Melchin et al.—Llandovery retiolitine graptolites from Arctic Canada
Remarks.—This taxon is represented by only two distal specimens, which have an unusual combination of features: zigzag ventral thecal floors as is typical of Pseudoretiolites; but long, paired spines on the margins of the thecal rims, typical of Pseudoplegmatograptus. The absence of information about the ancora umbrella, however, means it cannot assigned with confidence to either genus. Since the specimens possess thecal apertural spines, which our phylogenetic analysis indicates is an apomorphic trait for Pseudoplegmatograptus, we have questionably assigned them to that genus. The reticulum consists of moderately sized polygonal meshes, interrupted by ovate, raised stomata. Tubarium width 4mm exclusive of spines; 2 TRD 1.9–1.7mm (10.5–11.8 in 10mm).
Genus Rotaretiolites Bates and Kirk, 1992
Type species.—Rotaretiolites exutus Bates and Kirk, 1992, p. 56, pls.V–VI, figs. 54–119, 251, 252, by original designation.
Diagnosis (emended from Bates and Kirk, 1992).—Sicula represented by prosicular apertural ring. Ancora umbrella simple, with four radial ribs terminating in subcircular rim. Reverse zigzag list extends from prosicular ring. Nema attached to connecting rods. Thecae consist of everted apertural loops, dorsal transverse rods and mid-ventral lists arising from center of transverse rods of previous thecae. Mid-ventral lists inclined and slightly convex outward. Mid-ventral list of theca 11 attached directly to ancora umbrella rim, that of 12 attached to zigzag list close to prosicular ring. Ancora umbrella rim joined to mid-ventral list of 12 by unseamed list. Ancora sleeve not developed. Bandages linearly and coarsely striated.
Rotaretiolites exutus Bates and Kirk, 1992 Figure 12.6
[non] Retiolites sensu lato sp. Hutt, Rickards and Skevington, 1970 p. 7, pl. 1, figs. 19, 20.
1992 Rotaretiolites exutus Bates and Kirk, p. 56, pls. V– VI, figs. 54–119, 251, 252.
2001 Rotaretiolites exutus; Kozłowska-Dawidziuk and Lenz, fig. 3.11.
Materials.—A single, well-preserved, proximal end of an immature specimen.
Occurrence.—Snowblind Creek, Cornwallis Island. From S. guerichi or S. turriculatus Zone. This species is thus far known only fromsouthern Sweden and Arctic Canada, and occurs in the lower Telychian (S. guerichi and/or S. turriculatus Zone).
Remarks.—Although the present specimen is immature, it agrees closely with the holotype, the morphology of which was described in detail by Bates and Kirk (1992). Specimens of this species possess a very shallow, saucer-like, wheel-shaped ancora umbrella in which there are four radial lists joined to the virgella. The very simple thecal apertural lists are declined at 35o–45o. The mid-ventral list of theca 11 is attached directly to the ancora umbrella rim, whereas those of more distal thecae are attached to transverse rods. Pleural lists and lateral apertural
135
rods are absent. A robust prosicular ring marks the apertural region of the prosicula. List micro-morphology is of distinctive, coarse, parallel ridges (see Bates and Kirk, 1990, figs. 72, 73, 112, 113, 115, 117), a feature that appears to be unique to specimens of this species and those we have assigned to Rotaretiolites cf. exutus. The Arctic specimen differs from the type only in that the former is somewhat more flattened. The specimen of “Retiolites sensu lato sp.” illustrated in
Hutt et al. (1970, pl. 1, figs. 19, 20), considered by Bates and Kirk (1992) to belong to R. exutus, is assigned here to Rotaretiolites cf. exutus (see below). Bates and Kirk (1992) interpreted the ancora of
Rotaretiolites exutus to have a spiral mode of construction based on the pattern of fringes of increments in the seams of the ancora lists (see their fig. 81). None of the described specimens of this species, however, show continuous spiral lists in the ancora constructed of cortical bandages, as is seen in species of Pseudoretiolites (e.g., Fig. 8.3) and Aeroretiolites cancellatus (Fig. 14.3).
Rotaretiolites cf. exutus Bates and Kirk, 1992 Figure 12.1, 12.2, 12.5, 12.7
1970 Retiolites sensu lato sp. Hutt et al., p. 7, pl. 1, figs. 19, 20. 2014 Rotaretiolites sp. Maletz, p. 519, fig. 24E.
Materials.—Three moderately well-preserved but incomplete specimens, the longest with two thecal pairs; one specimen with incomplete ancora.
Occurrence.—Snowblind Creek, Cornwallis Island, from S. guerichi Zone.
Description.—Tubariumwidth 1.2–1.5mm. 2TRD estimated to be ~1.4mm (~14 in 10 mm). Length of prosicular interval 0.45mm, length of metasicular region and virgella 0.5mm. Prosicular ring present (Fig. 12.2, 12.7). Ancora umbrella only partially preserved in one specimen. Spines less than 1mm long present on mid-ventral lists and on lateral thecal rims (Figs. 12.1, 12.2, 12.7). Prominent parallel striae on all lists.
Remarks.—The Arctic specimens are similar in both dimen- sions and form of the thecal framework to the specimens assigned to Retiolites sensu lato sp. by Hutt et al. (1970) and the
specimen of Rotaretiolites sp. illustrated by Maletz (2014, fig. 24E). As with the present material, the specimen illustrated by Hutt et al. (1970, pl. 7, fig. 19) shows well-developed spines on both the mid-ventral lists and lateral apertural margins. The specimen illustrated by Maletz (2014, fig. 24E) also clearly shows spine bases on the mid-ventral lists of the first thecal pair and also has what may be a small, curved spine on the reverse, lateral margin of theca 11.We therefore consider that all of these specimens likely to belong to the same species. The specimens of Hutt et al. and Maletz show that this species has a simple, small ancora with four radial lists, and the incomplete ancora seen in our material (Fig. 12.7) is consistent with this. These specimens have most of the diagnostic characteristics of R. exutus, including similar width and thecal spacing, but differ in the presence of the spines on mid-ventral lists and on lateral
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172 |
Page 173 |
Page 174 |
Page 175 |
Page 176 |
Page 177 |
Page 178 |
Page 179 |
Page 180 |
Page 181 |
Page 182 |
Page 183 |
Page 184 |
Page 185 |
Page 186 |
Page 187 |
Page 188 |
Page 189 |
Page 190 |
Page 191 |
Page 192 |
Page 193 |
Page 194 |
Page 195 |
Page 196 |
Page 197 |
Page 198 |
Page 199 |
Page 200 |
Page 201 |
Page 202 |
Page 203 |
Page 204 |
Page 205 |
Page 206 |
Page 207 |
Page 208
