External Features of the Ammonitella

Ornament for all taxa consists of transverse lirae that cover the entire ammonitella except for a "bald spot" at the apex (Klofak et al., 1999). "Wrinkle-like" creases stretch between the lirae and are best defined on the initial chamber. These "wrinklelike" creases are most strongly expressed in the Mimagoniatitidae (Figs. 2.1b, 2.2e), less strongly expressed in the Anarcestidae (Fig. 2.3b), and least strongly expressed in the Agoniatitidae (Fig. 2.1f).

The lirae are a relief feature and are not preserved on the steinkern as shown in Latanarcestes where the shell is broken away on part of the initial chamber (Fig. 2.3a, c, d; see also Klofak et al., 1999: Figs. 3c, 8b). The lirae on the embryonic shell differ from those on the postembryonic shell in that the embryonic lirae are symmetrical, while the postembryonic lirae are asymmetrical and more steeply sloped on the apical side (Klofak et al., 1999).

Just adapical of the primary constriction, the lirae become much weaker and more closely spaced (Figs. 2.2b, 2.3d, 2.4b). The edge of the ammonitella is marked by a primary constriction, with no evidence of an accompanying varix in any speci-

Fig. 2.3 Latanarcestes sp. (AMNH 46646, Devonian, Morocco). (A) Lateral view of ammo-nitella showing initial chamber (IC) and part of apertural edge of ammonitella (arrow). Scale bar = 1.00mm. (B) Close-up of transverse lirae on initial chamber IC showing perpendicular "wrinkle-like" creases (arrows) stretched between them. Area of close-up is indicated by a box on 3A. Scale bar = 50.0mm. (C) Close-up near the end of the initial chamber IC where shell has broken away exposing the steinkern. Note that the surface of the steinkern is smooth, unaffected by the ornament on the shell. Scale bar = 500mm. (D) Close-up of ammonitella showing the apertural edge of the ammonitella (arrow). Scale bar = 80.0mm. (E) Close-up of the apertural edge of the ammonitella (arrow) showing the postembryonic shell emerging from beneath it and followed by a series of breaks (b) in the postembryonic shell, with new shell emerging from beneath the existing shell. Area of close-up marked by letter e in 3D. Scale bar = 50.0mm. (F) Close-up of smooth area of postembryonic shell to the right of3E showing a series of breaks (b) with new shell emerging from beneath the existing shell. Area of close-up is marked by letter f in 3D. Scale bar = 50.0mm.

Fig. 2.3 Latanarcestes sp. (AMNH 46646, Devonian, Morocco). (A) Lateral view of ammo-nitella showing initial chamber (IC) and part of apertural edge of ammonitella (arrow). Scale bar = 1.00mm. (B) Close-up of transverse lirae on initial chamber IC showing perpendicular "wrinkle-like" creases (arrows) stretched between them. Area of close-up is indicated by a box on 3A. Scale bar = 50.0mm. (C) Close-up near the end of the initial chamber IC where shell has broken away exposing the steinkern. Note that the surface of the steinkern is smooth, unaffected by the ornament on the shell. Scale bar = 500mm. (D) Close-up of ammonitella showing the apertural edge of the ammonitella (arrow). Scale bar = 80.0mm. (E) Close-up of the apertural edge of the ammonitella (arrow) showing the postembryonic shell emerging from beneath it and followed by a series of breaks (b) in the postembryonic shell, with new shell emerging from beneath the existing shell. Area of close-up marked by letter e in 3D. Scale bar = 50.0mm. (F) Close-up of smooth area of postembryonic shell to the right of3E showing a series of breaks (b) with new shell emerging from beneath the existing shell. Area of close-up is marked by letter f in 3D. Scale bar = 50.0mm.

Fig. 2.4 Latanarcestes sp. (AMNH50416, Devonian, Morocco). (A) Lateral view of ammonitella showing initial chamber (IC), with visible constriction (arrow). Scale bar = 1.00mm. (B) Close-up of apertural edge of ammonitella (arrow). Constriction is visible on dorsal edge (c). Scale bar = 300mm.

men examined (Figs. 2.2b, c, d, 2.3d, e, f, 2.4b). In previously studied specimens, the apertural edge of the ammonitella appears as a crease or break (Fig. 2.1a, c; see also Klofak et al., 1999). A perfectly preserved aperture is present in a specimen of Archanarcestes obesus (Fig. 2.2b, c, d) where the apertural edge parallels the ornament and forms a distinct line across the flank, slightly irregular in appearance. The postembryonic shell can be seen emerging from beneath the embryonic shell (Fig. 2.2c, d). This occurs in the area of smaller, finer lirae. In Latanarcestes sp., the area of small faint lirae is more extensive when compared to the specimens of Archanarcestes (Figs. 2.3d, 2.4b; see also Klofak et al., 1999: Fig. 8d). The apertural edge also appears in this area and, like Archanarcestes, is irregular in its course. Multiple breaks occur adoral of the apertural edge with successive shell layers emerging from beneath the previously secreted shell (Fig. 2.3e, f).

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