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1
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Llandoverian graptolite zonation in the northern Canadian Cordillera

100%
Lenz A. C.
Acta Palaeontologica Polonica
|
1979
|
tom 24
|
nr 1
A number of well exposed and structurally simple, graptolite sections, ranging in age from latest Ordovician through Wenlockian strata are present in the northern Canadian Cordillera. Recent more detailed sampling permits much finer zonation, than previously possible. The following lower and middle Llandoverian zones (following the zonal scheme of Hutt 1975) are recognized or tentatively recognized: acuminatus Zone, atavus Zone, acinaces Zone, gregarius Zone, triangulatus Zone, magnus Zone?, argenteus Zone, convotutus Zone, sedgwicki Zone, and turricutatus Zone. The crispus, griestoniensis and crenulata Zones are only tentatively and indirectly recognizable; instead, expanded and stratigraphically thick spiralis and Cyrtograptus sakmaricus - C. laqueus Zones appear to represent biofacies equivalents. The latter zone is correlated with the grandis and lapworthi Zone of some writers. The earliest Wenlockian centrifugus Zone may be present in a single section. The lowest Llandoverian persculptus Zone is only tentatively recognized, but there appears to be no evidence for a stratigraphic break between the Ordovician and Silurian. Lower and middle Llandoverian zones are thin, whereas those of the upper Llandoverian are considerably thicker, a fact probably related to the temporal duration of the zones.
2
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New and unusual upper Llandovery graptolites from Arctic Canada

63%
Lenz A. C., Kozlowska A.
Acta Palaeontologica Polonica
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2007
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tom 52
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nr 3
Three isolated and well−preserved retiolitid taxa, two of them new, and one taxon of unknown affinity, are described from the upper Llandovery of Arctic Canada. All taxa display unusual characteristics The three retiolitids display extraordinary morphological features: Pseudoplegmatograptus cf. obesus preserves very delicate and lacey or cobweb−like list structures external to the main skeletal lists, structures previously seen only rarely on flattened material; Pileograptus pileatus gen. et sp. nov. possesses a thecal framework typical of retiolitines, particularly Stomatograptus, but its thecal orifices possess broad, meshwork genicular hoods similar to those in the plectograptines; and Giganteograptus giganteus, formerly attributed to Pseudoplegmatograptus, with its very coarse and relatively simple skeletal framework, well−developed paired thecal apertural spines, and a pustulose micro−ornamentation on the lists. The latter feature suggests an assignment to the subfamily Plectograptinae, rather than to the Retiolitinae as has been previously presumed. The presence of pustulose lists in Giganteograptus adds a further complication to understanding of retiolitid evolution. Mirorgraptus arcticus gen. et sp. nov., the fourth taxon and of unknown systematic affinity, lacks the proximal end, but preserves lists with seams and some fusellar material suggestive of some type of new retiolitid. Unlike known retiolitids or other graptolites, however, the skeletal list development is apparently confined to a narrow region along one side of the nema.
3
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Convergent evolution of two Silurian graptolites

63%
Lenz A. C., Melchin M. J.
Acta Palaeontologica Polonica
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2008
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tom 53
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nr 3
The two graptolite speciesCochlograptus veles(Telychian, Upper Llandovery) and Testograptus testis(lower Homerian, upper Wenlock) are remarkably and uniquely similar in being strongly ventrally and planispirally coiled and in demonstrating an abrupt deflection in the immediate post−sicular regions of their rhabdosomes. The two species, however, are separated by a relatively large biostratigraphic gap and a global mass extinction, and they differ morphologically in the proportion of thecal overlap, different angles of inclination of the interthecal septa, relative proportions of the widths occupied by the free metathecae, the position of the sharp dorsal flexure relative to the tip of the sicula, and the presence of a distinctive, keel−like structure on T. testis. It is suggested therefore, that in spite of the strong proximal morphological parallelism between the two species, their origin is best explained as a remarkable example of convergent evolution. It is suggested that C. veles perhaps evolved from some modified monograptid such as Stimulograptus, whereas the small Testograptus group may have derived from some monograptid such as Monograptus flemingii. Cladistic analysis fully supports the independent derivation the two species. If correct, this hypothesis supports the validity of separate generic names for the two species, despite the close and unique rhabdosomal similarities, including proximal metathecal form.
4
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Silurian retiolitid graptolites: Morphology and evolution

51%
Bates D. E. B., Kozlowska A., Lenz A. C.
Acta Palaeontologica Polonica
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2005
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tom 50
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nr 4
705-720
Developmental mode and proximal structures are commonly accepted as the best for the recognition of high−level taxonomic categories within the Graptoloidea. The petalolithids and retiolitids are unique in possessing a virgellar ancora and in the latter, distal ancora development. The ancora structures are considered homologous, and the ancorate petalolithids are considered to be the direct ancestors to the retiolitids. The Retiolitidae are unique among the diplograptoids in possessing (1) outer, lateral, ancora sleeve walls (derived from distal extension of the ancora), and (2) a skeletal framework of bandaged lists between which are a succession of very thin and rarely preserved fusellar layers. Retiolitids possess different kinds of thecal profiles and two types of micro−ornamentation on the lists, and these have served to distinguish between the subfamilies Retiolitinae and Plectograptinae. Complete retiolitid morphological terminology is clarified and explained. Cladistic analysis of the retiolitids provides some measure of a better understanding of retiolitid evolution, but adds only modest support for the retention of the two subfamily categories.
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