126


Journal of Paleontology 91(1):116–145


Remarks.—See Melchin et al. (2011) for a full discussion of this taxon and the family Retiolitidae and subfamily Retioli- tinae, and their phylogenetic relationships, as well as descrip- tions of the characteristic synapomorphies of each of these taxa. Maletz (2014, fig. 17) appears to have accepted the


phylogeny proposed by Melchin et al. (2011) but not the classification that Melchin et al. derived from their phylogeny. Instead, Maletz stated that he prefers restricting both the superfamily Retiolitoidea and the family Retiolitidae to those taxa traditionally regarded as “retiolitids”, but did not provide any explanation or evidence that this preferred classification (Maletz 2014, fig. 17B) better reflects the phylogeny of these groups of Silurian biserial graptolites. In addition, he noted that the solution is preliminary and “might have to be revised in the light of the interpretation of characteristic homologous features in some Petalolithinae.” Melchin et al. (2011), however, had already taken into account the homologous features shared by some petalolithines and retiolitines and itwas partly on this basis that the Retiolitidea was expanded to include the petalolithines. The results in Melchin et al. (2011) suggest that, aside from the stem-family Normalograptidae, all Silurian graptolites belong to two major sister clades, which they named the Monograptoidea and the Retiolitoidea. We believe that this essential aspect of the phylogeny of Silurian graptolites should be reflected in the taxonomy and that the sister clade to the Monograptoidea should have a formal taxon name, which is not the case in the classification of Maletz (2014). As noted by Melchin et al. (2011), the rules of nomenclatural priority require that the sister superfamily to the Monograptoidea be named Retiolitoidea, despite the fact that it includes taxa that are not traditionally regarded as “retiolitids” (just as the Monograptoidea contains some biserial and uni-biserial species). Therefore, as noted above we follow the family-level classification proposed by Melchin et al. (2011).


Family Retiolitidae Lapworth, 1873


Definition.—The first species that acquired a unistipular biserial tubarium in the clade that contains Paramplexograptus mader-


nii and Retiolites geinitzianus and all of its descendants (Node 3, figs. 2 and 3 in Melchin et al., 2011).


Remarks.—This definition is slightly amended from that of Melchin et al. (2011) in that in the original definition the word “aseptate” was included in parentheses after the word “unistipular.” This has been omitted here to clarify the point that “aseptate” and “unistipular” are not directly synonymous. Some of the Silurian biserial taxa included in this clade by Melchin et al. (2011) possess a narrow partial median septum on the obverse


side, but the pattern of thecal development still indicates that there was no dicalyic and there was only a single stipe of alternately budding thecae. The earliest known member of this clade, Para- mplexograptusmadernii, is both unistipular and aseptate (Melchin et al., 2011), as are all known members of the Retiolitinae.


Subfamily Retiolitinae Lapworth, 1873


Definition.—The first ancora-bearing graptolite specieswithin the clade that includes Retiolites geinitzianus that acquired thecae constructed of a full framework of lists and reduced or absent fusellar walls, and all of its descendants (Melchin et al., 2011).


Genus Pseudoretiolites Bouček and Münch, 1944


Type species.—Retiolities perlatus Nicholson, 1868, by original designation.


Diagnosis (emended from Bouček and Münch, 1944).—Ancora umbrella deep, with up to five whorls of spiral lists; prosicula and part of metasicula commonly preserved. Nema attached to thecal framework by connecting rods. Mid-ventral thecal lists


present throughout, connecting distally to zigzag ventral thecal floors. Thecae with straight, outward-inclined ventral walls. Ancora sleeve reticulum composed of well-developed mesh- work covering entire lateral walls. Stomata present. List sur- faces smooth to weakly striated.


Remarks.—The most characteristic features of species of


this genus that are likely to be recognizable in well-preserved flattened specimens are the deep, spiral ancora and the zigzag lists of the apertural regions of the thecal ventral walls. All of the species of this genus known from isolated specimens possess stomata, although since the stomata do not possess well-developed rims, they cannot normally be seen in flattened material. Thus, stomata have not been described in specimens of Pseudoretiolites rete Richter, 1853, or Pseudoretiolites dentatus Bouček and Münch, 1944, but their otherwise very close similarity to P. decurtatus and P. perlatus suggests that they are likely to possess stomata as well. Pseudoretiolites rete and P. dentatus have never been recognized outside of Germany or the Prague Basin region, due either to poor preservation of the type material (potentially rendering the taxa unrecognizable elsewhere) or, possibly, biogeographical restrictions.


Pseudoretiolites perlatus (Nicholson, 1868) Figure 8.1–8.8


1868 1908


Retiolites perlatus Nicholson, p. 530, pl. 19, figs. 21, 22.


Retiolites (Gladiolites) perlatus; Elles and Wood, p. 338, pl. 34, figs. 10a–e (f?), text: figs. 221a–c.


Figure 9. Scanning electron microscopy images of Pseudoretiolites decurtatus Bouček and Münch, 1944 and Pseudoretiolites decurtatus Bouček and Münch, 1944?. (1, 2, 4, 5, 7, 8) Pseudoretiolites decurtatus Bouček and Münch, 1944: (1) GSC137619, mature specimen lacking proximal-most portion, note distal tapering of tubarium, unknown location on Cornwallis Island, L. convolutus Zone?; (2) GSC 137620, stereopair of nearly complete specimen with eight pairs of thecae, note thickened, spiralled cortical(?) tissue covering nema, section MRC02, R. orbitus Subzone or L. convolutus Zone; (4) GSC137625, immature specimen, enlargement of thecal floor retaining some fusellar bandaging, section ML64-85-2, R. orbitus Subzone; (5) GSC137618, stereopair of immature specimen, showing completely preserved prosicula with fine longitudinal rods, unknown location on Cornwallis Island, ?L. convolutus Zone; (7) GSC137622, stereopair of large specimen with nine pairs of thecae and four stomata, section MRC02, L. convolutus Zone; (8) GSC137623, stereopair of ventro-lateral view of specimen, with seven thecal pairs, at least one collar-like stomatal rim and well-developed prosicula, section ML64-85 #2, R. orbitus Subzone. (3, 6) Pseudoretiolites decurtatus Bouček and Münch, 1944?, 3, GSC85761, incomplete specimen with nine thecal pairs, prosicula preserved, but ancora umbrella missing, (3) whole specimen, (6) enlargement of sicula showing complete prosicula and some metasicular fusellae, sectionML64-85-2, upper R. orbitus Subzone.


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