classification of ancorate diplograptids
Alfred C. Lenz2
and Denis E.B. Bates3
Kozlowska-Dawidziuk, Instytut Paleobiologii PAN, ul. Twarda 51/55, PL-00-818
Warszawa, Poland. E–mail: firstname.lastname@example.org
Alfred C. Lenz email@example.com, Department of Earth Sciences, University
of Western Ontario, London, Ontario N6A 5B7, Canada. E–mail: firstname.lastname@example.org
3 Denis E. B. Bates, Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, Ceredigon SY23 3DB, UK. E–mail: email@example.com
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 rhabdosome
s (Bates and Kirk, 1997;
Kozlowska-Dawidziuk, 2001; Lenz and Kozlowska-Dawidziuk, 2001).
A new ancorate diplograptid classification
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 Bouček and Münch, 1952, based on stratigraphical criterion (Bouček 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 Bouček 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 Bouček and Münch, 1952
Figure 2. Stratigraphical ranges of new subfamilies of the new family Retiolitoidea. The dotted (uncertain) portion of the paraplectgraptine range relates to the inclusion of some informally identified taxa referred to in Lenz and Melchin (1997). The biozonal scheme is that used in the Generalized Graptolite Zonation of Koren’ et al. (1995).
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.
Bates, D.E.B. 1990. Retiolite nomenclature and relationships. Journal of the Geological Society, 147: 717-723.
Bates, D.E.B. and Kirk, N.H. 1987. The role of extrathecal tissue in the construction and functioning of some Ordovician and Silurian retiolitid graptolites. Bulletin of the Geological Society of Denmark, 35: 85-102.
Bates, D.E.B. and Kirk, N.H. 1992. The ultrastructure, mode and functioning of a number of Llandovery ancorate and retiolitid graptolites. Modern Geology, 17: 1-270.
Bates, D.E.B. and Kirk, N.H. 1997. The ultrastructure, mode of construction and functioning of the genera Stomatograptus and Retiolites, with an appendix on the incremental construction of the rhabdosome of Petalolithus, and its comparison with that of the thecal framework in Retiolites and Stomatograptus. Institute of Geography and Earth Sciences, University of Wales, Aberystwyth,49 – 53 10: 1-168.
Bouček, B. and Münch, A.1944. Die Retiolitien des mitteleuropäischen Llandovery und unteren Wenlock. Mitteilungen der tschechischen Akademie der Wissenschaften Jahrgang 51(41): 1-53.
Bulman, O.M.B. 1970. Graptolithina. In: C. Teichert (ed.), Treatise on Invertebrate Paleontology, Pt. V, second edition, Geological Society of America and University of Kansas Press.
Koren’, T.N., Lenz, A.C., Loydell, D.K., Melchin, M. J., Štorch, P., and Teller, L. 1995. Generalized graptolite zonal sequence defining Silurian time intervals for global paleogeographic studies, Lethaia, 29: 59-60.
Koren’ T.N. and Rickards R.B. 1996. Taxonomy and evolution of Llandovery biserial graptoloids from the Southern Urals, western Kazakhstan. Special Papers in Palaeontology, 54: 1-103.
Kozlowska-Dawidziuk, A. 1995. Silurian retiolitids of the East European platform. Acta Palaeontologica Polonica, 40: 261-326.
Kozlowska-Dawidziuk, A. 2001. Phylogenetic relationships within the Retiolitidae (Graptolithina) and a new Cometograptus genus. Lethaia, 34: 84-96.
Kozlowska-Dawidziuk, A. 2002. Agastograptus, a synonym of Plectograptus (Retiolitidae, Graptolithina). Acta Palaeontologica Polonica, 47: 459-467.
Lenz, A.C. 1993. Late Wenlock and Ludlow (Silurian) Plectograptinae (retiolitid graptolites), Cape Phillips Formation, Arctic Canada. Bulletins of American Paleontology, 104: 1-52.
Lenz, A.C. 1994. Uppermost Wenlock and lower Ludlow plectograptine graptolites, Arctic Islands, Canada: new isolated material. Journal of Paleontology, 68: 851-860.
Lenz, A.C. and Kozlowska-Dawidziuk, A. 2001. Upper Wenlock (Silurian) graptolites of Arctic Canada: pre-extinction lundgreni Biozone fauna. Palaeontographica Canadiana, 20: 1-61.
Lenz, A.C. and Melchin, M.J. 1987. Silurian retiolitids from the Cape Phillips Formation, Arctic Islands, Canada. Bulletin of the Geological Society of Denmark, 35: 161-170.
Lenz, A.C. and Melchin, M.J. 1997. Phylogenetic analysis of the Silurian Retiolitidae. Lethaia, 29: 301-309.
Melchin, M.J. 1998. Morphology and phylogeny of some Early Silurian ‘diplograptid’ genera from Cornwallis Island, Arctic Canada. Palaeontology, 41: 263-315.
Melchin, M.J. 1999. Origin of the Retiolitidae: insights from a new graptolite genus from the Early Silurian of Arctic Canada. Lethaia, 32: 261-269.
Obut, A.M. and Zaslavskaya, N. [Zaslavskaâ, N.] 1976. New data on the early stages of Retiolitidae develompent. [in Russian]. In: Kalio, D. and Koren’, T.N. (eds.), Graptolity i Stratigrafiâ, Izdatelstvo Nauka, Moskva,154-162.
Štorch P. and Serpagli, E.1993. Lower Silurian Graptolites from Southwestern Sardinia. Bolletino della Societŕ Paleontologica Italiana, 32: 3-57.
Received: February 15, 2003
Accepted: June 15, 2003