A new classification of ancorate diplograptids  

Anna Kozlowska-Dawidziuk¹, Alfred C. Lenz² and Denis E.B. Bates³  

¹Anna Kozlowska-Dawidziuk, Instytut Paleobiologii PAN, ul. Twarda 51/55, PL-00-818 Warszawa, Poland. E–mail: akd@twarda.pan.pl

² Alfred C. Lenz aclenz@uwo.ca, Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada. E–mail: aclenz@uwo.ca

³ Denis E. B. Bates, Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, Ceredigon SY23 3DB, UK. E–mail: deb@aber.ac.uk

Key words: Graptolites. Ancorate diplograptids. Phylogeny. Classification. Silurian.

The development mode and proximal structures are commonly accepted as the best for classification of higher taxa within Graptoloidea. The retiolitids and petalolithids are unique in possessing a virgellar ancora and/or distal ancora development. The ancora structures are considered homologous, and they are here regarded as synapomorphic features on which the new superfamily Retiolitoidea has been founded. The ancora is defined by Bulman (1970) as "anchor-shaped initial growth stage of retiolitids, apparently formed of virgella with two distal bifurcations" (emphases are ours). Now we know that the true ancora is developed in diplograptid Pattern I forms (Melchin, 1998) and retiolitids Pattern R. The first true ancora, beginning with a bifurcation, was developed in petalolithids representing Pattern I forms. Some evolution of the ancorae from small and simple to more complex is observed, although there has been no detailed study of the petalolithid group focusing on the astogenetic and historic development of ancorae. In Petalolithus and Pseudorthograptus there is sometimes considerable distal growth beyond the ancora umbrella to the point of partial envelopment of the post-sicular region of the rhabdosome (e.g., Koren’ and Rickards, 1996; see Figure 1 A, B). The most studied ancorate petalolithid Hercograptus Melchin, 1999; possesses an ancora umbrella connected to the thecae (Figure 1 C). The next and the most advanced stage of the ancora development occurs in the Retiolitidae in the form of an ancora sleeve (Figure 1 D, E). In this case the ancora sleeve is deeply integrated with thecal skeleton, which in addition, is built mostly by lists similar to those of the ancora. These two features: ancora sleeve and thecal framework are critical and unique for the retiolitids. The retiolitid ancora sleeve may form walls, outside the thecal skeleton, making an additional internal environment. The ancora sleeve wall may possess orifices and some additional structures as large stomas or long spines on the obverse and reverse sides of the rhabdosomes (Bates and Kirk, 1997; Kozlowska-Dawidziuk, 2001; Lenz and Kozlowska-Dawidziuk, 2001).

The intensive study of isolated, uncompressed retiolitids during the last two decades (e.g., Bates and Kirk, 1987, 1992, 1997; Bates, 1990; Lenz and Melchin, 1987, 1997; Lenz, 1993, 1994b; Kozlowska-Dawidziuk, 1995, 2001, 2002; Lenz and Kozlowska-Dawidziuk, 2001, 2002) has led to a much better understanding of retiolitid morphology, taxonomy, astogeny, and evolutionary history and relationships. These studies, together with the new data on biserial graptolites, especially petalolithids (Koren’ and Rickards, 1996; Melchin, 1998, 1999) leads us to propose the revised classification.

Figure 1. Ancorate petalolithids and retiolitids: A, Pseudorthograptus (Pseudorthograptus?) sp. C, cyphus or gregarius Biozone, proximal fragment of rhabdosome with thecae enveloped in ancora and its extension, CNIGR Museum 183/12879E. B, Pseudorthograptus (Pseudorthograptus) obuti (Rickards and Koren’), gregarius Biozone, sicula with the beginning of first theca and ancora umbrella, CNIGR Museum 162/12879. C, Hercograptus introvertus Melchin, GSC104936, fragment of proximal end with ancora umbrella and first theca, cyphus Biozone, after Melchin 1990. D, Stomatograptus sp. obverse view of specimen with well preserved fusellar thecal walls, GSC115529, after Lenz and Thorsteinsson 1997. E, Spinograptus praerobustus Lenz and Kozlowska-Dawidziuk, specimens with preserved thecal and ancora sleeve membranes, Arctic Canada, ABa3-98, 21 m, praedeubeli-deubeli Biozone. Specimens A, B after Koren’ and Rickards 1996.

We propose, therefore, that the ancorate petalolithids and retiolitids be subsumed under a new superfamily Retiolitoidea to emphasize their evolutionary affinities and their uniqueness, and to clearly separate them from the other diplograptoideans. A similar approach was advocated by Štorch and Serpagli (1993).

The previous retiolitids classification on two subfamilies Retiolitinae Lapworth, 1873 and Plectograptinae Bouèek and Münch, 1952, based on stratigraphical criterion (Bouèek and Münch, 1952) or two kinds micro-ornamentation on the lists (Lenz and Melchin, 1987). The retiolitid classification proposed herein modifies the approach of Kozlowska-Dawidziuk (2001) by formally elevating each of her lineages to subfamily status. This classification, which better reflects the evolutionary history of the group, IS generally based on synapomorphies earlier used for informal lineages.

Ten subfamilies are recognized: Retiolitinae Lapworth, 1873, Plectograptinae Bouèek and Münch, 1952, Gothograptinae Obut and Zaslavskaya, 1983, with seven new: Cometograptinae, Neogothograptinae, Paraplectograptinae, Pseudoretiolitinae, Rotaretiolitinae, Spinograptinae, and Sokolovograptinae (Figure 2).

Order Graptoloidea Lapworth, 1875

Suborder Virgellina Fortey and Cooper, 1986

Superfamily Retiolitoidea new

Family Petalolithidae Bulman, 1955

Family Retiolitidae Lapworth, 1873

Subfamily Pseudoretiolitinae new

Subfamily Rotaretiolitinae new

Subfamily Retiolitinae Lapworth, 1873

Subfamily Paraplectograptinae new

Subfamily Sokolovograptinae new

Subfamily Gothograptinae Obut and Zaslavskaya, 1983

Subfamily Cometograptinae new

Subfamily Neogothograptinae new

Subfamily Spinograptinae new

Subfamily Plectograptinae Bouèek and Münch, 1952

A virgellar ancora and/or distal ancora development structures as a primordial features are important for phylogenetic studies.

Ancora structures are homologues and characteristic only for petalolithids (Pattern I) and retiolitids (Pattern R).

Petalolithids are thought to be A paraphyletic group with ancorate petalolithids as possible retiolitid ancestors; together they are sister groups.

New classification of ancorate diplograptids with new superfamily Retiolitoidea and two families Petalolithidae and Retiolitidae better reflects their phylogenetic history.

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Received: February 15, 2003

Accepted: June 15, 2003