Laibl et al.—Post-embryonic development in Ellipsostrenua 92(6):1018–1027
granulation on the glabella, fixigenae, and thoracic tergites (in E. gripi and E. linnarssoni these parts of exoskeleton are gen- erally pitted); (2) longer genal spines; and (3) more distinct eye ridges (see Ahlberg 1984). Protaspides that were tentatively assigned to E. granulosa
by Cederström et al. (2009, pl. 4, figs. 8–10) most likely belong to Calodiscus lobatus because they closely resemble protaspides of the latter species.
Post-embryonic development of Ellipsostrenua granulosa
Post-embryonic development in trilobites is traditionally divi- ded into protaspid, meraspid, and holaspid periods based on the trunk articulation (for a comprehensive review seeWhittington, 1957; Chatterton and Speyer, 1997; Hughes et al., 2006). We follow this division herein, although we are aware that these periods probably do not represent corresponding life phases among all trilobite taxa (for a detailed discussion, see Chatterton and Speyer, 1997; Park and Choi, 2011).
Protaspid period.—Among protaspides of E. granulosa it is possible to recognize early and late protaspid stages that are distinguished mainly by the presence of a distinct trunk portion in the late protaspid stage. Based on cranidial length and width measurements (Fig. 3.1), we cannot exclude the possibility that the two smallest specimens (marked as P1? in Fig. 3.1) may belong to an even earlier protaspid stage. However, the com- parison of the cranidial length and intergenal spine separation (Fig. 3.2) shows slightly more distinct clusters. Because the intergenal spine separation undoubtedly reflects the width of the trunk more precisely than the cranidial width, we believe it is a more reliable indicator of the natural instars (provided that the development of the trunk is the most dynamic pattern during early development in all trilobites; see also Webster, 2007, for comparable results in Nephrolenellus from the uppermost Dyeran Stage of the Great Basin U.S.A.). Therefore, a two stage division of the protaspid period in E. granulosa is employed herein.
The early protaspid stage (Figs. 4.1–4.11, 6.1, 6.2) is
sub-circular to sub-elliptical in outline and gently vaulted in lateral view. The protocranidium is 0.54–0.60mm long and 0.60–0.75mm wide. The axial furrows are sub-parallel and diverge anteriorly around a distinct frontal axial lobe (LA). Well-developed anterior pits are present lateral to LA. The axis of the early protaspid stage has five morphologically expressed segments, designated as LA–LO. In some specimens there is a shallow medial depression in the anterior margin of LA. The anterior border protrudes forward to form a very short preglabellar field. The proximal parts of the eye ridges originate behind the anterior pits and are connected to the posterior part of LA. Distally, the eye ridges are terminated by well-developed palpebral lobes that reach approximately to the exoskeletal mid- length. Minute anterior fixigenal spines are developed near the posterior tip of each palpebral lobe. The fixigenae are sub-oval with a slightly swollen anterior part. The bacullae are distinct and situated lateral to SO. The posterior border furrow extends from the bacullae, curves forward abaxially, and reaches the
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posterior tip of the palpebral lobe. A pair of postero-ventrally directed intergenal spines is developed on the posterior margin of the exoskeleton. The late protaspid stage (Figs. 4.12–4.16, 6.3, 6.4) is
similar to the early protaspid stage with the following exceptions: the protocranidium is 0.60–0.69mm long and 0.72–0.86mm wide; the palpebral lobes are slightly longer (exsag.); the glabella is clearly divided into LA–LO. The most striking difference is, however, the presence of a trunk, which occupies the space between the intergenal spines. The trunk bears one pair of short postero-ventrally directed pleural spines.
Meraspid period.—The sclerites assigned to early meraspid stages of E. granulosa comprise early meraspid cranidia and pygidia.
Early meraspid cranidia (Figs. 5.1–5.5, 6.5) are sub- elliptical to sub-rectangular in outline, 0.81–0.91mm long,
cranidial width (mm)
intergenal spine separation (mm)
Figure 3. Plots of Ellipsostrenua granulosa (Ahlberg, 1984): (1) scatter plot of cranidial width versus cranidial length of protocranidia and cranidia of Ellipsostrenua granulosa (Ahlberg, 1984); (2) scatter plot of intergenal spine separation versus cranidial length of protocranidia and cranidia of Ellipsostrenua granulosa (Ahlberg, 1984). Abbreviations: M, meraspid stages; P1, early protaspid stage; P2, late protaspid stage.
cranidial length (mm)
cranidial length (mm)
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