Journal of Paleontology, 91(5), 2017, p. 960–967 Copyright © 2017, The Paleontological Society 0022-3360/17/0088-0906 doi: 10.1017/jpa.2017.29
Limulitella tejraensis, a new species of limulid (Chelicerata, Xiphosura) from the Middle Triassic of southern Tunisia (Saharan Platform)
Błażej Błażejowski,1 Grzegorz Niedźwiedzki,2 Kamel Boukhalfa,3 and Mohamed Soussi4
1Institute of Paleobiology, Polish Academy of Sciences, Twarda 51/55, 00-818 Warsaw, Poland ⟨
bblazej@twarda.pan.pl⟩ 2Department of Organismal Biology, Evolutionary Biology Center, Uppsala University, Norbyvägen 18A, 752 36 Uppsala, Sweden ⟨grzegorz.
niedzwiedzki@ebc.uu.se⟩ 3University of Gabes, Faculty of Sciences, City Riadh Zerig 6029 Gabes, Tunisia ⟨
boukhalfakamel@yahoo.fr⟩ 4University of Tunis El Manar, Faculty of Sciences, Department of Geology , C.P. 2092 Tunis, Tunisia ⟨
mohsou@yahoo.fr⟩
Abstract.—Numerous well-preserved remains of a new limulid species from the Anisian-lower Ladinian (Middle Triassic) of the Tejra section of southern Tunisia are described. Comparisons are made with limulids from the Triassic deposits of Europe and Australia. The new specimens are congeneric with the type species of Limulitella, but show some morphological differences. Here we describe Limulitella tejraensis new species, a small limulid with semicircular prosoma, small and triangular opisthosoma, well-defined axial ridge, and pleurae along both ridges of the opisthosoma. The Tunisian Limulitella fossils are associated with conchostracans, bivalves, gastropods, and microconchids. Sedimentological and paleontological data from the Tejra section suggest freshwater to brackish- water conditions during the formation of the fossil-bearing interval and the influence of marine transgression into a playa-like environment. Supposed adaptation to the stressful environment sheds new light on the origin and survival of the extant limulines. This is the first report of limulid body fossils from the Triassic of North Africa and the first documentation of Limulitella in the Middle Triassic of northern Gondwanaland.
Introduction
The chelicerate order Xiphosurida, generally known by the colloquial misnomer ‘horseshoe crabs,’ is among one of the rarest of invertebrate taxa, mostly owing to their unmineralized exoskeleton and predilection for marginal environments that are so rare in the stratigraphic record (Babcock et al., 2000; Love- land and Botton, 2015; Lamsdell, 2016). Thus, the discovery of Middle Triassic horseshoe crab material adds significantly to our understanding of the group. Xiphosurida have existed for ~480 Myr (Lamsdell, 2013, 2016), with the earliest unequi- vocal representatives found in the Upper Ordovician of
Manitoba, Canada (Rudkin et al., 2008), apparently preceded by finds of putative Xiphosurida from the Lower Ordovician of Morocco (Van Roy et al., 2010). Only four species of horseshoe crabs exist today, all of which are members of Limulidae Zittel, 1885 (= Mesolimulidae Størmer, 1952) and characterized by their large crescentic prosomal shield and the fusion of the opisthosomal tergites (Lamsdell and McKenzie, 2015). The recently published study of the Xiphosura (equivalent to total group Xiphosurida) phylogeny (Lamsdell, 2016) has interesting implications. First of all, they suggested that horseshoe crabs have independently invaded the non-marine realm at least five times during their long evolution. Secondly, horseshoe crabs have had rather a complex evolutionary history not only in the Paleozoic, but also in the Mesozoic.
The aim of this paper is to describe a new species of
horseshoe crab, Limulitella tejraensis n. sp., from the Middle Triassic of southern Tunisia (Saharan Platform). Limulitella represents the most basal representative of the family Limulidae (Lamsdell, 2016), which is a clade of Triassic to Recent horseshoe crabs that exhibits no vestige of segmentation dor- sally in the opisthosoma (Riek and Gill, 1971), and the axis of the thoracetron bears a dorsal keel (Lamsdell, 2016).
Geologic setting
Triassic deposits of southern Tunisia crop out widely in the Jeffara domain (Fig. 1.1, 1.2). The main outcrops are exposed along a NW-SE trending belt from Jebel Tebaga of Medenine, which includes the Tejra outcrops, to the Libyan border. The Triassic succession is very thick (>2000 m), especially in the Jeffara Basin, and is dominated in its lower part by red-beds, which usually lie unconformably on upper Permian rocks (Busson, 1967; Bouaziz, 1986; Kilani-Mazraoui et al., 1990; Kamoun et al., 1998, 1999, 2001; Dridi and Maazaoui, 2003). The Tunisian Triassic paleogeographic evolution results from the disassembly of Pangaea in the early Mesozoic, when the North African Platform became a broad Tethyan-facing passive continental margin (Soussi et al., 1998, 2001; Kamoun et al., 2001; Bouaziz et al., 2002; Courel et al., 2003).
960
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 |
Page 209 |
Page 210 |
Page 211 |
Page 212 |
Page 213 |
Page 214 |
Page 215 |
Page 216 |
Page 217 |
Page 218 |
Page 219 |
Page 220 |
Page 221 |
Page 222 |
Page 223 |
Page 224 |
Page 225 |
Page 226 |
Page 227 |
Page 228 |
Page 229 |
Page 230 |
Page 231 |
Page 232 |
Page 233 |
Page 234 |
Page 235 |
Page 236 |
Page 237 |
Page 238
