122


Journal of Paleontology 91(1):116–145 crispus


turriculatus guerichi sedgwickii convolutus


orbitus triangul. /pectin.


cyphus


Figure 6. Proposed phylogenetic tree based on cladogram depicted in Figure 5 and known taxonomic ranges of the taxa. Thick solid lines are ranges observed in Arctic Canada, thin solid lines are additional ranges observed in other regions. Dashed lines indicate proposed phylogenetic relationships. Note that Pseudoretiolites decurtatus was not included in the cladistics analysis because its coding was identical to that of P. perlatus. The two are therefore regarded as a sister species. See text for further discussion. Note that Pseudorthograptus and Hercograptus are members of the Petalolithinae of Melchin et al. (2011), the remaining taxa are assigned to the Retiolitinae.


Implications for family-level taxonomy


The present study brings into sharp focus the problemof the status of family-level subdivision within the “retiolitids”.Beginning with Bouček and Münch (1952), the retiolitids were divided into the subfamilies Retiolitinae (older) and Plectograptinae (younger) (see also Bulman, 1970), based mainly on the chronostratigraphic position of the known taxa and their overall morphological complexity. With the advent of electron microscopic studies of isolated material, Lenz and Melchin (1987a) continued with the twofold division, but focussed on the type of list micro-ornament on bandages; i.e., parallel striae for the former and pustules for the latter. Lenz andMelchin (1997) published the first cladistic study of all the then-known retiolitid genera, and proposed, based on phylogenetic branching relationships, that the Plectograptinae should be expanded to include some taxawith parallel striae on the lists rather than pustules. Unfortunately, those authors did not revise the diagnosis of the Plectograptinae to reflect this revised


concept of the taxon.Maletz (2014) hasmaintained the taxonomic distinction between the Retiolitinae and Plectograptinae based on their list ornamentation. The present cladistic study, focussing on only Llandovery


taxa (Fig. 5.1), arrives at essentially the same clade relationships as proposed by Lenz andMelchin (1997) and demonstrates that the simple twofold division into the subfamilies Retiolitinae and Plectograptinae, based on list ornamentation, is not consistentwith the phylogenetic relationships among the species. In particular, including all and only those specieswith non-pustulose lists in the Retiolitinae results in a taxon that contains the stem taxon Pseudoretiolites, the clade that includes Pseudoplegmatograptus, Stomatograptus,and Retiolites, and part (but not all) of the clade that includes Aeroretiolites, Rotaretiolites, Paraplectograptus, and Sokolovograptus (pustulose list ornamentation appears at node 13 in Fig. 5.1 and is only seen in Paraplectograptus eiseli and S. textor in our study). This is not a good reflection of the phylogeny of this group of genera. We propose therefore, that


Figure 7. Scanning electron microscopy images of Pseudoretiolites cf. tianbaensis (Ge, 1990): (1, 2) GSC137626, (1) stereopair of mature specimen with at least six thecae and three stomata, (2) enlargement showing partially preserved ancora umbrella with three or four spiral whorls, section LL1B, S. guerichi or S. turriculatus Zone; (3) GSC137627, stereopair of partially preserved specimen without ancora umbrella, LL1-77, S. guerichi Zone?; (4, 5) GSC137628, (4) proximal region, showing well-preserved umbrella with four spiral whorls, (5) enlargement of ventral wall of theca 11, showing robust thecal lip (a) and ‘pseudolip’ (b), section MSC07, 3m above 01G, S. guerichi Zone?; (6) GSC137631, proximal specimen with partially preserved umbrella with four (?) spiral whorls and two pairs of thecae, section SJF02, 1C, S. guerichi Zone?; (7, 8) GSC137630, (7) enlargement showing thecal lips underlain by a looped pseudolip (arrows), (8) stereopair of latero-ventral view of specimen with five pairs of thecae, showing robust, zigzag thecal ventral walls underlain by a somewhat lacy pseudolip, section MSC07, 3m above 01G, S. guerichi Zone?.


?


? ?


Rhud.


Aeronian curtus


Telychian


Pseudorthograptus


Hercograptus introversus Pseudoretiolites? sp.


Pseudoretiolites decurtatus Pseudoretiolites perlatus Pseudoretiolities hyrichus


Pseudoplegmatograptus obesus Aeroretiolites cancellatus


Stomatograptus Retiolites


Aeroretiolites? sp. Rotaretiolites cf. exutus Eorograptus spirifer


Rotaretiolites exutus


Paraplectograptus? sp. Paraplectograptus


Sokolovograptus


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