an ancestor of the Gothograptus lineage by Kozlowska-Dawidziuk (2001) and Kozlowska-Dawidziuk & Lenz (2001). According to the cladistic analysis (Bates et al. 2005), Eisenackograptus forms a separate group with the appendix independently derived from the appendix in Gothograptus. Eisenackograptus differs from gothograptids in a more complicated proximal end having a thecal ventral list on theca 1 (1), well-developed transverse rods and an ancora sleeve with seams facing out (Bates et al. 2005).

The Gothograptus lineage is regarded herein as a clade consisting of the genera Gothograptus, Baculograptus, Neogothograptus and Holoretiolites based on the cladistic analyses presented in Bates et al. 2005 (fig. 10A). The members of the Gothograptus lineage are characterized by tubaria tapering distally, most of them by a well-developed appendix, and singular genicular hoods as well as characteristic features of the proximal end, and thecae (e.g. Kozlowska-Dawidziuk 1995, 2004; Bates et al. 2005). Closely related to these forms are Cometograptus Kozlowska-Dawidziuk, 2001 from the C. lundgreni Biozone, Doliograptus Lenz & Kozlowska-Dawidziuk, 2002 and Papiliograptus Lenz & Kozlowska-Dawidziuk, 2002 from the Colonograptus praedeubeli-Col. deubeli Biozone (Bates et al. 2005).

The remnants of the sicula are rarely preserved, but the available data show that the length of the sicula is 1.2 mm in Gothograptus and 1.8 mm in the youngest genus Holoretiolites (Kozlowska-Dawidziuk 2004). The last form of the Gothograptus lineage is Holoretiolites mancki (Munch, 1931) known from the S. leintwardinensis Biozone (Kozlowska-Dawidziuk 1995; Dobrowolska 2013). Bigger reduction of the tubarium than that of H. mancki show H. helenaewitoldi Kozlowska-Dawidziuk, 2004 occurring in the Lobograptus progenitor Biozone and H. atrabecularis Kozlowska-Dawidziuk, 1995 in the Lobograptus parascanicus (Kozlowska-Dawidziuk 1995) and Cucullograptus hemiaversus biozones (author's unpublished data).

Genus Gothograptus

The oldest three species of the genus Gothograptus, G. kozlowskii Kozlowska-Dawidziuk, 1990 (Fig. 1A), G. obtectus Kozlowska-Dawidziuk, 1990 and G. storchi Lenz & Kozlowska, 2006, appeared in the C. lundgreni Biozone. They have densely reticulated tubaria, nema incorporated into the lateral wall and the largest and the most variable singular genicular processes, which may extend proximally covering the apertures and part of the theca (Kozlowska-Dawidziuk 1990). The sicula length is about 1.2 mm (author's unpublished data).

Gothograptus nassa Holm, 1890 has characteristic solid genicular hoods built of densely packed parallel bandages (Figs 1B, 2B). They are almost identical to the proximal thecal hoods of the older species G. kozlowskii, which may indicate their close relationship. Gothograptus nassa has the largest number of thecae of any species of the Gothograptus lineage, up to 18 pairs. This is the only retiolitid known to appear in the earliest post-extinction Pristiograptus dubius-G. nassa Biozone (Koren' 1991; Koren' & Urbanek 1994; Porebska et al. 2004) (see Introduction). However, the cladistic analysis suggests that the ancestor of the Plectograptus lineage could have spanned this interval as well (Bates et al. 2005, fig. 9).

Genus Baculograptus

Two species of Baculograptus Lenz & Kozlowska-Dawidziuk, 2002 are known from the Colonograptus praedeubeli Biozone, B. chainos Lenz, 1993 and B. batesi Lenz & Kozlowska-Dawidziuk, 2002. The genus is characterized by a reticulated rhabdosome with the nema incorporated in its distal part of the lateral wall, and a lack of genicular hoods. Given that the first occurrence of species of Baculograptus is from the Col. praedeubeli Biozone, as is the case with Neogothograptus Kozlowska-Dawidziuk, 1995, it is possible that these two genera may show an ancestor-descendant relationship, or be sister taxa, derived from Gothograptus (Kozlowska et al. 2009).

Genus Neogothograptus

Species of Neogothograptus flourished continuously from the Col. praedeubeli Biozone up to the Lobograptus scanicus Biozone (Lenz & Kozlowska-Dawidziuk 2002; Kozlowska et al. 2009). This is the first group of gothograptids with a free nema, which is a typical feature of Ludlow retiolitids.

Neogothograptus is the most diverse among gothograptid genera, with nine species. The great similarity of genicular solid hoods of N. eximinassa Maletz, 2008 (Figs 1C, 2D) suggests possible close relationship with G. kozlowskii and G. nassa, although the results of the cladistic analysis of Kozlowska et al. (2009) suggest that the hoods could have been derived independently. Neogothograptus balticus (Eisenack, 1951) and N. reticulatus Kozlowska, Lenz & Melchin, 2009 have similar hoods built of a reticulum (Figs 1E, 2C, E).

Neogothograptus thornsteinssoni Lenz & Kozlowska-Dawidziuk, 2004 (Fig. 1D) from the Neodiversograptus nilssoni and Lobograptus progenitor biozones exhibit a mix of characters of older gothograptids, having a dense reticulum, and of younger forms in the absence of genicular processes (Fig. 1D).

Neogothograptus alatiformis Lenz & Kozlowska-Dawidziuk, 2004 is the smallest form of this genus with only three thecae and a well-developed, dense reticulum, especially in mature colonies. This species possesses unique, well-developed singular genicular processes with a distal shelf-like projection. Similar processes are present in the most proximal part of the tubarium. These structures are unusual in the Gothograptus lineage (Lenz & Kozlowska-Dawidziuk 2004).

Species of Neogothograptus, like most of the Ludlow retiolitids, display a strong tendency to reduction of the reticulum and genicular hoods. The species N. purus Kozlowska-Dawidziuk, 1995 and N. melchini Lenz & Kozlowska-Dawidziuk, 2004, from the L. progenitor Biozone, possess tubaria with no reticulum or genicular structures.

Genus Holoretiolites

The greatest degree of rhabdosomal structural reduction occurs in the youngest gothograptids, species of Holoretiolites. The reduction of mid-ventral lists in H. atrabecularis shows the next step in reduction of the tubaria in this lineage. In addition, four of the species of this genus, H. mancki (Munch, 1931), H. erraticus Eisenack, 1951, H. atrabecularis and H. helenaewitoldi, have extremely reduced, small rhabdosomes with a simple proximal end, no reticulum and no genicular hoods (Fig. 1F). The two most reduced species of the genus, H. atrabecularis and H. helenaewitoldi (Fig. 1G), have a reduced appendix. The sicula in H. helenaewitoldi is about 1.8 mm long (Kozlowska-Dawidziuk 2004). Only H. manckoides Kozlowska-Dawidziuk, 1995 from the Cucullograptus hemiaversus Biozone has a larger tubarium and no reticulum.

According to the stratigraphical record, the species of Holoretiolites--H. mancki, known from the S. leintwardinensis Biozone, represents the last known species of the Gothograptus lineage, although two the most reduced forms H. helenaewitoldi and H. atrabecularis occur in the Lobograptus progenitor and L. parascanicus biozones.

SUMMARY

The Gothograptus lineage ranges through the upper Homerian to the lower Ludfordian of the Silurian, thus survived the lundgreni extinction event. Most of its members, the genera Gothograptus, Baculograptus, Neogothograptus and Holoretiolites, have a finite tubarium terminating with an appendix, except for some of the most reduced species of Holoretiolites. The appendix, regardless of its length, marks the end of colony growth and is considered to be a modified distalmost theca.

A characteristic feature of some gothograptids is the presence of singular hoods, developed on the genicular list of thecal apertures. They are built of a reticulum or densely packed parallel bandages, having different shapes and sizes in different species. Hoods are absent in species of Baculograptus, Holoretiolites and some species of Neogothograptus.

Reduction of tubaria complexity, size, numbers of zooids and increase in sicula length lead to the smallest forms represented by some species of Holoretiolites. A comparable degree of reduction, but with a different rhabdosomal structure, is known in Plectodinemagraptus gracilis Kozlowska-Dawidziuk, 1995 from the S. leintwardinensis Biozone (Kozlowska & Bates 2014).

doi: 10.3176/earth.2015.10

Acknowledgements. I thank Alf Lenz from the University of Western Ontario, Canada and Joerg Maletz from Institut fur Geologische Wissenschaften Freie Universitat, Germany for the kind donation of the pictures of Neogothograptus thorsteinssoni and Neogothograptus eximinassa. The reviewers Alf Lenz and Michael Melchin are thanked for their constructive comments which greatly improved this manuscript. This paper is a contribution to IGCP Project 591 'The Early to Middle Palaeozoic Revolution'.

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Received 10 July 2014, accepted 8 October 2014

Anna Kozlowska

Instytut Paleobiologii PAN, ul. Twarda 51/55, PL-00-818 Warszawa, Poland;

akd@twarda.pan.pl
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