Journal of Paleontology, 92(6), 2018, p. 1018–1027 Copyright © 2018, The Paleontological Society 0022-3360/15/0088-0906 doi: 10.1017/jpa.2018.25


Early post-embryonic development in Ellipsostrenua (Trilobita, Cambrian, Sweden) and the developmental patterns in Ellipsocephaloidea


Lukáš Laibl,1,2 Peter Cederström,3 and Per Ahlberg4


1National Museum Prague, Department of Paleontology, Cirkusová 1740, 193 00 Praha 9, Czech Republic ⟨lukaslaibl@gmail.com⟩ 2The Czech Academy of Sciences, Institute of Geology, Rozvojová 269, 165 00 Praha 6, Czech Republic ⟨lukaslaibl@gmail.com⟩ 3Axelvoldsvägen 27, SE-241 35 Eslöv, Sweden ⟨peter.cederstrom@telia.com⟩ 4Department of Geology, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden ⟨per.ahlberg@geol.lu.se⟩


Abstract.—This study documents the early post-embryonic developmental stages (protaspides and early meraspides) of the Cambrian trilobite Ellipsostrenua granulosa (Ahlberg, 1984) from the Gärdsjön Formation of Jämtland, Sweden. The early protaspid stage is characterized by a circular outline of the exoskeleton, two pairs of fixigenal spines, a short preglabellar field, a genal swelling, and prominent bacullae. The late protaspid stage differs only in having the trunk portion discernible. Early meraspid cranidia are sub-rectangular with prominent palpebral lobes, a wide anterior margin, a proportionally long anterior branch of the facial suture, and intergenal spines. Meraspid pygidia tentatively assigned to this species possess comparatively long macrospines. Small hypostomes associated with E. granulosa bear at least four pairs of marginal spines. A comparison of the early developmental stages of E. granulosa with some other species of Ellipsocephalidae and with species of the closely related Estaingiidae reveals several similarities. The conservative morphology of the early protaspid stage with only two pairs of fixigenal spines, the timing of the development of the trunk portion, and the presence of genal swellings and prominent bacullae could be phylogenetically informative. The range of size variation of the early protaspid stages in two families may be related either to taxonomical differences between Ellipsocephalidae and Estaingiidae, or to environmental differences in various paleogeographic settings.


Introduction


The Ellipsocephaloidea Matthew, 1887 (sensu Adrain, 2011) comprises morphologically diverse trilobites that were major faunal components of the Cambrian Epoch 2 and 3 ecosystems (e.g., Suvorova, 1956; Ahlberg and Bergström, 1978; Zhang et al., 1980; Geyer, 1990; Jell, 1990; Westrop and Landing, 2000; Żylińska and Masiak, 2007; Elicki and Geyer, 2013; Høyberget et al., 2015). The classification and phylogeny of this group is still not fully understood, and attempts to find its unique synapomorphies have not yet been successful (Geyer, 1990). This is partially caused by insufficient knowledge of post- embryonic development in many members of this superfamily. Developmental data are important for resolving phylogenetic


relationships at various taxonomic levels (Edgecombe et al., 1988; Fortey and Chatterton, 1988; Chatterton et al., 1990; Campbell and Chatterton, 2009) and for identifying homologous structures (Chatterton, 1971; Mabee, 2000). Detailed studies of post- embryonic development have the potential to be fundamental for a phylogenetic analysis of ellipsocephaloid trilobites, but our cur- rent knowledge of development in this group is based on a few immature specimens (e.g., Suvorova, 1956; Yi, 1988; Geyer, 1990; Geyer and Landing, 2004) or comparatively late develop- mental stages only (Cederström et al., 2011; Laibl et al., 2015). More comprehensive data are available only for Ellipsostrenua gripi (Kautsky, 1945), Ichangia ichangensis Chang, 1957, Strenuaeva? spinosa Ahlberg and Bergström, 1978, and Estaingia


sinensis (Chang, 1953) (see Kautsky, 1945; Zhang and Pratt, 1999; Cederström et al., 2012; Dai and Zhang, 2012; respectively). In this contribution we provide the first detailed description


of the early post-embryonic development (protaspid and early meraspid stages) in Ellipsostrenua granulosa (Ahlberg, 1984). Consequently, we compare the development of E. granulosa with some othermembers of EllipsocephalidaeMatthew, 1887 andwith members of the closely related Estaingiidae Öpik, 1975 in order to elucidate the developmental patterns in Ellipsocephaloidea.


Geological setting


The material was collected by Peter Cederström from an ~1m thick, gray calcareous mudstone at the top of the Gärdsjön Formation (Cambrian Series 2, Stage 4) in the Fånån area, south of Lake Sörsjön, northern Jämtland, Sweden (Fig. 1.1; see also Cederström et al., 2009 for details). Hundreds of sclerites were collected at two localities (localities 1 and 2 in Fig. 1.2). The majority of sclerites are from the eastern bank of the Fånån rivulet, ~300m south of Sörsjön (locality 1). This locality is situated ~300m north of the trilobite-bearing locality (locality 3) described by Asklund and Thorslund (1935) andAhlberg (1984). The second locality is a gravel pit situated 130meast of locality 1. All trilobite sclerites from the Fånån area are generally slightly distorted tectonically. Associated trilobites include Calodiscus lobatus (Hall, 1847); Luvsanodiscus cf. gammatus Korobov, 1980; Ellipsostrenua gripi (Kautsky, 1945); E. linnarssoni


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