Mimagoniatites Fecundus

The basic structure of the ammonitella or embryonic shell of the Ammonoidea has been well documented (Sandberger, 1851; Branco, 1879-1880, 1880-1881; Schindewolf, 1933; Erben, 1960). The ammonitella begins with a small egg-shaped or spherical initial chamber (=protoconch) (for discussion of terms see Schindewolf, 1933; House, 1996; and Landman et al., 1996; see Landman et al., 1996 for additional references) and extends as a straight shaft or coiled tube (called the ammonitella coil by House, 1996: 168). In most species the ammonitella extends approximately one whorl ending in a primary constriction (Landman et al., 1996; Klofak et al., 1999). Internally, the siphuncle originates in the initial chamber as a rounded caecum and is attached to the shell wall by a prosiphon (Landman, 1988: Figs. 1, 2 and Klofak et al., 1999: Figs. 1, 2).

N. H. Landman et al. (eds.), Cephalopods Present andfhst: NewInsights andFresh Perspectives, 15-56. © 2007 Springer.

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Fig. 2.1 Ammonitellas in the families Mimagoniatidae and Agoniatitidae. (A, B) Archanarcestes obesus (AMNH 45370, Devonian, Morocco). (A) Lateral view of ammonitella. The initial chamber (1C) is visible and the end of the ammonitella is marked by breakage of the shell (arrow). Scale bar = 1.00mm. (B) Close-up of transverse lirae on the initial chamber 1C showing well-developed lirae with "wrinkle-like" creases stretched perpendicular between them. Area of photograph is marked by a box in 1A. Scale bar = 50.0 ßm. (C) Archanarcestes obesus (AMNH45374, Devonian, Morocco). Lateral view of ammonitella showing the initial chamber 1C and primary constriction (arrow). Scale bar = 200ßm. (D) Agoniatites vanuxemi (NYSM 3545, Devonian, New York State). Lateral view ofammonitella. Scale bar = 200ßm. (E, F) Fidelites fidelis (AMNH 50417, Devonian, Morocco). (E) Lateral view of ammonitella showing the flattening on the ventral side of the ammonitella. Scale bar = 1.00mm. (F) Close-up oftransverse lirae showingfaint, perpendicular "wrinkle-like " creases (arrows) between them. Area of close-up is indicated by a box on 1E. Scale bar = 300ßm.

Fig. 2.2 Mimagoniatites fecundus (AMNH 46645, Devonian, Morocco). (A) Lateral view of ammonitella showing initial chamber (IC), the apertural edge of ammonitella (arrow), and about one-half whorl of postembryonic shell (PE). Scale bar = 1.00mm. (B) Close-up of aperture (arrow) showing the reduction in size and spacing of transverse lirae. Postembryonic shell is to the right. Scale bar = 500mm. (C) Close-up from B, rotated approximately 45° to the right, showing the apertural edge of the ammonitella (arrow). The postembryonic shell can be seen emerging from beneath the ammonitella edge on the right. Scale bar = 50.0mm. (D) Close-up of the apertural edge of the ammonitella from C (arrow). Scale bar = 20.0mm. (E) Close-up of the transverse lirae on the initial chamber IC showing the "wrinkle-like " creases stretched perpendicularly between them (arrows). Area of photograph is indicated by the small box on the initial chamber IC in 2A. Scale bar = 50.0mm. (F) Close-up of the postembryonic shell showing a healed break in the shell (arrow) which disrupted the production of the ornament. Area of close-up is indicated by box on the postembryonic shell in 2A. Scale bar = 300mm.

Fig. 2.2 Mimagoniatites fecundus (AMNH 46645, Devonian, Morocco). (A) Lateral view of ammonitella showing initial chamber (IC), the apertural edge of ammonitella (arrow), and about one-half whorl of postembryonic shell (PE). Scale bar = 1.00mm. (B) Close-up of aperture (arrow) showing the reduction in size and spacing of transverse lirae. Postembryonic shell is to the right. Scale bar = 500mm. (C) Close-up from B, rotated approximately 45° to the right, showing the apertural edge of the ammonitella (arrow). The postembryonic shell can be seen emerging from beneath the ammonitella edge on the right. Scale bar = 50.0mm. (D) Close-up of the apertural edge of the ammonitella from C (arrow). Scale bar = 20.0mm. (E) Close-up of the transverse lirae on the initial chamber IC showing the "wrinkle-like " creases stretched perpendicularly between them (arrows). Area of photograph is indicated by the small box on the initial chamber IC in 2A. Scale bar = 50.0mm. (F) Close-up of the postembryonic shell showing a healed break in the shell (arrow) which disrupted the production of the ornament. Area of close-up is indicated by box on the postembryonic shell in 2A. Scale bar = 300mm.

Most studies on ammonitellas are based on well-preserved Mesozoic specimens and it has been inferred that at least some of the same features could be extrapolated to the earliest Devonian forms (Schimansky, 1954; Erben et al., 1968; Kulicki, 1974, 1979; Drushschits et al., 1977; Bandel, 1982; Tanabe, 1989; Landman et al., 1996). The significance of these features and their implication for embryonic development has, however, been debated (see Klofak et al., 1999). Studies on Devonian taxa have demonstrated that many of the features defined for Mesozoic forms are present in their Devonian predecessors, for example, the prosiphon, caecum, and primary constriction (Klofak et al., 1999). There are critical differences, such as the ornament on the shell. In the advanced Paleozoic and Mesozoic ammonoids, the surface of the embryonic shell is either smooth or covered with fine tubercles (Tanabe et al., 1994; Landman et al., 1996; Sprey, 2002). In Devonian taxa the ammonitella is covered by transverse lirae (Clarke, 1899; Miller, 1938; Erben, 1960, 1964b; Clausen, 1969; House, 1996; Klofak et al., 1999). Another important difference occurs at the end of the ammonitella. In post-Devonian ammonoids the primary (or nepionic) constriction is accompanied by a thickening of the shell wall called the primary varix. Studies of well-preserved material have shown that this varix is absent in Devonian ammonoids (Klofak et al., 1999).

These differences have fueled much of the debate as to how the ammonitellas of Devonian ammonoids formed. Most models for post-Devonian ammonitellas call for a nonaccretionary mode of growth because these ammonitellas do not possess any ornament that might be interpreted as having formed by marginal accretion at the aperture. Devonian ammonoids, however, possess transverse lirae, which has suggested to some authors that these embryonic shells formed in an accretionary way (Erben, 1964b, 1966; Erben et al., 1968; Tanabe, 1989). The embryonic lirae are structurally different from those found on the postembryonic shell, however, which might suggest that the embryonic and postembryonic shell formed differently (Klofak et al., 1999). A similar situation has been described for Jurassic ammonites (Kulicki, 1974, 1979; Sprey, 2002) and Triassic ceratites (Landman et al., 2001). The authors in all of these studies describe tuberculate micro-ornament on both the embryonic and postembryonic shell. The morphology of the embryonic and postembryonic tubercles is different in each case, and, hence, it is likely that the two parts of the shell formed differently. Ultimately, the solution may lie in finding and examining shell microstructure, something we have not yet been able to do for Devonian ammonoids.

Previous descriptions of Devonian ammonitellas have been incorporated in numerous taxonomic descriptions (Miller, 1938; Erben, 1953, 1960, 1964b; Petter, 1959; House, 1962; Bogoslovsky, 1969; Chlupac and Turek, 1983; Bensaid, 1974; Goddertz, 1987, 1989; Wissner and Norris, 1991; Klug, 2001). Commonly, the size (diameter) and shape of the initial chamber and the degree of coiling of the ammo-nitella are given. Descriptions of lirae have generally been limited to noting their presence, largely due to the lack of well-preserved material. The exception is Erben (1964b) who described the pattern of the lirae in several Devonian taxa.

Our study entails an examination and quantification of the lirae of well-preserved ammonitellas from three families of Devonian ammonoids: the Mimagoniatitidae, the Anarcestidae, and the Agoniatitidae. The three families are all closely related phylogenetically, albeit of different taxonomic rank. Most workers have placed them within the same higher taxonomic group whether it be the superfamily Agoniatitacea (Miller, 1938), superfamily Anarcestaceae (Miller and Furnish, 1954; Petter, 1959; Erben, 1964b), suborder Agoniatitina (Bogoslovsky, 1969; House, 1981), order Anarcestida (Becker and House, 1994), or order Agoniatitida (Ruzhentsev, 1960, 1974; Becker and Kullmann, 1996; Korn, 2001; Korn and Klug, 2002). In most previously proposed phylogenies, the Mimagoniatitidae are viewed as ancestral to both the Agoniatitidae and the Anarcestidae, but generally more closely related to the Agoniatitidae. For example, within his superfamily Agoniatitacea, Miller (1938) placed the Mimagoniatitinae and Agoniatitinae in the family Agoniatitidae and the Anarcestinae in their own family, the Anarcestidae. More recently, the mimagoniatids and agoniatids were placed within the suborder Agoniatitina and the anarcestids were placed within the Anarcestina, both within the order Agoniatitida (Becker and Kullmann, 1996; Korn, 2001).

It is hoped that a study of the pattern of lirae spacing on the ammonitella may reveal data useful in defining both how the individual lirae as well as the ammo-nitella as a whole formed. By including data from three related families, the pattern of lirae spacing can then be compared among taxa. Differences that emerge may define useful taxonomic characters. They may also aid in our understanding of the early radiation of the Ammonoidea.

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